Analysis of the relaxing effect of dopamine on the isolated rat jejunum

The relaxing effect of dopamine on the rat isolated jejunum has been studied. Dopamine was found 170 times less potent than noradrenaline and 3 times more potent than tyramine. The relaxing effect of dopamine does not show the tachyphylaxis phenomenon, is present in preparations from rats pretreated with reserpine and is not influenced by cocaine. These results indicate that dopamine has a direct action. To characterize the receptor(s) through which dopamine causes intestinal relaxation the alpha-blocker phentolamine, the beta-blocker propranolol and the dopamine receptor blockers haloperidol and cis-alpha-flupenthixol, alone or in combination have been tested. 40% of inhibition of the response to dopamine was obtained with phentolamine, 25% with propranolol and 30% with haloperidol or cis-alpha-flupenthixol. Combining together three antagonists acting on three different receptors it was possible to obtain 70% of inhibition of the responses to dopamine. It is concluded that alpha and beta adrenoceptors and specific dopamine receptors are involved in the direct relaxing action of dopamine.