Agonist/antagonist activity of ergocristine at alpha-adrenoceptors in the rat

(1) The action of ergocristine at alpha-adrenoceptors was studied in vivo in the pithed rat, and in vitro on the rat isolated vas deferens. (2) In the pithed rat, the pressor response to ergocristine was reduced competitively by yohimbine, but not by prazosin. (3) Ergocristine decreased the tachycardia elicited by electrical stimulation of the cardioaccelerator sympathetic nerves, this effect being antagonized by yohimbine. (4) At postsynaptic alpha 1-adrenoceptors, on the vas deferens of the rat, ergocristine antagonized the contraction induced by phenylephrine in a competitive manner (pA2 = 7.85). (5) These results show that vasoconstriction due to ergocristine is mediated by alpha 2-adrenoceptors and that, in the rat, ergocristine acts as an alpha 2-adrenoceptors agonist, and an alpha 1-adrenoceptors antagonist.