Roles of calpastatin and ATP in regulation of the Ca channel

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Mechanisms of rundown of cardiac L-type Ca channel was investigated in guinea-pig vantricular cells using the patch-clamp method. Cytoplasm form bovine heart restored the Ca channel activity in the inside-out patch mode. Fractionation of active factor (s) by gel filtration and ion-exchange chromatography suggested that the factor co-purified with calpastatin, an endogenous inhibitor of the protease calpain. For the effect of cytoplasm, ATP was required, but non-hydrolyzable ATP such as AMP-PNP could be substituted partially for ATP.Cyclic AMP-dependent protein kinase alone had a minimal effect on the channel activity, but, in the presence of cytoplasm, the kinase exhibited and additional activation of the channel. These results syggest that cytoplasm contains factors that maintain the Ca channel activity in cardiac cells.