Dopamine-induced contractile responses of the rat anococcygeus muscle

In the rat anococcygeus muscle both dopamine and noradrenaline induced concentration-dependent contractile responses. The alpha 1-antagonist prazosin inhibited both dopamine and noradrenaline responses, whereas the alpha 2-antagonist yohimbine influenced noradrenaline-mediated responses only. The concentration-effect curves for dopamine were shifted to the right in presence of cocaine or after treatment with reserpine and alpha-methyl-p-tyrosine. When the tissues were previously exposed to 6-hydroxydopamine, the tyramine-induced contractile effect was abolished. Dopamine-induced concentration-effect curves were markedly shifted to the right. Treatment with 6-hydroxydopamine and reserpine did not modify the concentration-effect curves to noradrenaline. Our results indicate that dopamine has a dual effect: a partial effect due to an indirect sympathomimetic action and a partial effect due to the interaction with postjunctional receptors.