Calcium-dependent proteolysis in the myocardium of rats subjected to stress.

Activity of a calcium-dependent neutral protease (calpain II) and its specific endogenous inhibitor was investigated in the myocardium of rats subjected to different stressors: cold, anaesthesia, 24 and 48 h starvation and food restriction for 7 and 14 days. Enzyme and inhibitor activities were determined in the 37,200 g supernatant of homogenates prepared from the free left ventricular wall of the heart. The specific activity of the myocardial calcium-dependent proteinase increased in all rats exposed to stressful stimuli, reaching maximum values in animals starved for 48 hours. Decrease in the specific activity of the inhibitor accompanied the changes in enzyme activity. Differences from normal control values were statistically significant in the starved animals and in animals fed a restricted diet for 7 or 14 days. These observations suggest that interaction between calpain II and its specific inhibitor plays a role in the regulation of the enzyme activity and furthermore, that stressful stimuli lead to increased calcium-dependent proteolysis in the myocardium.