Alpha-adrenolytic properties of bromocriptine in dogs.

The effects of bromocriptine on dose-response curves to intravenously administered sympathomimetic agents (adrenaline, noradrenaline, phenylephrine and clonidine) were studied in normotensive anesthetized dogs. After pretreatment with bromocriptine (0.3 or 1 mg/kg i.v.) low doses of adrenaline showed a significant depressor action. In contrast, this drug failed to modify the variations in blood pressure induced by noradrenaline. Bromocriptine reduced the pressor effect of phenylephrine and the hypotensive action of clonidine. These results suggest that, in addition to dopaminergic stimulant properties, bromocriptine also possesses alpha-adrenolytic properties. The data obtained with phenylephrine and clonidine suggest that bromocriptine is a mixed alpha 1 and alpha 2 antagonist. The differences observed between adrenaline and noradrenaline are discussed in term of different subtypes (intra- and extra-synaptic) of alpha-adrenoceptors.