Ca2+ sensitivity change and troponin loss in cardiac natural actomyosin after coronary occlusion

Ca2+ sensitivity of natural actomyosin (NAM) isolated from both the intact left ventricular free wall and an area of myocardial infarction (MI) was analyzed by use of superprecipitation response from 2 to 48 h after left anterior descending coronary artery ligation in the dog. NAM from the intact tissue showed normal superprecipitation and normal Ca2+ sensitivity. Four hours after coronary ligation, Ca2+ sensitivity was lowered only in the endocardial half of MI region; it was markedly decreased both in the epicardial and endocardial halves at 6 h and completely lost at 24 and 48 h. A superprecipitation response was, however, demonstrated in all samples, indicating that both myosin and actin preserved their functions in the course of MI. With polyacrylamide gel electrophoresis in sodium dodecyl sulfate, NAM from the MI region revealed moderate decrease of the tropomyosin-binding subunit of troponin(TN-T) and the Ca2+-binding subunit of troponin(TN-C) and drastic decrease of the inhibitory subunit of troponin(TN-I). This resulted in the formation of extra bands of low molecular weights. These results suggest that degradation of troponin subunits occurs relatively early (4 h after coronary artery occlusion) and from the endocardial half of MI region. This degradation may be caused by one or several proteases that preferentially degrade the regulatory proteins among myofibrillar proteins.