The influence of halothane and isoflurane on [alpha]-adrenergic-mediated vasoconstriction before and following calcium channel modulation was investigated in chronically instrumented dogs. After ganglionic, cholinergic, and [beta]-adrenergic blockade, systemic hemodynamic responses following equieffective pressor doses of phenylephrine (0.6 [micro] g/kg iv), a selective [alpha]1agonist, and azepexole [B-HT 933] (20 [micro]/kg iv), a selective [alpha]2 agonist, were obtained. The calcium channel stimulator Bay k 8644 (0.5 and 1 [micro]g[middle dot]kg-1[middle dot] min-1) was infused intravenously for 10 min and phenylephrine and azepexole administered at the end of each infusion. On different days, each dog was subsequently anesthetized with equihypotensive concentrations of halothane (1.7%) or isoflurane (2%) in oxygen and the same pharmacologic interventions were repeated in the presence of halothane or isoflurane. Twenty-one experiments (three groups) using seven chronically instrumented dogs were completed. Halothane and isoflurane produced significant (P < 0.05) attenuation of the increase in arterial pressure after bolus administration of phenylephrine and azepexole. Bay k 8644 augmented the pressor responses mediated by both phenylephrine and azepexole in all three groups. Thus, halothane and isoflurane nonselectively reduced the pressor response to both [alpha]1- and [alpha]2-adrenergic receptor stimulation and this was probably not mediated by inhibition of transmembrane calcium flux through dihydropyridine sensitive channels.
(C) 1990 American Society of Anesthesiologists, Inc.