Halothane, But Not Isoflurane, Impairs the β-adrenergic Responsiveness in Rat Myocardium

BACKGROUND: The aim of this study was to identify the mechanisms by which halothane and isoflurane change the myocardial beta-adrenergic signal transduction pathway. METHODS: The authors investigated the influence of volatile anesthetics on the isometric force of contraction of rat papillary muscles. Concentration-response curves for isoproterenol and epinephrine were studied under control conditions and in the presence of halothane or isoflurane. In radioligand receptor-binding studies, the beta-adrenoceptor affinities for isoproterenol and epinephrine were investigated with and without guanosine triphosphate. In addition, the isoproterenol-induced cyclic adenosine monophosphate accumulations in viable cardiomyocytes in the absence and in the presence of halothane were determined by radioimmunoassays. RESULTS: The half-maximal positive inotropic effect of isoproterenol was reached at a half-maximal effective concentration (EC50 value) of 68 nM (33-141 nM; n = 10). A minimum alveolar concentration of 1.3 halothane reduced the positive inotropic potency of isoproterenol (EC50 = 158 nM [118-214 nM; n = 10; P < 0.01 vs. control]), whereas isoflurane did not changed it. This observation held true when the force of contraction was stimulated with epinephrine. Halothane (1.3 minimum alveolar concentration) depressed beta-adrenoceptor high-affinity binding and beta-adrenoceptor agonist affinity in radioligand binding assays, an effect not seen with isoflurane. Halothane shifted the intracellular cyclic adenosine monophosphate response curve of isoproterenol to the right. CONCLUSION: Halothane, but not isoflurane, impairs the beta-adrenergic responsiveness in rat myocardium by reducing the agonist affinity of the beta-adrenoceptors.