Abstract
Calcineurin (CaN) has been reported as a critical mediator for cardiac hypertrophy and cardiac myocyte apoptosis. Ischemia is associated with multiple alterations in the extracellular and intracellular signaling of cardiomyocytes and may act as an inducer of apoptosis. Rat ischemic heart showed significant increase in CaN activity. In ischemic-reperfused hearts, the expression of CaN A was significantly low and immunoreactivity was observed in proteolytic bands of 46 kDa. Immunohistochemical studies showed strong staining of immunoreactivity in rat hearts that had undergone 30 minutes of ischemia followed by 30 minutes of reperfusion, similar to that found in human ischemic heart. To elucidate the mechanism of proteolysis of CaN A in ischemic-reperfused rat heart, in vitro proteolysis of bovine cardiac CaN by m -calpain was carried out. In the presence of Ca2+ , the 60−kDa subunit (CaN A) was degraded to a 46-kDa immunoreactive fragment, whereas in the presence of Ca2+ /CaM, immunoreactive fragments of 48 and 54 kDa were observed. Calpains are Ca2+ -dependent cysteine proteases that regulate various enzymes, transcription factors, and structural proteins through limited proteolysis. The increase in CaN activity and strong immunostaining observed in ischemic-reperfused rat heart may be due to the calpain-mediated proteolysis of this CaM-dependent phosphatase. These studies remain an important area of in-depth investigation.
