Selectivity for catecholamines of presynaptic alpha-receptors involved in feedback control of sympathetic neurotransmitter secretion in guinea-pig vas deferens.

The aim of the study was to quantitatively compare the relative affinities of noradrenaline, adrenaline, dopamine and isoprenaline for the, probably neural, receptors mediating feedback control of sympathetic neurotransmitter secretion. The experiments were carried out in isolated superfused field stimulated guinea-pig vas deferens, in which the noradrenaline stores had been labelled by preincubation with tritiated (-)-noradrenaline. Desipramine and normetanephrine were added to prevent rebinding of transmitter. Exogenous noradrenaline was found to cause a dose-dependent and reversible depression of the secretion of tracer noradrenaline evoked by field stimulation. Since the depressing effect was not affected by a ten-fold rise in the desipramine concentration, it seems likely that it was not due to uptake and preferential secretion of unlabelled exogenous noradrenaline, but was truly due to depression of the secretory mechanism. Adrenaline was significantly more potent than noradrenaline, as inhibitor of the secretion of tracer transmitter, while dopamine, at the same molar concentration, was without effect. The beta-agonist isoprenaline did not depress, but rather tended to enhance, the secretion of tracer noradrenaline. It is concluded that the receptors controlling the secretion of noradrenaline from the sympathetic nerves of guinea-pig vas deferens quantitatively-with regard to sensitivity-as well as qualitatively-with regard to order of preference for different catecholamines-resemble the "classical" alpha-receptors of e.g. smooth muscle in the same tissue.