THE EFFECTS OF VOLATILE ANESTHETICS ON L-TYPE AND T-TYPE CALCIUM-CHANNEL CURRENTS IN CANINE CARDIAC PURKINJE-CELLS

The effects of halothane (0.45 and 0.9 mM, equivalent to 0.7 and 1.5%, respectively), isoflurane (0.54 and 1.23 mM, equivalent to 0.9 and 2.0%, respectively) and enflurane (0.65 and 1.48 mM, equivalent to 1.2 and 2.5%, respectively) on macroscopic L- and T-type Ca2+ channel currents were compared in single canine cardiac Purkinje cells using the whole-cell voltage-clamp technique. Cells were dialyzed with pipette solution containing CsCl and superfused with an external solution containing 10 mM BaCl2 and tetraethylammonium chloride. The long-lasting (L) and transient (T)-type Ca2+ channel currents were measured by depolarizing the membrane from different holding potentials (HPs). Voltage steps from an HP of either -80 or -70 mV elicited a low threshold, rapidly inactivating inward current at -40 to -30 mV, which maximally activated at -14 +/- 0.9 mV. This current was reduced by Ni2+ (100-mu-M) but not by nifedipine (1-mu-M), therefore resembling T-type Ca2+ channel current. In contrast, depolarizing steps from an HP of -40 mV elicited a sustained inward current that maximally activated at +4.1 +/- 0.8 mV and was nifedipine-sensitive, showing the characteristics of an L-type Ca2+ channel current. Halothane, isoflurane, and enflurane produced a concentration-dependent suppression of total Ca2+ channel current in every cell studied. Separation of Ca2+ channel types showed that both L- and T-type Ca2+ channel currents were depressed to a similar extent by anesthetic administration. These agents reduced peak L- and T-type current elicited at each pulse potential but did not shift the current-voltage (I-V) relationship for either T- or L-type current activation. There was no significant difference between the sensitivities of either L- or T-type currents to halothane, isoflurane, or enflurane at equianesthetic concentrations. This suppression of L- and T-type currents may help to explain the negative inotropic and chronotropic effects of these agents on the heart.