Activation of peripheral dopamine presynaptic receptors lowers blood pressure and heart rate in dogs

Role of dopamine and selective dopamine receptor agonists on mouse ductus arteriosus tone and responsiveness

BACKGROUND

Indomethacin treatment for patent ductus arteriosus (PDA) is associated with acute kidney injury (AKI). Fenoldopam, a dopamine (DA) DA₁-like receptor agonist dilates the renal vasculature and may preserve renal function during indomethacin treatment. However, limited information exists on DA receptor-mediated signaling in the ductus and fenoldopam may prevent ductus closure given its vasodilatory nature.

METHODS

DA receptor expression in CD-1 mouse vessels was analyzed by qPCR and immunohistochemistry. Concentration-response curves were established using pressure myography. Pretreatment with SCH23390 (DA₁-like receptor antagonist), phentolamine (α -adrenergic receptor antagonist) or indomethacin addressed mechanisms for DA-induced changes. Fenoldopam's effects on postnatal ductus closure were evaluated in vivo.

RESULTS

DA₁ receptors were expressed equally in ductus and aorta. High-dose DA induced modest vasoconstriction under newborn O₂ conditions. Phentolamine inhibited DA-induced constriction, while SCH23390 augmented constriction, consistent with a vasodilatory role for DA₁ receptors. Despite this, fenoldopam had little effect on ductus tone nor indomethacin- or O₂-induced constriction and did not impair postnatal closure in vivo.

CONCLUSION(S)

DA receptors are present in the ductus but have limited physiologic effects. DA-induced ductus vasoconstriction is mediated via α-adrenergic pathways. The absence of DA₁-mediated impairment of ductus closure supports the study of potential role for fenoldopam during PDA treatment.

Association of race, heart failure and chronic kidney disease

Heart failure and kidney disease are two important emerging epidemics. The importance of pre-end stage kidney disease was introduced in the 2002 publication of the National Kidney Foundation's Chronic Kidney Disease Guidelines. One in nine US adults has some degree of kidney disease, many of whom also have heart failure. Among all patients with heart failure, approximately half have significant kidney disease. The distribution of etiologies of these conditions varies among races; blacks tend to have heart and kidney disease predominantly due to hypertension, while whites tend to be affected by ischemic heart disease and Hispanics by diabetic kidney disease. The burden of disease is disproportionately borne by minorities, the cause of which remains to be fully elucidated. The bulk of knowledge of these diseases is based on studies involving predominantly white subjects. Recent studies have suggested that there are racial differences in patients' responsiveness to various classes of drugs. Designs of future studies should take into account these differences.

Transforming growth factor- 1 hyperexpression in African-American hypertensives: A novel mediator of hypertension and/or target organ damage

Hypertension, a remediable risk factor for stroke, cardiovascular disease, and renal failure, affects 50 million individuals in the United States alone. African Americans (blacks) have a higher incidence and prevalence of hypertension and hypertension-associated target organ damage compared with Caucasian Americans (whites). Herein, we explored the hypotheses that transforming growth factor-beta(1) (TGF-beta(1)) is hyperexpressed in hypertensives compared with normotensives and that TGF-beta(1) overexpression is more frequent in blacks compared with whites. These hypotheses were stimulated by our recent demonstration that TGF-beta(1) is hyperexpressed in blacks with end-stage renal disease compared with white end-stage renal disease patients and by the biological attributes of TGF-beta(1), which include induction of endothelin-1 expression, stimulation of renin release, and promotion of vascular and renal disease when TGF-beta(1) is produced in excess. TGF-beta(1) profiles were determined in black and white hypertensive subjects and normotensive controls and included circulating protein concentrations, mRNA steady-state levels, and codon 10 genotype. Our investigation demonstrated that TGF-beta(1) protein levels are highest in black hypertensives, and TGF-beta(1) protein as well as TGF-beta(1) mRNA levels are higher in hypertensives compared with normotensives. The proline allele at codon 10 (Pro(10)) was more frequent in blacks compared with whites, and its presence was associated with higher levels of TGF-beta(1) mRNA and protein. Our findings support the idea that TGF-beta(1) hyperexpression is a risk factor for hypertension and hypertensive complications and provides a mechanism for the excess burden of hypertension in blacks.

Hypertensive cardiovascular disease in African Americans

Hypertension is the most common public health challenge in the United States because of its prevalence and associated increase in comorbid cardiovascular diseases. Yearly expenses related directly or indirectly to the treatment and detection of hypertension in the United States are approximately $10 billion, excluding the enormous yearly financial burden of $259 billion and the social burden from heart disease and stroke, which remain the first and third leading causes of death, respectively, in the United States. Despite the importance of these observations, blood pressure is poorly controlled in the United States.

Hemodynamic characterization of YM435, a novel dopamine DA1 receptor agonist, in anesthetized dogs

The cardiovascular effects of YM435, a dopamine DA1 receptor agonist, were compared with those of dopamine in open-chest anesthetized dogs. Intravenous infusion of YM435 (0.1-3 microg/kg/min) increased renal blood flow and cardiac output and reduced renal vascular resistance and total peripheral vascular resistance, with a decrease in mean blood pressure, in a dose-dependent manner, with little change in heart rate. At 1 microg/kg/min i.v., renal blood flow increased by 20 +/- 7%, cardiac output increased by 14 +/- 6%, renal vascular resistance decreased by 22 +/- 4%, total peripheral vascular resistance decreased by 18 +/- 4%, and mean blood pressure decreased by 7 +/- 1%. The striking difference between the cardiovascular effects of YM435 and those of dopamine was that YM435 caused no vasoconstriction or increase in heart rate, even at high doses. The cardiovascular effects of YM435 (1 microg/kg/min i.v.) were almost completely inhibited by treatment with SCH 23390, a selective dopamine DA1 receptor antagonist. Furthermore, intravenous infusion of YM435 (0.1-3 microg/kg/min) dose-dependently reversed the increase in blood pressure and renal vascular resistance induced by angiotensin II or norepinephrine in closed-chest anesthetized dogs. Our results demonstrate that intravenous infusion of YM435 produces dose-dependent renal vasodilating and hypotensive effects by stimulation of dopamine DA1 receptors and suggest that YM435 may be useful for the parenteral treatment of acute elevation of blood pressure.

Hemodynamic responses during psychological stress: Implications for studying disease processes

Investigation of the physiological correlates of psychological stress is of interest in relation to the putative impact of stress in the etiology of cardiovascular disease. Although the assessment of blood pressure and heart rate responses to psychological stress has been very informative, the addition of cardiac output measurement has added a further dimension to this research field. In recent studies, a more complete hemodynamic picture of the stress response has been documented in terms of cardiac output and systemic vascular resistance components of blood pressure changes. Different stressors have been shown to produce similar blood pressure increases due to quite different hemodynamic mechanisms. Furthermore, when faced with the same stressor, different individuals may exhibit pressor responses that are very different hemodynamically. There is growing evidence that these hemodynamic response patterns to psychological stress are stable individual traits. Response stability is a prerequisite for considering how stress-related hemodynamic changes may be implicated in the pathophysiology of cardiovascular diseases. Observations that hemodynamic response patterns in individuals at higher risk for the development of hypertension differ from those of lower risk individuals show that specific patterns of hemodynamic response are associated with disease processes. Although it is as yet unclear whether they represent markers or mechanisms. Overall. hemodynamic studies appear to he helping to refine our understanding of how stress can impact cardiovascular disease processes.

The role of the sympathetic nervous system in promoting liver cirrhosis induced by carbon tetrachloride, using the essential hypertensive animal (SHR)

The effects of the sympathetic nervous system on liver injury induced experimentally by carbon tetrachloride (CCl4) were examined in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). It was found that the SHR had an elevated catecholamine (CA) content in the adrenal gland without any treatment, and fluorescence histochemistry also revealed dense adrenergic innervations in the liver. Moreover, the SHR showed greater sensitivity to CCl4 stimulation in the sympathetic nervous system than the WKY, resulting in a decreased hepatic blood flow in the acute stage and a depleted CA in the adrenal gland, a lowered blood pressure (BP) and a released non-esterified fatty acid (NEFA) from peripheral adipose tissue in the chronic stage. Upon repetition of the CCl4 treatments twice a week for 4 weeks, the liver injury was more severe in the SHR than in the WKY. Plasma glutamate-pyruvate transaminase (GPT) activity was increased in both strains but more significantly in the SHR than in the WKY. Histological examination of the liver in the SHR showed established cirrhosis, whereas only bridging fibrosis was seen in the WKY. These results suggest that the pathogenesis of the liver damage induced by CCl4 in the SHR, is attributable to the enhanced response of the sympathetic nervous system that releases massive amounts of CA which then lead to vasoconstriction and metabolic changes that promote liver damage.

Role of dopaminergic mechanisms in the kidney for the pathogenesis of hypertension

1. To estimate the role of renal dopaminergic mechanisms in the pathogenesis of hypertension, patients with essential hypertension and animal models of hypertension were investigated. 2. Impaired dopaminergic activity in kidneys for natriuresis was observed in patients with 'salt-sensitive' hypertension and with low-renin hypertension. 3. Decreased dopaminergic activity in kidneys was observed in the Dahl S-rats without salt loading. 4. In spontaneously hypertensive rats, renal dopamine synthesis was enhanced whereas there was a decrease of adenylate cyclase activity in renal tubules. 5. Demonstration of impaired dopaminergic mechanisms in kidneys of human and animal hypertension suggests that renal dopaminergic mechanisms play an important role in development of hypertension.

Effects of dopamine D1 and dopamine D2 receptor agonists on coronary and peripheral hemodynamics

This study evaluated the coronary dopamine receptors by using the dopamine D1 receptor agonist fenoldopam, dopamine D2 receptor agonist propylbutyldopamine, and their selective antagonists SCH23390 and domperidone. Left circumflex coronary flow (CF), coronary perfusion pressure at constant flow, left ventricular hemodynamics, and total peripheral vascular resistance (TPR) were measured in pentobarbital-anesthetized dogs at constant arterial pressures. At doses of 200, 500 and 5000 nM, both fenoldopam and propylbutyldopamine induced dose-related inotropic effects, as evidenced by maximal dp/dt and cardiac output, an increase in CF, decrease in coronary vascular resistance and a decrease in TPR. Fenoldopam was more potent in its cardiac and coronary effects while propylbutyldopamine was more potent peripherally. On the basis of dosage used, the positive inotropic effects of fenoldopam and propylbutyldopamine were much weaker than dopamine. After beta-receptor blockade, the inotropic and coronary effects of fenoldopam and propylbutyldopamine were extremely attenuated. Domperidone could largely antagonize the propylbutyldopamine-induced inotropic and coronary effects while SCH23390 showed no significant effect. In addition, under our experimental conditions, the fenoldopam- and propylbutyldopamine-induced decreases in TPR were markedly reduced by SCH23390 and domperidone, respectively. The results indicate that the coronary effects of fenoldopam and propylbutyldopamine result not from a primary coronary vasodilating action, but from vasodilation secondary to positive inotropic effects. Both dopamine D1 and dopamine D2 receptors are involved in the peripheral vascular hemodynamics.

Systemic and renal hemodynamic effects of celiprolol in essential hypertensives

To evaluate the humoral and hemodynamic (both systemic and renal) effects of celiprolol and to assess whether these effects are at least partially due to the activation of dopamine (DA) receptors, 9 out-patients with mild to moderate uncomplicated essential hypertension, without any therapy for at least 3 weeks, received celiprolol (400 mg once daily) and placebo, each for 1 month, according to a double-blind randomized trial pattern. At the end of each treatment period, blood pressure, heart rate, renal plasma flow, glomerular filtration rate, plasma renin activity, plasma aldosterone and plasma norepinephrine were measured after administration of saline solution and intravenous metoclopramide. Compared with placebo, celiprolol significantly reduced mean blood pressure, heart rate and plasma norepinephrine. Plasma renin activity showed a tendency toward a reduction during celiprolol treatment, which was not associated with changes in plasma aldosterone. Despite the decrease in mean blood pressure, renal plasma flow did not change, so that renovascular resistances were significantly reduced. Glomerular filtration rate was unchanged and the filtration fraction showed a trend toward a reduction during celiprolol treatment. Percent decrements of renovascular resistances and of mean blood pressure induced by celiprolol tended to correlate with changes in plasma norepinephrine. Metoclopramide did not influence the hemodynamic (systemic and renal) effects of celiprolol nor plasma renin activity and plasma norepinephrine, and it increased aldosterone levels to a similar extent before and after administration of celiprolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Sustained hemodynamic improvement during long-term therapy with levodopa in heart failure: role of plasma catecholamines

Long-term therapy with oral sympathomimetic amines in patients with heart failure has been limited by the eventual development of diminished pharmacologic efficacy. However, a previous investigation in five subjects with heart failure suggested that long-term ingestion of levodopa, which is decarboxylated endogenously to dopamine, produces a sustained improvement in cardiac function. In the present study, levodopa was administered orally (1.5 to 2.0 g) to 14 patients with heart failure while hemodynamic responses and plasma catecholamines were monitored. Initially, an increase in cardiac index and stroke volume index was accompanied by a decline in systemic vascular resistance, mean pulmonary capillary wedge pressure and mean right atrial pressure. Heart rate and mean arterial pressure were unchanged. Plasma concentrations of dopamine rose substantially after drug ingestion and correlated significantly with changes in cardiac index (r = 0.73, p less than 0.05). After 12 weeks of treatment with levodopa, the changes in cardiac index, stroke volume index, systemic vascular resistance and plasma dopamine levels persisted (n = 12 patients). Moreover, a significant decrease occurred in the heart rate at rest. Although there was an initial tendency for plasma norepinephrine concentrations to increase, a return to control levels was documented after long-term treatment. Thus, tolerance to the hemodynamic actions of levodopa did not develop during long-term administration of the drug. The hemodynamic responses observed can be ascribed to the activation of beta 1-adrenoceptors and dopamine1 receptors by dopamine generated from levodopa. The dopamine2 activity of dopamine does not appear to be responsible for the improvement in cardiac performance produced by levodopa.

Role of dopamine2-receptors in mediating renal vascular response to low dose dopamine infusion in the rat

The effects of the interaction of dopamine (DA) and the DA2-receptors on regional blood flows and cardiac output have been studied in the rat. By means of the microsphere technique the blood flow (BF) and vascular resistance (VR) were determined in the kidney, duodenum, spleen, liver, and lung during infusion of DA in the absence and presence of selective DA2-receptor blockade with S-Sulpiride (S-SP), and during infusion of a selective DA2-receptor agonist (LY-171555, LY). In order to evaluate the role of the presynaptic DA2-receptor, the experiments were performed without alpha- and beta-adrenergic blockade. Dopamine was given in such low doses that stimulation of the adrenergic receptors should be negligible. Dopamine, LY and DA + S-SP did not significantly influence BF and VR in the spleen, liver and lung. Dopamine significantly increased BF and decreased VR in the kidney and the duodenum; LY significantly increased BF in the the kidney but not in the duodenum and decreased VR in both the kidney and the duodenum. In the presence of selective DA2-receptor blockade, DA did not significantly influence BF or VR in the kidney but in the duodenum BF increased and VR decreased to the same extent as in the absence of blockade. In conclusion; the kidney and the intestine are more abundantly supplied with vascular DA-receptors than other organs. In the kidney the interaction between DA and the DA2-receptors significantly contributes the the DA-induced vasodilation. The interaction between DA and the DA2-receptors is of less importance for the DA-induced vasodilation in the intestine.

Synthesis and pharmacological evaluation of N,N-di-n-propyldopamine congeners containing phenolic bioisosteres

A series of analogues of N,N-di-n-propyldopamine (DPDA) in which the 3-hydroxyl group was replaced by bioisosteric groups was prepared and evaluated for D1- and D2-receptor affinity. The 3-methane-sulfonamide analogue (18) had a higher affinity for the D2 receptor than DPDA and was more selective for the D2 receptor. The 3-formamide derivative (15) also retained significant D2 affinity. Both of these compounds demonstrated in vivo cardiovascular and renal profiles in an anesthetized rat model that were consistent with selective D2-receptor agonism.

Involvement of dopamine in development of hypertension in spontaneously hypertensive rat: effect of carbidopa, inhibitor of peripheral dopa decarboxylase

The demonstration of acceleration of hypertension was investigated in spontaneously hypertensive rats (SHR) treated with carbidopa, inhibitor of peripheral dopa decarboxylase. Oral administration of carbidopa to young SHR for 4 weeks accelerated significantly (P less than 0.05) development of hypertension as compared to SHR treated with vehicle. Urinary excretion of dopamine (DA) (P less than 0.01) and renal content of DA (P less than 0.02) were significantly decreased by carbidopa treatment. Urinary excretion of sodium (P less than 0.05) was significantly decreased and renal content of norepinephrine (NE) (P less than 0.01) was significantly increased by carbidopa. Urinary excretion of NE and epinephrine (E) did not change during the experimental period. Negative correlation between systolic blood pressure and urinary excretion of sodium (P less than 0.05) or dopamine (P less than 0.01) and positive correlation between systolic blood pressure and renal content of NE (P less than 0.05) were significantly observed in both groups of SHR treated with carbidopa and with vehicle for 4 weeks. These results suggest that decreased DA biosynthesis in peripheral tissues accelerates development of hypertension mediated by decrease of natriuresis and enhanced release of NE in the kidneys of SHR. DA plays an important role in regulation of blood pressure, and reduced dopaminergic mechanisms enhance blood pressure in SHR.

Potential use of DA1 and DA2 receptor agonists in the treatment of hypertension

Will dopamine (DA) agonists have a role in the treatment of hypertension? Recent advances of medicinal chemistry and receptor pharmacology have suggested a positive answer. First, the division of DA receptors into two subtypes, DA1 and DA2, and the differentiation of these receptors from other receptors have resulted in the synthesis of relatively selective agonists and antagonists. Second, agonists of DA1 and DA2 receptors have been shown to decrease blood pressure in experimental animals and hypertensive patients. Review of clinical data with DA1, DA2, and combination of DA1 and DA2 agonists not only has demonstrated efficacy, but has revealed problems in the use of these compounds. Finally, possible solutions of these problems will be discussed.

Selective antagonism of the hypotensive effects of dopamine agonists in spontaneously hypertensive rats

Agonists of dopamine receptors can lower blood pressure by vasodilation through action on dopamine1 receptors, inhibition of sympathetic nerve activity by action on dopamine2 receptors, or actions in the central nervous system. Fenoldopam, a selective dopamine1 agonist, piribedil, a selective dopamine2 agonist, and dipropyl dopamine, a mixed dopamine1 and dopamine2 agonist, were injected intravenously in pentobarbital-anesthetized, spontaneously hypertensive rats (SHR). The mechanism for the antihypertensive effect was evaluated by administration of the selective dopamine1 antagonist SCH 23390 and the selective dopamine2 antagonist domperidone. While SCH 23390 only antagonized the hypotensive effects of fenoldopam, domperidone abolished the fall in blood pressure produced by dipropyl dopamine and piribedil but not by fenoldopam. Increments in heart rate and plasma norepinephrine levels accompanied the hypotensive effects of fenoldopam. The increase in heart rate was abolished by a dose of SCH 23390 sufficient to completely block the hypotensive effects and was significantly attenuated by the ganglionic blocking agent hexamethonium, which suggests that the increase in heart rate was due to a baroreceptor reflex. Fenoldopam does not cross the blood-brain barrier, which suggests that its hypotensive effect was mediated by peripheral dopamine1 receptors. Since domperidone does not cross the blood-brain barrier and significantly antagonized the hypotensive and bradycardic effects of dipropyl dopamine and piribedil, these effects were mediated primarily by peripheral dopamine2 receptors. These results indicate that SCH 23390 and domperidone are useful agents to identify the receptor subtype mediating the action of dopamine agonists in SHR.

Cardiovascular regulation during administration of co-dergocrine to normal subjects

Whether and to what extent activation of peripheral presynaptic dopamine2-receptors may modulate the release of norepinephrine (NE) and so affect blood pressure (BP) in normal or hypertensive man is not clear. The hydrogenated ergotoxine derivative, co-dergocrine, given in effective antihypertensive rather than excessive experimental doses, has recently been shown to act predominantly as a peripheral dopamine2-receptor agonist in several species. Accordingly, BP regulation assessed has been in 8 normal men on placebo and after 3 weeks on codergocrine 4 mg/day. Co-dergocrine significantly reduced urinary NE excretion from 43 to 33 micrograms/24 h, supine and upright plasma NE 21 to 16 and 49 to 36 ng/dl, respectively, heart rate (-8 and -5%, respectively) and upright systolic BP, 115 to 102 mm Hg; upright diastolic BP also tended to be lower. A standard pressor dose of infused NE was lowered from 131 to 102 ng/kg/min, and the relationship between NE-induced changes in BP and concomitant NE infusion rate or plasma NE concentration was displaced to the left. Exchangeable sodium and plasma volume tended to be slightly decreased. Plasma and urinary electrolytes and epinephrine, plasma renin activity and aldosterone levels, pressor responsiveness to angiotensin II, the chronotropic responses to isoproterenol, and the NE-induced rise in BP, plasma clearance of NE, glomerular filtration rate and effective renal plasma flow were not consistently modified. The findings are consistent with effective peripheral dopamine2-receptor agonism by co-dergocrine in humans. Peripheral presynaptic dopaminergic activation may modulate sympathetic activity and BP in normal man.

The effects of dopexamine on the cardiovascular system of the dog

The cardiovascular effects of dopexamine and dopamine were compared in the anaesthetized and conscious dog by the use of intravenous infusions over the dose range 3 X 10(-9) - 10(-7)mol kg-1 min-1. In the anaesthetized dog, dopexamine produced a dose-related fall in blood pressure due to peripheral vasodilatation and a small rise in heart rate and contractility. By contrast, dopamine did not significantly reduce blood pressure but produced a larger dose-related increase in contractility. At the highest infusion rate (10(-7)mol kg-1 min-1) blood pressure and heart rate were increased by dopamine. Dopexamine dilated the renal and mesenteric vascular beds with a potency similar to that of dopamine. Femoral vascular responses produced by both agents were inconsistent but the highest infusion rate of dopamine did produce vasoconstriction. With the aid of selective receptor antagonists (haloperidol, propranolol and bulbocapnine) the vasodepressor activity of dopexamine was shown to be mediated by stimulation of DA2-, beta- and DA1-receptors. The cardiac stimulation and renal vasodilatation produced by both compounds were due to stimulation of beta-adrenoceptors and DA1-receptors respectively. In the conscious dog, intravenous infusion of dopexamine caused a dose-related fall in blood pressure, renal vasodilatation and an increase in cardiac contractility and heart rate. Dopamine also increased cardiac contractility, and renal blood flow due to renal vasodilatation but without affecting heart rate. At the highest infusion rate, blood pressure was increased. Dopexamine and dopamine produced a similar incidence of panting and repetitive licking at 3 X 10(-8)mol kg-1 min-1 and emesis at 10(-7)mol kg-1 min-1, due to stimulation of dopamine receptors in the chemoreceptor trigger zone. Dopexamine produces a different cardiovascular profile from dopamine in the anaesthetized and conscious dog. Both compounds reduce renal vascular resistance, but in contrast to dopamine, dopexamine reduces afterload and produces only mild inotropic stimulation. These differences reflect contrasting activity at adrenoceptors.

Effects of the dopaminergic agonist cianergoline on blood pressure, the renin-angiotensin-aldosterone axis and the sympathetic nervous system in patients with essential hypertension

Cianergoline is a new dopaminergic agonist with a predominant cardiovascular action. Its effects on blood pressure, the renin-angiotensin-aldosterone axis, the sympathetic nervous system and lipid metabolism were assessed in 20 patients with benign essential hypertension. Cianergoline given in increasing doses for 4 weeks (maximum daily dose 12 +/- 2 mg (SD)) and placebo both caused a slight decrease in arterial pressure, (from 159/104 to 152/98 mm Hg and from 154/104 to 149/103 mm, respectively; difference not significant). Supine and upright plasma renin activity, plasma aldosterone, norepinephrine, epinephrine and dopamine levels, urinary catecholamine excretion rates as well as serum prolactin, low and high density cholesterol and triglyceride concentrations were not changed after cianergoline or placebo. Total serum cholesterol and triglyceride levels decreased significantly after placebo, but were unchanged after cianergoline. 3 out of 10 patients in the cianergoline group complained of nausea. The findings indicate that the new dopaminergic agonist cianergoline exerts only a mild blood pressure lowering effect in patients with essential hypertension and does not modify the release of prolactin, lipid metabolism or the basal activity or postural responsiveness of the renin-angiotensin-aldosterone axis and of the sympathetic nervous system.

Dopamine receptors and hypertension. Physiologic and pharmacologic implications

The endogenous catecholamine dopamine lowers blood pressure by acting on two receptor subtypes: dopamine 1 and dopamine 2. Dopamine 1 receptors subserve vasodilation, especially in the renal, coronary, mesenteric, and cerebral vascular beds. Dopamine 2 receptors have been located at the endings of postganglionic sympathetic nerves and, when activated, inhibit norepinephrine release. Inhibition of emesis and inhibition of prolactin release also appear to be dopamine 2-mediated phenomena. The receptor subtypes have been classified by differences in chemical structure of agonists and by specific antagonists. Dopamine also acts on beta 1 receptors to stimulate the heart and alpha 1 and alpha 2 receptors to cause vasoconstriction. Alpha adrenergic activity and lack of oral availability limit the use of dopamine in the treatment of hypertension. However, studies with the selective dopamine 1 agonist, fenoldopam, and dopamine 2 agonists such as LY 141865 and bromocriptine, indicate that agonists of both receptor subtypes can lower blood pressure in experimental animals and in hypertensive patients. Initial use of dopamine agonists in the treatment of hypertension and its possible involvement in the etiology and maintenance of hypertension are discussed.

DL-3,4-threo-DOPS in Parkinson's disease: effects on orthostatic hypotension and dizziness

Parkinsonian patients with orthostatic hypotension and dizziness due to usual antiparkinson therapy have been treated with the precursor amino-acid of noradrenaline, DL-3,4-threo-dihydroxyphenylserine (DL-3,4-threo-DOPS). Oral and intravenous administration improved these side effects significantly. A combined treatment of L-dopa, peripheral decarboxylase inhibitor and DL-3,4-threo-DOPS seems to be of benefit with respect to akinesia and orthostatic hypotension.