Prolonged hyporesponsiveness of vascular smooth muscle contraction after halothane anesthesia in rabbits

Halothane diminishes smooth muscle contractility in vascular tissue. In order to further characterize this phenomenon we undertook a series of in vivo and ex vivo experiments. Pressor dose-response curves to the selective alpha 1-adrenergic agonist, phenylephrine, were constructed in groups of rabbits before, during and 2 hr after halothane anesthesia and the dose of phenylephrine that induced a 25 torr increase in mean arterial pressure (ED25) was derived by polynomial regression analysis. ED25 torr increased significantly during halothane anesthesia, and rabbits remained in this insensitive state when the ED25 was assessed 2 hr after anesthesia. The halothane-induced loss of responsiveness was corroborated by ex vivo experiments utilizing aortic rings from halothane-anesthetized rabbits. The maximal contraction to norepinephrine (NE) was significantly lower in halothane-treated aortic rings and only slowly returned to normal by 4 hr. The EC50 (the dose causing a 50% maximal contraction) for NE was significantly greater in aortic rings from halothane-anesthetized rabbits. This loss of sensitivity, reflected by the higher EC50 was not restored by 4 hr of ex vivo incubation in a halothane-free medium. We conclude that halothane induces loss of sensitivity to adrenergic agonists that persists for several hours after termination of the halothane anesthetic.