α-Adrenergic agonist and antagonist activity of dihydroergotoxine in rats

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Abstract
The α-adrenergic activity of dihydroergotoxine had been studied in both pithed and urethane or pentobarbitone anaesthetized rats. In anaesthetized rats, blood pressure effects varied with the anaesthetic agent: hypotension with urethane, hypertension with pentobarbitone. This latter pressor response was a peripheral effect. In pithed rats, the vasopressor response to dihydroergotoxine was reduced competitively by yohimbine, and noncompetitively by nifedipine, but not by prazosin or methysergide, showing that the vasoconstriction is mediated by α2-adrenoceptors. Dihydroergotoxine decreases the tachycardia elicited by stimulation of the cardioaccelerator nerves, this effect being antagonized by yohimbine. It also reduced the pressor response to (-)-phenylephrine. These results indicated that, on the peripheral vascular system of the rat, dihydroergotoxine acts as an α1-adrenoceptor blocker and an α2-adrenoceptor agonist.

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