Insensitivity of P- Type Calcium Channels to Inhalational and Intravenous General Anesthetics

BACKGROUND: Voltage-gated Ca2+ channels long have been considered plausible targets for general anesthetics. Previous anesthetic studies have focused on L-, T-, or N-type channels, but there have been no studies on channels identified as P-type. Since P-type channels may be the most important voltage-gated Ca2+ channels involved in synaptic transmission in mammalian brain, it is important to establish their sensitivity to clinically relevant concentrations of general anesthetics. METHODS: Acutely dissociated cerebellar Purkinje neurons were obtained from 7-14-day-old Sprague-Dawley rats. P-type currents were measured using the whole-cell version of the patch-clamp technique, with Ba2+ as the current carrier. General anesthetics were applied to the neurons in aqueous solution at room temperature (20-23 degrees C). RESULTS: P-type Ca2+ channels were found to be very insensitive to a variety of general anesthetics and ethanol. Inhibitions of less than 10% were produced by 0.35 mM halothane, 0.35 mM isoflurane, 32 microM thiopental, 50 microM pentobarbital, 2 microM propofol, and 200 mM ethanol. Substantial anesthetic inhibition was found only at free aqueous concentrations much greater than those that are clinically relevant. For halothane, the dose-response curve showed an IC50 concentration of 1.17 +/- 0.02 mM and a Hill coefficient of 2.02 +/- 0.04 (mean +/- SEM). CONCLUSIONS: The relatively small inhibitions of P-type Ca2+ channels produced by volatile and intravenous anesthetics at their free aqueous EC50 concentrations for general anesthesia in mammals suggest that these channels do not play a major role in the induction of general anesthesia.