Halothane and isoflurane preferentially depress a slowly inactivating component of Ca2+ channel current in guinea-pig myocytes

1. The effects of the inhalational anaesthetics halothane and isoflurane on the high-voltage-activated Ca2+ channels were determined in isolated guinea-pig ventricular myocytes using the patch-clamp technique. 2. Recording solutions were equilibrated with inhalational anaesthetic vapour delivered from a calibrated vaporizer set at clinically relevant ranges of partial pressure. Anaesthetic concentrations in solution were determined using gas chromatography. 3. Halothane (0.9 mM in solution) and isoflurane (0.8 mM in solution) decreased peak whole-cell Ca2+ current (I-Ca) by similar to 40 and similar to 20%, respectively, while increasing the apparent rate of inactivation. 4. The sum of fast and slow exponential decay functions was required to fit the inactivation phase of I-Ca. The anaesthetics preferentially affected the slow component of inactivation while also increasing the rate of slow inactivation. The physiological significance of these effects was addressed by examining I-Ca evoked by a ventricular action potential waveform. 5. Measurement of the current carried by Ba2+ through Ca2+ channels (I-Ba) permitted the isolation of the slow component of inactivation. Halothane and isoflurane diminished peak I-Ba at 0 mV by similar to 45 and similar to 20%, respectively, with similar changes in rate and magnitude of the slowly inactivating component as with I-Ca. 6. Cell-attached patch-clamp measurements of Ca2+ channel activity revealed that halothane did not alter single-channel conductance. Instead, the anaesthetic reduced channel open probability to the same extent as observed during the whole-cell recording, an effect partially due to an increase in null sweeps. In patches with a single channel present, the open-time distribution, fitted by a single exponential, showed a decrease in mean open time. The closed-time distribution, fitted by the sum of slow and fast exponential components, revealed an anaesthetic-induced increase in the duration of the slow component with no effect on the fast component. Results are presented in terms of a channel-gating model, and model predictions are examined with a computer simulation.