Effect of dopamine on dog distal colon in-vitro

The effects of dopamine, isoprenaline, adrenaline and noradrenaline in dog isolated colon strips were studied. All these drugs produced in the preparation relaxations which were inhibited by propranolol and sotalol or, in the case of noradrenaline, by a mixture of propranolol and phentolamine. A selective beta 1-adrenergic antagonist, practolol, had the same inhibitory effects on the relaxations induced by all the agonists, suggesting the existence of beta 1- and beta 2-adrenoceptor subtypes in the dog distal colon. There is no evidence for the presence of specific dopaminergic receptors in this preparation.