Alpha 2-adrenergic agonist and alpha 1-adrenergic antagonist activity of ergotamine and dihydroergotamine in rats.

The effect of ergotamine and dihydroergotamine on peripheral alpha-adrenoceptors was examined in pithed rats. The pressor response to ergot alkaloids was reduced competitively by yohimbine but not by prazosin or methysergide. These results show that the vasoconstriction induced by ergot alkaloids is mediated by alpha 2-adrenoceptors. Ergot alkaloids decreased the tachycardia elicited by stimulation of the cardioaccelerator sympathetic nerves, this effect being antagonized by yohimbine. The pressor response to (-)-phenylephrine was reduced by ergot alkaloids, suggesting competitive antagonism. It is suggested that in the pithed rat both ergotamine and dihydroergotamine act at the periphery as competitive alpha 1-adrenoceptor blockers and partial alpha 2-adrenoceptor agonists.