Carrlinvascirlur Research, 1979, 13, 22-30

Sodium excretion in normal conscious dogs W. J . O’CONNOR A N D R . A . SUMMERILL Frnm

Thc Dcppcirtmetit nf Physinlo~~y, The University of Lcc~rls

Bitches maintained on a low Na intake, were given doses of saline (0.125 mol.litre NaCI, 0.025 mol.litre NaHCO,, 0.004mo1,litr-e * KCI) by stomach tube. Doses of 100 and 200 cm3 produced only minor increases in Na excretion; after 300 cm3? Na excretion rose from about 2 to about 60 pmol.min-’. Plasma protein fell by 1.8 litre for each 100 cm3 of saline retained. Within normal ranges of Na excretion there is a threshold of plasma protein concentration above which Na is retained and below which Na is excreted. Changes in exogenous creatinine clearance were measured allowing calculation of the filtered load of Na, which shows that the absolute tubular reabsorption of Na and water is increased in volume expansion by isotonic saline. Meat produced large increase in glomerular filtration rate without much increase in Na excretion and mechanisms are discussed by which Na reabsorption is more effective after meat than after doses of saline. Creatinine clearance increased by 0.67 cm3.min for each fall of I g.litre in plasma protein; this is predicted by a theory that the glomerular capillary blood pressure is 9.3 kPa (70 mmHg) rather than 6.7 kPa (50 mmHg).

SUMMARY

after large doses of saline, with the rate of Na excretion rising far beyond the rates which occur in normal life. Based on the Na content of foods, O’Connor (1977) indicated that the rates of excretion of Na required by normal feeding of dogs ranged from practically zero in dogs fed meat, up to 50 to 100 pmolmin-1 in dogs fed commercial dog foods containing added NaCI. I t is within this range that normal control of Na excretion must be studied. In the experiments of this paper, the range has been covered by using dogs maintained on a diet of low salt content and recording the effects of 100, 200, and 300 cm3of saline. O’Connor ( 1962) gave reasons for believing that increased excretion of Na after sufficient doses of saline is due to the direct effect on the kidneys of fall in plasma protein without the intervention of any hormonal or neural mechanisms. The intrarenal mechanisms have remained in doubt because there have been no adequate measurements of the changes in creatinine or inulin clearance after small doses of saline in normal conscious dogs. O’Connor (1962) could only argue indirectly that fall in plasma protein would cause increase in glomerular filtration rate, increase in filtered load of Na and increased excretion. Without sufficient measurements of glomerular filtration rate the belief has developed that increased excretion of Na after saline is due to decreased tubular reabsorption. Glomerular and

O’Connor (1977) has pointed out that since Na is the ion of extracellular fluids, the Na content of the body determines the extracellular volume, thereby influencing blood volume, cardiac output and under certain circumstances heart failure. The mechanisms regulating body Na, therefore, are fundamental to a consideration of the control of blood volume. Most reviewers (for references, O’Connor, 1977) seek hormonal mechanisms which control Na excretion by controlling tubule reabsorption; O’Connor (1977) on the other hand emphasises the role of plasma protein. The experiments reported here were devised to investigate the role of plasma protein and changes in glomerular filtration rate as factors determining Na excretion. These relationships have been most studied by administration of test doses of isotonic saline (e.g. 0.1 25 mol4itre-l NaCI, 0.025 mol.litre-‘ NaHCO,), an experiment which is often referred to as ‘volume expansion.’ Although earlier experiments by O’Connor ( 1958) and Matthews and O’Connor (1968) have shown the general relationships, they were incomplete in that the relationship between the amount of saline retained in the body and fall in plasma protein was not accurately plotted. Also the earlier work mainly described the urinary changes ‘Address for reprints: Dr R. A . Summerill, T h e Department of Physiology, The University of Leeds, Leeds LSZ 9JT.

22

tubular contributions to increased excretion of Na can only be a assessed by sufficiently accurate measurements of creatinine o r inulin clearance. In this paper exogenous creatinine clearance has been measured by the method described by O'Connor and Summerill (1976a). The method uses no loading doses of water o r saline, so that creatinine clearances are measured at the normal low urine flows and without disturbing the normal rates of excretion of Na.

Methods The experiments were performed on four trained mongrel bitches weighing about 13 kg. The dogs were prepared and maintained as previously described (O'Connor and Summerill, 1976a). During the period when the animal was being used for experiments the daily meal consisted of 150 g of lean raw beef, 30 g of shredded suet and 30 g of glucose, providing approximately 6 mmol per day Na and 5 mmol per day CI.

P K O C E D U K ES

These were similar to those described previously (O'Connor and Summerill, 1976a). A typical example is shown in Fig. I . At 08.20, 50 cm:' of 2"/, creatinine (8.8 mmol) was given by stomach tube and the animal allowed free in the exercise yard for 10 min before being brought into a cage in the laboratory. At 09.00 the bladder was catheterised and washed o u t and the animal placed on its side on the experimental table, where it lay with loose restraint for the duration of the experiment. At 09.20, the bladder was again washed with 4 x 10 cm:l of distilled water to begin the first of six urine collection

( g IitrP')

6462-

Crealinine

-

As described by O'Connor and Summerill (1976a) creatinine clearance was determined on the falling curves for plasma creatinine and creatinine excretion shown in Fig. 1. In periods 2, 4,and 6, the clearance (cm:' plasmaemin I ) was directly calculated as creatinine excretion (pmol.min') divided by plasma creatinine (mmol-cm :I).To avoid undue blood loss, no blood samples were taken in periods 3 and 5 but a value for plasma creatinine was obtained from the measurements of periods 2. 4 and 6 by interpolation

-58

40-

.

20-

l a 0900 , ,----,

0820

.

-6 3

mOcm3 soline

-06 r----1 I

n 00

- 0 L Urine mlurne .02 (crn3 min" 1

.___

_____ 10 00 Time

H 0 I) S

CALCULATIONS

66-

60 Na excreiwn (,umol min-')

A N A L Y T I C A L MET

Sodium and potassium were measured by manual flame photometer, all other determinations were by autoanalyser (O'Connor and Summerill 1976a,b,c).

E X P E R I M E N T A L P R 0 T O C O L A N I)

Plasma protein

periods, each of 30 min and ended by a 4 x 10 cm:' washout. During the second, fourth and sixth periods, at 09.54, 10.54, 1 1.54, i t 10 to I2 min before the midpoint of the period, blood saniples were taken from the carotid artery. The test dose was given by stomach tube immediately after the end of the second collection period. This protocol was strictly followed in each of the 48 experiments reported here. The effects of six different doses were tested: I . In nine control experiments, either no dose o r 20 cni:l water (Fig.2a). 2. In nine experiments. 300 cm:' of 'saline' (0.125 niol.litre I NaCI, 0.025 mol4itre ' NaHCO,, 0.004 niol4itre I KCI) (Fig. 2a). 3. In nine experiments, 200 cm:' of 'saline' (Fig. 2b). 4. In nine experiments, 100 cm:' of 'saline' (Fig. 2b). 5 . In six experiments, 300 cm3 of water (Fig. 5a). 6. In six experiments meat, I0g.kg was quickly eaten (Fig. 6).

12 00

W. J. O‘Connor, urrtl R. A . Sunnnerill

24

on a semi-logarithmic plot. I n Fig. 1 the open circles show the interpolated values for plasma creatinine and the values in italics are the clearances calculated from them. The results in each experiment are stated as the change in creatinine clearance from the initial value on each day (the lower figure at the top of Fig. I ) ; this avoids the variability in the initial absolute values. Plasma protein is similarly expressed as the difference from the initial values. The results for each dose of ‘saline’ in Fig. 2 are shown as the mean of nine experiments on four dogs; three experiments on two of the dogs, two on one animal and one on the fourth. The simple means are justifiable because the random variations were much larger than any differences between individual dogs. Since the initial values for Na and CI excretion were always small and varied little in comparison with the effects of the doses, Na and CI excretion are expressed as the absolute rates of excretion. At low rates of excretion, the distribution of values must be skew in that higher rates are possible whereas Na excretion cannot fall below 0. The means stated for low rates of excretion of Na and CI have therefore been calculated using logarithms; these better represent the predominance of low values. After 300 cm3 ‘saline’ when Na excretion increased to 10 to 88 pmol.mirl, the scatter of the results is approximately equal about the arithmetic means, which are therefore stated in figures and tables.

Results EFFECTSOF DOSES OF ‘SALINE’

Full in plasma proteiti In 48 experiments on four dogs the mean plasma protein in period 2 before any dose was 63.1 *2.l

601

(SD) g-litre-Iand there was no change in the control experiments (Fig. 2a). The increasing doses of ‘saline’ produced increasing falls in plasma protein (Fig. 2a, b). The blood collected 3 4 min after the dose always showed considerable fall in plasma protein. Detailed investigation by more frequent sampling (O’Connor, 1958; Matthews and O’Connor, 1968). showed that the greatest fall in plasma protein was about an h o u r after the dose. The sample in period 6, 94 min after the dose of ‘saline’ by stomach tube allowed time for absorption and the fall of plasma protein can therefore be related to the distribution of the added ‘saline’ in the fluid compartments. In the bottom frame of Fig. 3 the fall in plasma protein is plotted against the unexcreted volume of ‘saline’, allowance having been made on the abscissa scale for the excreted Na and water. The mean result was a fall of 1.8 gelitre in plasma protein for each 100 cm3 of ‘saline’ retained, ia a dilution by 3%. I f the plasma volume of dogs of 13 kg is taken as 700 cm3 (Hopper et al., 1944), 3 % fall in plasma protein therefore indicates an increase of 20 cm3 in plasma volume. I t seems therefore that of 100 cm3of ‘saline’ retained, 20 cm3 remains in the plasma and 8 0 cm3 in the interstitial fluid. The extracellular fluid volume of dogs of 13 kg is about 3 litre (Swan r t NI., 1954). Nu excrrtioti I n 48 experiments on four dogs, Na excretion in period 2 before the dose was in the range 0.5 to 8.5, logarithmic mean 2.2 pmol-min I , and did not change in the control experiments (Fig. 2a). The mean Na excretion in period 6 of the control experiments was 1.8 pmol-min I, range of 0.6 to 3. I . There was similarly low rate of excretion of CI. (Table I).

(9

@

Na excretion ( ,urn01 min”

4D

00

Fig. 2 The effi.cts on plusntu protein, creatinine clearance, urine volume and the excretion of Nu. ( a ) in control experiments

Volume (cm’ miri’

Change in creatinine clearance (cm3 mri’ ) Change n plasma protein ( g Iitrci’

0.-

0920

-

y

10.20 Time

1120

p.20

09.20

~OZO Time

n 20

1220

The changes in the lower two graphs are front the initial value in each individital e.uperinient.

Sorliurii excretioti

25

Table I Rate of excretion of ions and volunies of urine in period 6 in control experiments and 14 to 2 h after doses of 'saline'. Mean and standard error or range; tiuniher of experiments- 9 except where otlierwise staiecl.

N a ~ ~ m o l ~ n i i n1.8 - ' (0.6-3.1). K ,. 1.1 f0.4 NH, 10.5 f 4 . 4 ( 3 ) CI 2.1 (1.1-3.4)' 1'0, ., 3.9 f2.8 ( 3 ) so, 3 . I f 1.6 ( 3 ) PH 5.5-6.0

.. 1.

..

Urea pmol.rnin I 73 f 14 (6) Volume (crn3.min-') 0.1 I f0.02 *logarithmic mean and range

4.5(1.8-20.3)' 3.2 f0.6 8.1 f 1.8 (6)

3.3 f0.5 (6) 2.7 f I .0( 3 ) 5.7-6.7

56.0f10.4 9.7 f 2 . 1 7.3 f 4 . 2 (3) 72.2*4.S 6.8 f 1.7 ( 3 ) 3.9 f 0 . 4 (3) 5.5-8.0

69 f 8 ( 6 )

94 f I4 ( 5 )

O.IOf0.02

0.33 f 0 . 0 3

2.8(1.9-4.4)*

After 300 cm3 of 'saline' Na excretion increased (Fig. 2a). Na excretion rose in period 4 together with the fall in plasma protein, with the maximum rate 1 to 14 hours after the dose, as described by O'Connor (1958). The mean rate of excretion in period 6 was

P

T

i

T

P

Retaned saline

Fig. 3 The effect of different doses of 'saline'. Data of period 6 in Fig. 2. Abscissa shows the amount of 'saline' retained (ie dose - excretion). Ordinates: change iti plasma protein and creatine clearancd: rate of excretion of Na: meat1 and standard error is shown. The nutnbers are the mean and statrrlaril deviation in rhe initial control periods.

56 pmolmin - I , range 10 to 88. The excretion of other ions in this period is shown in Table 1. C1 excretion rose to 22 prnolmtin-l and the collected diluted urine was usually alkaline to test papers (pH 5.5 to 8.0) indicating that it contained some HCO, as anion accompanying the increased excretion of Na (Matthews and OConnor, 1968). The excretion of K, PO, and SO, perhaps increased a little but any change was small in comparison with the large increases in Na and C I . Urine volume increased to a mean of 0.8 cm"*min I in period 4 and in period 6 was 0.33 cm3vnin I with urinary Na concentration increased to 0.17 molditre I as compared to 0.02 mol-litre in control experiments. The effects of I00 and 200 cm:l 'saline' is shown in Fig. 2b as almost no effect, Na excretion increasing to the logarithmic means of 4.2 and 4.5 iAmolmin I, After 100 cm:l in eight of nine experiments, Na excretion was in period 6 in the range 1.1 to 6.4 pmolmin '; in the remaining experiment it rose to 42.5 pmolmin ' accompanied by CI, 20.4 p o l min l. After 200 ern:', in seven of nine experiments Na excretion was in the range 1.8 to 4.1 pnolmin and in the other two it rose to 20.3 and 16.7 pmol. min I . Fig. 3 shows the relationship between Na excretion and the amount of 'saline' retained in the body (ir expansion of extracellular fluid) while Fig. 4a shows Na excretion plotted against fall in plasma protein. In comparison with the earlier work of O'Connor (1958) and Matthews and O C o n n o r (l968), the present results emphasise the flat lower part of the curve and this may be due to differences in the N a content of the food. In the earlier papers the animals ingested about I5 mmol Na per day; in this paper only 6 mmol per day. With the dogs on the lower Na intake, doses of 'saline' of 200 cm3 o r less (30 mmol N a or less) usually produce only minor increase in Na excretion. 200 cni3 'saline', with fall of 3.5 gelitre-' in plasma protein, is a threshold dose above which there is steep increase in Na excretion, accompanied by CI and HCO, and increased urine volume. Sodium intake of 6 mmol per day is equivalent to 4 pmolmin - I . The control urines thus have the low rate of Na excretion by which an animal can remain in Na balance on the low Na intake of unsalted food. The highest rate of excretion in Figs. 3,4a, 56 pmol. min I, is equivalent to 80 mmol per day, an intake which would only be reached by heavy salting of artificial foods. Figs. 3 and 4a thus cover therange of Na excretion required by normal life. Itrcreasc in creurinitie clcrirrrrire In the initial period 2 in experiments on four dogs, the mean creatinine clearance was 31.6 I 2.2 (SD,

26

W . J . O’Cotrrror. uiirl K. A . Sritrrttrerill ‘C

E

Fig. 4 ( a ) Na e.ucretiotr nnrl (1)) iticrcw.w itr crcwtitiitie clearance plotted rrgcririst full itt plaatia protein. The open .s,twrhol.r( ‘ , ’,, , ) circ the tireatis ant1 standard w r o r .frottr pcviocl 6 Of Fig. 2 atid the ckr~seds,twrhols H , A,e) /iotri period 4. Burs show otrc siuri(1urtl error. ’\

+l

0

-3

Change in plasma pTotein ( g litre‘ )

n

4); 31.6 I 2.2 (16); 35.5.13.2 (16) and 38.5 12.5 I . The mean of the 48 experiments on four dogs was 34.6 1 3 . 8 (SD). There was no significant change in the nine control experiments (Fig. 2a); from the mean initial value of 34.8 ! I.7 (SE, 9) creatinine clearance in periods 4 and 6 changed by -0.5 ! 0.8 and -0.2 :t0.9 cm3-min I. After 300 cm3 ‘saline’ (Fig. 2a) from the mean initial clearance of 34.0 1 1.0 (SE, 9) cm:’*min I , creatinine clearance increased in periods 4 and 6 by f 3 . 5 10.9 and -t 4.1 1-0.7 cm:’.min I and by t 4.2 I 1 .O in the value estimated by interpolation in period 5 . Creatinine clearance increased as plasma protein fell in the first half hour after the dose and thereafter changed little in the period 4 to 2 hours after the dose. In Fig. 2a the increased creatinine clearance after 300 cm” ‘saline’ is significantly different from the results in control experiments ( P .-0.003). I n Fig. 2b smaller increases after 200 and 100 cmJ of ‘saline’ apparently followed the same time course, the increases in period 6,90 to 120 min after the doses, being 2.3 f 0 . 9 (SE, 9) and 0.5 I 1 .O (SE, 9). The middle frame of Fig. 3 displays the indicated increase in creatinine clearance with increasing retention of ‘saline’. In Fig. 4b increase in creatinine clearance is plotted against fall in plasma protein; the open

( 12) cni”-niin

Urine volume (cm’ min-’)

-6

1 0920 Time

Errors in m e a m r e m e i i t s of‘ crcn tiriirri, c l i w r m c c Random errors were mininiised because the stated results are the means of nine experiments identical in their procedures. The statistics and comparison with controls d o not cover the possibility of systematic distortion of creatinine excretion which might arise from doses of ‘saline’, particularly because of changes in the volume of the urine. After 300 cm:’ o f ’saline’ in Fig. 2a, urine volume rose in period 4 to 0.82 ! 0.25 cm:’ emin I and then fell to 0.33 10.03 in period 6. Changes in this time sequence occurred in nearly all of the experiments but were larger in two where maximum flows of about 3 c m J m i n I were reached 45 min after the dose. However, in Fig. 2a, the rising urine volume in period 4 and its subsequent fall did not affect creatinine clearance, now was any such effect more apparent in the two experiments with high urine flow.

7 D20

1120

12 20

(v,

symbols show the mean points from period 4 and the closed symbols those of period 6. The calculated regression coefficient was an increase of 0.67 cni:’.niin I for each fall of 1 gslitre in plasma protein. The mean points for the intermediate doses lie close to the regression line, indicating that thc data can be represented by a linear relationship.

05

Change in plasma -2 protm ( g litre’)

.

0 -2 Change in plasma protein ( g IilrC’ )

Fig. 5 ( c i ) Elfi,ct qj’300 cttr” wwier h,t*stoniach t i t h e . The tiieatis and .sratirlarrl error of six e.uperitrriwts on thr1.e ( I o ~ s plotted as in Fig. 2. ( h ) Changes it1 rrealitritie r/ecircrtirc crfi1.r 300 ctri:‘ wciter plotted ngairist fall in pla.sniu proteitr: data of periocls 4, 6 of’ Fig. 50. The reRressioti h i e is tlrcrf afier ‘salitrc’ itt Fig. 41).

27

-

No excretion

20

( p n o l mn?)

10-

T

,+

1

Volume (cm’ miri’)

T

057

..j.

...... -.......

+loChange in crealinine clearance ( cmj miri’ 1

‘;ti’

*5-

I

Change n plasma pTotmn ( g IitrP’ )

;

1 ,

+2-0 - 1 09 20

...... ....... ....

2

3

D 20

5

L

6

11 20

n120

Fig. 6 Tlir r f l k i oJiirefit (l0g.X.g I ) r a m ai the arrow. Ploiiing us in Fin. 2. Means aid .viaiidard error o/.si.v c,.vperinieiits on iIirre cIoKs.

Time

To investigate directly the effect o f changing urine volume 300 cm3 of water was given in six experiments on three dogs with the mean results shown in Fig. 5a. O’Connor and Potts (1969) and O’Connor (1975) have reported that under our conditions a single dose o f water does not produce a large diuresis; in Fig. 5a urine flow rose to I .S c m 3 m i n I . 300 cm.l of water caused a small fall in plasma protein (Fig. 5a) and there were small increases i n creatinine clearance of the magnitude to be expected from the changes in plasma protein. This i s shown in Fig. 5b where the points after water are plotted in comparison with the regression line of Fig. 4b. The points after water were with urine volumes of 0.7 and I .5 cm:’.min whereas the nearest corresponding points after ‘saline’ in Fig. 4b were those after 100 cm3 o f ‘saline’ with urine volumes of 0.19 and 0.1 I cm:’min I . Thus the changes in creatinine clearance after ’saline’ are not a secondary effect o f the changes in urine volume and would seem to measure increase in glomerular filtration rate. Shannon (1936) showed that creatinine and inulin clearances are identical in dogs at low urine flows. EFFECT OF EATING MEAT

OConnor and Summerill (1976a) showed that a meal of meat (10 g-kg I ) increased glomerular filtration rate without change in plasma protein. Fig. 6 i s the mean result o f six experiments in which meat was given under the exact conditions of these experiments; in period 6 glomerular filtration rate was increased by 13.0 1 1.4 cm3.min ] and Na excretion to 17.5 14.1 pnol.min I . Meat caused no fall in

plasma protein. These findings are similar to those o f O’Connor and Summerill (1976 a,b,c). I n comparison, after 300 cm:’ ‘saline’ in Fig. 2a, creatinine clearance increased by 4.1 c m 3 m i n I and N a excretion to 56 molmin I . Comparison between the effects of ‘saline’ and protein in causing increased N a excretion has also been made by Lindheimer et a/. 1967) and Massry and Kleeman (1972) who raised creatinine clearance in dogs by about 20 cm:’-min I , either by infusion of 1.8 litre o f ‘saline’ or by feeding casein; Na excretion increased to about 900 mole min after saline and to about 50 m o l m i n I after casein.

Discussion C O N T R O L OF VOLUME O F EXTHACELLULAK F L U I D

After ingestion of 300 cm:’ ‘saline’ the composition of the urine rapidly changed from that given i n column 1 of Table I to that given in column 3. This latter represents an excretion of approximately 80 mmol NaCl per day. The dose o f 300 cm:’ provides an increase o f approximately 97‘: in the volume o f extracellular fluid. I n one narrow range the mechanism i s highly sensitive; practically the whole change in the urine composition occurred when the dose was increased from 200 cmS (column 2) to 300 cm:’ of ‘saline’, a change of 3 in the extracellular fluid volume. From the experiments o f O’Connor (1958) and Matthews and O’Connor (1968) small additional doses o f saline increased Na excretion still further, accompanied by CI, HCO,,, and water.

28

W. J. O'Connor. and R . A. Summerill

Further characteristics of renal function in the acting directly on the kidneys, is the agent which two states of Na retention and Na excretion have causes increased excretion of Na; there is no need to been defined by these experiments. Firstly, there is postulate an intervening hormonal or neural only a small difference in glomerular filtration rate, mechanism. Fig. 4a plots this proposed causal rethe mean rate of 34.6 cmS.min-' in control experi- lationship with an abrupt threshold of plasma proments being increased after 300 cm3 'saline' by tein concentration below which Na excretion in4.1 cms*min-' (12%). Possibly because of the lower creases steeply. With different dietary intakes of glomerular filtration rate there is somewhat lower NaCI, plasma protein could thus remain in a range excretion of K, POo, SO, in the state of Na retention of about 2 gelitre-' on either side of the threshold, so in column 1 compared with column 3 of Table I . regulating the Na content of the body, and the However Bayliss and OConnor ( 1976a,b) and volume of the extracellular fluid within a range about O'Connor and Summerill (1976b,c) have shown that 3 % on either side of a 'normal' volume of about 3 administered doses of these electrolytes and of urea litres. are adequately excreted by dogs on the low dietary intake of Na. Secondly, 300 cms of water in Fig. 5a Plasma protein andglomerular filtration rate only slowly increased urine flow to 1.5 ~ m ~ * m iinn - ~The critical point in the following discussion is the the animals on low salt intake, whereas in the relationship between fall in plasma protein and inclassical experiments on water diuresis (eg Verney, creased glomerular filtration rate, as expressed by 1947) a rate of 4 to 5 cmS.min-' was reached about the slope of the regression line of Fig. 4b showing 40 min after the dose and practically all of the in- that creatinine clearance increased by 0.67 cm3*min gested water was excreted in 2 h; in Fig. 5a 2 h after (2%) for each fall of 1 g-litre-' in plasma protein. the dose of 300 cm3 water only 110 cm3 had been ex- Although change in plasma protein must be a creted. Similar results have been reported by possible cause for change in glomerular filtration O'Connor and Potts (1969) and OConnor (1975). rate, there is very little data in the literature relating The changes in renal function in man in severe Na these two measurements. Massry and Kleeman deficiency reported by McCance (1 936) and (1972) caused serum solids of dogs to fall from 66 to McCance and Widdowson (1937) were similar but 49 galitre-' by infusion of 1.8 litre of saline and creatinine clearance increased from 63 to 85 cm3* much larger. Analysis of the distribution of retained 'saline' by min-* (35%). Wesson ef al. (1950) with a similar O'Connor (1977) indicated each retention of large infusion in two dogs, caused plasma protein to 100 cm3 of 'saline' produces no change in the intra- fall from 57 to 46 and 66 to 45 gslitre-' with increase cellular fluid, increase of 100 cms ( 3 %) in the extra- in creatinine clearance from 47 to 60 and 40 to cellular fluid, of which about 20cm3is increase in the 65 cms.min-l. In neither of these studies was the revolume of the plasma, giving an increase of 1.5 % in lationship between plasma protein and glomerular the circulating blood volume. These are the volume filtration rate a particular object of study but their changes which occur concurrently with the observed findings over a much larger range agree with the fall in plasma protein by 1.8 gelitre-' shown at the present result that a fall in plasma protein of 1 gbottom of Fig. 3. According to the arguments set out litre-' is associated with an increase of 2 % in creaby O'Connor (1977) the fall in plasma protein, tinine clearance. In man, McCance and Widdowson Table 2 Calculated absolute reabsorption of water and sodiuni, and estimated protein concentration in glomerular efferent blood in period 6 in control experiments, and 1 & to 2 h afer doses of 'saline' and meat.

(no dose)

After 'saline' 200 rnP

n= 9 GFR Urine volume Reabsorbed

No (pmol*min-') filtered excreted rea bsorbed

Control

After meni

n= 9

300 m i 3 n= 9

n- 6

34.6f 1.6 0.11 f0.02 34.5

35.8 f1.7 0.10f0.02 35.7

38.1 f1.4 0.33 f0.03 37.8

47.3 f2.5 0.18 fO.O1 47.1

5190 1.8 f0.3 5188

5370 4.5 f2.3 5365

5715 56.0f 10.4 5659

7095 17.5 f4.1 7077

Volrrme ( ~ n r ~ ~ i i i i i i - ~ )

Mean f SE is given for the values actually measured, not for those estimated.

Sodium excretion

29

of Na and water thus increased to match increased filtration. Since the urine contained little C1 and no HCO, (Table I), the increased reabsorption of Na was accompanied by CI and HCO,. With further increase in filtration rate and filtered load of Na after 300 cm, of ‘saline’, absolute reabsorption of volved in producing glomerular filtration according Na and water continued to increase but the increase to two existing theories and reviewed the evidence. was insufficient to prevent excretion of Na, with CI The first theory proposes that the blood pressure in and HCO, in an increased volume of urine. With the glomerular capillaries is high, 9.3 kPa much larger doses of ‘saline’ with Na excretion in(70 mmHg). According to this theory, after 300 cm3 creased to about loo0 pmolvnin-l. Wesson et a/. ‘saline’ the fall of 5.4 gvlitre-1 in plasma protein, (1950), Lindheimer et al. ( I 967) and Massry and by causing a fall in oncotic pressure, would increase Kleeman (1972) all found that the increase in the net filtration pressure by I2 % and an increase of filtered load due to increase in glomerular filtration glomerular filtration by 12% would therefore be ex- rate exceeded the increase in Na excretion ie absolute pected. The second theory developed in Munich- reabsorption of Na was increased after ‘saline’. The Wistar rats proposes that the capillary blood results with large doses of ‘saline’ and in this paper pressure is low, 6.7 kPa (50 mmHg) and the fall in within a more normal range, deny the statement oncotic pressure after 300 cm3 ‘saline’ would pro- often made by reviewers that following doses of duce increase in net filtration pressure of 50%. The ‘saline’ (‘volume expansion’), increased excretion of observed increase in creatinine clearance was 12%. Na and water is due to decreased absolute reAs discussed in detail by O’Connor (1977), the absorption (eg de Wardener rt al., 1961). There are present results are explained by the first theory as no reported experiments on conscious animals in directly due to the fall in plasma protein without which it has been established that infusion of need to postulate any other effect of saline. If the ‘saline’ produced decreased absolute reabsorption. I n contrast to the effect of ‘saline’, meat produced glomerular capillary blood pressure were low, as in the second theory, it must be supposed that the a large decrease in creatinine clearance with only saline also caused reduced permeability of the small increases in the excretion of Na, CI, HCO,, glomerular capillary walls or changes in capillary and water. Analysis in the last column of Table 2 blood pressure or flow by unexplained mechanisms. shows an increase in filtered load of water and Na However it is most probable that the glomerular far beyond the load which in the ’saline’ experiments capillary pressure is different in the dog from the was associated with progressive increase in the excretion of Na, CI, HCO,, and water. The increase in rat (Ott et a/., 1976). The measurements of creatinine clearance appear glomerular filtration after meat is accompanied by adequate to sustain this argument. Increase of 50% much more effective tubular reabsorption of Na and in glomerular filtration rate after 300 cmS ‘saline’ water than after ‘saline’. Similar results were ob- ~beyond , the tained by Lindheimer et ul. (1967) and Massry and would be an increase 17.0 ~ m ~ m i nfar range of the observed increase of 4.1 10.7 (SEM, Kleeman ( I 972). O’Connor (1977) has indicated two mechanisms n =9) and there seems no likelihood of a systematic error of this magnitude. Also the present results are which might explain the differences in tubular rein line with the effects of large doses of saline given absorption of Na after meat and after saline. On the to dogs and Na deficiency in man (references above); one hand, saline produces a fall in plasma protein the same arguments could be applied to these concentration in the peritubular blood and a theory proposes that this would limit proximal tubular reextreme circumstances. absorption. Alternatively it was suggested that substances produced during the metabolism of protein Filtered load and excretion of Nu Accepting the observed values for glomerular filtra- might not only increase glomerular filtration rate but tion rate allows the analysis of tubular reabsorption also increase tubular reabsorption of Na. Both these of Na and water presented in Table 2. Since plasma mechanisms remain speculative at this time. Na (150 mmol.litre-l) remained unchanged by the doses of ‘saline’, the amount of N a filtered at the The,furictiorr oj’clistulpurts qf‘the twphroii In the control experiments where there is virtually no glomeruli is calculated as G F R x ISO(pmol*inin I ) and increases proportionally with increase in glomer- excretion of Na, CI, HCO,, and water, O’Connor ular filtration rate. With doses of 100 and 200 cm3 (1977) has suggested that reabsorption of Na accom‘saline’ there was almost no increase in Na excretion panied by CI, HCO, and water is probably comor urine volume, and calculated tubular reabsorption pleted well before the end of the nephron, and some

(1937) reported that in severe Na deficiency with plasma protein increased from 65 to 80 gelitre-’, glomerular filtration rate fell by about 30%, again a change of 2 % in filtration rate for each change of 1 gslitre-I in plasma protein. O’Connor (1977) has tabulated the pressures in-

IY. J. O'Currrror., a t r d

30

of the facts reported here would be explained by this hypothesis. Doses of 100 and 200 cm3 of 'saline' would cause reabsorption of Na, CI, HCO, and water to continue further into the distal parts of the tubule and the collecting duct until with 300 cm, or more the process is incomplete and Na, CI, HCO, appear in urine of increasing volume. Entry of only small amounts of Na, CI, and HCO, into the distal tubule would explain the poor diuresis recorded in Fig. 5a. According to the accepted explanation ( Jamison, 1976) water diuresis results when reabsorption of Na, CI, and HCO, occurs in the distal parts of the tubules and collecting ducts without reabsorption of water; in the absence of antidiuretic hormone the wall becomes impermeable to water. Water diuresis would be limited if only small amounts of Na, CI, and HCO:, were reaching the diluting segments. We wish to thank Mrs S. Snack for her help with the experiments, particularly in the operation of the Auto-Analysers.

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628-635. Jamison, R. L. (1976). Urinary concentration and dilution. I n This Kirlni.r, Vol. I, Chapter I I pp. 391-441. Edited hy B. M. Brenner and R. C. Rector. W. B. Sunders: Philadelphia. Lindheimer, M. D.. Lalonc, R. C.. and Levinsky, N. G. (1967). Evidence that an acute increase in glonierular filtration has little effect on N a excretion in the dog unless extracellular volume is expanded. Jorrrncrl of Clinicd I n wstiguf ion, 46, 2 5 6 2 6 5 .

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McCance, R. A. (1936). Experimental sodium chloride deficiency in man. Proceivlings ( I / thc R o w 1 Societ>. o / London. E , 119, 245-268. McCance, R . A,, and Widdowson, E. M. (1937). The secretion o f urine in man during experimental salt deficiency. Jortrnul 11f Physiology, 91, 222-23 I. Massry, S. G., and Kleeman, C. R . (1972). Calcium and magnesium excretion during acute rise in glomerular filtrat ion rate. Jortmul (if Laborator). wid Clinicirl Mrdii,iirr,,

80,654-664. Matthews, D. L., and O'Connor, W. J. (1968). The effect on blood and urine o f the ingestion o f sodium bicarbonate. Qrrar/wlr Joirrnul of Exprrirncwtal Plr.~~.siolog.~~. 53, 399-444. O'Connor. W. J. (1958). The effect on urinary volume and composition o f the ingestion o f 0.9 per cent sodium chloride and o f occlusion of the carotid arteries. Qrrurtcrlj~ Jortrnul of Espiviririwtal Plrj..sioIiig~',43, 367-383. O'Connor, W. J . (1962). R i v d Funi,riou. London: Edward Arnold. O'Connor, W. J. (1975). Drinking by dogs during after running. Jorrrnal (if Ph.wio1og.r. 250, 247-259. O'Connor, W. J . (1977). Normal sodium blanace in dogs and in man. Currliovascrtlur Rrseurch, I I , 375-408. O'Connor, W. J . , and Potts, D. J. (1969). The external water exchanges of normal laboratory dogs. Qiturrcrlv Jorrrnol 11f E.\-pc~rinimtulP h . ~ . ~ i o l ~54, g . ~244-265. , O'Connor, W. J., and Summerill, R. A. (1976a). The effect o f a meal o f nieat on glomerular filtration rate in dogs at normal urine flows. Jorrrnul ofPhj,siolog.r, 256, 8 1-91. O'Connor, W. J., and Summerill, R. A. (1976b). The excretion of urea by dogs following a nieat meal. Jortrnirl of Phy.TioI(Ig).,256, 93 I-102. O'Connor, W. J., and Summerill, R . A . ( 1 9 7 6 ~ ) Sulphate . excretion by dogs following ingestion of anlnioniuni sulphate or nieat. Jortrnul ofPhysiologv, 260, 597-607. Ott, C. E., Marchard, G . R.,Diaz-Buxo, J . A,, and Knox, F. G . (1976). Determinants o f glonierular filtration rate in the dog. AnrisricanJorrrnul uf Ph.~~siolog.r, 231, 235-239. Shannon, J. A. (1936). The excretion of inulin and creatinine at low urine flows by the normal dog. Anrrricun Joirrnd of P h y . ~ i d ~ g114. ~ . , 362-365. Swan, R. C., Madisso, H., and Pitts. R. F. (1954). Measurement o f extracellular fluid volume in nephrectoniised dogs. Jorrrnnl ~ J C l i n i c aInwstiRution. l 33, 1447-1453. Verney, E. B. (1947). The antidiuretic hormone and the of thc, factors which determine i t s release. Proi~i~~i1ing.s R o ~ , uSiicic,t.v l ufLontlon, E , 135, 25- 106. de Wardener. H. E., Mills, I . H., Claphani, W. F., and Hayter, C. J . (1961). Studies on the eITerent mechanism o f the sodium diuresis which follows the administration o f intravenous saline in the dog. Clinicul S(,icwcc,.21, 249-258. Wesson. L. G.. Anslow. W. P.,Raisz, L. G . , Bolomey. A. A.. and Ladd, M . (1950). Effect o f sustained expansion o f extracellular fluid volume upon filtration rate, renal plasma flow and electrolyte and water excretion in the dog. Ainrricirn Jorrrnul o f Phj~.sio/ogj~, 162, 677-686.