Alterations of alpha 1 and alpha 2 adrenoceptor-mediated pressor responses by halothane and isoflurane anesthesia

The mechanism by which halothane and isoflurane interfere with catecholamine-mediated vasoconstriction was investigated, utilizing selective agonists of postjunctional alpha1 and alpha2 adrenoceptors in chronically instrumented dogs. After ganglionic, cholinergic, and beta adrenergic blockade, dose responses to phenylephrine (0.3-1.2 micrograms/kg, iv), a selective alpha1 adrenoceptor agonist, and azepexole [B-HT 933] (5-20 micrograms/kg, iv), a selective alpha2 adrenoceptor agonist, were obtained in conscious dogs. Each dog was subsequently anesthetized with either halothane (1.7%) or isoflurane (2%) in oxygen in equihypotensive concentrations. After a 1 h equilibration period, the dose response curves were repeated. Twenty experiments in ten chronically instrumented dogs were completed. Halothane and isoflurane produced significant (P less than 0.05) attenuation of both the increase in systolic and diastolic arterial pressure after bolus administration of all doses of phenylephrine and azepexole. No specific selectivity of either volatile anesthetic for alpha1 or alpha2 mediated pressor responses was found. Therefore, in chronically instrumented dogs, alpha1- and alpha2-mediated pressor responses were similarly influenced by halothane and isoflurane. The present results suggest that both halothane and isoflurane act as functional antagonists to alpha adrenergic mediated vasoconstriction.