Jai D. Kohli

Peripheral dopamine receptors: A classification based on potency series and specific antagonism

Abstract In this paper evidence is presented, based on the order of potency of agonists and specificity of antagonists, for the existence of two subtypes of dopamine receptors in the periphery. This classification may provide a basis for clearing up the confusion which prevails in this area resulting from the multiplicity of schemes and variety of terminology used for dopamine receptors on the basis of binding assays, biochemical or anatomical criteria, or on the type of response (excitatory or inhibitory).

Selective DA2 versus DA1 antagonist activity of domperidone in the periphery

Relative activity of domperidone as an antagonist of the two peripheral dopamine (DA) receptors, DA1 and DA2, was studied in pentobarbital-anesthetized dogs. Renal vasodilation produced by intra-arterial injection of DA into the phenoxybenzamine-pretreated renal vascular bed was the DA1-mediated response, while femoral vasodilation induced by dipropyl dopamine injected into the femoral artery with intact nerve supply was the DA2-mediated response. Domperidone, 0.5-5 micrograms/kg intravenously, inhibited the DA2 response by 15-75%. In contrast, doses up to 5 mg/kg had no effect on DA1-mediated or bradykinin-induced renal vasodilation. Domperidone has thus proved to be more selective than other DA2 antagonists, differentiating between the two peripheral DA receptors by a margin of greater than 10(4). Furthermore, domperidone was found to be selective as a DA2 versus alpha 2-adrenergic antagonist as studied on the dog cardioaccelerator nerve, thus, enhancing its value as a selective DA2 antagonist.

Regional variation in the alpha-adrenergic receptors in the canine resistance vessels.

SummaryVasoconstrictor effects of non-selective and relatively selective alpha1- and alpha2-adrenergic agonists (noradrenaline, phenylephrine, methoxamine, clonidine, and alpha-methylnora drenaline) and the inhibition of their vasoconstrictor effects by prazosin or yohimbine were studied in vivo in the renal and femoral vascular beds of pentobarbital anesthetized, beta-adrenergic and ganglion-blocked dogs. Some animals were additionally pretreated with methylphenidate to block uptake of the agonists by the nerves. In the femoral vascular bed the following results were obtained: 1. Clonidine was equipotent with noradrenaline and about 15-fold more potant than phenylephrine in reducing the blood flow. 2. Yohimbi while prazosin had practically no effect on, the induced by clonidine. 3. Vasoconstriction induced by phenylephrine or methoxamine was antagonized by prazosin but not by yohimbine. The results obtained in the renal vascular bed were as follows: 1. Clonidine was about 12-fold weaker than noradrenaline and nearly equipotent with phenylephrine in reducing blood flow. 2. Both yohimbine and prazosin were weak antagonists of the vasoconstriction induced by clonidine and tended to be additive. 3. Prazosin effectively antagonized the effects of phenylephrine or methoxamine. Noradrenaline and alpha-methylnoradrenaline caused vasoconstriction in both beds and their effects were inhibited by both antagonists which tended to be additive in either vascular bed.These results provide further evidence that the vasoconstrictor effects of noradrenaline and other alpha-adrenergic agonists are mediated by both subtypes of alpha-adrenergic receptors in the blood vessels of the dog. Higher potency of clonidine in the femoral bed and its selective inhibition by yohimbine indicate that alpha2 subtype of adrenergic receptors predominate in this bed. Similar considerations in the reverse suggest that alpha1-adrenergic receptors predominate in the renal vascular bed of the dog. Thus, these results show that there is regional variation in the distribution of the subtypes of alpha-adrenergic receptors in the resistance vessels of the dog.

Relative activities of SCH 23390 and its analogs in three tests for D1/DA1 dopamine receptor antagonism

Inhibitory activities of a series of analogs of SCH 23390 ((R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine) in which the 7-chloro group was substituted by bromo, fluoro, methyl and methoxy groups, respectively, were compared in three tests for D1 and DA1 dopamine (DA) receptor antagonism: inhibition of DA-induced renal vasodilation in the anesthetized dog (DA1 receptor model), inhibition of DA-stimulated adenylate cyclase in the striatum of adult female rats (D1 receptor model) and displacement of [3H]SCH 23390 in the striatal homogenates of male rats. In addition the D2 receptor affinity of each of the compounds chosen was assessed via displacement of [3H]spiperone binding from rat striatum. S-enantiomers of the Cl and CH3 analogs were 200- to 700-fold weaker than the respective R-enantiomers in all three tests. The activity of all the R-enantiomers was in the nanomolar range and varied no more than 8-fold in all three tests. The F analog in the ligand binding test was the only exception, which was 30-fold weaker than the C1 analog. All of the R-enantiomers studied showed much weaker affinity for the D2 receptor, as assessed by displacement of [3H]spiperone binding. Similar enantiomer selectivity and parallel affinities of the R-enantiomers in the prototype models for D1 and DA1 receptors strengthen the evidence in support of identity between the D1 and DA1 dopamine receptors. These results further indicate that displacement of SCH 23390 in the ligand binding test reflects affinity of a compound for D1 and DA1 dopamine receptors.

Sulpiride: A weak antagonist of norepinephrine and 5-hydroxytryptamine

Antagonism of norepinephrine (NE) and 5-hydroxytryptamine (5-HT) by sulpiride was studied in vitro on rabbit aortic strips. Concentrations of sulpiride ranging from 3 X 10(-5) to 3 X 10(-3) M caused a progressive shift of the dose response curves of both NE and 5-HT to the right without inhibiting the responses to KCl. pA2 values of sulpiride calculated from Schild plots against NE and 5-HT were 4.6 and 4.4, respectively. Thus, sulpiride, which has been previously shown to be the most potent antagonist of the dopamine vascular receptors, is a very weak alpha-adrenergic or 5-HT antagonist.

Relative DA1-Dopamine-Receptor Agonist and α-Adrenoceptor Antagonist Activity of Fenoldopam in the Anesthetized Dog

Relative DA1-dopamine-receptor agonist and alpha-adrenoceptor antagonist activities of fenoldopam were determined in pentobarbital anesthetized dogs. The renal vasodilating effect of intravenous infusions of fenoldopam was used as an index of its DA1-agonist activity. Inhibition by fenoldopam of femoral vasoconstriction produced by intraarterial administration of phenylephrine (a relatively selective alpha 1 agonist) and UK 14,304 ( a relatively selective alpha 2 agonist) was used to determine its alpha-adrenoceptor antagonist activity. Intravenous infusions of 0.1 and 0.2 microgram/kg/min of fenoldopam caused dose-related reductions in renal vascular resistance and mean arterial pressure. Higher rates of infusions of fenoldopam (2 and 5 micrograms/kg/min) were given after pretreatment with 50 micrograms/kg/min SCH 23390 (a DA1-selective antagonist) to attenuate DA1-mediated hemodynamic effects. After SCH 23390, the infusion of 5 micrograms/kg/min of fenoldopam produced approximately the same hypotensive and renal vasodilating actions as the 0.2 microgram/kg/min infusion without SCH 23390. Alpha-adrenoceptor blocking activity was not observed with the 0.1, 0.2, and 2 micrograms/kg/min infusions of fenoldopam. The 5 micrograms/kg/min infusion, however, produced 14 and 25% inhibition of the vasoconstrictor effects of phenylephrine and UK 14,304, respectively. These data indicate that fenoldopam decreases renal vascular resistance in doses which are 25 to 50 times less than the dose needed to antagonize alpha adrenoceptors.

Modification of cardiovascular actions of 2-amino-5,6-dihydroxytetralin by N,N-Di-n-propyl substitution

Effects of dopamine (DA) and the N,N-di-n-propyl derivative of 2-amino-5,6-dihydroxytetralin (dipropyl-A-5,6-DTN) were compared on renal blood flow in phenoxybenzamine pretreated dogs. Equivalent increments in renal blood flow were observed in the dose range of 12-190 nmol (ED50 of DA, 1.4 x 10(-8) mol; ED50 of dipropyl-A-5,6-DTN, 1.2 x 10(-8) mol, n = 7). Effects of 47 nmol of DA and dipropyl-A-5,6-DTN were reduced equally by simultaneous injections of 0.5 mg sulpiride (76 +/- 4 and 59 +/- 6%, respectively, n = 5). Intravenous injections of dipropyl-A-5,6-DTN in doses of 4-16 micrograms/kg elevated arterial blood pressure and decreased heart rate and cardiac contractility in open chest, vagotomized dogs, confirming its neuronal inhibitory effect mediated by DA2-presynaptic receptors. Dipropyl-A-5,6-DTN also caused dose-related femoral artery vasoconstriction in the hexamethonium pretreated dog. The hypertensive effect of intravenous dipropyl-A-5,6-DTN and its femoral vasoconstrictor effect were abolished by phenoxybenzamine. These results show that dipropyl-A-5,6-DTN possesses DA1-, DA2- and alpha-adrenergic activities. It is significant that by introducing di-n-propyl substitution on the nitrogen of 5,6-dihydroxy-2-aminotetralin, potent DA1-agonist activity can be conferred upon a molecule which is inactive in that respect. Further, beta-adrenergic activity of the primary amine is replaced by potent alpha-adrenergic activity by this substitution.

Hypotensive effects of N,N-din-n-propylodopamine in the anesthetized dog: comparison with sodium nitroprusside.

Summary N,N-Di-n-propyldopamine (DPDA), a dopamine (DA) vascular agonist without beta1-adrenergic activity, was compared with nitroprusside in pentobarbital-anesthetized dogs. DPDA was infused intravenously at 20, 40, and 80 μg/kg/min. DPDA caused dose-related reductions in mean arterial pressure when infused at 20 and 40 μg/kg/min; no further decrease occurred at 80 μg/kg/min. Sodium nitroprusside, 1–6 μg/kg/min, approximately equaled the hypotension produced by 40 μg/kg/min of DPDA. DPDA differed from sodium nitroprusside in causing a more rapid fall in mean arterial pressure and a more rapid recovery upon discontinuation. DPDA had no effect on pulmonary arterial pressure, but sodium nitroprusside reduced it. DPDA reduced heart rate; sodium nitroprusside increased heart rate. Hexamethonium, 10 mg/kg, significantly reduced the hypotension produced by DPDA. The DA antagonist, sulpiride, completely eliminated the reduction in blood pressure caused by DPDA. Neither hexamethonium nor sulpiride affected the hypotension produced by sodium nitroprusside. These studies suggest that DPDA reduces blood pressure in part by inhibiting the sympathetic nervous system and in part by vasodilation secondary to action on DA vascular receptors.

Relative contracting adn relaxing potencies of a series of prostaglandins on isolated canine mesenteric artery strips

The contracting and relaxing potencies of and interactions between a number of prostaglandins (PGs) were studied in vitro on spiral strips of small canine mesenteric arteries (outside diameter less than 1 mm). PGF2 alpha and PGE2, the most potent contracting PGs, were nearly equal in potency (EC50 4 x 10(-7) M) and did not cause relaxation under our experimental conditions. PGI2 and PGE1 were equal and the most potent relaxing PGs (EC50 3 x 10(-9) M). PGE1 also caused contraction, but this effect was not consistent. PGI2 did not cause contraction in concentrations up to 3 x 10(-6) M. In higher concentrations, however, it caused abrupt and near maximal contraction. PGD2 was weak in both respects, causing incomplete relaxation and contraction or biphasic effects. Interaction studies showed that PGE1 and PGI2 mutually excluded the relaxing effects of each other. PGE1 also reversed the relaxing effect of isoproterenol. However, pre-exposure to PGD2 did not attenuate the relaxing effect of PGE1 or PGI2 nor was the relaxing effect of PGD2 changed by pre-exposure to PGE1. Two different orders of potency of PGs suggest two PG receptors subserving contraction and relaxation, respectively. Further, it appears that several PGs can act upon both receptors which may explain unusual interactions between the PGs and some of their atypical effects. Finally, the data also suggest that there may be subtypes of the PG receptors subserving contraction and relaxation.

Absence of postsynaptic DA2 dopamine receptors in the dog renal vasculature

In experiments designed to look for functional postsynaptic DA2 dopamine receptors in the dog renal vasculature, the vascular effects of two selective DA2 receptor agonists, bromocriptine and quinpirole, were studied in pentobarbital-anesthetized dogs. Intra-arterial (i.a.) injections of bromocriptine reduced renal blood flow before and after blockade of alpha-adrenoceptors. I.a. quinpirole produced dose-related increases in renal blood flow which were not altered by blockade of alpha- or beta-adrenoceptors or DA1 dopamine receptors. Neither of the selective DA2 dopamine receptor antagonists, domperidone or YM 09151-2, altered the renal vascular effects of quinpirole, but these were reduced by combined H1 and H2 histamine receptor blockade. The results of these experiments do not support the existence of postsynaptic DA2 dopamine receptors mediating vasodilation in the canine renal vasculature.