Degradation of sarcoplasmic reticulum calcium-pumping ATPase in ischemic-reperfused myocardium: role of calcium-activated neutral protease.

In an attempt to clarify the mechanism of sarcoplasmic reticulum (SR) dysfunction during the genesis of irreversible damage in the ischemic-reperfused myocardium, the changes in SR Ca2(+)-pumping ATPase (Ca2(+)-activated, Mg2(+)-dependent ATPase; Ca2(+)-ATPase) activity were studied during ischemia and subsequent reperfusion in the isolated perfused guinea pig heart preparation and correlated with the accumulation of calcium in the myocardium. Although the SR Ca2(+)-ATPase activity was not affected by ischemia of 40 min, reperfusion of the ischemic myocardium resulted in a definite time-dependent decrease in the enzyme activity. The reduction of SR Ca2(+)-ATPase activity was associated with a concomitant decrease in the enzyme concentration in the isolated SR and was in a good correlation with a substantial accumulation within the myocardium. As the results indicated the possibility that proteolytic degradation by a calcium-activated protease(s) was responsible for the reduction of enzyme activity, we examined for the possible involvement of calcium-activated neutral protease (CANP). However, SR Ca2(+)-ATPase obtained either from the normal hearts or from the hearts after 40-min ischemia was found to be quite resistant to proteolytic actions of the two forms of CANP, i.e., microCANP and mCANP partially purified from the guinea pig heart. These results suggest that a destructive process leading to the degradation of SR Ca2(+)-ATPase is activated by reperfusion, but not by ischemia per se, and that CANP is not implicated in the degradation of SR Ca2(+)-ATPase.