Volatile anesthetic inhibition of neuronal Ca channel currents expressed in Xenopus oocytes

The genes encoding the alpha(1A), alpha(1B), alpha(1C) and alpha(1E) subunits of neuronal high voltage-gated Ca channels (HVGCCs) were separately expressed with beta(1B) and alpha(2)/delta subunits in Xenopus oocytes to determine the effects of volatile anesthetics (VAs) on currents through each specific channel. VA effects were determined on currents carried by Ba2+ (I-Ba) using the two electrode voltage clamp technique. Although time to peak was unaffected, both halothane (0.59 mM) and isoflurane (0.70 mM) reversibly inhibited peak I-Ba by 25-35% and late current (at 830 ms) by 50-60%. A hyperpolarizing shift in steady-state inactivation of alpha(1E)-current was found which could contribute up to one third of observed decrease in the peak current. The rate of inactivation of I-Ba seen with alpha(1A), alpha(1B) and alpha(1E)-type Ca channels was consistently increased by halothane and isoflurane. To more clearly quantify these effects, I-Ba inactivation was fit by a single exponential function. The anesthetics depressed both the: inactivating and non-inactivating residual components of I-Ba and decreased the time constant of inactivation. In the case of I-Ba through alpha(1C)-type channels, inactivation was minimal; however, the average current was inhibited by VAs. Similar inhibition of all these HVGCCs by halothane and isoflurane suggests that a common structural component may be involved. Furthermore, the inhibition of such neuronal HVGCCs in situ could alter synaptic neurotransmitter release and contribute to the anesthetic state. (C) 1999 Elsevier Science B.V. All rights reserved.