Run-down of the cardiac L-type Ca2+ channel: partial restoration of channel activity in cell-free patches by calpastatin.

We have found previously that run-down of cardiac Ca2+ channels in cell-free patches is reversed by cytoplasm plus adenosine triphosphate (ATP). Characterization of the factor in cytoplasm revealed that it is likely to be calpastatin (CS), an endogenous inhibitor of calpain (Ca2+-activated neutral protease). We therefore investigated the possible restoring effect of CS obtained from various tissues (activity 1.3-23 U/ml) on Ca2+ channel activity after run-down in inside-out patches. Although CS from porcine erythrocytes (plus 3 mM ATP) had only a minimal effect in restoring channel activity (to 4% of the control level recorded before the run-down), CS from porcine heart restored channel activity to 19% of control. The product of recombinant complementary deoxyribonucleic acid (cDNA) of human heart CS, a membrane-bound CS partially purified from bovine heart and CS from rabbit skeletal muscle (Sigma) restored channel activity to 28%, 23% and 10% of control levels, respectively. These results suggest that tissue-type CS, but not erythrocyte-type (truncated) CS, seems to have an effect on the cardiac Ca2+ channel to maintain its activity. Purified CS had relatively small effects compared to that of crude cytoplasm, implying that some other factor(s) might contribute also to the regulation of Ca2+ channel activity.