Volatile anesthetics depress calcium channel blocker binding to bovine cardiac sarcolemma

Volatile anesthetics produce their negative inotropic effect on the heart mainly by interference with calcium homeostasis in the myocardial cell. In order to elucidate the mechanism of the depression, we have evaluated the effect of the volatile anesthetics on the binding of the calcium channel blocker [3H]nitrendipine to purified bovine cardiac sarcolemma. The radioligand binding studies were carried out at 25 degrees C, with increasing concentrations of [3H]nitrendipine (0.01-1 nM), in the presence or absence of unlabeled nitrendipine to determine specific binding, and with or without 1.9% halothane, 2.3% isoflurane, and 4.8% enflurane. Separately, [3H]nitrendipine was measured in the presence of increasing doses of halothane (0.78, 1.33, 1.90, and 2.57%). Kinetic studies of association and dissociation rate were performed with 1.90% halothane and 1 nM [3H]nitrendipine at different time intervals. All three volatile anesthetics produced depression of [3H]nitrendipine binding to the isolated cardiac sarcolemma. Only halothane produced a significant depression in binding, ranging between 59 and 66% (P less than 0.05), depending on the concentration of [3H]nitrendipine used. Isoflurane produced 29-38% depression, and enflurane produced 5-22% depression. Halothane also produced a significant (P less than 0.01) dose-dependent decrease in [3H]nitrendipine-specific binding. The kinetic binding experiments demonstrated that the time course for halothane's effect on association and dissociation of [3H]nitrendipine was 5 min for the half-maximum effect; the maximal reduction in binding capacity was at 15-30 min (P less than 0.05). Scatchard analysis revealed that all three volatile anesthetics produced reduction in the maximal number of binding sites; however, they varied in their effect on binding affinity. Only halothane produced a homogenous increase in the dissociation constant, signifying reduced affinity of the Ca2+ blocker to the channel. We suggest that the volatile anesthetics produce conformational changes in these channels consistent with their ability to depress channel-mediated Ca2+ influx into myocytes.