Background: Volatile anesthetics exert profound effects on the heart, probably through their effect on Ca2+ movements during the cardiac cycle. Ca2+ movements across the sarcolemma are thought to involve mainly Ca2+ channels and the Na+/Ca2+ exchanger. We have therefore investigated the action of halothane, isoflurane, and enflurane on Na+/Ca2+ exchange and Ca2+ channel activity to assess the contribution of these pathways to the observed effect of the anesthetics on the myocardium.
Methods: Sarcolemmal ion fluxes were investigated using radioisotope uptake by isolated adult rat heart cells in suspension. Na+/Ca2+ exchange activity was measured from 45Ca2+ uptake by Na(+)-loaded cells. Ca2+ channel activity was measured from verapamil-sensitive trace 54Mn2+ uptake during electric stimulation.
Results: Halothane, isoflurane, and enflurane inhibited Na+/Ca2+ exchange completely, with similar potency when concentrations were expressed in millimolar units in aqueous medium but not when expressed as minimum alveolar concentration (MAC). The inhibition by enflurane was particularly strong, > 50%, at 2 MAC. In contrast, the three anesthetics inhibited Ca2+ channels with similar potency when concentrations were expressed as MAC but not when expressed in millimolar units in aqueous medium. Hill plots of pooled data with all three anesthetics showed a slope of -3.87 +/- 0.50 for inhibition of Na+/Ca2+ exchange and -1.73 +/- 0.19 for inhibition of Ca2+ channels.
Conclusions: Halothane, isoflurane, and enflurane inhibit both Na+/Ca2+ exchange and Ca2+ channels at concentrations relevant to anesthesia, although they exhibit differences in potency and number of sites of action. At 1.5 MAC, halothane inhibits Ca2+ channels more than Na+/Ca2+ exchange, whereas enflurane inhibits Na+/Ca2+ exchange more than Ca2+ channels. Isoflurane inhibited both systems equally. The inhibition of Ca2+ influx by these agents is likely to contribute to their negative inotropic effect in the heart. The inhibition of Na+/Ca2+ exchange by enflurane may account for its observed action of delaying relaxation in species lacking sarcoplasmic reticulum.