General anesthetics and vascular smooth muscle: direct actions of general anesthetics on cellular mechanisms regulating vascular tone.

General anesthetics threaten cardiovascular stability by causing changes in cardiac function, vascular reactivity, and cardiovascular reflexes and significantly alter distribution of cardiac output to various organs. Their overall impact is often systemic hypotension, which is attributable to myocardial depression, peripheral vasodilation, and attenuated sympathetic nervous system activity. However, one could be more causative than the others, depending on anesthetic agents and cardiovascular factors inherent in patients (e.g., coexisting heart disease). It is generally believed that most general anesthetics attenuate sympathetic nervous system outflow from the central nervous system, thereby decreasing vascular resistance in peripheral circulations. Indeed, in previous in vivo studies, during administration of various general anesthetics, vascular resistance was decreased in most peripheral circulations; however, it was unaffected or increased in some peripheral circulations. General anesthetics may act directly on vascular smooth muscle and/or endothelial cells in various vascular beds, influencing total peripheral and/or regional vascular resistance, and hence organ blood flow. This article reviews previously reported direct (i.e., nonneural) vascular actions of general anesthetics and discusses their underlying mechanisms, their in vivo relevance, and the future of research for general anesthetic vascular pharmacology.