The inactivation of Ca channels during the action potential plateau of guinea-pig ventricular myocytes was investigated by interrupting action potentials with voltage clamp pulses to assess Ca channel availability. The influence of the bulk cytosolic calcium [( Ca]i) transient on Ca channel inactivation was also studied by impaling cells with microelectrodes containing the Ca chelator BAPTA (1,2 bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; 125-200 mM). Ca channel availability decreased progressively with action potential duration, reaching approximately 20% of maximum availability after 100 ms and falling close to zero at the end of the plateau. When membrane potential became more negative than -40 mV Ca channel availability increased. Elevation of the action potential plateau to more positive levels increased Ca channel availability (even though this was expected to increase peak [Ca]i). When the cytosol was loaded with BAPTA Ca channel availability during the plateau increased. Inactivation of Ca channels was not, however, abolished. The observations are consistent with the hypothesis that in guinea-pig ventricular myocytes the majority of Ca channels are inactivated during the plateau and recovery does not occur until repolarization is almost complete. It may be that while the cytosolic Ca transient (that is generated in part by release of Ca from the intracellular Ca stores) modulates Ca channel availability, significant inactivation of the Ca channel during the action potential plateau is due to voltage dependent inactivation and to Ca-induced inactivation resulting from the Ca which enters the myocyte via Ca channels.

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