Endogenous digitalis-like factors.

Clinical and Experimental Hypertension. Part A: Theory and Practice

ISSN: 0730-0077 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/iceh19

Endogenous Digitalis-Like Factors Wilhelm Schoner To cite this article: Wilhelm Schoner (1992) Endogenous Digitalis-Like Factors, Clinical and Experimental Hypertension. Part A: Theory and Practice, 14:5, 767-814, DOI: 10.3109/10641969209036220 To link to this article: http://dx.doi.org/10.3109/10641969209036220

Published online: 03 Jul 2009.

Submit your article to this journal

Article views: 6

View related articles

Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=iceh19 Download by: [Australian National University]

Date: 14 March 2016, At: 08:29

CLIN. AND EXPER. HYPER.-THEORY

AND PRACTICE, A14(5), 767-814 (1992)

ENDOGENOUS DIGITALIS-LIKE FACTORS

Downloaded by [Australian National University] at 08:29 14 March 2016

Wilhelm Schoner Institut fur Biochemie und Endokrinologie, Fachbereich Veterinarmedizin, Justus-Liebig-Universitat GieBen, Frankfurter Str. 100, D-6300 Giessen (Federal Republic of Germany) Tel. 0641/702-4840

Key words: Endogenous digitalis-like factor, ouabain, digoxin, cardiac glycosides, natriuretic factor, essential hypertension.

ABSTRACT The postulate of a natriuretic factor inhibiting the sodium pump in the kidney led to the detection of increased concentrations of endogenous digitalis-like factors in blood after salt loading, in essential hypertension, in pregnancy-induced hypertension and in chronic hypervolaemia. The recent isolation of ouabain or a close isomer thereof from human plasma and the demonstration of a compound similar if not identical to digoxin in adrenals and human urine shows that mammals like non-vertebrates and toads may synthesize cardiac glycosides in their adrenals and possibly in hypothalamus. The hypothalamus also forms other compounds of unknown structure which bind to the cardiac ylycoside receptor site. The differential functions of endogenously formed ouabain and of a digoxin-like substance are unclear. The detailed knowledge of the physiological role of both endogenously formed cardiac glycosides in the regulation of blood pressure has still to be worked out.

767 Copyright 0 1992 by Marcel Dekker, Inc.

768

SCHONER


BACKGROUND

Since the early 1960s increasing evidence has accumulated supporting the existence of a natriuretic hormone [ l l . Subsequently, in studies with salt-induced hypertension, Dahl [ 2 1 and Haddy & Overbeck [ 3 1 accumulated evidence that a circulating, low molecular weight inhibitor may exist which inhibits the sodium pump like an endogenous digitalis. It has therefore been termed endogenous digitalis-like factor (EDLF). This concept gained further support by the recent isolation of pure [ 4 1 or almost pure substances 1 5 1 with cardiac glycoside-like action. When in 1785 Withering first described the therapeutic action of extracts of foxglove against dropsy in great detail, the diuretic efficacy was, in his opiof prominent interest. Later on it was found nion, that the main therapeutic principle of these extracts is the increase in heart contractility which gave the present denomination for the group of steroid derivatives as cardiac glycosides. Besides their inotropic action on the heart, the increase of peripheral resistance by vasoconstriction and the natriuretic and diuretic action - especially of the water soluble ouabain - is well documented 1 6 1 . Digitalis also affects the cardiovascular system via the nervous system at therapeutic doses [ 7 1 . Cardiac glycosides in therapeutic use are mostly of plant origin, where they are probably synthesized via progesterone as an intermediate in a biosynthetic pathway [ 8 1 thus far unknown; Cardiac glycosides, however, are also synthesized in some beetles of the Chrysemelid species [ 9 1 and probably also in toads E l 0 1 to protect these animals against predators.



The biosynthetic pathway in these animals has not been investigated.

769

yet

Cardiac glycosides act by binding to the specific receptor site of Na+/K+-ATPase and thus inhibit the sodium pump. The receptor site for this drug has been conserved in all Na+/K+-ATPases detected so far, but their affinity varies widely. One may wonder why a specific receptor site for a toxin from a plant or an animal, whose occupancy may kill the individuum, is part of the sodium pump and is conserved over long distances of evolution. The advantage conferred by the stability of this site over the millennia must indicate some specific purpose. The question thus arises whether an endogenous cardiac glycoside-like factor as a circulating inhibitor of the sodium pump may exist which binds with the same or an higher affinity to it.

CARDIAC GLYCOSIDES OF VERTEBRATES

The occurence of cardiac glycosides in non-vertebrates and vertebrates raises the question whether they may only fulfill the needs of a protecting agent against predators or whether they may have additional physiological functions. It is well known that toads synthesize bufotoxin, a member of the family of bufadienolides, in their parotis glands and skin [lo]. The skin is the major organ of the regulation of sodium and water homeostasis in this species [111 and rich in When toads are subjected to NaCl Na+/K+-ATPase t121. adaptation the concentration of EDLF increased gradually with the salt concentration in skin, decreased in brain but remained unchanged in plasma 1131. A bufadie-


170

SCHONER

nolide, mono-hydroxy-14,15-epoxy-20,22-dienolide (resibufogenin) is found in the plasma of toads [141. Positive inotropic effects were seen by this compound, but unfortunately such studies have not been performed with the toad heart [151. Therefore the question of whether resibufogenin acts in the toad as an endogenous cardiac glycoside under physiological conditions comes up, especially as it is known that the sodium pump of toad hearts are rather insensitive to cardiac glycosides [161. Interestingly, bufalin (another cardiac glycoside of the toad) increases the systemic arterial blood pressure in the dog, but no natriuresis or diuresis was observed [171.

ENDOGENOUS DIGITALIS-LIKE FACTORS IN MAMMALS Much effort has been made to show the existence of endogenous digitalis-like substances in mammals. It is evident now that in normal subjects acute volume expansion or salt loading leads to the production of an inhibitor of the sodium pump detectable in serum and urine, which may contribute to reduced renal sodium reabsorption and to the re-establishment of the sodium balance [18,191. The demonstration of elevated intracellular sodium and abnormal Na+/K+-ATPase activity in patients with essential hypertension and the relationship between salt intake and and essential hypertension led to the theory [3,191 that increased concentrations of a natriuretic hormone may exist in patients with essential hypertension. The importance of this hypothesis is that inhibition of the sodium pump may theoretically lead to an increase in vascular tone and thus to hypertension. This could result from depolari-



771

sation of vascular smooth muscle subsequent to inhibition of the sodium transport, leading to an influx of Ca2+ through voltage-dependent Ca2+-channels, or alternatively, by activation of the Na+-Caz+-exchanger resulting from the inhibition of the sodium pump 1201. This hypothesis gained further credence when it was found that a circulating inhibitor exists in blood, whose concentration correlates with the blood pressure [21-241. Additionally a sodium pump inhibitor has been found in increased concentrations in forms of chronic hypervolemia, in pregnancy-induced hypertension, in neonates, in chronic renal failure and hepatic cirrhosis [1,3,18,22-291. However, in spite of enormous efforts and the demonstration of inhibitory activity in fractions of serum, urine and a variety of tissues from normotensives and hypertensive animals and humans, the chemical nature of these substances has only recently started to emerge.

How t o measure endogenous d i g i t a l i s - l i k e f a c t o r s ? It became obvious while attempting to purify EDLF that the type of assays used to follow the purification procedure is of critical significance. 4 Types of assays have been used to detect endogenous digitalis-like activity: (a) biological assays in measuring natriuresis in rats, toad bladder short currency, inotropy, vascular reactivity; (b) inhibition of the sodium pump or ouabain binding to human red and white blood cells, smooth muscle and heart ventricular cells, MDCK and LLC-PK1-cells; (c) inhibition of Na+/K+-ATPase activity and [3Hlouabain binding with purified Na+/K+-ATPase; (d) displacement of cardiac glycosides from digoxin and


772

SCHONER

ouabain antibodies. It turned out that measuring of inhibition of the sodium pump in intact cells or of 13Hlouabain displacement from its receptor site on intact red blood cells is the most specific assay to detect endogenous digitalis-like acting factors on a cellular level 130,311. Antibodies against ouabain or digoxin are also useful to look for substances with structures similar or identical to these substances and to measure their concentrations [51, provided care is taken to avoid interference by other impurities in the assay mixture. Considering all these types of assays, the tests with isolated Na+/K'-ATPase are the easiest to perform. However, since the isolated enzyme is susceptible at its cytosolic site to a great number of substances, all the results worked out with this assay have to be controlled carefully by other methods, using at least one assay with intact cells. Assays with complex biological systems like those mentioned above in a) are full of pitfalls as pointed out by Hamlyn [321. Such assays should be used to test isolated, pure substances. They are misleading in the search for a purification procedure of EDLF. In addition, a proper physiological concentration range has to be kept in mind. Many people working in the field did not consider this aspect carefully enough. Therefore substances like saturated and unsaturated fatty acids 133-351, a variety of phospholipids [341, bile salts 1361, lignans 1371 ascorbic acid [38,391, dehydroepiandosterone sulfate [401, dopamine 1411 and hemin [421 have been isolated which do not behave digitalis-like.



Concentrations in blood:

of endogenous digitalis-like

773

factorfs)

The therapeutic concentration of digoxin in plasma giving a favourable hemodynamic effect is in the range of 1 - 2 nM, but higher concentrations have also been reported. Toxic concentrations range from 3 . 3 - 5.6 nM and above [ 4 3 1 . By analogy one would anticipate that the concentrations of an endogenous digitalis will be at least in the therapeutic range or even lower. Estimations of the plasma concentrations of EDLF in normotensive man are in the range of 2 0 - 100 pM ouabain equivalents, when biological assays like the inhibition of e6Rb+-uptake into human red blood cells or the competition with [JHlouabain for the cardiac glycoside receptor site of the sodium pump of these cells are used 1 4 4 , 4 5 1 . This assay measures any substance competing with the cardiac glycoside binding site at the outer cell surface which inhibits the sodium pump (Table 1). Competition with cardiac glycosides in immunoassays, if carefully controlled, measures substances similar to the compound against which the antibody was raised. Most antibodies to digoxin cross-react poorly with ouabain, for instance. Using the crossreactivity in such immunoassays, concentrations of digitalis-like and ouabain-like immunoreactive substances have been found in almost the same concentration range (Table 2 , 3 ) . The digoxin-like immunoreactivity was found in a concentration range of 5 0 - 720 pM (Table 2 ) [ 5 0 - 5 2 1 . The concentrations of ouabain-like immunoreactivity are in the range of 100 141 or 6 0 0 1100 pM 1 4 8 , 4 9 1 (Table 3 ) . The differences between in concentrations of ouabain-like immunoreactive material reported by the two groups have been ascribed to diffe-

2 0.5 2 27.4

B

A

A

B

B

Method used

ouabain equivalents (picomoles/ day)

1000

80 - 100

40 - 80 40 2 20 ( 5 ) 320 2 5 0 ( 5 )

NaCl treatment of man €or 5 days: control ( 5 1 mMol NaCl/day) 1.8 NaCl Diet ( 3 4 2 mMol/day) 69.2

MESUREMENTS IN URINE:

Human plasma: control after intravenous infusion of 0.9% NaCl

Dog plasma

Human, bovine and rat plasma: control volume expansion

MEASUREMENTS IN PLASMA:

ouabain equivalents (pmoles/liter)

[471

1461

1451

Ref.

The concentrations were either determined by ( A ) inhibition of e6Rb+-uptake into human red blood cells or ( B ) [3Hlouabain displacement from human red blood cells.

Effect of volume expansion and NaC1-treatment on EDLF-concentration.

TABLE 1


U

U

*

138 37 80 36 105 975 t 2 2 2 2 2

43 7 18 12 43 79

*Received 0.9 % NaCl in the drinking water

Hamlyn et al. [ 4 1 : Human (11) Dog ( 6) Rat (11) Rat, adrenalectomized* ( 8 ) Rat, uni-nephrectomy ( 6 ) Rat, DOCA-salt ( 5 )

1060 t 860 2590 2 1 390

300 300

experiment 2: normotensive man ( 7 ) primary aldosteronism ( 7 )

2 2

660 1720

experiment 1 : normotensive man (10) essential hypertension ( 1 6 )

Masugi et al. [48,491:

ouabain equivalents (pmoles/liter)

~~~

113 156

2 2

4 12

87.2 f 3.7 111.9 2 12.2

mean arterial blood pressure (mm Hg)

Ouabain-like immunoreactivity in plasma of different species

TABLE 2


I

w

rn

H

CJ

6 67

lower vena cava adrenal artery adrenal vein

87 f 85 f 195 2

23 5 53

3) 3)

( ( (

3) 3) 3)

(25) (24) (20) (20) (20)

(25) (24)

( (

FPIA, Fluorescence polarisation immunoassay

Dog:

50 f 198 f

pregnant woman 3rd trimester: 250 f 34 normotensive 404 f 37 hypertensive amniotic fluid: normotensive 690 f 38 974 f 6 7 hypertensive 250 f 70 cord serum 720 f 330 160 2 110

Human : -plasma of adult + 0.5 mg ACTH ( 3 0 min)

digoxin equivalents (pmoles/liter)

1511

[501

Reference

RIA, NEN

1501

FPIA, Abbot. Lab [521 RIA, Clin. Assays RIA, Pharmacia

RIA, NEN

RIA, NEN

assay system

Digoxin-like immunoreactivity in plasma of different species

TABLE 3




777

rences in the properties of the antibodies and variations in the extraction procedure 1531. On the one hand an antibody to ouabain scarcely interacts with digoxin, likewise cross-reactivity of digoxin antibodies with ouabain is negligible, but the sodium pump does not discriminate such between ouabain and digoxin (although ouabain is bound with greater affinity). Since the concentrations of the ouabain-like and digoxin-like immunoreactive material are almost the same (Tables 2,31 one might wonder why EDLF concentrations determined with the e6Rb'-uptake assay into human red blood cells are not double in the concentrations of each individual assay. Apparently, the assays used so far do not have yet this discriminatory power. The higher values for EDLF reported earlier with - as we know now - insufficient methods of up to 80 nM or greater [24,54-571 are probably only relevant as an internal scale for the comparison of changes in the concentrations of the endogenous digitalis-like substance under physiological and pathological conditions. Using the more specific assays with intact human red blood cells, Hamlyn 1461, when measuring e6Rb+-uptake, found that volume expansion in man increased the concentrations of endogenous digitalis-like factor almost 10-fold from 80 pM to 900 pM. By measuring [3Hlouabain displacement high-salt diet from human red blood cells after a lasting 5 days, Goto et al. 1471 found a 38-fold increase of the urinary excretion of a polar digitalislike compound from 1.8 t 0.5 to 69.2 f 27.4 pmol ouabain equivalents per day [471 (Table 1). Using immunological cross-reactivity with ouabain antibodies, Masugi et al. [48,491 found 10 - 3 0 times higher values of endogenous ouabain-like immunoreacting substance.

778


This

SCHONER

parameter was increased 3-fold

in patients

with

essential hypertension (which is consistent with earlier reports [22,24,56,571) and doubled in patients with primary aldosteronism (Table 2). It is also evident from tables 2, 3 that the adrenal is an important organ for the synthesis of ouabain-like and digoxinlike immunoreactive material and that ACTH seems to control it (Table 3 ) . These data also show that very low concentrations of EDLF circulate and that huge amounts of tissue or body fluids have to be worked up to yet enough material to determine the chemical structure. This may also mean that the proper selection of the source to isolate EDLF is of prime importance to obtain a structure of the substance.

PROPERTIES

OF HIGHLY P U R I F I E D PREPARATIONS OF

DIGITA-

L I S - L I K E FACTORS

The most important progress in the field is the development of reproducible purification schemes that can produce active material near or at homogeneity [4,30,31,45,58-731. This progress is to some extent in part the result of an increased awareness of the limitation of assays using purified Na+/K+-ATPase in the initial steps of purification and the use of assays with intact cells or antibodies to cardiac glycosides. All the compounds with EDLF activity described so far belong to one of the 4 types of substances described below:


779

1. Endogenous cardiac qlycosides:


a. Ouabain-like compound(s) Similar procedures have been used for the isolation of this type of factor from human plasma [30,31,451, hemofiltrate [611, human urine [601, hypothalamus [621, and bovine adrenals [651. These substances elute on a C-18 HPLC reversed phase column around 18% acetonitrile. Recently Hamlyn's group published a series of highly interesting papers on the isolation and characterization of an ouabain-like compound from human plasma [4,30,53,72-741 (Fig. 1): 53 nmol of the substance was purified almost 0.6 billionfold from approximately 300 1 human plasma by several HPLC procedures and an affinity adsorbtion to isolated Na+/K+-ATPase. In 4 different HPLC systems the compound was found to have identical retention times to ouabain or to comigrate. The molecular mass as determined by fast atom bombardment was the same as ouabain, namely 585.296 Da. Derivatisation with acetic acid anhydride of the compound did not reveal any differences to authentic ouabain. But the possibility was not ruled out that the compound might be a closely related isomer of ouabain. Furthermore, the purified compound cross-reacted with antibodies to ouabain. Moreover, bovine adrenocortical cells in culture produced immunoreactive ouabain-like substance and the production was decreased when the cells were depolarized by 40 mM KC1. When the antibodies were used to determine the plasma concentrations in an immunoassay in normotensive men, a concentration of 138 t 43 pMol/l was found. Although ouabain is almost not resorbed from intestine [61 and the exogenous nature of the isolated material unlikely, the idea

SCHONER

7 80


0

ouabain

HO

HO

OH

OH digoxin

Fiqure 1: Structures of ouabain and digoxin a as putative endogenously formed cardiac glycosides.

that ouabain might be taken up via the gut was avoided. Therefore the finding is of interest that four individuals on total parenteral nutrition for 1 week had plasma concentrations of 108 2 4 3 pMol/l. In the rat, adrenals had the highest concentrations of endogenous digitalis (approximately 500-fold higher than in plasma) and plasma levels fell almost by half after 3 days of adrenalectomy. In DOCA salt-treated rats there was a 10-fold rise of endogenous ouabain-like factor in the plasma concomited by an increase of mean arterial blood pressure. It should also be kept in mind, that in patients with primary aldosteronism the level of ouabain-like immunoreactivity and the blood pressure were reduced after removal of adrenal adenomas 1751. In studies on the effects of the purified endogenous oua-


781


bain-like substance and plant-derived ouabain in guinea pig atria and aorta rings, there was no difference in cardiotonic and vasotonic actions; but relatively high concentrations of 80 - 170 nM of the compounds had to be applied. Consistent with the finding of Hamlyn's group we recently succeeded in purifying an ouabain-like compound from 2 0 0 0 g pig adrenals by affinity chromatography on a Sephadex column containing ouabain antibodies and subsequent HPLC chromatographies. The compound behaved like ouabain in 2 different HPLC systems [unpublished work]. Additional more hydrophobic compounds were visible after affinity purification. Moreover, we purified 16 picomoles of a substance inhibiting the sodium pump from 6 0 0 0 1 hemofiltrate and the substance was not identical to ouabain in 2 different HPLC-systems, but slightly more hydrophohic. Its molecular mass determined by mass spectrometry was 582. The substance raised the intracellular calcium in low and high [Ca2+li cells 161,771 like ouabain. It is unclear so far, whether this substance is a degradation product of endogenously formed ouabain or a separate compound. Some of the factors that have been isolated earlier in small amounts in almost pure form by eluting at 18% acetonitrile from a C-18 HPLC column may be identical or very similar: Goto et al. showed that human, rat and bovine plasma contains a which displaces [3Hlouabain from its receptor site at the sodium pump of human red blood cells and that the hypothalamus contains a factor that elutes in 3 different chromatographic systems in the same way as the one derived from plasma and urine t45,621. These findings strongly suggest that the fac-


782

SCHONER

tor in mammalian hypothalamus is identical to the circulating ouabain-like factor [621. A detailed study on the stability and of the conditions for inhibition of the factors revealed: The inhibitors from plasma, hemofiltrate, urine and adrenal are uncharged or slightly anionic [30,61,651. The inhibitors from human plasma and beef adrenals were destroyed under acid and alkaline conditions [30,651, and are not affected by several proteases and phospolipase [60,651. The polar factors from human plasma [301, urine [ G O 1 and bovine adrenal [651 have been shown to be specific inhibitors' of Na+/K+-ATPase, i.e. they do not affect Ca2'-ATPase from sarcoplasmic reticulum. Goto's urine-derived [3Hlouabain displacing activity inhibits the sodium pump in a number of different cell types [78-801. In cultured rat vascular smooth muscle A 10 cells this not completely purified material lowers the Ca2+-efflux, increases the Ca2+-influx and raises the cytosolic free calcium concentration [78,791. Ouabain and bufalin showed no effect on free cytosolic calcium in these cells [791. The urine-derived [3Hlouabain displacing activity may therefore have vasoactive properties. It still has to be clarified how its chemical nature differs from that of the ouabain-like activity.

b. Digoxin-like compound(s) substance cross-reacting with digoxin antibodies and eluting between 33 - 40% acetonitrile from an Octadecyl HPLC column can be detected in a number of body A



783

fluids and tissues [5,Glf69,7O,79,81-831. Cloix's urine-derived factor is considered to be an aminoglycosteroid containing a carboxyl group and 3 methyl groups [811. It inhibited the sodium pump reversibly by binding to the cardiac glycoside binding site of canine renal Na+/K+-ATPase. Binding was observed under type I and type I1 conditions for cardiac glycoside binding. The factor differed, however, in 3 aspects from ouabain: (a) It inhibited to a smaller extent MgZ'-ATPase and Ca2+-ATPase from rabbit brain microsomes; (b) No binding was detected in the presence of Mg2' + ATP. (c) In a bioassay the factor was diuretic and natriuretic and not kaliuratic like ouabain [701. Nothing is known about the affinity of the compound. Goto's group isolated a digitalis-like factor indistinguishable from digoxin from human urine (Fig. 1 ) [831. It behaved like digoxin in 3 HPLC- and 3 thin-layer chromatography systems. The substance which bound to anti-digoxin antibodies and inhibited esRb+ uptake into human red blood cells had a molecular mass in FAB mass spectometry of 781.7 m/z and a proton NMR spectrum similar to digoxin. The authors claim that the plasma levels of this compound should be around 10-50 pM (see also Table 3 ) and therefore 100 times lower than those found in digoxin therapy. The group previously reported that this factor is unable to raise intracellular free calcium concentrations in cultured rat vascular smooth muscle A 1 0 cell [791, but the factor isolated by Dasgupta et al. does so in simian smooth muscle cells [G91. The digitalis-like factor is most probably synthesized in the adrenals 15,841: Valdes' group recently reported the isolation of a compound from bovine adrenal cortex which cross-reacted with digoxin antibodies

784

SCHONER


t51.

The substance, which they believe is similar to digoxin, shows a single peak in the HPLC running slightly more hydrophobic than digoxin. FAB mass spectrometry shows a molecular mass of 780 m/z comprised of one 390 dalton aglycone plus several sugar moieties. The compound shows a maximum in UV absorbance at 215 nm (cardiac glycosides at 220 nM). Using a molecular absorption coefficient of cardiac glycosides, the cross reactivity of the isolated digitalis-like factor was calculated to be 947-fold less than digoxin itself. Hence, the concentration of the digitalis-like factor was estimated to be 1.2 nMol per g adrenal. The cortex contained 7 times more of the substance then the medulla. Serum concentration in humans was 0.6 ng digoxin equivalent/g ( 2 728 nM using the converting factor given above). The concentrations of the compound in different tissues was adrenal > serum > liver > kidney > heart = brain = lung.

2 . The hypothalamic inhibitor

In 1979 Haupert and Sancho described the existence of an inhibitor of the sodium pump in the hypothalamus t851; this report was soon followed by others describing endogenous digitalis-like material in brain and other tissues. The hypothalamic inhibitor has since been purified to a considerable extent t63,64,661 and the data known so far clearly show that it is not identical to the endogenously formed cardiac glycosides reported above. Its chemical nature is unknown. It is acid resistant [63,64,661, unaffected by proteases and phospholipases t64,661 and shows zwitterionic properties t 8 5 1 . There are no reports so far that it circulates in blood plasma.

785



The affinity of the bovine hypothalamic inhibitor for purified Na+/K+-ATPase from dog kidney is 1.4 nM and thus 5 times higher than that of ouabain 1671. In reconstituted liposomes containing renal rabbit Na'/K+ATPase, the factors affinity is even 30 times higher than that of ouabain 1861. Unlike cardiac glycosides, the hypothalamic factor also inhibits Ca2+-ATPase from sarcoplasmic reticulum but not of plasma membrane t 6 6 1 . The inhibitor binds to the outside of cells [ 6 6 1 but, like digoxin, may transverse artificial liposomal membranes 1861. Contrary to cardiac glycosides, the hypothalamic inhibitor shows allosteric properties in inhibiting the sodium pump in LLC-PK1 renal tubular cells 1881; hyperbolic properties would be expected. The factor inhibits the ouabainsupported backwards phosphorylation by stabilizing a E,-type conformation [ 6 6 1 . Since the factor traverses liposomal membranes, it is conceivable that this activity represents a supplementary pathway to increase intracellular Ca2+-concentrations. In fact, studies with beating rat heart cells reveal that the hypothalamic inhibitory factor shows inotropic effects since it increases the force of contraction and intracellular Ca2'-concentrations which exceed those where ouabain shows toxic effects 1871. The hypothalamic factor also leads reversibly to contractitons of vascular rings of the rat aorta 1891.

3 . Derivatives of unsaturated fatty acids

Although not synthesized in mammals, most potent animal toxin from corals

Palytoxin, the of the genus


786

SCHONER

Palythoa with a unique structure resembling a polyhydroxy and unsaturated fatty acid derivative has been shown to inhibit the sodium pump (Fig. 2). It does so at concentrations of 1 pM or less, and inhibits from outside the cells by binding to a site overlapping with the cardiac glycoside binding site. It thereby opens a pore which facilitates the diffusion of small ions which, among others, lead to the contraction of arteries [901. It is interesting in this context that unsaturated and saturated fatty acids have been isolated in the search for EDLF 133-351. But the concentrations needed to inhibit the sodium pump were too high to assume any physiological significance. Schwartzman et al. [911 showed that the arachidonic acid derivative 1 2 ( R ) - h y d r o x y - 5 , 8 , 1 0 , 1 4 - i c o s a t e t r a e n o i c acid [12(R)HETEI, which is synthesized from arachiacid via the cytochrome P450 system in the corneal epithelium, is a very effective inhibitor of Na+/K'-ATPase from beef cornea ( K i = 50 nM). The inhibitory effect is stereospecific for the R-isomer. The half-maximal inhibitory effect was seen at 50 nM. It was speculated that the substance is involved in the regulation of ocular transparency and intraocular pressure [911. In fact, 12(R)HETE applied to the rabbit eye lowers it [92,931 probablyy due to the inhibition of the sodium pump in ciliary epithelium [931. 12(S)HETE, however did neither affect intraocular pressure nor corneal or ciliary Na'/K+-ATPase activity [91,931. Another derivative of arachidonic acid, 11,12-dihydroxy-eicosatrienoic acid, is synthesized by cells of the thick ascending limb of rabbit kidney and inhibits at a concentration of 60-200

OH


OH

dH

palytoxin

OH

HO 11, 13-dihydroxy-14-octadecaenoic acid

12(~)-hydroxy-5,8, 10, 14-eicosatetraenoic acid Figure 2: Structures of palytoxin, 11,13-dihydroxy-14octadecanoid acid and 12(R)-hydroxy-5,8,10,14-icosatetraenoic acid as inhibitors of Na+/K+-ATpase.


7 88

SCHONER

nM half-maximally 1941. Interestingly, 19(S)hydroxyeicosatetraenoic acid is a potent stimulator of renal Na+/K+-ATPase 1951. So far, the function of these arachidoic acid derivatives in the regulation of blood pressure is unclear, especially since derivatives of eicosanoides, which may be found in increased concentrations in hypertension, are considered to be vasodilatators, but there are also opposing views [ 9 6 1 . Lichtstein et al. ( 5 9 1 recently purified 11,13dihydroxy-14-octadecaenoic acid from 1500 liters of bovine plasma (Fig. 2 ) . It inhibited rat brain microsoma1 Na+/K+-ATPase and competed with 13Hlouabain binding to rat brain synaptosomes. It has not been reported whether this compound inhibits the sodium pump in intact cells, at which concentrations and whether the inhibitory effect is reversible. These data need to be shown, however, to verify that the substance indeed works in a digitalis-like manner. Although the present information speaks against a role of such derivatives as circulating regulators of the sodium pump, a possible paracrine function cannot be ruled out.

4 . Low molecular weiqht peptidic inhibitors

3 Groups report that the endogenous digitalis-like factor may be a small peptide [71,97-991. This conclusion is based on the observation that the activity is lost upon incubation with proteases. The low molecular weight inhibitor of the cerebrospinal fluid is specific for the sodium pump, it decreases the affinity to K+ [711. The inhibitor derived from the supernatant of cultured cells of rat hypothalamus consists of 13 amino acids and has a blocked N-terminus. It is bound


789

reversibly to the ouabain binding site and causes vasoconstrictor effects in a dose-dependent way 197,981.


The diqitalis-like and ouabain-like immunoreactive factors and their physiological siqnificance: Using commercial digoxin antibodies false-positive cross-reacting material in body fluids may be detected. Such material tends not to show any linear relationship between endogenous digitalis-like factors and equivalents of digoxin [ l o 0 1 - a parameter often not looked for. The reason for this is unclear and there is so far no agreement in literature how endogenous digitalisimmunoreactive material has to be measured best [for a review see 1001. Gruber et al. [101,1021 were the first to show increased concentrations of a digoxin-like immunoreactive material in the plasma of volume expanded dogs and in hypertensive monkeys. This material was also found in the serum of 5 4 patients with renal impairment who were not on digoxin therapy by Graves et al. [1031. More than 60% of these digoxin-free patients had false positive digoxin values. There are repeated reports that the digoxin-like immunoreactive material is increased in pregancy-induced hypertension and neonatal serum [25,511. But the pathophysiological importance of this observation is unclear: Only in women with proteinuria did the hypertony of pregnancy-induced hypertension correlate with the concentration of an inhibitor of the sodium pump [251. Increased concentrations of immunologically cross-reacting material is also found in amniotic fluid and in the plasma of newborn


790

SCHONER

babies [511. Liu et al. [lo41 report a positive relation of endogenous digoxin-like immunoreactivity with blood pressure, sodium content of red blood cells, plasma and urinary output in hypertensive children. They therefore believe that a circulating inhibitor of the sodium transport may play an important role in the early stage of hypertension evolution. Such an interpretation would be consistent with the knowledge that cardiac glycosides 1171 - especially ouabain - may increase peripheral vascular resistance in anesthetized animals and man (for a review see [61). Recently, Buhler's group reported that ouabain increases vasoconstrictor activity in the forearm of man 11051. Halfmaimal effects were seen at 8 pg ouabain/100 ml tissue ( a 13 nM) wich were not associated with an increase in the regional noradrenalin release. The vasoconstrictor response was greater ( p < 0.05) in patients with mild essential hypertension than in normotensives. On the contrast, digoxin given to man for 4 days produced no significant changes in blood pressure, urinary electrolyte, aldosterone excretions and plasma renin activity but significantly augmented pressure responsiveness to both norepinephrine and angiotensin [1061. It has also been reported that ouabain in nanomolar concentrations has profound counteracting effects on acetylcholineinduced relaxation of human resistance arteries [1511. In conclusion: Ouabain may raise blood pressure. Ouabain has also central hypertensive effects 1107-1091: When infused into the third cerebral ventricle of anesthesized rats consistently an initial bradycardia was seen followed by a progessive increase in blood pressure and heart rate. The hypertension was associated with a rise of potassium in the cerebrospinal fluid. In the free moving rat seizures are observed



791

upon intraventricular injection [107,1101. Preferably the Na'/K'-ATPase of hyothalamus and hippocampus is inhibited under these conditions [1101. Also increased neurotransmitter efflux has been observed under this conditions [111,1121. The pressure response of intracerebroventricularly administered ouabain was abolished by bilateral nephrectomy, adrenalectomy, pretreatment with the B-adrenergic blocker propanolol and the inhibitor of the angiotensin converting enzyme, captopril, the angiotensin I1 antagonist saralasin [1081. Plasma renin and epinephrine levels were found about 2.5-fold elevated and it was concluded that the hypertensive effect does not primarily involve peripheral a-adrenergic receptors but appears to be due to angiontensin I1 produced by renin of renal origin. It was, furthermore, speculated that activation of B receptor of juxtaglomerular cells are involved in the renin release [1081. Since cardiac glycoside-like substances have been found in the brain 1621 and and since it is known that even under therapeutic concentrations of cardiac glycosides neuronal effects are seen [71, the above observations are of considerable interest. The observation of central hypertensive effects of ouabain in rats is consistent with reports that cardiac glycosides may produce hypertensive effects in cats and dogs, but also the opposite has been found [71. Digoxin is long in u s e to treat heart insufficiency, but a rise in blood pressure is not a regularly observed symptom, rather a decrease in arterial and venous tone via effects on the nervous system are reported 171. Provided there is a connection between the level of endogenous digoxin-like immunoreactive material and blood pressure, such a correlation ought to be able to demonstrate by infusion of digoxin anti-


792

SCHONER

bodies and their Flb fragments which are commercially available. There are a number of reports on this topic: Goodlin 11141 reports on a case of a 31-year old woman in the 25.5th week of pregnancy and with severe pregnancy-induced hypertension who was treated with 10 mg Fa, fragments of digoxin antibodies. Within 30 minutes her mean blood pressure had fallen from 140 mm Hg to 80 mm Hg and fell again immediately after infusion 12 hours later. A decrease of blood pressure has also been found in DOCA-salt rat 11151, in rats with chronic coarctation of the aorta and its subsequent hypertension 11161 and in spontaneous hypertensive rats (SHR) 11171 upon injection of digoxin antibodies. But nonspecific actions of the antibodies via complement activation or histamine release cannot be excluded under these conditions. Therefore, Mann et al. 11181 studied the effect of Fa, fragments of digoxin antibodies. They found no alteration in blood pressure, cardiac output and total peripheral resistance in normal and volume expanded SHR, uremic SHR and DOCA-salt hypertensive rats. The Flb fragments, however, prevented in v i v o the increase of total peripheral resistance induced by i.v. injection of digoxin. The authors confirmed that intact digoxin antibodies - bearing the F, domains with complement activating properties - lowered the blood pressure in SHR and in deoxycorticosterone hypertension 11181. When cross circulation was studied between spontaneous hypertensive and normotensive rats, transmission of the hypertension to the normotensive acceptor did not take place when the donor had been pretreated by 0 . 2 5 mg Fa,-fragments of digoxin antibodies 11191. In the one-kidney, one clip hypertensive rat a negative relation between mean arterial pressure and serum digoxin-like immunoreactivity was found by Kobayashi et al. 11201. They therefore suppose that the increased



793

concentration of the digoxin-like immunoreactive material is not a cause of hypertension, but rather a compensatory factor against the development of severe hypertension in one-kidney, one clip hypertensive rat. In summary, no clear cut final conclusion can be drawn so far from the experiments infusing antibodies. Apparently, more properly designed and controlled experiments have to be carried out to get a definitive answer. Takahashi et al. [121,1221 observed that a material crossreacting with digoxin- and ouabain-antibodies is released in the urine by sodium loading of rats and humans. In humans it relates to the blood pressure. This observation is consistent with Masugi's observation of a correlation of blood pressure with the concentrations of substances which crossreact with antibodies against ouabain [48,491. Takahashi et al. [ 1 2 1 1 also tried to localize the site of synthesis in the brain. Immunoreactivity to digoxin antibodies is restricted to the paraventricular und supraoptic nucleus, the areas where water and electrolyte control takes place. Moreover, Goto and coworkers [ 1 2 3 1 and Scott et al. [1241 find that the material cross-reacting with digoxin antibodies responds in neither man, rat nor rabbit to a high salt diet whereas a polar circulating inhibitor of Na+/K+-ATPase (in his human red blood cell competition assay which is most probably endogenously synthesized ouabain) does so [57,1231. This dissociation of the biological response of both factors could indicate that both probably have different biological tasks and that the digoxin-like immunoreactive

794

SCHONER

material

may

not represent t h e

putative

natriuretic

hormone [ 1 2 4 1 . The m o l a r c o n c e n t r a t i o n s o f EDLF i n b l o o d a r e r a t h e r low

(Tables

posed,

1-3).

whether

The q u e s t i o n

could

therefore

be

s u c h low c o n c e n t r a t i o n s s u f f i c e t o oc-

to


cupy t h e c a r d i a c g l y c o s i d e r e c e p t o r s i t e a d e q u a t e l y induce a physiological response i n arteries, kidneys.

The

h e a r t and

a p p a r e n t a f f i n i t y of ouabain r e p o r t c d t o

t h e human v e n t r i c u l a r m u s c l e i s 2 . 5 nM [ 7 6 1 . T h i s would mean t h a t u n d e r p h y s i o l o g i c a l c o n d i t i o n s o n l y 3% o f t h e cardiac

glycoside

concentration

are

occupied.

t h e EDFLs i n c r e a s e s u p t o

1) by i n t r a v e n a l i n f u s i o n of 0 . 9 %

(Table This

of

receptor sites

would t h e n o c c u p y c a .

The

10000

pM

NaCl

[4Gl.

30% o f t h e r e c e p t o r

sites

a n d lead t o i n o t r o p i c r e s p o n s e . The r a b b i t is almost a s s e n s i t i v e t o c a r d i a c g l y c o s i d e s a s humans.

[1251

Barlet

along

determined t h e s e n s i t i v i t y

r a b b i t n e p h r o n a n d came t o

the

Doucet for

the

ouabain

conclusion

t h e n a t r i u r e t i c f a c t o r should act a t t h e

that

t i n g duct,

collec-

because t h e y found a n i n c r e a s e of t h e a f f i -

n i t i e s o f t h e s o d i u m pumps f o r o u a b a i n f r o m t h e mal

t o t h e collecting duct.

tubule

e f f e c t o n Na'/K'-ATPase

was

seen

and

half-maximal

a c t i v i t y of t h e c o l l e c t i n g d u c t

5 . 9 t 0 . 7 x 10-8 M

at

The

proxi-

and

a

dissociation

c o n s t a n t of t h e o u a b a i n r e c e p t o r c o m p l e x o f 1 . . 2 x M.

Provided

mined

in

t h e s e half-maximal c o n c e n t r a t i o n s

vitr-o are i d e n t i c a l w i t h t h o s e

concentration

o f EDLF o f 1 nM a f t e r

in

deter-

vim,

a

volume

expansion

would i n h i b i t o n l y 1%o f t h e s e s o d i u m pumps.

Unfortuna-

t e l y , a l s o t h e c o n c e n t r a t i o n s o f EDLF i n t h e r a b b i t a r e unknown,

but

suggests

t h a t t h e y w i l l p r o b a b l y n o t d i f f e r much

other

species.

a

comparison of t h e d a t a i n Tables Provided

all these

asssumptions

1-3 from

are

v a l i d , t h e e n d o g e n o u s l y f o r m e d o u a b a i n may t h e n n o t a c t



795

natriuretically even under conditions of volume expansion. Since Na+/ K'-ATPase from rat kidney shows halfmaximal inhibition at around 10 V M this argument is even more worthwile to consider for this species. In fact, Wechter et al. [1261 and Meyer-Lehnert et al. t1271 do not find any correlation between Na'/K+-ATPase inhibitory fractions in urine and natriuresis. Is there an additional natriuretic factor [11? The fact that vertebrates may synthesize cardiac glycosides and secrete them into their plasma raises the question whether such endogenously synthetized cardiac glycosides may show different functions. Ouabain is eliminated relatively fast and may therefore perform regulatory tasks more easily. The rate of degradation of digoxin, however, is much slower [1281. It is therefore not suited for rapid regulations of blood pressure. But it may serve long term adaptations.

Interaction of endoqenous digitalis-like substances with hormonal systems involved i n blood pressure regulation

All the present information favours the concept that cardiac glycosides can be synthesized in mammals preferably in the adrenals [4,51 but probably also in the hypothalamus [621 (by a yet unknown biosynthetic pathway which has to be proved to exist). In analogy to the cerebrally-mediated [7,107-1101 and the peripheral effects of cardiac glycosides [61 also endogenously formed cardiac glycosides or cardiac glycoside-like factors may act via neuronal and humoral effects. Consistent with a cerebral regulation is the finding that the onset of deoxycorticosterone acetate (DOCA)-salt


796

SCHONER

induced hypertension which is at least in part due to the raise of a circulating inhibitor of the sodium pump, is inhibited by hypothalamic lesions of the anterioventral (AV3V) 1 1 2 9 1 ; also the hindrance of a decrease of lNa+li in the denervated slow tonic soleus muscle by setting AV3V lesions was interpreted in this way 1 1 3 0 1 . It was therefore proposed that the release of a circulating inhibitor of the sodium pump may be regulated by the central nervous system 1 1 2 9 , 1 3 1 1 . Consistent with this idea a number of manipulations in the cerebral ventricle increase the concentration of the circulating inhibitor of the sodium pump in blood, like infusion of hyperosmotic NaCl concentrations t 1 3 6 3 of a dopamine agonist 11371 and A N F 1 1 4 5 , 1 4 7 1 : When cerebral ventricles are infused with hypertonic Na+ solutions a release of antidiuretic hormone ( A D H ) , an increase of plasma levels of catecholamines, an elevation of arterial pressure and a promotion of natriuresis are seen 1 1 3 2 - 1 3 4 1 . The natriuretic action is not due to increases in arterial blood pressure nor mediated by ADH, renal nerves, aldosterone or angiotensin I1 t 1 3 5 1 . In the dog, infusion of cerebral ventricles with artificial cerebrospinal fluid containing 1 5 0 or 300 mM NaCl resulted in a significant increase of a component in the blood plasma that inhibited ouabainsensitive s6Rb+-uptake into red blood cells [ 1 3 6 1 . Blood vessels appeared to be less sensititive for the circulating inhibitor of the sodium pump since only blood plasma of dogs whose ventricles were perfused with 300 mM NaCl showed inhibitory action 1 1 3 6 1 . When dogs received intracysternally the dopamine-2 receptor agonist pergolide also increased concentrations of a ciculating inhibitor of the sodium pump were detected; it inhibited the ouabain-sensitive 86Rb+-uptake into



797

blood vessels from the saphenous veins. Although no significant alterations in the contraction of arteries were seen by measuring the basal perfusion pressure of denervated hind limb preparations, pressure-effect effect of noradrenaline were significantly increased t1441. It was therefore proposed that a central dopaminergic mechanism may be involved in the release of Na'-pump inhibitor(s1 but an unspecific effect of the drug could not be ruled out [1441. When angiotensin I1 was infused into the third ventricle of rats an increase of an inhibitor of (Na'/K')-ATPase in serum was' noted, which was prevented by the angiotensin I1 antagonist saralasin t1381. Can anyiotensin lead to an increased release of endogenously formed cardiac glycosides? The atrial natriuretic factor (ANF) is a family of peptides with potent natriuretic, diuretic and selective vasodilatory properties t1391. It interacts with the renin-angiotensin-aldosterone systems [140-1421. ANF is also localized in areas of cardiovascular regulation [143,1441. Centrally administered ANF appears to inhibit aldosterone secretion 11411 and inhibits the angiotensin II-stimulated water intake t1421 and salt appetite 11401. Since angiotensin I1 increases the concentration of a circulating inhibitor of the sodium pump t1381, it was of interest to learn how ANF applyed intraventricularly may affect the levels of the circulating inhibitor of the sodium pump. A bolus injection of [Arg101-Tyr1261ANF into the lateral ventrical of anesthetized rats induced the occurence of a peripheral Na'-pump inhibitor and the blood pressure was unchanged (Similar conditions lead to a significant reduction of plasma vasopressin t1411). But intravenious injections of ANF had no effect on the inhibi-


7 98

SCHONER

tor's concentration as assayed by the esRb+ uptake into aortic smooth muscle cells [ 1 4 5 1 . In human essential hypertension and in the DOCA-salt rat hypertension increased concentrations of plasma ANF are found t 1 4 6 1 . Such conditions are also characterized by increased plasma concentrations of an inhibitor of the sodium The plasma atrial natriuretic peptide pump 1 2 1 - 2 4 , 6 8 1 . and the digoxin-like immunoreactive factor are also elevated in women in the peripartum period and correlate positively 1 1 4 7 1 . Unfortunately it is not known whether the endogenous digitalis-like immunoreactive factor may lead to a release of the atrial natriuretic peptide. Hamlyn found an increase of the plasma concentration of his factor in deoxycorticosterone-induced hypertension which correlates to the blood pressure and the concentration of atrial peptide [ G a l . In this context it is interesting to note that ouabain may stirnulate the secretion of atrial natriuretic peptide ( A N P ) in vivo when ouabain 30 ug/kg is injected as a bolus into mongrel dogs [ 1 4 8 1 and it affects the secretion in v i t m [ 1 4 9 1 and the synthesis of A N P 1 1 5 0 1 in primary atrial myocytes [ 1 4 9 1 and superfused rat atria [1501 only at concentrations lo-" M ouabain and above. It still remains inconclusive, whether the natriuretic action of ouabain in volume expanded rats [ 7 0 1 is not due to the increased synthesis and secretion of A N P . Ouabain as an endogenous factor and other stil badly defined circulating inhibitors of the sodium pump may therefore affect a number of hormone systems. The interplay with all the hormones and factors connected to the water and salt metabolism and their connection to blood pressure and natriuresis has only marginally studied. Unfortunately, much more more work is necessary to understand the interaction of these hormones at

799



physiological glycosides.

concentrations

of

endogenous

cardiac

Acknowledgement: The authors own work has been supported by the Deutsche Forschungsgemeinschaft, the SonderPharmakolog ie bio log ischer forschungsbereich 249 Makromolekule" Giessen and the Fonds der Chemischen Industrie Frankfurt/M.

REFERENCES

1. DE WARDENER, H.E., CLARKSON, E.M. Concept of natriuretic hormone. Physiol. Rev. 65: 658-759, 1985. 2. DAHL, L.K., KNUDSEN, K.D., IWAI, J. Humoral transmission of hypertension: Evidence from parabiosis. Circ. Res. 25/26 (suppl. I): 121-133, 1969. 3. HADDY, F.J., OVERBECK, H.W. The role of humoral agents in volume expanded hypertension. Life Sci. 19: 935-948, 1976.

4. HAMLYN,

J.M., BLAUSTEIN, M.P., BOVA, S., DU CHARME, D.W., HARRIS, D.W., MANDEL, F., MATHEWS, W.R., LUDENS, J.H. Identification and characterization of a ouabain-like compound from human plasma. Proc. Natl. Acad. Sci., USA 88: 6259-6263, 1991.

5. SHAIKH, J.M., LAU, B.W.C., SIEGFRIED, B.A., VALDES, Jr., R. Isolation of digoxin-like immunoreactive factors from mammalian adrenal cortex. J. Biol. Chem. 266: 13672-13678, 1991. 6. HAUSTEIN,

K.-0.

Digitalis. Pharmac. Ther. 18: 1-

89, 1983. 7 . GILLIS, R.A.,

QUEST, J.A. The role of the nervous system in the cardiovascular effect of digitalis Pharmacol. Rev. 31: 19-97, 1980.

.

8. GXRTNER, D.E., WENDROTH, S . , SEITZ, H.U. A stereospecific enzyme of the putative biosynthetic pathway of cardenolides. Characterization of progesterone 5B-reductase from leaves of Digitalis p u r p u r e a L. F E B S Lett. 271: 239-242, 1990.

8 00

SCHONER


9. PASTEELS, J.M., DALOZE, D., VAN DORSSER, W., ROBA, J. Cardiac glycosides in the defensive secretion of Chrysolina herbacea (coleoptera, chrysomelidae). Identification, biological role and pharmacological activity. Comp. Biochem. Physiol. 63C: 117-121, 1979.

10. MEYER, K., LINDE, H. Collection of toad venoms and chemistry of the toad venom steroids. In: Venomous animals and their venoms, Vol. 2 (Bucherl, W. & Buckley, E.E., eds.) pp. 521-556, Academic Press, New York, 1971. 11. SHOEMAKER V.H., NAGY, amphibians and reptiles. 449-471, 1977.

K.A. Ann.

Osmoregulation in Rev. Physiol. 39:

12. BONTING, S . L . , CARAVAGGIO, L.L. Studies on sodiumV. potassium-activated adenosinetriphosphatase. Coorrelation of enzyme activity with cation flux in six tissues. Arch. Biochem. Biophys. 101: 3745, 1963. 13. LICHTSTEIN, D., GATI, J., BABILA, T. HAVER, E., KATZ, U. Effect of salt acclimation on digitalislike compounds in the toad. Biochim. Biophys. Acta 1073, 65-68, 1991. 14. LICHTSTEIN, D., KACHALSKY, S., DEUTSCH, J. Identification of a ouabain-like compound in toad skin and plasma as a bufadienolide derivative. Life Sci. 38: 1261-1270, 1986. 15. SHIMONY, Y., GOTSMAN, M., EPSTEIN, M., KACHALSKY, S., DEUTSCH, J., LICHTSTEIN, D. Further characterization of the inotropic effect of a bufadienolide glycoside: An endogenous ouabain-like compound. Cardiac. Res. 20: 229-239, 1986. 16. REPKE, K., EST, M., PORTIUS, H.J. Uber die Ursache der Speziesunterschiede in der Digitalisempfindlichkeit. Biochem. Pharmacol. 14: 1785-18022, 1965. 17. ELIADES, D., SWINDALL, B., JOHNSTON, J., PAMNANI, M., HADDY, F. Hemodynamic Effects of bufalin in the anesthetized dog. Hypertension 13: 690-695, 1989. 18. BUCKALEW, V.M., GRUBER, K. Natriuretic Hormone. Ann. Rev. Physiol. 46: 343-358, 1984.


801

19. DE WARDENER, H.E., MacGREGOR, G.A. Dahl's hypothesis that a saliuretic substance may be responsible for a sustained rise in arterial blood pressure: its possible role in essential hypertension. Kidney Int. 18: 1-9, 1980.


20. BLAUSTEIN, M.P. Sodium ions, calcium ions, blood pressure regulation and hypertension: A reassessment and hypothesis. Am. J. Physiol. 232: C165C173, 1977.

L., SEWELL, R.B., WILKINSON, S.P., 21. POSTON, RICHARDSON, P.J., WILLIAMS, R., CLARKSON, E.M., MacGREGOR, G.A., DE WARDENER, H.E. Evidence for a circulating sodium transport inhibitor in essential hypertension. Br. Med. J. 282: 847- 849, 1981. 22. MORETH, K., KUSKE, R., RENNER, D., SCHONER, W. Blood pressure in essential hypertension correlates with the concentration of a circulating inhibitor of the sodium pump. Klin. Wochenschr. 64: 239-244, 1986. 23. HAMLYN, J.M., RINGEL, R., SCHAEFFER, J. LEVINSON, P.D., HAMILTON, B.P., KOWARSKI, A.A., BLAUSTEIN, M.P. A circulating inhibitor of (Na' + K+)-ATPase associated with essential hypertension. Nature 300: 650-652, 1982. 24. MORETH, K., RENNER, D., SCHONER, W. A quantitative receptor assay for "digitalis-like" compounds in serum. Demonstration of raised concentrations in essential hypertension and correlation with arterial blood pressure. Klin. Wochenschr. 65: 179184, 1987. 25. POSTON, L. Sodium transport inhibitors in pregnancy-induced hypertension. Cardiovascular Drugs and Therapy 4: 351-356, 1990.

Endogenous digitalis-like factor or 26. HADDY, F.J. factors. IS1 Atlas Pharmacol, 1: 119-123, 1987. 27. GRAVES, S.W. Endogenous digitalis-like factors. Crit. Rev. Clin. Lab. Sci. 23: 177-200, 1986. 28. HAUPERT, G.T. Physiological inhibitors of Na,KATPase: Concept and status. Prog. Clin. Biol. Res. 268B: 279-320, 1988.

802

SCHONER

29. BUCKALEW, V.M., HADDY, F.J., Endocrine Mechanisms in Hypertension (Laragh, J.H., Brenner, B.M. & Kaplan, N.M., eds.), pp. 306-334, Raven Press, New York, 1989.


30. HAMLYN, J.M., HARRIS, D.W., LUDENS, J.H. Digitalis-like activity in human plasma. J. Biol. Chem. 264: 7395-7404, 1989. 31. HAMLYN, J.M., HARRIS, Dew., CLARK, M.A., ROGOWSKI, A.M., WHITE, R.J., LUDENS, J.H. Isolation and characterization of a sodium pump inhibitor from human plasma. Hypertension 13: 681-689, 1989. 32. HAMLYN, J.M. Endogenous Digitalis: Where are we? IS1 Atlas Pharmacol. 2: 339-344, 1988. 33. TAMURA, M., KUWANO, He, KINOSHITA, T., INAGAMI, T. Identification of linoleic and oleic acids as endogenous Na+,K+-ATPase inhibitors from acute volume-expanded hog plasma. J. Biol, Chem. 260: 9672-9677, 1985. 34. KELLY, R.A., O'HARA, D.S., MITCH, W.E., SMITH, T.W. Identification of NaK-ATPase inhibitors in human plasma as nonesterified fatty acids and phospholipids. J. Biol. Chem. 261: 11704-11711, 1986.

J.-N., ROSSI, B., RENAUD, J.-F., 35. BIDARD, LAZDUNSKI, M. A search for a 'ouabain-like' substance from the electric organ of Electrophorus electricus which led to arachidonic acid and related fatty acids. Biochim. Biophys. Acta 769: 245-252, 1984. 36. VASDEV, S., LONGERICH, L.L., ITTEL, T.H., JOHNSON, E., BARROWMAN, J.A., GAULT, H. Bile salts as endogenous digitalis-like factors. Clin. Invest. Med. 9: 201-208, 1986. 37. BRAQUET, P., SENN, N., ROBIN, J.P., ESANU, A . GARAY, R.P. Endogenous lignans: a potential endogenous digitalis. J. Hypertens. 4(Suppl 5 ) : 5161164, 1986. 38. NG, Y.-C., AKERA, T., HAN, C.S., BRASELTON, W.E., KENNEDY, R.H., TEMMA, K., BRODY, T.M., SATO, P.H. Ascorbic acid: An endogenous inhibitor of isolated Na+,K'-ATPase. Biochem. Pharmacol. 34: 2525-2530, 1985.

803



39. KUSKE, Re, MORETH, K., RENNER, D., WIZEMANN, V., SCHONER, W. Sodium pump inhibitor in the serum of patients with essential hypertension and its partial purification from hemofiltrate. Klin. Wochenschr. 65 (Suppl. VIII): 53-59, 1987. 40. VASDEV, S., LONGERICH, Le, JOHNSON, E., BRENT, D., GAULT, M.H.G. Dehydroepiandrosterone sulfate as a digitalis-like factor in plasma of healthy human adults Res. Commun. Chem. Pathol. Pharmacol. 49: 387-399, 1985. 41. CLARKSON, E.M., DE WARDENER, H.E. Observation on a low molecular weight natriuretic and Na+,K+-ATPase inhibitory material in urine. Clin. Exptl. Hypertension A7: 673-683, 1985.

42. YASUHARA, T., MORI, M., WAKAMATSU, K., KUBO, K. Isolation and identificatiom of hemin as an endogenous Na'/K+-ATPase inhibitor from porcine blood cells. Biochem. Biophys. Res. Commun. 178: 95-103, 1991. 43. SMITH, T. Pharmacokinetics, bioavailability and serum levels of cardiac glycosides. J. Am. Coll. Cardioal. 5: 43a-50a, 1985.

44. HAMLYN, J.M., HARRIS, D.W., LUDENS, J.H. A digitalis-1i.ke compound from human plasma: purification and characterization. FASEB J. 2(4): A239, Abstr. 194, 1988. 45. GOTO, A * , YAMADA, K., ISHII, M., YOSHIOKA, Me, ISHIGURO, T., EGUCHI, C., SUGIMOTO, T. Presence of digitalis-like factor in mammalian plasma. Biochem. Biophys. Res. Commun. 152: 322-327, 1988.

46. HAMLYN, J.M., Diuretics I11 (Puschett, J.B. & Greenberg, A., eds.), pp. 648-655, Elsevier Science Publishing Co, Amsterdam, 1990

.

47. GOTO, A., YAMADA, K., Ishii, M., SUGIMOTO, T. Does dogoxin-like immunoreactivity really represent natriuretic hormone? Nephron 54: 99-100, 1990. 48. MASUGI, F., OGIHARA, T., HASEGAWA, T., KUMAHARA, Y. Ouabain and non-ouabain-like factors in plasma of patients with essential hypertension. Clin. Exp. Hypertension A9: 1233-1242, 1987.

8 04

SCHONER

F., OGIHARA, T., HASEGAWA, T., TOMII, A, NAGANO, M, HIGASHIMORI, K., KUMAHARA, K., TERANO, Y. Circulating factor with ouabain-like immunoreactivity in patients with primary aldosteronism. Biochem. Biophys. Res. Commun. 1 3 5 : 41-45, 1 9 8 6 .

49. MASUGI,


50.

GAULT, M.H., E., FARID, N., Evidence for digitalis-like 253-261,

VASDEV, S . , LONGERICH, L., JOHNSON, LEGAL, Ye PRABHAKARAN, V., FINE, A. an adrenal contribution to plasma factors. Clin. Physiol. Biochem. 6 :

1988.

51.

GRAVES, S.W. The possible role of digitalis-like factors in pregnancy-induced hypertension. Hypertension 1 0 [Suppl. I1 1 - 8 4 - 1 - 8 6 , 1987.

52.

SCHERRMANN, J.M., SANDOUK, P., GUEDENEY, X. Endogenous digitalis-like immunoreactive substances in cord serum characterized by anti-digitoxin and anti-digoxin antibodies. Effect of modulated incubation conditions. Clin. Biochem. 1 9 : 201-204, 1986.

53

HARRIS, Dew., CLARK, M.A., FISHER, J.F., HAMLYN, J.M., KOLBASA, K.P., LUDENS, J.H., DU CHARME, D.W. Development of an immunoassay for endoyenous digitalislike factor. Hypertension 1 7 : 936-943, 1 9 9 1 .

5 4 . RAUCH,

A.L., BUCKALEW, V.M. Jr. Tissue distribution of an endogenous ligand to the Na,K ATPase molecule. Biochem. Biophys. Res. Commun. 1 5 2 : 8 1 8 824,

1988.

55.

VASDEV, S., JOHNSON, E., LONGERICH, L., PRABHAKARAN, V.M., GAULT, M.H. Plasma endogenous digitalis-like factors in healthy individualls and in dialysis dependent and kidney transplant patients. Clin. Nephrol. 2 7 : 1 6 9 - 1 7 4 , 1 9 8 7 .

56.

FAVRE, He, SIEGENTHALER, G., MARTIN, B e , ROTH, D., GRANGES, G., LOUIS, F. Receptor assay for endogenous inhibitors of Na-K-ATPase. Klin. Wochenschr. 65: (Suppl. 8 ) , 49-52, 1 9 8 7 .

57.

DEVYNCK, M.A., PERNOLLET, M.G., CLOIX, J.-F., De THE, H., KAMAL, L., ELGHOZI, J.L., ROSENFELD, J., MEYER, P. Circulating digitalis-like compounds in essential hypertension. Clin. Exptl. Hypertension A6:

441-453,

1984

.


805

58. KHATTER, J.C., AGHANYO, M., HOESCHEN, R.J. Endogenous digitalis-like substance in pig left ventricle. Life Sci. 39: 2483-2492, 1986.


59. LICHTSTEIN, D., SAMUELOV, S., GATI, I, FELIX, A.M., GABRIEL, T.F., DEUTSCH, J. Identification of 11,13-dihydroxy-14-octadecaenoic acid as a circulating Na+/K+-ATPase inhibitor. J. Endocrinol. 128: 71-78, 1991. 60. GOTO, A * , YAMADA, K., ISHII, M., YOSHIOKA, Me, ISHIGURO, T., EGUCHI, C., SUGIMOTO, T. Purification and characterization of human urine-derived digitalis-like factor. Biochem. Biophys. Res. Commun. 154: 847-853, 1988. 61. HEIDRICH-LORSBACH, E. Isolierung und Characterisierung eines endogenen Hemmstoffes der (Na++K+)ATPase mit Herzglykosid-ahnlicher Wirkungsweise aus dem hamofiltrat und Dialysat des Menschen. Dissertationsarbeit. Fachbereich Ernahrungswissenschaften Universitat Giessen 1991. 62. GOTO, A., YAMADA, K., ISHII, M., YOSHIOKA, M., ISHIGURO, T., EGUCHI, C., SUGIMOTO, T. Existence of a plar digitalis-like factor in mammalian hypothalamus. Biochem. Biophys. Res. Commun. 161: 953958, 1989. 63. ILLESCAS, M., RICOTE, M., MbNDEZ, E., ROBLES, R.G., SANCHO, J.M. Partial purification of a sodium pump inhibitor from bovine adenohypophysis. Its comparison with the natriuretic factor isolated from hypothalamus. Clin. Exp. Hypertension AlO(Supp1. 1): 301-307, 1988. 64. ILLESCAS, M., RICOTE, M., R.G., SANCHO, J. Complete identical Na+-pump inhibitors hypothalamus and hypophysis. 440, 1990.

MENDEZ, E., ROBLES, purification of two isolated from bovine FEBS Lett. 261: 436-

65. TAMURA, M., LAM, T.-T., INAGAMI, T. Isolation and characterization of a specific endogenous Na+,K+ATPase inhibitor from bovine adrenals. Biochemistry 27: 4244-4253, 1988. 66. CARILLI, C.T., BERNE, M., CANTLEY, L.C., HAUPERT, G.T. Jr. Hypothalamic factor inhibits the (Na,K)ATPase from the extracellular surface. J. Biol. Chem. 260: 1027-1031, 1985.

806

SCHONER

6 7 . HAUPERT,

G.T., CARILLI, C.T., CANTLEY, L.C. Hypothalamic sodium-transport inhibitor is a highaffinity reversible inhibitor of Na+,K+-ATPase. Am. J. Physiol. 2 4 7 : F919-F924, 1 9 8 4 .

6 8 . HAMLYN, J.M. Increased levels of a humoral digita-

lis-like factor in deoxycorticosterone acetateinduced hypertension in the pig. J. Endocrinol.


122: 409-420,

1989.

6 9 . DASGUPTA,

A., YEO, K.-T., MALIK, S., SANDU, P., AHMAD, S . , KENNY, M. Two novel endogenous digoxinlike immunoreactive substances isolated from human plasma ultrafiltrate. Biochem. Biophys. Res. Commun. 1 4 8 : 6 2 3 - 6 2 8 , 1 9 8 7 .

7 0 . CRABOS,

M., GRICHOIS, M.-L., GUICHENEY, P., Further biochemical WAINER, I.W., CLOIX, J . - F . characterization of an Na+ pump inhibitor purified from human urine. Eur. J. Biochem. 1 6 2 : 1 2 9 - 1 3 5 , 1987.

7 1 . HALPERIN,

J.A., RIORDAN, J . F . , TOSTESON, D.C. Characteristics of an inhibitor of the Na+/K' pump in human cerebrospinal fluid. J. Biol. Chem. 2 6 3 : 646-651,

1988.

7 2 . LUDENS, J.H., CLARK, M.A.,

DU CHARME D.W., HARRIS, Dew., LUTZKE, BeS,, MANDEL, F . , METHEWS, W.R., SUTTER, D.M., HAMLYN, J.M. Purification of an endogenous digitalislike factor from human plasma for structural analysis. Hypertension 1 7 : 9 2 3 - 9 2 9 , 1991.

7 3 . MATHEWS,

W.R., DU CHARME, D.W., HAMLYN, J.M., HARRIS, Dew., MANDEL, F . , CLARK, M.A., LUDENS, J.H. Mass spectral characterization of an endogenous digitalislike factor from human plasma. Hypertension 1 7 : 9 3 0 - 9 3 5 , 1 9 9 1 .

7 4 . BOVA,

S., BLAUSTEIN, M.P., LUDENS, J.H., HARRIS, D.W., DU CHARME, D.W., HAMLYN, J.M. Effect of an endogenous ouabainlike compound on heart and aorta. Hypertension 1 7 : 9 4 4 - 9 5 0 , 1 9 9 1 .

F . , OGIHARA, T., HASEGAWA, T., SAKAGUCHI, K., KUMAHARA, Y. Normalization of high plasma level of ouabain-like immunoreactivity in primary aldosteronism after removal of adrenals. J. Hum. Hypertens. 2 : 4 0 9 - 4 2 0 , 1 9 8 8 .

7 5 . MASUGI,


807

76. ERDMANN, E. Influence of cardiac glycosides on their receptor. in: Handbook of Experimental Pharmacology 56/I, 337-380, 1981.


77. HEIDRICH-LORSBACH, E.,, KIRCH, U., SICH, B., AHLEMEYER, B., SCHONER, W. Purification and properties of an endogenous digitalis-like factor from hemofiltrate of uraemic patients. Clin. Exptl. Hypertension 1992 (submitted). 78. GOTO, A., YAMADA, K., ISHII, M., YOSHIOKA, M., ISHIGURO, T., SUGIMOTO T. The effect of urinary digitalislike factor on cultures vascular smooth muscle cells. Hypertens on 11: 645-650, 1988. 79. GOTO, A., YAMADA, K., ISHII, M., YOSHIOKA, M., ISHIGURO, T., EGUCHI, C., SUGIMOTO, T. Urinary sodium pump inhibitor r ises cytosolic free calcium concentration in rat aorta. Hypertension 13: 916-921, 1989. 80. GOTO, A., YAMADA, K., ISHII, M., YOSHIOKA, M., ISHIGURO, T., EGUCHI, C., SUGIMOTO, T. Effect of human urine-derived digitalis-like factor on cultures renal tubular epithelial cells. J. Hypertension 6 (Suppl. 4): S287-5289, 1988. 81. CLOIX, J.-F., CRABOS, M., WAINER, I.W., RUEGG, U., SEILER, M., MEYER, P . High yield-purification of a urinary Na+-pump inhibitor. Biochem. Biophys. Res. Commun. 131: 1234-1240, 1985. 82. FAGOO, M . , GODFRAIND, T. Further characterization of cardiodigin, Na+,K+-ATPase inhibitor extracted from mammalian tissues. FEBS Lett. 184: 150-154, 1985. 83. GOTO, A., ISHIGURO, T., YAMADA, K., ISHII, M., YOSHIAKA, M . , EGUCHI, C., SHIMORA, M., SIJGIMOTO, T. Isolation of a urinary digitalis-like factor indistinguishable from diyoxin. Biochem. Biophys. Res. Commun. 173: 1093-1101, 1990. 84. DORIS, P.A., STOCCO, D.M. An endogenous digitalislike factor from the adrenal gland: studies from adrenal tissue from various sources. Endocrinology 125: 2573-2579, 1989. 85. HAUPERT, G.T., SANCHO, J.M. Sodium transport inhibitor from bovine hypothalamus. Proc. Natl. Acad. Sci. USA 76: 4658-4660, 1979.

808

SCHONER

86. ANNER, B.M., REY, H.G., MOOSMAYER, M., MESZOELY, I., HAUPERT, G.J. Hypothalamic Na+-K+-ATPase inhibitor characterized in two-sided liposomes containing pure renal Na+-K+-ATPase. Am. J. Physiol. 258: F144-Fl53, 1990.


87. HALLAQ, H . A . , HAUPERT, G.T. Positive inotropic effect of the endogenous Na+/K+-transporting ATPase inhibitor from the hypothalamus. Proc. Natl. Acad. Sci. 86: 10080-10084, 1989. 88. CANTIELLO, H.F., CHENr E., REY, S . 1 HAUPERT, G.T. Na+ pump in renal tubular cells is regulated by endogenous Na+, K+-ATPase inhibitor from hypothalamus. Am. J. Physiol. 255: F574-F580, 1988. 89. HAUPERT, G.T., KACHORIES, C., HALLAQ, H . A . Physiologic regulation of the Na,K-ATPase: positive inotropic and vasoconstrictive effects of the hypothalamic Na,K-ATPase inhibitor. In: Proc. IUPHAR Satellite Meeting "Endogenous DigitalisLike Factors - Purification, Properties, Physiological Functions - Giessen, 1990, p . 35. 90. HABERMANN, E. Palytoxin acts through ATPase. Toxicon. 27: 1171-1187, 1989.

Na+,K+-

91. SCHWARTZMAN, M.L., BALAZY, M., MASFERRER, J., ABRAHAM, N.G., MCGIFF, J.C., MURPHY, R.C. 1.2(R)Hydroxyicosatetraenoic acid: A cytochrome P450dependent arachidonate metabolite that inhibits Na+, K+-ATPase in the cornea. Proc. Natl. Acad. Sci. USA 84: 8125-8129, 1987. 92. MASFERRER,

J.L.

DUNN,

M.W.

SCHWARTZMAN, M.L. an endogenous corneal arachhidonate metabolite lowers corneal pressure in rabbits. Invest. Ophthalmol. Vis. Sci. 31: 535, 1990. 12(R)-Hydroxyeicosatetraenoic acid,

9 3 . DELAMERE,

N.A. SOCCI, R.R., KING, K.L., BHATTACHERJEE, P. The influence of 12(R)hydroxyeicosatetraenoic acid on ciliary epithelial sodium, potassium adenosine triphosphatase activity and intraocular pressure in the rabbit. Invest. Ophthalmol. Vis. Sci. 32: 2511-2514, 1991.

94. SCHWARTZMAN, M., FERRERI, N.R., CARROLL, M.A., SONGU-MIZE, E., McGIFF, J.C., Renal cytochrome P450-related arachidonate metabolite inhibits (Na++K+)ATPase. Nature 314: 620-622, 1985.


809

95. ESCALENTE, B., FALCK, J.R., YADAGIRI, P., SUN, L., SCHWARTZMAN, M.L. 19(S)-Hydroxyeicosatetraenoic acid is a potent stimulator of Na+-K+-ATPase. Biochem. Biophys. Res. Commun. 152: 1269-1274, 1988.


96. QUILLEY, J., QUILLEY, C.P., McGIFF, J.C., Hypertension Pathophysiology, Diagnosis and Management (Laragh, J.H. & Brenner, B.M., eds.), pp. 829-840, Raven Press, New York, 1990. 97. MORGAN, K., LEWIS, M.D., SPURLOCK, G., COLLINS, P.A., FOORD, S.M., SOUTHGATE, K., SCANLON, M.F., MIR, M.A. Characterization and partial purification of the sodium-potassium-ATPase inhibitor released from cultured ra’t hypothalamic cells. J. Biol. Chem. 260: 13595-13600 1985. 98. MIR, M.A. The release, partial purification and characterisation of a sodium potassium ATPase inhibitor from rat hypothalamic cells in culture. Proc. XIth IUPHAR Satellite Meeting on Endogenous Digitalis-like Factors - Purification, Properties, Giessen, Physiological Functions - July 8-10, 1990, p. 32. 99. KRAMER, H.H., HEPPE, M., WEILER, E., BACKER, A., LlIDDIARD, C., KLINGMULLER, D. Further Characterization of the endogenous natriuretic and digoxinlike immunoreacting activities in human urine: Effects of changes in sodium intake. Renal Physiology 8: 80-89, 1985. 100. VINGE, E., EKMAN, R. Evaluation of cross-reactivity of urinary digoxin-like substance in different radioimmunoassays. Therapeutic drug monitoring 10: 316-320, 1988.

101. GRUBER, K.A., WHITAKER, J.M., BUCKALEW, V.M.Jr. Endogenous digitalis-like substance in plasma of volume expanded dogs. Nature 287: 743-745, 1980.

102. GRUBER, K.A., RUDEL, L.L., BULLOCK, B.C. Increased circulating levels of an endogenous digoxin-like factor in hypertensive monkeys. Hypertension 4: 348-354, 1982. 103. GRAVES, S.W., BROWN, B., VALDES, R. An endogenous digoxin-like substance in patients with renal impairment. Ann. Internal Med. 99: 604-608, 1983.

810

SCHONER

104. LIU, Z., YAO, R., YANG, D. Endogenous digoxin-like substance as an important factor in the development of hypertension in children. Int. J. Cardiol 29: 343-348, 1990.


105. HULTHEN, U.L, BOLLI, P., KIOWSKI, W., BUHLER, FIR. Forearm Vasoconstrictor response to ouabain: Studies in Patients with mild and moderate essential hypertension. J. Cardiovasc. Pharmacol. 6: S75-S81, 1984. 106. GUTHRIE, Jr, G.P. Effect of Digoxin on Responsiveness to the pressor actions of angiotensin and norepinephrine in man. J. Clin. Endocrinol. Metab. 58: 76-80, 1984. 107. JACOMINI, L.C.L., ELGHOZI, J.L., DAGHER, G. DEVYNCK, M.A., MEYER, P. Central hypertensive effect of ouabain in rats. Arch. int. Pharmacodyn. 267: 310-318 (1984). 108. CALDWELL, R.W., SONGU-MIZE, E., BEALER, S.L. The vasopressor response to centrally administered ouabain. Circ. Res. 55: 773-779, 1984 109. SAXENA, P.R., BHARGAVA, K.P. The importance of a central adrenergic mechanism in the cardiovascular Eur. J. Phramacol. 31: 332responses to ouabain 346, 1975.

.

110. DONALDSON, J.,ST-PIERRE, T., MINNICH, J., BARBEAU, A. Seizures in rats assoziated with divalen cation inhibition of Na+-K+-ATPase. Canad. J. Biochem. 49: 1217-1224, 1971. 111. PATON, D.M. Neuronal transport of noradreanaline and dopamine. Pharmacology 21: 85-92, 1980. 112. VIZI, E.S. Na+-K+-activated adenosine triphosphatase as a trigger in transmitter release. Neuroscience 3: 367-384, 1978. 113. ELGHOZI, J.L., LE QUAN-BUI, K.H., DEVYNCK, M.A., MEYER, P. Momifensine antagonizes the ouabaininduced increase in dopamine metabolites in cerebrospinal fluid of the rat. Eur. J. Pharmacol. 9: 279-282, 1983. 114. GOODLIN, R.C. Antidigoxin antibodies in eclampsia. N. Engl. J. Med. 318: 518-518, 1988.


811

115. KOJIIMA, I., YOSHIHARA, S., OGATA, E. Involvement of endogenous digitalis-like substance in the genesis of deoxycorticosterone-salt substance. Life Sci. 30: 1775-1781, 1982.


116. HUANG, C.T., SMITH, R.M. Lowering of blood pressure in chronic aortic coarctate hypertensive rats with anti-digoxin antiserum. Life Sci. 35, 115, 1984. 117. DIETZ, R., SCHOMIG, A., DART, A.M., MAYER, E., KUBLER, W. Modulation of sympathetic vasoconstriction by potassium. J. Cardiovasc. Pharmacol 6: S230-S235, 1984.

J.F.E., MIEMITZ, R., GANTEN, U., RITZ, E. 118. MA", Haemodynamic effects of intact digoxin antibody and its Fab fragments in experimental hypertension. J. Hypertension 5: 543-549, 1987. 119. ZIDEK, W., OTTENS, E., HECKMANN, U. Transmission of hypertension in rats by cross circulation. Hypertension 14: 61-64, 1989. 120. KOBAYASHI, Ye SHIMODA, K., KUSHIRO, T., NOZAWA, K., OTSUKA, Y., KAJIWARA, N. The role of endogenous digitalis-like substance in one-kidney, one clip hypertensive rats. J. Hypertension 4 (suppl. 3): S353-S354, 1986. 121

TAKAHASHI, H., MATSUSAWA, M., OKABAYASHI, H. SUGA, K., IKEGAKI, I., NISHIMURA, M., YOSHIMURA, M. Endogenous digitalis-like substance in an adult population in Japan. Am. J. Hypertens. 1: 168s172S, 1988.

122. TAKAHASHI, H., MATSUSAWA, M. SUGA, K., IKEGAKI, I, NISHIMURA, M., YOSHIMURA, M., IHARA, N., YAMADA, H,, SANO, Y. Hypothalamic digitalis-like substance is released with sodium loading in rats. Am. J. Hypertens. 1: 146-151, 1988. 123. YAMADA, K., GOTO, An, ISHII, Me, YOSHIOKA, Ma, SUGIMOTO, T. Dissociation of digoxin-like immunoreactivity and Na+, K'-ATPase inhibitory activity in rat plasma. Experientia 44: 992-993, 1988. 124. SCOTT, P.J., LITTLE, P.J., BOBIK, A. Circulating digoxin-like immunoreactivity in renal hypertensive rabbits: Lack of modulation by alterations in dietary sodium intake. J. Hypertension 6: 205-209, 1988.

8 12

SCHONER


125. DOUCET, A., BARLET, C. Evidence for differences in the sensitivity to ouabain of Na,K-ATPase along the nephron of rabbit kidney. J. Biol. Chem. 261: 993-995, 1986. 126. WECHTER, W.J., BENAKSAS, E.J., MURRAY, E.D., RODGERS, C., PHAM, T., GARI, D., GIEBEL, R. The lack of correlation between sodium pump inhibition in cells and the natriuretic activity of partially purified uremic isolates in the rat. Proc. IUPHAR Satellite Meeting "Endogenous Digitalis-Like Factors - Purification, Properties, Physiological Functions - Giessen, 1990, pp. 24. 127. MEYER-LEHNERT, H., MICHEL, H., BACKER, A., KRAMER, H.J. Natriuretic hormone (NH) and ouabain-like factor (OLF): Different compunds with different cellular effects. Proc. IUPHAR Satellite Meeting "Endogenous Digitalis-Like Factors - Purification, Properties, Physiological Functions - Giessen, 1990, pp. 25. 128. THOMAS, R., GRAY, P., ANDREWS, J. Digitalis: Its mode of action, receptor, and structure-activity relationships. Adv. Drug Res. 19: 311-562, 1989. 129. SONGU-MIZE, E., BEALER, S.L., CALDWELL, R.W. Effect of AV3V lesions on development of DOCA-salt hypertension and vascular Na+-pump activity. Hypertension Dallas 4: 575-580, 1982. 130. KATAFUCHI, T., OOMURA, Y., MARUYAMA, T., AKAIKE, N. Hypothalamus and sodium-potassium pump activity in skeletal muscles of DOCA-hypertensive rats. Am. J. Physiol 253: R396-R401, 1987. 131. SONGU-MIZE, E., BEALER, S.L., CALDWELL, R.W. Effect of DOCA-salt treatment duration and anteroventral third ventricle lesions on a plasma-borne sodium pump inhibitor in rats. J. Hypertens. 5: 461-467, 1987. 132. ANDERSON, B. Regulation of water intake. Physiol. Rev. 58: 582-603, 1978. 133. ERICSON, A.C., SJOQUIST, M. Efferent renal nerve activity during intracarotid and intracerebroventricular infusions of hypertonic sodium chloride solutions and isotonic volume expansion in the rat. Acta Physiol. Scand. 114: 9-15, 1982.


813

134. BEASLEY, D., MALVIN, R.L., MOUW, D.R. CNS-induced natriuresis and renal hemodynamics in conscious rats. Am. J. Physiol. 14: F763-F771, 1983.


135. KAWANO, Y., FERRARIO, C.M. Neurohormonal characteristics of cardiovascular response due to intraventricular hypertonic NaC1. Am. J. Physiol. 247, H422-H428, 1984. 136. JANDHYALA, B.S., ANSARI, A.F. Elevation of sodium levels in the cerebral ventricles of anaesthetized dogs triggers the release of an inhibitor of ouabain-sensitive sodium, potassium-ATPase into the circulation. Clin. Sci. 70: 103-110, 1986. 137. JANDHYALA, B.S., LOKHANDWALA, M.F., KIVLIGHN, S.D., ANSARI, A.F., DE FEO, M.L. Intracisternal administration of pergolide, a dopamine receptor agonist, triggers the release of an inhibitor of ouabain-sensitive sodium,potassium-dependent adenosine triphosphatase and enhances vascular reactivity in anaesthetized dogs. Clin. Sci. 73: 183-188, 1987. 138. BALDA, Mas., PIROLA, C.S., DABSYS, S.M., FINKIELMAN, S., NAHMOD, V.E. Saralasin blocks the effect of angiotensin I1 and extracellular fluid saline expansion on the Na-K-ATPase inhibition release in 997-1008, rats. Clin. Exptl. Hypertension A 8 : 1986. 139. CURRIE, M.G., GELLER, D.M., COLE, B.R., BOYLAN, J.G., YUSHENG We, HOLMBERG, Sew., NEEDLEMAN, P. Bioactive cardiac substances: Potent vasorelaxant activity in mammalian atria. Science Wash. DC 221: 71-73, 1983. 140. FITTS, D.A., THUNHORST, R.L., SIMPSON, J.B. Diuresis and reduction of salt appetite by lateral ventricular infusions of atriopeptin 11. Brain Res. 348: 118-124, 1985. 141. IITAKE, K., SHARE, L., CROFTON, J.T., BROOKS, D.P., OUCHI, Y, BLAINE, E.H. Central atrial natriuretic factor reduces vasopressin secretion in the rat. Endocrinology 119: 438-440, 1986. 142. MASOTTO, C., NEGRO-VILAR, A. Inhibition of spontaneous or angiotensin 11-stimulated water intake by atrial natriuretic factor. Brain Res. Bull. 15: 523-526, 1985.

SCHONER

8 14

143. SAMSON, W.K. Atrial natriuretic factor and the central nervous system. Endocrinol. Metab. Clin. N. Am. 16: 145-161.


C.B., STANDAERT, D.B., CURRIE, M.G., 144. SAPER, SCHWARTZ, D., GELLER, D.M., NEEDLEMAN, P. Atriopeptin-immunoreactive neurons in the brain: Presence in cardiovascular regulatory areas. Science Wash. DC 227: 1047-1049, 1985. 145. SONGU-MIZE, E., BEALER, S.L., HASSID, A.I. Centrally administered ANF promotes appearance of a circulating sodium pump inhibitor. Am. J. Physiol. 258: H1655-Hl659, 1990. 146. SUGIMOTO, T., ISHII, M., HIRATA, Y., MATSUOKA, H., SUGIMOTO, T., MIYATA, A * , TOSHIMORI, T., MASUDA, H., KANGAWA, K., MATSUO, H. Increased release of atrial natriuretic polypeptides in rats with DOCAsalt hypertension. Life Sci. 38: 1351-1358, 1986. 147. SHILO, L . , DOLEV, S . , SHENKMAN, L. Atrial natriuretic peptide and digoxin-like factor in the peripartum period. J. Clin. Pharmacol. 29: 1106-1107, 1989. 148. YAMAMOTO, A , , SHOUJI, T., TAMAKI, T., FUKUI, K., IWAO, H., ABE, Y. Effects of calcium and ouahain on the release of atrial natriuretic factor. J. Hypertension 6 (Suppl. 4): S317-5319, 1988. 149. BLOCH, K.D., ZAMIR, N., LICHTSTEIN, D., SEIDMAN, C.E., SEIDMAN, J.G. Ouakxiin induces secretion of proatrial natriuretic factor by rat atrial cardiocytes. Am. J. Physiol. 255: E383-E387, 1988. 150. MORISE, T., TAKEUCHI, Y., OKAMOTO, S . , TAKEDA, Re Stimulation of atrial natriuretic peptide secretion and synthesis by Na-K-ATPase inhibitors. Biochem. Biophys. Res. Commun. 176: 875-881, 1991. 151. WOOLFSON, R.G., POSTON, L. Effect of ouabain on endothelium-dependent relaxation of human resistance arteries. J. Hypertension 17: 619-625, 1991.

Submission date Date accepted

10/28/91 2/1/92

Mild exercise decreases plasma endogenous digitalislike substance in hypertensive individuals.

Cancerogenic risk by endogenous factors and processes.

Immunoreactivity of endogenous digitalis-like factors.

Fighting fire with fire: endogenous retrovirus envelopes as restriction factors.

Re: Endogenous and exogenous factors affecting the levels of carboxyhemoglobin.

Endogenous and exogenous factors affecting the levels of carboxyhemoglobin.

NO., CO and .OH. Endogenous soluble guanylyl cyclase-activating factors.

Induction of B cell responsiveness to growth factors by Epstein-Barr virus conversion: comparison of endogenous factors and interleukin-1.

Distribution of endogenous albumin in the rat glomerulus: role of hemodynamic factors in glomerular barrier function.

Risk factors predictive of endogenous endophthalmitis among hospitalized patients with hematogenous infections in the United States.

The association of reproductive and lifestyle factors with a score of multiple endogenous hormones.

Endogenous factors with immunological and biological activity similar to cardiac glycosides: biochemical and pathophysiological implications.

Tobacco smoking, chewing, and snuff dipping: factors contributing to the endogenous formation of N-nitroso compounds.

Human Endogenous Retroviruses as Pathogenic Factors in the Development of Schizophrenia.

Transcriptional regulation of gene expression by microRNAs as endogenous decoys of transcription factors.

Adentylate cyclase activity in myocardium of spontaneously hypertensive rat: effect of endogenous factors and solubilization.

Lifetime endogenous reproductive factors and severe depressive symptoms in postmenopausal women: findings from the E3N cohort.

Regulation of endogenous human gene expression by ligand-inducible TALE transcription factors.

pH-dependent column fractionation for characterization of endogenous digoxin-like immunoreactive factors in pregnant urine.

Prevalence of and risk factors for endogenous endophthalmitis in patients with pyogenic liver abscesses.

Endogenous and exogenous factors modifying the activity of human liver cytochrome P-450 enzymes.

Scedosporium prolificans endogenous endophthalmitis.

Immunoglobulins in endogenous uveitis.

Endogenous candida endophthalmitis.