Abstract
On isolated rabbit mesenteric artery - preincubated with phenoxybenzamine (10-5M) and contracted with prostaglandin F2α- dopamine(DA, 10-6-3xl0-4M) produced dose-dependent relaxations, which were only antagonized by the DA-receptor antagonists metoclopramide (pA2 5.18) droperidol (pA2 6.05) and d-butaclamol (pA2 6.77) in a competitive manner. l-Butaclamol, however, had no effects. Atropine (10-6M), metiamide (10-5M) and pretreatment of the rabbits with 6-hydroxy-DA (3×30 mg/kg i.v.) did not modify DA-caused relaxations thus excluding cholinergic, histaminergic and noradrenergic mechanisms. Hence, DA produces its relaxing effect on rabbit mesenteric artery through direct stimulation of postsynaptic vascular DA-receptors. Various DA-receptor agonists were tested on their ability to stimulate the vascular DA-receptor. Among these the most potent were epinine, A-6,7-DTN and bromocriptine, while apomorphine was only a weak (partial) agonist. The existence of vascular DA-receptors could be further demonstrated by binding studies with 3H-spiroperidol, which binds to membranes from rabbit mesenteric and renal artery with high affinity (KD=13.1 and 15.9 nM, respectively). DA-receptor antagonists inhibited binding much more potently than α-adrenolytic drugs and DA was much more potent than noradrenaline. Thus, the characteristics of 3H-spiroperidol binding sites are indistingui shable from those of DA-receptors.
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