European Heart Journal (1992) 13 (Supplement D), 129-135
Pharmacological basis for antihypertensive therapy with a novel dopamine agonist G. HAEUSLER, I. LUES, K.-O.
MINCK, P. SCHELLING AND C. A. SEYFRIED
Department of Pharmaceutical Research and Development, E. Merck, Frankfurter Strasse 250, D-6100 Darmstadt, Germany KEY WORDS: Antihypertensive drugs, carmoxirole, dopamine agonists, dopamine2-receptors, presynaptic dopamine receptors. In the past, nearly all major mechanisms involved in the regulation of blood pressure have become targets of antihypertensive drugs. They include the brain stem with its neuronal circuits of central cardiovascular regulation, the sympathetic neuro-effector system, the kidney, the renin angiotensin aldosterone system and the vascular smooth muscle cell. There are various ways of influencing the function of the sympathetic nervous system, but the clinical potential of one mechanism of action has not yet been explored in detail. Drugs that inhibit noradrenaline release through stimulation of inhibitory receptors located at adrenergic nerve terminals in the cardiovascular system (inhibitory presynaptic receptors) are not available for the treatment of hypertension. Among the multiple presynaptic receptors, dopamine receptors which belong to the dopamine2 subtype, are of particular interest. Carmoxirole is a novel indole derivative with a potent agonist action selective for dopaminei-receptors of the periphery. Experimental evidence shows that carmoxirole lowers blood pressure in various models of hypertension mainly or exclusively through inhibition of noradrenaline release from sympathetic nerve endings. This effect of carmoxirole is mediated by presynaptic dopamine receptors with the characteristic that release inhibition is restricted to low rates of sympathetic nerve discharge. Novel mechanisms of potential antihypertensive drags The mechanisms by which antihypertensive drugs lower blood pressure can be subdivided into three categories, namely interference with the function of (a) the sympathetic neuro-efTector system, (b) the kidney and the renin angiotensin aldosterone system and (c) vascular smooth muscle (Table 1). Within these categories possibilities for the development of new types of antihypertensive drugs are few. Potential new drugs are indicated by asterisks in Table 1. As far as the renin angiotensin system is concerned we have not yet developed clinically viable renin inhibitors, however, angiotensin II receptor blocking agents are already under clinical investigation. In the past, nearly every opportunity was used to impair the function of the sympathetic nervous system in order to reduce high blood pressure. Drugs such as or-methyldopa and clonidine act on neuronal circuits in the brain stem to centrally reduce sympathetic tone, and ganglionic blocking agents inhibit ganglionic transmission. Reserpine and adrenergic neurone blocking drugs act on the presynaptic site of the adrenergic neuroeffector junction by depleting adrenergic nerve terminals of the physiological transmitter, noradrenaline, and alphaand beta-blockers prevent the interaction of noradrenaline with its respective receptors at the postsynaptic site. There appears to be only one target left that has not been used for the development of antihypertensive drugs, Address for correspondence: Dr. Guenther Haeusler, Department of Pharmaceutical Research and Development, E. Merck. Frankfurter Strasse 250, D-6100 Darmstadt, Germany.
0195-668X/92/OD0129 + 07 $08.00/0
namely the presynaptic or prejunctional receptors of postganglionic sympathetic nerves. Presynaptic receptors It is well established that the terminal region of peripheral adrenergic nerves is endowed with a variety of receptors1'"". They are believed to be located at or near the varicosities (Fig. 1). Such presynaptic receptors are subdivided into those which, upon stimulation, reduce the amount of noradrenaline released per action potential (inhibitory presynaptic receptors) and those which augment this amount (facilitatory presynaptic receptors) (Fig. I). For obvious reasons only the former lend themselves to high blood pressure treatment. Inhibitory presynaptic receptors include the well-known alpha2adrenoceptor, but also adenosine, muscarine, opiate, prostagJandin and dopamine receptors'1"7'. An antihypertensive drug intended to inhibit the release of noradrenaline through a presynaptic mechanism has to be an agonist at one of the above receptors. However, few of them appear to be well suited for drug development because of a number of unwanted effects. Dopamine receptors appear apropriate for several reasons'8 10'. They do not constitute a homogeneous population, either in the periphery or in the central nervous system'""131. The presynaptic inhibitory dopamine receptors at adrenergic nerve terminals belong to the dopamine2-receptor subtype. Other dopamine2-receptors include those in the central nervous system, the pituitary gland, the sympathetic ganglia and the adrenal cortex. 1992 The European Society of Cardiology
130 G. Haeusler et al.
Table I
Target-based classification of antihypertensive drugs
Sympathetic neuro-eflector system Brainstem Alpha-methyldopa Clonidine Ganglia Ganglionic blocking agents Adrenergic neurone Reserpine Adrenergic neurone blocking agents Presynaptically acting agents* Effector system Alpha and beta blockers Kidney and renin-angiotensin system Diuretics Aldosterone antagonists Renin inhibitors* ACE inhibitors Angiotensin II receptor blockers* Vascular smooth muscle Calcium antagonists Potassium channel activators Hydralazine * Not available or not yet approved for the treatment of hypertension.
Dopamine r receptors occur in the central nervous system, some vascular beds, the kidney and sympathetic ganglia. Thus, by constructing a rather selective dopamineireceptor agonist the number of target organs could be narrowed with some focus on presynaptic inhibitory receptors of postganglionic sympathetic neurones. Further selectivity could highlight a molecule that barely penetrates the blood-brain barrier and, therefore, spares
Inhibitory prttynapilc receptors adenosine alpha2 mutcarint opiate* prostaglandin doponlxlO'5 lx |0" 6 >1 x 1 0 s ND
4x 10"" 4x10"' I x 10~7 6x 10 7 4x 10" 1 x 10"3 2x 10 * 2x 10"6 1 x 10"'
D, Da a, a2 O-2
fi
5HT, A 5HT 2
M,/M 2
Pergolide
5
5x|0" 5x10 ' 7x 10 6 5x 10 7 1 x 10~7 ND
7x I0" 8 3x 10 7 > l x 10 '
ADTN •= amino-6.7-dihydroxy-1,2,3,4-tetrahydronaphthalene; DH A = dihydroalprenolol; 8-OH-DPAT" 8hydroxy-2(di-N-propylamine)tetralin; QNB = quinuclidinyl-benzilate; ND = not determined.
•2 HI, S 4 / S 2
• 0 - 3 Hi, S J / S ,
i
340 300 J 240' |
• »
200
§ t
160 120 80 40 10 9 8 7 6
7 7
10
9 8 7 6
7
7
Conctntration, -log (mol.l"')
Figure 3 The effects of carmoxirole (Bj), clonidine (0) and rauwolscine (D) on the release of 3H-noradrenaline in response to transmural stimulation of the perfused/superfuscd rabbit ear artery previously loaded with 3H-noradrcnaline. Transmural stimulation was performed with alternating frequencies of 0-5 Hz (1st and 3rd stimulation) and 2 Hz (2nd and 4th stimulation) for 1 min each, with supramaximal voltage and a pulse width of 0-5 ms. The ear arteries were stimulated twice before the administration of drugs (Si and Sj) and twice during their continuous infusion (Sj and Sj). The evoked overflow of 3H-NA was calculated as the difference of the electrically induced release minus baseline release. The effect of the drugs is expressed as percent of controls (means ±SEM, n = 4-8).
provide convincing evidence that the in vivo effect of carmoxirole is also mediated by presynaptic dopamine receptors, as suggested by the in vitro results. In anaesthetized vagotomized cats, the tachycardia, in response to stimulation of the cardioaccelerator nerve with 2 Hz, was inhibited in a dose-dependent manner by carmoxirole (0-3-300 (ig . kg"1 i.v.). Under control conditions, the stimulation-induced increase in heart rale was by approximately 50 beat, min"1 and maximal inhibition
025
OS
I
Z
4
Eltcirlcal cumulation {Hz)
Figure 4 The effect of carmoxirole (0-1 mg . kg"' i.v.) on the pressor responses induced by electrical stimulation of the total sympathetic outflow in pithed spontaneously hypertensive rats. Electrical stimulation was performed through the pithing rod introduced into the vertebral canal and was carried out with supramaximal voltage and a pulse width of 1 ms. Carmoxirole reduced the pressor responses to electrical stimulation of the total sympathetic outflow, an effect which was antagonized by (-)-sulpiride. Shown are mean values ±SEM of 10 pithed rat preparations. O = control; • =carmoxirole (0-1 mg . kg ',i.v.); • —carmoxirole (0-1 mg . kg ', i.v.) and (-)-sulphide (0-1 mg . kg"'. i.v.).
by carmoxirole was 50-60% (results not shown). This degree of maximal inhibition agrees well with the findings obtained in 3H-noradrenaline release experiments (see Fig. 3). From the results shown in Figs 3-5 it can be concluded that carmoxirole affects both vascular and cardiac adrenergic nerve endings. Antihypertensive effect of carmoxirole
Carmoxirole is a potent and efficacious antihypertensive agent in both spontaneously hypertensive rats and
Pharmacological basis for a new dopamine agonist 133
100
-10 -15 -20 -25 0-25
A-
0-5 Electrical stimulation (Hz)
16
Figure 5 A replot of the results given in Fig. 4 and showing the percent inhibition of the pressor response. Note that the inhibitory effects of carmoxirole are particularly pronounced at low rates of electrical stimulation. Sj = carmoxirole ( 0 1 m g . kg"' i.v.); D=carmoxirole (0-1 mg . kg"'i.v.) and (-)-sulphide (01 mg.kg~'i.v.).
renal hypertensive dogs (cellophane perinephritis hypertension). In spontaneously hypertensive rats with severe hypertension (mean arterial blood pressure before treatment >200 mmHg) carmoxirole produced a dose-dependent reduction in blood pressure by maximally 100 mmHg over the dose range of 0 1 to 10 mg . kg"1 p.o. Carmoxirole was particularly potent in renal hypertensive dogs with the relevant dose range between 0005 and 005 mg . kg"' p.o. and a maximal reduction of mean arterial blood pressure from 140 to 120 mmHg. Figure 6 shows the dose-dependent decreases in mean arterial blood pressure and total peripheral resistance produced by i.v. injection of carmoxirole to chronically instrumented, conscious, spontaneously hypertensive rats. There appears to be little doubt that the fall in blood pressure is due, mostly or exclusively, to a decrease in total peripheral resistance. Similar to the 3H-noradrenaline release experiments (Fig. 3), sulpiride also antagonized the antihypertensive effect of carmoxirole. The same holds true for the dopamine2-receptor antagonist domperidone (results not shown). The use of spontaneously hypertensive rats with chronically implanted Doppler ultrasonic devices allowed the measurement of regional blood flows in the lower aorta, mesenteric artery and renal artery. In these animals carmoxirole, administered i.v., produced a dosedependent fall in mean arterial blood pressure. While blood flow in the renal artery was maintained or slightly elevated, it was reduced in the mesenteric vascular bed. The most conspicuous finding was a pronounced
Figure 6 Dose-dependent reduction in mean arterial blood pressure (MAP) and total peripheral resistance (TPR) by carmoxirole ( • ) and antagonism of the effect of carmoxirole by (-)-sulpiride ( 0 ) in chronically instrumented, conscious spontaneously hypertensive rats. Shown are the mean values ±SEM of eight animals.
increase of blood flow in the lower aorta, indicating that the neuro-effector system of arteries to skeletal muscle is particularly sensitive to carmoxirole. Discussion of the mechanism of the antihypertensive effect So far, our results have demonstrated that carmoxirole inhibits adrenergic neurotransmission and lowers blood pressure. The question arises whether the antihypertensive effect of carmoxirole can be attributed entirely, or overwhelmingly, to the peripheral presynaptic effect of the drug. Since binding experiments revealed some affinity of carmoxirole for alpha-adrenoceptors and 5HT IA receptors, effects related to blockade and stimulation, respectively, of these receptor sites could contribute to the fall in blood pressure. However, carmoxirole over the dose range relevant for an antihypertensive effect did not antagonize the pressor responses to alpha r or alpha2-adrenoceptor agonists either in rats or monkeys. Furthermore, pharmacological 5-HT,A receptor blockade did not antagonize the antihypertensive effect of carmoxirole. The penetration of carmoxirole into the central nervous system appears to be very poor. This is indicated by the results of experiments measuring striatal DOPA accumulation in rats pretreated with reserpine and a decarboxylase inhibitor. Inhibitory effects of carmoxirole were seen only at doses beyond those necessary for the reduction of high blood pressure. The same holds true for the influence of carmoxirole on rotational
134 G. Haeusler et al.
MAP
Lower oorto
Mesenteric artery
I Row I Resistance
Renal artery
-50 40
160
640 ', i.v.
Figure 7 Cardiovascular effects of carmoxirole in chronically instrumented, conscious, spontaneously hypertensive rats. Regional blood flows in the lower aorta, mesenteric artery and renal artery were measured with previously implanted. Doppler ultrasonic devices. Concomitant with a dose-dependent fall in mean arterial blood pressure (MAP) by carmoxirole was an increase of flow in the lower aorta, which suggests that elevation of blood flow to skeletal muscle is a prominent effect of the drug. Shown are the mean values ±SEM of eight animals.
behaviour in rats with unilateral lesions of the nigrostriatal pathway. Furthermore, autoradiographic studies provided direct proof for minor access of the drug to the central nervous system. From this one may infer that any centrally mediated antihypertensive action of carmoxirole, be it dopaminergic or through 5-HT,A-receptors, is not likely. It is, therefore, concluded that most of the antihypertensive effect of carmoxirole can be assigned to the stimulation of inhibitory presynaptic dopamine2-receptors. It cannot be excluded, however, that ganglionic dopamine2-receptors may play some role. References [1J Langer SZ. Presynaptic regulation of the release of catecholamines. Pharmac Rev 1981; 32: 337-62. [2] Starke K. Presynaptic receptors. A Rev Pharmac Toxicol 1981; 21: 7-30. [3] Starke K, Gothert M, Kilbinger H. Modulation of neurotransmitter release by presynaptic autoreceptors. Physiol Rev 1989; 69: 864-989. [4] Lokhandwala MF. Presynaptic receptor systems on cardiac sympathetic nerves. Life Sci 1979; 24: 1823-32. [5] Dahlof C. Studies on /?-adrenoceptor mediated facilitation of sympathetic neurotransmission. Acta Physiol Scand (Suppl) 1981; 500: 1-147.
[6] Long JP, Heintz S, Cannon JG, Kim J. Inhibition of the sympathetic nervous system by 5,6-dihydroxy-2-dimethylaminotctralin (M-7), apomorphine and dopamine. J Pharmacol Exp Ther 1975; 192: 336-42. [7] Wilffert B, Smit G, de Jonge A, Thoolen MJMC, Timmermans PBMSM, van Zwieten PA. Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat. Characterization and role in relation to presynaptic ar-adrenoceptors. NaunynSchmiedeberg's Arch Pharmacol 1984; 326: 91-8. [8] Cavero I, Massingham R, Lefivre-Borg F. Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents. Part II: sites and mechanisms of action of dopamine receptor agonists. Life Sci 1982; 31: 1059-69. [9] Clapham JC, Hamilton TC. Presynaptic dopamine receptors mediate the inhibitory action of dopamine agonists on stimulation-evoked pressor responses in the rat. J Auton Pharmacol 1982; 3: 181-8. [10] Clark BJ. Is stimulation of prejunctional dopamine receptors an antihypertensive principle? Clin Exp Hypertens, Part A, Theory and Practice 1987; A9: 1045-68. [11] Creese 1. Biochemical properties of CNS dopamine receptors. In: Meltzer HY, ed. Psychopharmacology. The third generation of progress. New York: Raven Press, 1987: 257-64. [12] Hilditch A, Drew GM. Subclassification of peripheral dopamine receptors. Clin Exp Hypertens, Part A, Theory and Practice 1987; A9: 853-72. [13] Willems JL, Buylaert WA, Lefebvre RA, Bogaert MG. Neuronal dopamine receptors on autonomic ganglia and sympathetic nerves and dopamine receptors in gastrointestinal system. Pharmacol Rev 1985; 37: 165-216. [14] Sved AF, Fernstrom JD. Reduction in blood pressure in normal and spontaneously hypertensive rats by lergotrile mesylate. J Pharm Pharmacol 1979; 31: 814-7. [15] Sved AF, Fernstrom JD. Evidence for a peripheral dopaminergic mechanism in the antihypertensive action of lergotrile. Life Sci 1980; 27: 349-54. [16] Hahn RA, Farrell SK. Inhibition of peripheral adrenergk neurotransmission by lergotrile in spontaneously hypertensive rats. Life Sci 1981; 28: 2497-504. [17] Yen TT, Stamm NB, Clemens JA. Pergolide, a potent dopaminergic antihypertensive. Life Sci 1979; 25: 209-16. [18] Hahn R. Inhibitory effects of pergolide on peripheral adrenergic neurotransmission in spontaneously hypertensive rats. Life Sci 1981; 29: 2501-9. [19] Barrett RJ, Lokhandwala MF. Dopaminergic inhibition of cardiac sympathetic nerve function by pergolide. Eur J Parmacol 1982; 77: 79-83. [20] Laubie M, Schmitt H. Inhibitory effects of piribedil on adrenergic neurotransmission. Eur J Pharmacol 1978; 52: 99-107. [21] Hahn RA, MacDonold BR, Martin MA. Antihypertensive activity of LY 141 865, a selective presynaptic dopamine receptor agonist. J Pharmacol Exp Ther 1983; 224: 206-14. [22] Memo M, Sagheddu G, Carruba MO, Spano PF. Dihydroergotoxine decreases blood pressure in spontaneously hypertensive raU by interacting with peripheral dopamine receptors. Life Sci 1985; 36: 1515-22. [23] Roquebert J, Grenie B. Presynaptic inhibition by dihydroergotoxine of the response to peripheral sympathetic nerve stimulation in rats. Eur J Pharmacol 1987; 138: 433-7. [24] Lokhandwala MF. Analysis of the effect of bromocriptine on blood pressure and sympathetic nerve function. Eur J Pharmacol 1979; 56: 253-6. [25] Ziegler MG, Lake CR, Williams AC, Teychenne PF, Shoulson I, Steinsland O. Bromocriptine inhibits norepinephrine release. Cli Pharmacol Ther 1979; 25: 137-42. [26] Van Loon GR, Sole MJ, Bain J, Russe JL. Effects of bromocriptine on plasma catecholamines in normal men. Neuroendocrinology 1979; 28: 425-34.
•4
-i
Pharmacological basis for a new dopamine agonist 135
[27] Carey RM, Van Loon GR, Baines AD, Kaiser DL. Suppression of basal and stimulated noradrenergic activities by the dopamine agonist bromocriptine in man. J Cli Endocrinol Metab 1983; 56: 595-602. [28] Mercuro G, Rossetti ZL, Tocco L, Rivano CA, Cherchi A, Gessa GL. Bromocriptine reduces plasma noradrenaKne and 3,4-dihydroxyphenylacetic acid in normal and hypertensive subjects. Eur J Clin Pharmacol 1985; 27: 671-5. [29] Mercuro G, Rossetti ZL, Rivano CA, Ruscazio M, Tocco L, Gessa GL, Cherchi A. Peripheral dopamine receptors in the antihypertensive action of dihydroergotoxine in humans. Hypertension 1987; 9: 35-40. [30] Lombardi C, de Cotiis R, Spedini C, Missale C, Memo M, Spano PF, Reversal by the selective D-2 dopamine receptor blocker sulpiride of the hypotensive effect of co-dergocrine in elderly hypertensives. Eur J Clin Pharmacol 1987; 33: 519-21. [31] Brodie BB, Chang CC, Costa E. On the mechanism of action of guanethidine and bretylium. Br J Pharmacol 1965; 25: 171-8. [32] Haeusler G, Haefely W, Huerlimann A. On the mechanism of the adrenergic nerve blocking action of bretylium. NaunynSchmiedeberg's Arch Pharmacol 1969; 265: 260-77. [33] Haeusler G, Haefely W. Modification of release by adrenergic neuron blocking agents and agents that alter the action potential. In: Paton DM ed. The release of catecholamines from adrenergic neurons. Oxford: Pergamon Press, 1979: 185-216.
r
r
[34] files P. Mechanisms of receptor-mediated modulation of transmitter release in noradrenergic, cholinergic and sensory neurones. Neuroscience 1986; 17: 909-28. [35] Bottcher H, Gericke R. Synthese von 3-[4-(1.2,3,6-Tetrahydro-4-phenyH-pyridinyl)butyl]-5-indolcarbonsaure, eine blutdrucksenkende Verbindung mit neuartigem Wirkprinzip. Liebigs Ann Chem 1988: 749-52. [36] Haase AF, Greiner HE, Seyfried CA. In vitro and in vivo dopaminergic properties of EMD 45 609. Naunyn-Schmiedeberg's Arch Pharmacol 1988; 337 (Suppl): R71. [37] Schelling P, Bergmann R, Bottcher H, Minck K.O, Schulze E. A new dopamine agonist selective for DA^receptors as an antihypertensive drug. Naunyn-Schmiedeberg's Arch Pharmacol 1988; 337 (Suppl): R71. [38] Haase AF, GreineT HE, Seyfried CA. Neurochemical profile of EMD 45 609 (carmoxirole), a dopamine DAj-receptor agonist. Naunyn-Schmiedeberg's Arch Pharmacol 1991; 343: 588-94. [39] Aguilera G, Mendelsohn FAO, Catt KJ. Dopaminergic regulation of aldosterone secretion. In: Martini L, Ganong WF, eds. Frontiers in Neuroendocrinology. New York: Raven Press, 1984; 8: 265-93. [40] Carey RM. Dopaminergic control of aldosterone secretion. In: Hieble JP, ed. Cardiovascular function of peripheral dopamine receptors. New York: Marcel Dekker, 1989: 297-314.
Pharmacological basis for antihypertensive therapy with a novel dopamine agonist G. HAEUSLER, I. LUES, K.-O.
MINCK, P. SCHELLING AND C. A. SEYFRIED
Department of Pharmaceutical Research and Development, E. Merck, Frankfurter Strasse 250, D-6100 Darmstadt, Germany KEY WORDS: Antihypertensive drugs, carmoxirole, dopamine agonists, dopamine2-receptors, presynaptic dopamine receptors. In the past, nearly all major mechanisms involved in the regulation of blood pressure have become targets of antihypertensive drugs. They include the brain stem with its neuronal circuits of central cardiovascular regulation, the sympathetic neuro-effector system, the kidney, the renin angiotensin aldosterone system and the vascular smooth muscle cell. There are various ways of influencing the function of the sympathetic nervous system, but the clinical potential of one mechanism of action has not yet been explored in detail. Drugs that inhibit noradrenaline release through stimulation of inhibitory receptors located at adrenergic nerve terminals in the cardiovascular system (inhibitory presynaptic receptors) are not available for the treatment of hypertension. Among the multiple presynaptic receptors, dopamine receptors which belong to the dopamine2 subtype, are of particular interest. Carmoxirole is a novel indole derivative with a potent agonist action selective for dopaminei-receptors of the periphery. Experimental evidence shows that carmoxirole lowers blood pressure in various models of hypertension mainly or exclusively through inhibition of noradrenaline release from sympathetic nerve endings. This effect of carmoxirole is mediated by presynaptic dopamine receptors with the characteristic that release inhibition is restricted to low rates of sympathetic nerve discharge. Novel mechanisms of potential antihypertensive drags The mechanisms by which antihypertensive drugs lower blood pressure can be subdivided into three categories, namely interference with the function of (a) the sympathetic neuro-efTector system, (b) the kidney and the renin angiotensin aldosterone system and (c) vascular smooth muscle (Table 1). Within these categories possibilities for the development of new types of antihypertensive drugs are few. Potential new drugs are indicated by asterisks in Table 1. As far as the renin angiotensin system is concerned we have not yet developed clinically viable renin inhibitors, however, angiotensin II receptor blocking agents are already under clinical investigation. In the past, nearly every opportunity was used to impair the function of the sympathetic nervous system in order to reduce high blood pressure. Drugs such as or-methyldopa and clonidine act on neuronal circuits in the brain stem to centrally reduce sympathetic tone, and ganglionic blocking agents inhibit ganglionic transmission. Reserpine and adrenergic neurone blocking drugs act on the presynaptic site of the adrenergic neuroeffector junction by depleting adrenergic nerve terminals of the physiological transmitter, noradrenaline, and alphaand beta-blockers prevent the interaction of noradrenaline with its respective receptors at the postsynaptic site. There appears to be only one target left that has not been used for the development of antihypertensive drugs, Address for correspondence: Dr. Guenther Haeusler, Department of Pharmaceutical Research and Development, E. Merck. Frankfurter Strasse 250, D-6100 Darmstadt, Germany.
0195-668X/92/OD0129 + 07 $08.00/0
namely the presynaptic or prejunctional receptors of postganglionic sympathetic nerves. Presynaptic receptors It is well established that the terminal region of peripheral adrenergic nerves is endowed with a variety of receptors1'"". They are believed to be located at or near the varicosities (Fig. 1). Such presynaptic receptors are subdivided into those which, upon stimulation, reduce the amount of noradrenaline released per action potential (inhibitory presynaptic receptors) and those which augment this amount (facilitatory presynaptic receptors) (Fig. I). For obvious reasons only the former lend themselves to high blood pressure treatment. Inhibitory presynaptic receptors include the well-known alpha2adrenoceptor, but also adenosine, muscarine, opiate, prostagJandin and dopamine receptors'1"7'. An antihypertensive drug intended to inhibit the release of noradrenaline through a presynaptic mechanism has to be an agonist at one of the above receptors. However, few of them appear to be well suited for drug development because of a number of unwanted effects. Dopamine receptors appear apropriate for several reasons'8 10'. They do not constitute a homogeneous population, either in the periphery or in the central nervous system'""131. The presynaptic inhibitory dopamine receptors at adrenergic nerve terminals belong to the dopamine2-receptor subtype. Other dopamine2-receptors include those in the central nervous system, the pituitary gland, the sympathetic ganglia and the adrenal cortex. 1992 The European Society of Cardiology
130 G. Haeusler et al.
Table I
Target-based classification of antihypertensive drugs
Sympathetic neuro-eflector system Brainstem Alpha-methyldopa Clonidine Ganglia Ganglionic blocking agents Adrenergic neurone Reserpine Adrenergic neurone blocking agents Presynaptically acting agents* Effector system Alpha and beta blockers Kidney and renin-angiotensin system Diuretics Aldosterone antagonists Renin inhibitors* ACE inhibitors Angiotensin II receptor blockers* Vascular smooth muscle Calcium antagonists Potassium channel activators Hydralazine * Not available or not yet approved for the treatment of hypertension.
Dopamine r receptors occur in the central nervous system, some vascular beds, the kidney and sympathetic ganglia. Thus, by constructing a rather selective dopamineireceptor agonist the number of target organs could be narrowed with some focus on presynaptic inhibitory receptors of postganglionic sympathetic neurones. Further selectivity could highlight a molecule that barely penetrates the blood-brain barrier and, therefore, spares
Inhibitory prttynapilc receptors adenosine alpha2 mutcarint opiate* prostaglandin doponlxlO'5 lx |0" 6 >1 x 1 0 s ND
4x 10"" 4x10"' I x 10~7 6x 10 7 4x 10" 1 x 10"3 2x 10 * 2x 10"6 1 x 10"'
D, Da a, a2 O-2
fi
5HT, A 5HT 2
M,/M 2
Pergolide
5
5x|0" 5x10 ' 7x 10 6 5x 10 7 1 x 10~7 ND
7x I0" 8 3x 10 7 > l x 10 '
ADTN •= amino-6.7-dihydroxy-1,2,3,4-tetrahydronaphthalene; DH A = dihydroalprenolol; 8-OH-DPAT" 8hydroxy-2(di-N-propylamine)tetralin; QNB = quinuclidinyl-benzilate; ND = not determined.
•2 HI, S 4 / S 2
• 0 - 3 Hi, S J / S ,
i
340 300 J 240' |
• »
200
§ t
160 120 80 40 10 9 8 7 6
7 7
10
9 8 7 6
7
7
Conctntration, -log (mol.l"')
Figure 3 The effects of carmoxirole (Bj), clonidine (0) and rauwolscine (D) on the release of 3H-noradrenaline in response to transmural stimulation of the perfused/superfuscd rabbit ear artery previously loaded with 3H-noradrcnaline. Transmural stimulation was performed with alternating frequencies of 0-5 Hz (1st and 3rd stimulation) and 2 Hz (2nd and 4th stimulation) for 1 min each, with supramaximal voltage and a pulse width of 0-5 ms. The ear arteries were stimulated twice before the administration of drugs (Si and Sj) and twice during their continuous infusion (Sj and Sj). The evoked overflow of 3H-NA was calculated as the difference of the electrically induced release minus baseline release. The effect of the drugs is expressed as percent of controls (means ±SEM, n = 4-8).
provide convincing evidence that the in vivo effect of carmoxirole is also mediated by presynaptic dopamine receptors, as suggested by the in vitro results. In anaesthetized vagotomized cats, the tachycardia, in response to stimulation of the cardioaccelerator nerve with 2 Hz, was inhibited in a dose-dependent manner by carmoxirole (0-3-300 (ig . kg"1 i.v.). Under control conditions, the stimulation-induced increase in heart rale was by approximately 50 beat, min"1 and maximal inhibition
025
OS
I
Z
4
Eltcirlcal cumulation {Hz)
Figure 4 The effect of carmoxirole (0-1 mg . kg"' i.v.) on the pressor responses induced by electrical stimulation of the total sympathetic outflow in pithed spontaneously hypertensive rats. Electrical stimulation was performed through the pithing rod introduced into the vertebral canal and was carried out with supramaximal voltage and a pulse width of 1 ms. Carmoxirole reduced the pressor responses to electrical stimulation of the total sympathetic outflow, an effect which was antagonized by (-)-sulpiride. Shown are mean values ±SEM of 10 pithed rat preparations. O = control; • =carmoxirole (0-1 mg . kg ',i.v.); • —carmoxirole (0-1 mg . kg ', i.v.) and (-)-sulphide (0-1 mg . kg"'. i.v.).
by carmoxirole was 50-60% (results not shown). This degree of maximal inhibition agrees well with the findings obtained in 3H-noradrenaline release experiments (see Fig. 3). From the results shown in Figs 3-5 it can be concluded that carmoxirole affects both vascular and cardiac adrenergic nerve endings. Antihypertensive effect of carmoxirole
Carmoxirole is a potent and efficacious antihypertensive agent in both spontaneously hypertensive rats and
Pharmacological basis for a new dopamine agonist 133
100
-10 -15 -20 -25 0-25
A-
0-5 Electrical stimulation (Hz)
16
Figure 5 A replot of the results given in Fig. 4 and showing the percent inhibition of the pressor response. Note that the inhibitory effects of carmoxirole are particularly pronounced at low rates of electrical stimulation. Sj = carmoxirole ( 0 1 m g . kg"' i.v.); D=carmoxirole (0-1 mg . kg"'i.v.) and (-)-sulphide (01 mg.kg~'i.v.).
renal hypertensive dogs (cellophane perinephritis hypertension). In spontaneously hypertensive rats with severe hypertension (mean arterial blood pressure before treatment >200 mmHg) carmoxirole produced a dose-dependent reduction in blood pressure by maximally 100 mmHg over the dose range of 0 1 to 10 mg . kg"1 p.o. Carmoxirole was particularly potent in renal hypertensive dogs with the relevant dose range between 0005 and 005 mg . kg"' p.o. and a maximal reduction of mean arterial blood pressure from 140 to 120 mmHg. Figure 6 shows the dose-dependent decreases in mean arterial blood pressure and total peripheral resistance produced by i.v. injection of carmoxirole to chronically instrumented, conscious, spontaneously hypertensive rats. There appears to be little doubt that the fall in blood pressure is due, mostly or exclusively, to a decrease in total peripheral resistance. Similar to the 3H-noradrenaline release experiments (Fig. 3), sulpiride also antagonized the antihypertensive effect of carmoxirole. The same holds true for the dopamine2-receptor antagonist domperidone (results not shown). The use of spontaneously hypertensive rats with chronically implanted Doppler ultrasonic devices allowed the measurement of regional blood flows in the lower aorta, mesenteric artery and renal artery. In these animals carmoxirole, administered i.v., produced a dosedependent fall in mean arterial blood pressure. While blood flow in the renal artery was maintained or slightly elevated, it was reduced in the mesenteric vascular bed. The most conspicuous finding was a pronounced
Figure 6 Dose-dependent reduction in mean arterial blood pressure (MAP) and total peripheral resistance (TPR) by carmoxirole ( • ) and antagonism of the effect of carmoxirole by (-)-sulpiride ( 0 ) in chronically instrumented, conscious spontaneously hypertensive rats. Shown are the mean values ±SEM of eight animals.
increase of blood flow in the lower aorta, indicating that the neuro-effector system of arteries to skeletal muscle is particularly sensitive to carmoxirole. Discussion of the mechanism of the antihypertensive effect So far, our results have demonstrated that carmoxirole inhibits adrenergic neurotransmission and lowers blood pressure. The question arises whether the antihypertensive effect of carmoxirole can be attributed entirely, or overwhelmingly, to the peripheral presynaptic effect of the drug. Since binding experiments revealed some affinity of carmoxirole for alpha-adrenoceptors and 5HT IA receptors, effects related to blockade and stimulation, respectively, of these receptor sites could contribute to the fall in blood pressure. However, carmoxirole over the dose range relevant for an antihypertensive effect did not antagonize the pressor responses to alpha r or alpha2-adrenoceptor agonists either in rats or monkeys. Furthermore, pharmacological 5-HT,A receptor blockade did not antagonize the antihypertensive effect of carmoxirole. The penetration of carmoxirole into the central nervous system appears to be very poor. This is indicated by the results of experiments measuring striatal DOPA accumulation in rats pretreated with reserpine and a decarboxylase inhibitor. Inhibitory effects of carmoxirole were seen only at doses beyond those necessary for the reduction of high blood pressure. The same holds true for the influence of carmoxirole on rotational
134 G. Haeusler et al.
MAP
Lower oorto
Mesenteric artery
I Row I Resistance
Renal artery
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640 ', i.v.
Figure 7 Cardiovascular effects of carmoxirole in chronically instrumented, conscious, spontaneously hypertensive rats. Regional blood flows in the lower aorta, mesenteric artery and renal artery were measured with previously implanted. Doppler ultrasonic devices. Concomitant with a dose-dependent fall in mean arterial blood pressure (MAP) by carmoxirole was an increase of flow in the lower aorta, which suggests that elevation of blood flow to skeletal muscle is a prominent effect of the drug. Shown are the mean values ±SEM of eight animals.
behaviour in rats with unilateral lesions of the nigrostriatal pathway. Furthermore, autoradiographic studies provided direct proof for minor access of the drug to the central nervous system. From this one may infer that any centrally mediated antihypertensive action of carmoxirole, be it dopaminergic or through 5-HT,A-receptors, is not likely. It is, therefore, concluded that most of the antihypertensive effect of carmoxirole can be assigned to the stimulation of inhibitory presynaptic dopamine2-receptors. It cannot be excluded, however, that ganglionic dopamine2-receptors may play some role. References [1J Langer SZ. Presynaptic regulation of the release of catecholamines. Pharmac Rev 1981; 32: 337-62. [2] Starke K. Presynaptic receptors. A Rev Pharmac Toxicol 1981; 21: 7-30. [3] Starke K, Gothert M, Kilbinger H. Modulation of neurotransmitter release by presynaptic autoreceptors. Physiol Rev 1989; 69: 864-989. [4] Lokhandwala MF. Presynaptic receptor systems on cardiac sympathetic nerves. Life Sci 1979; 24: 1823-32. [5] Dahlof C. Studies on /?-adrenoceptor mediated facilitation of sympathetic neurotransmission. Acta Physiol Scand (Suppl) 1981; 500: 1-147.
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