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First published online on October 4, 2002.
 Copyright © 2002 by The Physiological Society
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Received July 26, 2002
Accepted after revision September 6, 2002

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

I. Findlay1*

1 CNRS UMR 6542, Faculté des Sciences, Parc de Grandmont, 37200 Tours, France

* To whom correspondence should be addressed. E-mail: findlay{at}univ-tours.fr.

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 {tau}f ~30 ms and {tau}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 ß-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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