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Voltage-dependent inactivation of L-type Ca²⁺ currents in guinea-pig ventricular myocytes

Ian Findlay

The objective of this study was to describe the kinetics of voltage-dependent inactivation of native cardiac L-type Ca²⁺ currents. Whole-cell currents were recorded from guinea-pig isolated ventricular myocytes. Voltage-dependent inactivation was separated from Ca²⁺-dependent inactivation by replacing extracellular Ca²⁺ with Mg²⁺ and recording outward currents through Ca²⁺ channels. Voltage-dependent inactivation accelerated from slow monophasic decay at -30 mV to maximal rapid biphasic decay at +20 mV. Maximal voltage-dependent inactivation occurred with _f ~30 ms and _s ~300 ms, the fast component of decay accounted for 70 % of the current amplitude. In basal conditions Ca²⁺ current availability was sigmoid. Isoproterenol (isoprenaline) evoked a large increase in a time-independent component of the Ca²⁺ current which also increased with depolarisation. This was responsible for the apparent recovery of Ca²⁺ channel current availability at positive membrane potentials and thus a U-shaped availability-voltage (A-V) relationship. It is concluded that -adrenergic stimulation altered the reaction of native cardiac L-type Ca²⁺ channels to membrane voltage. In basal conditions, voltage accelerated inactivation. In isoproterenol, voltage could also reduce inactivation.

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