Potentiation of dopamine-induced contractions of the rat vas deferens by low concentrations of its antagonists.

Pimozide (5--10 ng/ml), haloperidol (0.5--5 ng/ml) and yohimbine (2--5 ng/ml) selectively potentiated dopamine-induced and methoxamine-induced contractions of the rat vas deferens. The concentration-response curves were significantly shifted to the left and the maximal responses were significantly increased. Responses of the vas deferens to noradrenaline were not so influenced. Concentrations of the antagonists in excess of 50 ng/ml produced concentrations-related parallel shifts of dopamine concentration-response curve to the right. These antagonists behaved in a similar manner towards methoxamine, a directly acting sympathomimetic amine which has a low affinity for neuronal uptake, suggesting that the mechanism of this potentiation may be independent of neuronal uptake inhibition.