Dopamine-induced depression of adrenergic nerve-mediated contraction of smooth muscle

1 Dopamine (0.25-1.0 micrometer) applied extraluminally depressed vasoconstrictor responses of the perfused rabbit ear artery to low frequency adrenergic nerve stimulation. 2 The depressant effect of dopamine was prevented in the presence of haloperidol (0.1 micrometer) but not in the presence of phentolamine (0.03-0.14 micrometer). 3 Extraluminal dopamine (1 micrometer) or intraluminal injection of dopamine in amounts up to 0.025 micronmol had no dilator effect on arteries partially constricted by infusion of noradrenaline. 4 Dopamine also depressed contractile responses of guinea-pig vas deferens to low frequency adrenergic nerve stimulation. However, in this tissue the effect of dopamine was prevented by phentolamine but not affected by haloperidol. 5 In neither the rabbit ear artery nor the guinea-pig vas deferens did haloperidol increase the magnitude of responses to nerve stimulation or affect muscle sensitivity to noradrenaline. 6 We conclude that the adrenergic axons in the rabbit ear artery, but not the guinea-pig vas deferens, possess specific receptors for dopamine whose activation depresses axonal conduction or transmitter release. However, our results do not favour the view that these receptors are activated during normal transmission at physiological frequencies.