Neuronal and postjunctional components in the blood pressure effects of dopamine and bromocriptine in rabbits

We have studied the contribution of neuronal and postjunctional dopamine (DA) receptors and of the DA1 and DA2 receptor subtypes to the blood pressure effects of DA and bromocriptine in the rabbit. The norepinephrine release rate, i.e., the rate of entry of endogenous norepinephrine into the plasma, was derived from the plasma level of endogenous norepinephrine and the plasma [3H]norepinephrine clearance. Bromocriptine (40 micrograms kg-1) lowered the norepinephrine release rate and the arterial blood pressure both in anesthetized rabbits and in pithed rabbits with electrically stimulated sympathetic outflow. These effects were antagonized by the selective DA2 antagonist domperidone but not by the selective DA1 antagonist SCH 23390. DA (10-160 micrograms kg-1 min-1) dose-dependently increased the norepinephrine release rate and caused only transient hypotension in anesthetized rabbits. However, after treatment with desipramine, DA did not change the norepinephrine release rate and produced a persistent fall in blood pressure. When DA and domperidone were given simultaneously to desipramine-treated rabbits, the hypotensive effect of DA was unchanged, but now DA increased the norepinephrine release rate. When DA and SCH 23390 were given simultaneously to desipramine-treated rabbits, DA failed to lower blood pressure and decreased the norepinephrine release rate. Propranolol did not change the effects of DA in desipramine-treated rabbits. These results suggest that bromocriptine decreases blood pressure by activating ganglionic and/or prejunctional, inhibitory DA2 receptors in the peripheral sympathetic nervous system. DA also activates these receptors, but in addition releases norepinephrine in the manner of an indirectly acting sympathomimetic amine and activates postjunctional vascular DA1 receptors, and the latter seems to be the main component in DA-induced hypotension.     

Dopamine: pharmacologic and therapeutic aspects

Dopamine is a biogenic amine synthesized in the hypothalamus, in the arcuate nucleus, the caudad, and various areas of the central and peripheral nervous system. It has been widely established that dopamine and its agonists play an important role in cardiovascular, renal, hormonal, and central nervous system regulation through stimulation of alpha and beta adrenergic and dopaminergic receptors. There are several agonists of dopamine-2 (DA 2 ) dopaminergic receptors, such as bromocriptine, pergolide, lisuride, quinpirole, and carmoxirole, which inhibit norepinephrine release and produce a decrease in arterial blood pressure; in some cases, bromocriptine and pergolide also reduce heart rate. From a therapeutic point of view, the above-mentioned agonists are used for treating Parkinson's disease, acting over DA 2 dopaminergic receptors of the nigrostriatal system. Bromocriptine and the other dopaminergic agonists mentioned act over DA 2 receptors of the tuberoinfundibular system, inhibiting prolactin release and decreasing hyperprolactinemia and tumor size. Among DA 1 receptor agonists, we can mention fenoldopam, piribedil, ibopamine, SKF 3893, and apomorphine (nonspecific). Activation of these receptors decreases peripheral resistance, inducing lowering of arterial blood pressure and increases in heart rate, sympathetic tone, and activity of the renin aldosterone system. Among DA 2 receptor antagonists, we can mention metoclopramide, domperidone, sulpiride, and haloperidol. From a therapeutic point of view, metoclopramide and domperidone are used in gastric motility disorders, and haloperidol is used in psychotic alterations. Antagonists of DA 1 receptors are SCH23390 and clozapine. Clozapine is used for treating schizophrenia.     

Mechanism of the pressor action of LY171555, a specific dopamine D2 receptor agonist, in the conscious rat

Administration of LY171555, a specific dopamine D2 receptor agonist, (10-1000 micrograms/kg i.v.) produced dose-related increases in mean arterial pressure in conscious Sprague-Dawley rats. Pretreatment with metoclopramide (5 mg/kg i.v.) abolished the pressor action of LY171555, whereas pretreatment with domperidone (2.5 mg/kg i.v.) and propranolol (10 mg/kg i.p.) did not affect the pressor action of LY171555. The vasopressor antagonist of arginine vasopressin (AVP), d(CH2)5Tyr(Me)AVP (10 micrograms/kg i.v.) and phenoxybenzamine (1 mg/kg i.v.) partly blocked and hexamethonium (25 mg/kg i.v.) enhanced the pressor action of LY171555. After combined treatment with both d(CH2)5Tyr(Me)AVP and phenoxybenzamine, LY171555 induced a depressor response which was completely blocked by pretreatment with domperidone. LY171555 administration induced a rapid, short-acting depressor response followed by a pressor response in conscious adrenomedullectomized Sprague-Dawley rats which was smaller in magnitude than that seen in intact Sprague-Dawley rats. LY171555 administration increased plasma norepinephrine, epinephrine and AVP in conscious Sprague-Dawley rats. These results suggest that the pressor action of LY171555 in conscious rats is dependent on activation of sympathetic outflow and AVP release through the central D2 dopaminergic system and that the central pressor effects of LY171555 could mask a depressor effect of LY171555 at the peripheral D2 dopamine receptor.     

Dopaminergic Influence on the Cardiovascular and Hormonal Responses to Cold Stress in Hypertensive Patients

Immersion of one hand into ice water (cold pressor test) in eight hypertensive subjects induces elevation of mean arterial pressure, increase in heart rate, and no significant changes of plasma renin activity and plasma aldosterone concentrations. Domperidone, a DA2 dopaminergic antagonist, attenuates heart rate increase induced by the cold pressor test, and the combination of bromocriptine, a known DA2 dopaminergic agonist, with domperidone again provoked a heart rate increase during the cold pressor test. Domperidone caused an increase of both plasma renin activity and plasma aldosterone concentrations, which were reversed by bromocriptine. These results suggest that a dopamine-receptor stimulation is taking place during the cold pressor test.     

Effect of Domperidone on the Bromocriptine-Induced Antihypertensive Action in Hypertensive Patients

Dopaminergic receptors have been involved in the cardiovascular and renin-angiotensin systems (RAS). We have recently reported that bromocriptine is an effective antihypertensive drug by stimulating DA(2) dopaminergic receptors. However, the nature of the dopaminergic receptors in RAS has not been established. Ten outpatients with essential hypertension were treated at the Vargas Hospital with bromocriptine (BR) (11.25 mg day(minus sign1)), a DA(2) dopaminergic agonist, for a 2-week period, after which an oral dose of 30 mg day(minus sign1) of domperidone (DO), a peripheric DA(2) dopaminergic antagonist, was added for 2 additional weeks. The active period was preceeded by a 2-week placebo period. Bromocriptine decreased blood pressure (BP) significantly by 19/9 mm Hg (systolic/diastolic BP). Bromocriptine did not cause heart rate (HR) changes. Bromocriptine decreased plasma aldosterone (ALD) without altering plasma renin activity (PRA). Domperidone partially blocked bromocriptine-induced antihypertensive submaximal treadmill effects and reversed ALD decrease. Exercise response was not significantly altered by BR + DO. We conclude the following: (1) BR is an effective antihypertensive agent; (2) BR seems to be acting at both the central and peripheric nervous systems, and (3) the nature of the dopaminergic receptor involved in renin secretion does not seem to be DA(2).     

Effects of Metoclopramide and Bromocriptine on the Renin-Angiotensin-Aldosterone System in Man: DOPAMINERGIC CONTROL OF ALDOSTERONE

This study was designed to investigate the possible role of dopaminergic mechanisms in the control of the renin-angiotensin-aldosterone system in normal man. Six normal male subjects in metabolic balance at 150 meq sodium, 60 meq potassium constant intake received the specific dopamine antagonist, metoclopramide, 10 mg i.v. or placebo followed by angiotensin II infusion 1 h later on 2 consecutive days. Metoclopramide increased plasma aldosterone concentration from 8.2+/-2.2 to 21.0+/-3.3 ng/100 ml (P < 0.005) and plasma prolactin concentration from 18.0+/-4.0 to 91.7+/-4.0 ng/ml (P < 0.001) within 15 min of its administration. At 1 h, plasma aldosterone and prolactin concentrations remained elevated at 16.8+/-2.1 ng/100 ml (P < 0.01) and 86.8+/-15.9 ng/ml (P < 0.005), respectively. Angiotensin II at 2, 4, and 6 pmol/kg per min further increased plasma aldosterone concentration to 27.2+/-3.4, 31.9+/-5.7, and 36.0+/-6.7 ng/100 ml (P < 0.02), respectively. Placebo did not alter plasma aldosterone or prolactin concentrations, but angiotensin II increased plasma aldosterone concentration to 13.7+/-2.4, 19.0+/-1.9, and 23.3+/-3.2 ng/100 ml (P < 0.005). The increment of plasma aldosterone concentration in response to angiotensin II was similar after metoclopramide or placebo.The six subjects also received the dopamine agonist, bromocriptine, 2.5 mg or placebo at 6 p.m., midnight, and 6 a.m. followed by angiotensin II infusion on 2 consecutive d. Bromocriptine suppressed prolactin to <3 ng/ml. After placebo, plasma aldosterone concentration increased from 5.2+/-1.4 to 12.3+/-1.7, 17.2+/-2.2, and 21.8+/-3.5 ng/100 ml (P < 0.01) and after bromocriptine from 7.2+/-1.0 to 14.7+/-3.0, 19.8+/-3.2, and 23.4+/-1.6 ng/100 ml (P < 0.001) with each respective angiotensin II dose. No difference in the response to angiotensin II after bromocriptine or placebo was observed. Plasma renin activity, free 11-hydroxycorticoid concentration, and serum potassium concentration were unchanged by metoclopramide or bromocriptine.The results suggest that aldosterone production is under maximum tonic dopaminergic inhibition which can be overridden with stimulation by angiotensin II in normal man.     

Effect of bromocriptine treatment on prolactin, noradrenaline and blood pressure in hypertensive haemodialysis patients

Bromocriptine (2.5 mg/day orally) produced a significant fall in supine mean arterial pressure in nine hypertensive haemodialysis patients with high serum prolactin levels, without causing significant changes in heart rate. On bromocriptine, there was a significant decrease in the mean value of both serum prolactin and plasma noradrenaline, without significant changes in the mean value of plasma renin activity. A significant relationship was found between the changes in supine plasma noradrenaline and the changes in supine mean arterial pressure induced by bromocriptine. The increase in mean arterial pressure in response to the tilt test was greater on bromocriptine than on placebo although the changes in plasma noradrenaline were reduced by bromocriptine. Similar results were observed during the cold pressor test. These findings suggest that the arterial pressure-lowering effect of bromocriptine is related to the reduction in sympathetic out-flow. The parallel decrease in serum prolactin raises the question of the possible involvement of dopaminergic mechanisms in the development of hypertension in our patients. Moreover, bromocriptine seems to enhance the vascular response to endogenous noradrenaline.     

Pharmacological, hemodynamic and autonomic nervous system mechanisms responsible for the blood pressure and heart rate lowering effects of pergolide in rats

In conscious spontaneously hypertensive rats, pergolide (50.0 micrograms/kg s.c.) produced a sustained decrease in tail artery pressure which was blocked by haloperidol (1.0 mg/kg s.c.) pretreatment. In anesthetized spontaneously hypertensive rats this effect was accompanied by a fall in total peripheral resistance inasmuch as pergolide did not significantly change cardiac output. In anesthetized normotensive rats, pergolide (30.0 micrograms/kg i.v.) also lowered blood pressure. This effect was not significantly modified by adrenalectomy, methysergide, idazoxan (alpha-2 adrenoceptor antagonist), vagotomy alone or plus ligation of carotid arteries or plus atenolol, but was entirely prevented by domperidone or sulpiride pretreatment and was reverted to a pressor response (due to stimulation of alpha adrenoceptors and 5-hydroxytryptamine receptors) by blockade of ganglionic transmission with chlorisondamine. Pergolide given either i.v. or into the cisterna magna or the lateral cerebral ventricle produced changes in blood pressure of the same magnitude. In intact or adrenalectomized rats, i.v. pergolide significantly lowered plasma norepinephrine concentration. Furthermore, in saline but not sulpiride-pretreated pithed rats, pergolide reduced the pressor responses and the accompanying increases in plasma norepinephrine evoked by electrical stimulation of the spinal cord. However, pergolide failed to modify the vascular reactivity to several pressor agents and lacked beta-2 and DA-1 dopamine receptor agonist properties. These results indicate that the decrease in blood pressure produced by pergolide can be accounted for by an inhibition of sympathetic tone resulting from stimulation of peripheral neuronal dopamine receptors. A possible central contribution remains to be substantiated. The pronounced bradycardia produced by pergolide (30.0 micrograms/kg i.v.) in anesthetized intact rats was partly reduced by vagotomy, methylatropine, domperidone, sulpiride, idazoxan, phentolamine or atenolol. The effects of pergolide in vagotomized rats were further diminished by domperidone but they were blocked by the combination of phentolamine or idazoxan plus domperidone. In rats pretreated with atenolol or in rats with the cervical section of spinal cord and the low level of heart rate increased with an isoprenaline infusion, the decrease in heart rate produced by pergolide was abolished by domperidone, methylatropine or idazoxan. In pithed rats, pergolide changed neither the base-line heart rate nor the tachycardia to exogenous norepinephrine nor the bradycardia evoked by carbachol or electrical stimulation of the peripheral cervical vagus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Metoclopramide and domperidone block the antihypertensive effect of bromocriptine in hypertensive patients

This study was conducted in normotensive and hypertensive subjects at the Vargas Hospital of Caracas. Normotensive subjects received, in a cross-over fashion, placebo, metoclopramide (MTC), or domperidone (DOMP), 40 mg of each drug, daily for 1 week. The first group of patients under placebo for 1 week received a single 2.5-mg oral dose of bromocriptine (Br). The second group of patients received 30 mg MTC daily (divided into three doses) for 1 week. At the end of the period a single dose of 2.5 mg Br was administered to each patient. The third group of eight hypertensive patients received DOMP for 1 week at 30 mg/d and then a single 2.5-mg Br dose. Cardiovascular and biochemical parameters including arterial pressure, heart rate, plasma renin activity, and plasma aldosterone were evaluated during the 6-hour period before and after the administration of Br. Neither DOMP nor MTC significantly modified blood pressure and heart rate in normotensive patients. Br reduced both systolic and diastolic arterial pressure in hypertensive subjects. The peak of the antihypertensive effect appeared 3 hours after drug administration, but reduction of arterial pressure lasted approximately 6 hours. At the same time, Br reduced plasma aldosterone levels and plasma renin activity. MTC and DOMP reversed the antihypertensive effect of Br and its effect on aldosterone levels and plasma renin activity. We conclude from these findings that Br acts as an antihypertensive agent at peripheral and central levels by stimulating dopamine-2 receptors, which are involved in the aldosterone and renin secretion.     

Cardiovascular responses to intrathecal dopamine receptor agonists in conscious DOCA-salt hypertensive rats

Previous studies have demonstrated that in conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rats, the hypotensive action of intravenous (i.v.) bromocriptine, a selective dopamine D2 receptor agonist, was mediated partly by peripheral and partly by spinal dopamine D2 receptor stimulation, and that this effect was greater and longer-lasting than that in uninephrectomized control rats. To determine whether this amplification results partly from a putative spinal hypersensitivity phenomenon, cardiovascular responses to intrathecal (i.t.) administration of apomorphine and quinpirole were studied in conscious, 4-week DOCA-salt hypertensive rats and compared with those in uninephrectomized control rats. In both groups, upper thoracic (T2-T4) i.t. injections of apomorphine (9.1, 45.5 and 91.1 microg/rat) induced immediate and dose-dependent decreases in mean aortic pressure (MAP) and heart rate (HR), while i.t. quinpirole (38.4 microg/rat) induced only bradycardia. Neither magnitude nor duration of these responses was enhanced in DOCA-salt hypertensive rats when compared to control rats. In DOCA-salt hypertensive rats, apomorphine-induced hypotension and bradycardia remained unaffected by i.v. (500 microg/kg) pretreatment with domperidone, a selective dopamine D2 receptor antagonist that does not cross the blood-brain barrier. However, i.t. (40 microg/rat at T2-T4) pretreatment with domperidone significantly reduced apomorphine-induced hypotension, but fully suppressed bradycardia elicited by either apomorphine or quinpirole. These results demonstrated that in conscious DOCA-salt hypertensive rats, intrathecally-injected apomorphine or quinpirole decreased MAP and/or HR through a spinal D2 dopaminergic mechanism, as previously demonstrated in normotensive intact rats. Since both magnitude and duration of these responses were unchanged with respect to uninephrectomized control rats, enhancement of the hypotensive effect of intravenously-administered bromocriptine in DOCA-salt hypertensive rats does not appear to involve spinal dopamine D2 receptors.     

Hypotensive action of bromocriptine in the DOCA-salt hypertensive rat: Contribution of spinal dopamine receptors

Summary— To assess the role of spinal dopamine receptors in mediation of hypotension induced by systemic administration of the dopamine D₂ receptor agonist, bromocriptine, conscious deoxycorticosterone acetate (DOCA)-salt hypertensive rats were pretreated with either intravenous (iv; 500 μg/kg) or intrathecal (it; 40 μg/rat at T₉-T₁₀) domperidone, a selective dopamine D₂ receptor antagonist that does not cross the blood-brain barrier. In DOCA-salt hypertensive rats, iv administration of a sub-maximal dose of bromocriptine (150 μg/kg) induced a significant decrease in mean aortic pressure (MAP) which was greater and longer lasting than that in uninephrectomized control rats. Intravenous or it pretreatment with domperidone reduced partially, but significantly, the hypotensive effect of bromocriptine (reduction of about 57% and 45% of the maximal effect, respectively). The remaining responses observed during the 60 min postinjection period were still statistically significant as compared with vehicle injection. In contrast, the bromocriptine-induced hypotension was fully abolished by iv pretreatment with metoclopramide (300 μg/kg), a dopamine D₂ receptor antagonist that crosses the blood-brain barrier, or by combined pretreatment with iv and it domperidone. These results suggest that, in DOCA-salt hypertensive rats, the hypotension induced by iv bromocriptine is mediated partly through a peripheral D₂ dopaminergic mechanism and partly through stimulation of spinal dopamine D₂ receptors, as has been demonstrated in conscious normotensive rats.     

Mechanism of the depressor action of LY171555, a selective dopamine D2 receptor agonist, in the anesthetized rat

Administration of LY171555 (1 mg/kg i.v.) decreased mean arterial pressure (MAP) and heart rate in both pentobarbital- and urethane-anesthesized Sprague-Dawley rats. The depressor response to LY171555 in pentobarbital-anesthetized rats was sustained for at least 30 min, but in urethane-anesthetized rats lasted only approximately 3 min after LY171555 injection. In pentobarbital-anesthetized rats, pretreatment with domperidone (0.5 mg/kg) or metoclopramide (5 mg/kg) attenuated the depressor action of LY171555, whereas pretreatment with d(CH2)5Tyr(Me)arginine vasopressin (AVP) (10 micrograms/kg) only delayed the recovery phase of the depressor response to LY171555. In contrast, LY171555 administered to urethane-anesthetized rats after domperidone pretreatment induced a pressor response which was blocked completely by d(CH2)5Tyr(Me)AVP. Metoclopramide pretreatment in urethane-anesthetized rats prevented the decreases in MAP and heart rate induced by LY171555, whereas pretreatment with d(CH2)5Tyr(Me)AVP delayed the recovery phase of the depressor response. Pretreatment with d(CH2)5Tyr(Me)AVP per se decreased basal MAP in the urethane-anesthetized group, but not in pentobarbital-anesthetized rats. Basal plasma norepinephrine, epinephrine and AVP levels were higher in urethane-anesthetized rats than in the pentobarbital-anesthetized group. LY171555 administration decreased plasma norepinephrine without altering plasma epinephrine in both groups and induced a significant increase in plasma AVP which was greater in the urethane-anesthetized rats than in pentobarbital-anesthetized animals. These results suggest that LY171555 decreases MAP and heart rate in anesthetized rats by inhibiting norepinephrine release from nerve endings through the peripheral dopamine D2 receptor and that the time course of the depressor response may be altered by LY171555-induced AVP release, the magnitude of which appears to be dependent on the anesthetic agent.     

Effect of quinpirole, a specific dopamine DA2 receptor agonist on the sympathoadrenal system in dogs

The effects of quinpirole, a specific dopamine DA2 receptor agonist, were investigated on both cardiovascular responses in conscious dogs and catecholamine release from the adrenal medulla in anesthetized dogs. In conscious normal dogs, i.v. quinpirole (30 micrograms/kg) elicited a decrease in blood pressure and a marked increase in heart rate associated with a rise in plasma catecholamine levels. The increase in heart rate is due to both baroreflex and central mechanisms because a slight but significant positive chronotropic effect persists in sinoaortic denervated dogs (i.e., animals deprived of baroreflex pathways). The central origin of this excitatory effect was confirmed by two subsequent protocols: intracisterna magna injection of quinpirole (5 micrograms/kg) increased blood pressure, heart rate and plasma catecholamines; i.v. domperidone reversed the hypotensive effect of i.v. quinpirole into a pressor response. The rise in plasma catecholamines was associated with an increase in plasma vasopressin levels. In anesthetized dogs, i.v. quinpirole (10 micrograms/kg/min during 12 min), which also decreased blood pressure, failed to modify epinephrine (and norepinephrine) release from the adrenal medulla whatever the stimulation frequencies (1, 3 and 5 Hz) of the sectioned splanchnic nerve. Similar results were obtained with apomorphine (5 micrograms/kg/min during 12 min). These results show that two mechanisms are involved in the action of quinpirole: first, a peripheral depressor action (which elicits the decrease in blood pressure) and secondly, a central pressor component involving an increase in both sympathetic tone and vasopressin release. They also demonstrate clearly that peripheral DA2 receptors are not involved in the control of catecholamine release from the adrenal medulla under in vivo conditions.     

The role of the fourth cerebroventricle in nicotine-stimulated prolactin release in the rat: involvement of catecholamines.

The central mechanism(s) whereby peripherally administered nicotine (N) stimulates prolactin secretion has not been clarified. The current studies showed that an i.c.v. injection of N into the fourth ventricle (IV) produced a significant dose-dependent elevation of plasma prolactin [buffer less than N 0.125 micrograms = N 0.25 micrograms (P less than .05) less than N 0.5 micrograms = N 2.5 micrograms (P less than .01)]. Injecting the nicotinic cholinergic (NAch) antagonist, mecamylamine, into the IV (20 or 40 micrograms) or i.v. (0.5 mg/kg b.wt.) before the administration of N by the alternate route indicated that nicotine was activating NAch receptors accessible from the IV. Because brain stem catecholaminergic cell groups, adjacent to the IV, project to hypothalamic regions involved in modulating prolactin release, the involvement of IV catecholaminergic neurons in N-stimulated prolactin release was investigated. Ablation of central catecholaminergic neurons by 6-hydroxydopamine abolished the prolactin response to N injected i.c.v. (1 or 2.5 micrograms; P less than .05) or i.v. (0.03 or 0.05 mg/kg b.wt.; P less than .05). To assess the role of norepinephrine and epinephrine, LY 10853, an inhibitor of dopamine-beta-hydroxylase, was given i.p. 4 h before i.v. nicotine; the prolactin response was attenuated (P less than .01). Selective inhibitors of epinephrine synthesis, SKF 64139 or 2,3-dichloro-alpha-methylbenzylamine, administered on the same schedule, reduced (P less than .01) the prolactin response to N without altering responsiveness to the dopamine receptor antagonist, domperidone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acute and chronic administration of idazoxan on blood pressure and plasma catecholamine concentrations of rats

To test the hypothesis that alpha-2 adrenoceptor antagonists can modulate sympathetic nerve release of norepinephrine in vivo through blockade of peripheral prejunctional alpha-2 adrenoceptors, acute and chronic effects of the alpha-2 adrenoceptor antagonist idazoxan on mean arterial pressure (MAP), heart rate and plasma catecholamine concentrations have been investigated in conscious and anesthetized rats. In normotensive rats, a single i.v. dose of idazoxan (300 micrograms kg-1) caused an immediate 2-fold increase in plasma concentration of norepinephrine and epinephrine, a transient increase in heart rate but no significant change in MAP. Plasma norepinephrine concentration of conscious normotensive rats increased significantly during a 4-hr i.v. infusion of idazoxan (300 micrograms kg-1 hr-1) with no concomitant changes in MAP or heart rate. In anesthetized spontaneously hypertensive rats, the increases in plasma norepinephrine concentration and heart rate caused by i.v. idazoxan (300 micrograms kg-1) were accompanied by a significant decrease in MAP. The increase in plasma norepinephrine after idazoxan in spontaneously hypertensive rats was much greater than that produced by an equihypotensive dose of the vasodilator hydralazine. Normotensive rats treated continuously for 7 days with s.c. idazoxan (7.5 mg kg-1 day-1) had similar blood pressures and plasma catecholamine concentrations to vehicle-treated rats. These results suggest that idazoxan causes a greater increase in plasma norepinephrine concentration than that which can be attributed to baroreceptor stimulation. Blockade of prejunctional alpha-2 adrenoceptors by idazoxan may, therefore, increase release of norepinephrine from peripheral sympathetic nerves of anesthetized and conscious rats. This effect is short-lived and does not influence blood pressure of normotensive rats.     

The value of plasma prolactin levels in the prediction of the responsiveness of growth hormone secretion to bromocriptine and TRH in acromegaly

Abstract. The mechanism of the inhibition of growth hormone secretion in response to bromocriptine and the ability of thyrotropin releasing hormone to stimulate growth secretion in acromegaly is unknown. In the present study the relationship between the plasma prolactin concentration of untreated acromegalic patients and the reaction of growth hormone to thyrotropin releasing hormone and bromocriptine was investigated.
Plasma prolactin levels were elevated in thirty-three (42%) of seventy-nine untreated acromegalic patients. Seventeen patients had mildly elevated prolactin levels, but in sixteen the plasma prolactin concentration was higher than 30 ng/ml. Bromocriptine (2.5 mg) inhibited growth hormone secretion by more than 50% in 22% of the normoprolactinaemic, in 53% of the mild hyperprolactinaemic and in 88% of the patients with a prolactin level above 30 ng/ml ( P <0.01 v. normoprolactinaemic; P <0.01 v. mildly elevated prolactin levels). An increase of growth hormone secretion by more than 100% of the basal value in response to thyrotropin releasing hormone was observed in 44% of the normoprolactinaemic, in 59% of the mildly hyperprolactinaemic and in 75% of the clearly hyperprolactinaemic patients; ( P <0.01 v. normo-and mildly hyperprolactinaemic patients).
Conclusion: An increased plasma prolactin concentration in patients with acromegaly is accompanied in most patients by a higher sensitivity of growth hormone secretion to bromocriptine.     

Nonpeptide angiotensin II receptor antagonists. IX. Antihypertensive activity in rats of DuP 753, an orally active antihypertensive agent

In conscious renal artery-ligated rats, a high renin hypertensive rat model, DuP 753, a p.o. active nonpeptide angiotensin II (AII) receptor antagonist, decreased blood pressure at 0.1 to 3 mg/kg given i.v. or at 0.3 to 10 mg/kg given p.o. with an i.v. ED30 of given i.v. or at 0.3 to 10 mg/kg given p.o. with an i.v. ED30 of 0.78 mg/kg and a p.o. ED30 of 0.59 mg/kg. The antihypertensive efficacy of DuP 753 was similar to that of captopril. Unlike the peptide AII antagonist saralasin, DuP 753 did not cause a transient increase in blood pressure, suggesting absence of agonistic activity. At 3 mg/kg p.o., DuP 753 lowered blood pressure for at least 24 hr and did not change heart rate, suggesting a long duration of antihypertensive effect. At 3 mg/kg i.v., DuP 753 inhibited the pressor response to AII but not to norepinephrine or vasopressin. Pretreatment of renal artery-ligated rats with captopril, saralasin or propranolol, but not with prazosin, hydralazine or indomethacin, abolished or reduced the antihypertensive effect of DuP 753. In the deoxycorticosterone acetate hypertensive rat, a low renin model, DuP 753 did not lower blood pressure. These results suggest that DuP 753 is a p.o. active, antihypertensive agent in renal artery-ligated rats with a similar antihypertensive efficacy as captopril. The antihypertensive effect of DuP 753 is most likely related to the blockade of the vasoconstrictor effect of AII. Unlike saralasin, DuP 753 does not have agonistic activity.     

Role of vasopressin in the cardiovascular effects of LY171555, a selective dopamine D2 receptor agonist: studies in conscious Brattleboro and Long-Evans rats

To elucidate the role of central and peripheral arginine vasopressin (AVP) in the cardiovascular action of LY171555 in conscious rats, we have examined the effects of LY171555 on mean arterial pressure, heart rate, plasma AVP and catecholamine levels in conscious, congenitally AVP-deficient Brattleboro (DI) rats and Long-Evans (LE) control rats. Administration of LY171555 (1 mg/kg i.v.) increased heart rate without altering mean arterial pressure in DI rats but increased both mean arterial pressure and heart rate in LE rats. After pretreatment with domperidone, LY171555 induced both a pressure response and a tachycardia in DI rats. Domperidone pretreatment enhanced the pressor action of LY171555 and attenuated the LY171555-induced tachycardia in LE rats. After pretreatment with phenoxybenzamine, LY171555 induced a depressor and bradycardic response that could be blocked by pretreatment with domperidone in DI rats. Pressor and bradycardic responses to LY171555 were attenuated by phenoxybenzamine pretreatment in LE rats. LY171555 administration increased plasma norepinephrine and epinephrine levels in both DI and LE rats but increased plasma AVP only in LE rats. The vasopressor effect of the alpha-1 adrenoceptor agonist phenylephrine was significantly attenuated in DI rats compared with LE rats, whereas the pressor action of angiotensin II was similar in both groups. These results suggest that the pressor action of LY171555 in conscious LE rats is mediated by an increase in plasma AVP and by activation of sympathetic outflow through the central D2 dopaminergic system but not through the central vasopressinergic system.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of dopamine and dopamine-antagonist infusion on blood platelet count, size and release reaction in hypertensive and normotensive subjects

To examine the effect of dopamine on in vivo platelet function and a possible platelet contribution to the clearance of dopamine we measured platelet count, platelet size and plasma concentration of the platelet specific protein beta-thromboglobulin (BTG) in groups of 40 year old untreated hypertensive and normotensive men. One hypertensive (n = 10) and one normotensive (n = 10) group received dopamine infusion at doses from 0.5 to 2.0 micrograms/kg/min which increased plasma dopamine 100-fold from baseline. Two other groups of hypertensive (n = 10) and normotensive (n = 11) subjects received 10 mg of the dopamine antagonist metoclopramide intravenously, upon which serum prolactin concentration increased 10-fold. No significant effect on platelet function in any group was observed during these interventions. Platelet phenol-sulphotransferase may contribute to dopamine conjugation. However, the selected platelet parameters correlated only weakly with dopamine kinetics during the infusion. Neither dopamine nor a dopamine antagonist altered the selected platelet parameters, nor did these parameters influence the clearance of dopamine during a short-lasting pharmacological infusion.     

Dopaminergic regulation of aldosterone secretion: assessment in different subtypes of primary aldosteronism and in essential hypertension.

The inhibitory effect of dopamine on aldosterone secretion was investigated in patients with different types of primary aldosteronism, six with idiopathic hyperaldosteronism (IHA) and four with dexamethasone-suppressible hyperaldosteronism (DSH), and in 10 patients with essential hypertension. The effects of 10 mg metoclopramide given intravenously, 10 mg bromocriptine given orally and 100 micrograms adrenocorticotrophic hormone given intravenously on plasma aldosterone and renin activities were investigated in all patients. Metoclopramide induced a rise in plasma aldosterone activity only in patients with IHA and not in those with DSH and essential hypertension. After bromocriptine plasma aldosterone concentrations decreased in patients with IHA only, and after adrenocorticotrophic hormone plasma aldosterone concentrations increased in patients with DSH only. Plasma renin activity was unaffected in all cases. These results provide evidence of increased endogenous dopaminergic inhibition of aldosterone secretion in IHA and of a blunted aldosterone response in both DSH and essential hypertension.