Dopamine-sensitive cAMP generating system in rat extracerebral arteries

Light microscope autoradiography of peripheral dopamine receptor subtypes

Radioligand binding assay techniques associated with light microscope autoradiography were used for investigating the pharmacological profile and the micro anatomical localization of peripheral dopamine receptor subtypes. In systemic arteries, the predominant dopamine D1-like receptor belongs to the D5 (or D1B) subtype. It is located within smooth muscle of the tunica media. In pulmonary arteries, dopamine D1-like receptors have primarily an endothelial localization and belong to the dopamine D1 (or D1A) receptor subtype. Both systemic and pulmonary arteries express a dopamine D2-like receptor belonging to the D2 receptor subtype. It has a prejunctional localization in the majority of vascular beds investigated. In cerebral, coronary and mesenteric arteries, it has also an endothelial localization. In the heart, a dopamine D4 receptor was identified. It is expressed by atrial tissue and has a widespread distribution overall atrial musculature. The kidney expresses both dopamine D1-like and D2-like receptors. Renal dopamine D1-like receptors have a vascular and tubular localization. The majority of these sites belongs to the D5 receptor subtype. A smaller D1 receptor population has primarily a tubular localization. Renal dopamine D2-like receptors belong to the dopamine D3 subtype and in lesser amounts to the D2 and D4 receptor subtypes. Renal dopamine D3 receptor has to a greater extent a tubular localization, whereas the D4 receptor is located within glomerular arterioles. The above results suggest that radioligand binding assay and autoradiographic techniques, if performed in the presence of compounds displaying specific receptor subtype selectivity, may contribute to characterize, mainly from a quantitative point of view, peripheral dopamine receptors.

Autoradiographic localization of dopamine receptors in rat cerebral blood vessels

Combined in vitro radioreceptor binding and autoradiographic techniques were used to analyze the pharmacological profile and the anatomical localization of dopamine (DA) DA1 and DA2 receptor sites in the arteries and arterioles of the pial-arachnoid membrane in the rat. [3H]SCH 23390, used as a ligand of DA1 receptors, was bound by sections of rat cerebral vessels in a manner consistent with the labeling of DA1 receptors, with a value of 2.2 nM for the dissociation constant (Kd) and of 68.4 fmol/mg protein for the density of binding sites (Bmax). The ligand was bound primarily by the medial layer of cerebral arteries. The density of silver grains developed after exposure of cerebral vessel sections to [3H]SCH 23390 was unchanged after chemical sympathectomy with 6-hydroxydopamine (6-OHDA) and was not significantly different in either the circle of Willis arteries or in the pial-arachnoid arteries and arterioles. [3H]Spiroperidol was used as ligand of DA2 receptors in the presence of ketanserin to block the possible binding to 5-HT2 receptors. [3H]Spiroperidol was bound by sections of rat cerebral vessels in a manner consistent with labeling of DA2 receptors, with Kd and Bmax values of 1.9 nM and 39.8 fmol/mg protein, respectively. The ligand was bound primarily by the adventitia and the adventitial-medial border as well as by the intimal layer of cerebral arteries. The accumulation of adventitial and adventitial-medial silver grains developed after exposure of cerebral vessels sections to [3H]spiroperidol did not occur in 6-OHDA-treated animals. In contrast, chemical sympathectomy was without effect on the density of intimal silver grains. The density of adventitial silver grains was higher in the anterior than in the posterior circle of Willis and pial-arachnoid arteries and arterioles, as well as in the pial-arachnoid arteries and arterioles than in circle of Willis arteries. These findings indicate the existence of both DA1 and DA2 receptor sites in the rat cerebrovascular tree.

Cardiovascular dopamine receptors: role of renal dopamine and dopamine receptors in sodium excretion

Research efforts in the area of peripheral dopamine have now established the presence of two distinct subtypes--DA1 and DA2--of DA receptors, and have identified a potential role for dopamine produced within the kidney in the control of renal sodium excretion. Selective DA1 and DA2 receptor agonists are being developed because they exhibit therapeutic potential for treatment of cardiovascular and renal disorders. Furthermore, basic research efforts are aimed towards identifying the stimulus and/or stimuli for the production of dopamine within the kidney and characterizing the cellular signalling processes involved in mediating the renal effects of dopamine and selective DA receptor agonists.

Vasodilator responses to dopamine in rat perfused mesentery are age-dependent

1. Dose-dependent vasodilator responses to dopamine, isoprenaline, noradrenaline, 3-isobutyl-1-methylxanthine (IBMX) and sodium nitroprusside were obtained in isolated perfused mesentery preparations, taken from reserpine-treated rats of different ages. The preparations were pretreated with phenoxybenzamine (1 microM) and perfused with physiological salt solution containing cocaine (10 microM), additional KCl (20 mM) and vasopressin (0.1 microM). 2. Vasodilator responses to dopamine were abolished by the dopamine1 (DA1)-selective antagonist SCH 23390 (10 nM) and those to isoprenaline by propranolol (1 microM), but the vasodilator responses to noradrenaline were abolished only when SCH 23390 and propranolol were used together. This indicated that dopamine was acting via DA1-receptors, isoprenaline via beta-adrenoceptors and that noradrenaline could act via DA1-receptors and beta-adrenoceptors in this preparation. 3. Responses to all the vasodilator drugs decreased in magnitude between the ages of 1 and 2 months. Responses to dopamine declined further in 4 month-old rats and were negligible at 6 or 22-24 months of age. Responses to isoprenaline were well maintained up to 6 months of age, but were negligible at 22-24 months. 4. It is concluded that, in the rat mesenteric vasculature, there is a non-specific decline in responses to vasodilator drugs during development (1 to 2 months). Subsequently there is a specific decline in DA1-receptor-mediated and beta-adrenoceptor-mediated responses; the former are lost at an earlier age than the latter. This different time course suggests that age influences receptor numbers, or their coupling to adenylate cyclase, rather than a post-receptor event in the adenylate cyclase/cyclic AMP pathway.

Presence of dopamine-dependent adenylate cyclase activity in human renal cortex

The effects of dopamine (DA) and of two selective DA DA1 agonists (SKF 38393 and SKF 82526) on adenylate cyclase activity were studied with human kidney cortex membrane preparations. DA elicited a dose-related stimulation of adenylate cyclase activity with an EC50 of 60 microM. The selective DA DA1 antagonist SCH 23390 behaved as a competitive antagonist, shifting the dose-response curve to the right. The non-selective beta-adrenoceptor antagonist (-)-propranolol did not affect the EC50 of the dose-response curve to DA but attenuated the maximal stimulatory effect of DA at concentrations higher than 100 microM. (+)-Sulpiride inhibited DA-induced adenylate cyclase stimulation in a dose-dependent manner with an IC50 of 4.6 X 10(-8) M but (-)-sulpiride was without effect. Both SKF 38393 and SKF 82526 stimulated the adenylate cyclase activity of human kidney cortex; this effect was completely antagonized by SCH 23390. Our results, demonstrating the presence of DA-sensitive adenylate cyclase activity, strongly suggest the presence of a DA receptor of the DA1 subtype in human kidney cortex.

Involvement of cyclic-AMP in the hypotensive response to fenoldopam but not to quinpirole in the anaesthetized rat

1. The objective of the present investigation was to ascertain whether cyclic-AMP acts as a mediator in the hypotensive responses to fenoldopam, a selective dopamine DA-1 receptor agonist and quinpirole, a selective DA-2 receptor agonist, in the anaesthetized rat. 2. The hypotensive response to two doses of fenoldopam (10 and 40 micrograms kg-1, i.v.), when administered in separate groups of animals was reproducible following the administration of saline. 3. Following the i.v. bolus administration of theophylline, IBMX or forskolin, the hypotensive response to fenoldopam (10 micrograms kg-1, i.v.) was significantly potentiated. In addition, the hypotensive response to the higher dose (40 micrograms kg-1, i.v.) was augmented by IBMX. 4. Neither theophylline nor forskolin influenced the hypotensive response to quinpirole. 5. Prior administration of theophylline or forskolin did not alter the hypotensive response to acetylcholine. On the other hand, the hypotensive response to histamine which is reportedly mediated through cyclic-AMP accumulation was significantly potentiated after theophylline administration. 6. These results suggest that cyclic-AMP may be acting as a mediator in the vasodilation and the resultant hypotension following DA-1 receptor activation by fenoldopam. However, the hypotensive response seen with DA-2 receptor activation does not appear to involve cyclic-AMP.

Dopamine-stimulated adenylate cyclase in human term placenta

The effect of dopamine on adenylate cyclase activity was investigated in slices of human term placentas. Dopamine elicited a dose-dependent stimulation of cAMP formation with a ED50 value of about 1 X 10(-6)M dopamine and an increase of 110% over the control with 1 X 10(-4)M dopamine. (-)-Epinephrine and (-)-norepinephrine also increased placental cAMP formation. Apomorphine displayed a slight but non-significant stimulatory effect while bromocriptine was not effective. SCH 23390, a selective antagonist of dopamine D1 receptors caused a dose-dependent decrease of the dopamine activation. In contrast, the dopamine increase of cAMP was unaffected by beta- and alpha-adrenergic blocking drugs and by the D2 selective antagonist, (-)-sulpiride. These data indicate that dopamine stimulates cAMP formation in human term placenta through a specific mechanism via D1 dopaminergic receptors positively coupled to adenylate cyclase.

Vasomotor responses of cerebral arterioles in situ to putative dopamine receptor agonists

The vasomotor responses of individual cerebral pial arterioles on the convexity of the cerebral cortex to subarachnoid perivascular micro-injections of dopamine and the putative dopamine receptor agonists, apomorphine, SKF 38393 and LY 141865, have been examined in 38 anaesthetized cats. The perivascular microapplication of dopamine (10(-9)-10(-3)M) effected dose-dependent reductions in pial arteriolar calibre, with the maximum reductions in calibre (22 +/- 2% from preinjection levels: mean +/- s.e.) being observed at 10(-3)M. The cerebrovascular constriction produced by dopamine (10(-5)M) could be significantly attenuated by the concomitant perivascular administration of phentolamine (10(-6)M) or methysergide (10(-6)M). The perivascular microapplication of apomorphine (10(-8)-10(-4)M) effected dose-dependent increases in arteriolar calibre, with the maximum increase (31 +/- 6%) being observed with apomorphine (10(-5)M). The perivascular administration of the putative dopamine D1-receptor agonist, SKF 38393 (10(-9)-10(-4)M) increased arteriolar calibre, with the maximum response (24 +/- 3%) being observed with injection of 10(-7)M. The putative dopamine D2-receptor agonist, LY 141865, also increased cerebral arteriolar calibre, but only at high concentrations (maximum calibre increase 25 +/- 6.1 with 10(-4)M). The cerebrovascular dilatations elicited by apomorphine and by SKF 38393 were markedly attenuated by the concomitant perivascular microapplication of the putative dopamine D1-receptor antagonist, SCH 23390 (10(-8)M). The perivascular administration of SCH 23390 (10(-9)-10(-5)M) per se did not alter arteriolar calibre nor the arteriolar dilatation provoked by microinjections of acidic cerebrospinal fluid. These results point to the presence on cat cerebral arterioles of dopamine receptors (probably of D1 subtype) mediating dilation.

Dopamine-sensitive adenylate cyclase in rabbit portal vein

The effect of dopamine (DA) on 3',5'-cyclic adenosine monophosphate (cAMP) generation in the rabbit portal vein was investigated. Dopamine added to homogenates of rabbit portal vein increased the concentration of cAMP. DA-elicited cAMP increase was exclusively inhibited by DA antagonists fluphenazine and haloperidol suggesting the existence of DA receptors (of the DA1 subtype) in the portal vein.

Age-associated decrease of dopamine-sensitive adenylate cyclase in rabbit renal artery

Using renal arteries from young (6 months), mature (18 months) and old (70 months) rabbits, the effect of dopamine on 3',5'-cyclic adenosine monophosphate (cAMP) generation was studied. No significant differences in basal adenylate cyclase values were found in homogenates of renal artery from young, mature or old rabbits. The protein content of the renal artery was also unchanged during the lifespan. Dopamine stimulated the production of cAMP. The stimulatory effect of dopamine was similar in the renal artery of young and mature rabbits but was lower by about 40% in old rabbits. The present results led us to hypothesize an impairment of the peripheral dopaminergic system in old age similar to that described for the central dopaminergic system.