Calcium-dependent proteolysis and its inhibition in the ischemic rat myocardium.

Activity of calcium-dependent neutral proteinase and its specific inhibition was investigated in the acutely ischemic myocardium after ligation of the left descending coronary artery in anaesthetized open chest rats. In experiments where the mean arterial blood pressure could be maintained above 70 mm Hg the hearts were quickly removed 5 to 30 min after ligation and homogenized in ice-cold buffer. The activity of the calcium-dependent neutral proteinase and of its endogenous inhibitor were determined in the 10,000 g supernatant of ventricular and septal tissues. Hearts from normal anaesthetized and sham operated animals left on the respirator for 10, 20 and 30 min, were used as controls. Only traces of proteinase activity could be found in the supernatants obtained from normal controls, while in the sham-operated animals the specific activity of the Ca2+-dependent proteinase increased with time, reaching significantly elevated values after 30 min on the respirator. In the ischemic groups enzyme activity also increased with increasing duration of ischemia and was substantially elevated in the ventricular myocardium 20 min after ligation. The increased calcium-dependent proteinase activity was accompanied by significantly reduced activity of its endogenous inhibitor. The concomitant changes in the activities of the myocardial calcium-dependent neutral protease and its endogenous inhibitor suggest that interaction between the enzyme and its inhibitor play a role in the regulation of intracellular calcium-dependent proteolysis.