An ischemic β-dystroglycan (βDG) degradation product:: Correlation with irreversible injury in adult rabbit cardiomyocytes

A loss of sarcolemmal dystrophin was observed by immuno-fluorescence studies in rabbit hearts subjected to in situ myocardial ischemia and by immuno-blotting of the Triton soluble membrane fraction of isolated rabbit cardiomyocytes subjected to in vitro ischemia. This ischemic loss of dystrophin was a specific event in that no ischemic loss of sarcolemmal alpha-sarcoglycan, gamma-sarcoglycan, alphaDG, or betaDG was observed. The maintenance of sarcolemmal betaDG (43 Kd) during ischemia was interesting in that dystrophin binds to the C-terminus of betaDG. However, during late in vitro ischemia, a 30 Kd band was observed that was immuno-reactive for betaDG. Additionally, this 30 Kd-betaDG band was observed in rabbit myocardium subjected to autolysis. Finally, the 30 Kd-betaDG was observed in the purified sarcolemmal fraction of rabbit cardiomyocytes subjected to a prolonged period of in vitro ischemia, confirming the sarcolemmal localization of this band. The potential patho-physiologic significance of this band was indicated by the appearance of this band at 120-180 min of in vitro ischemia, directly correlating with the onset of irreversible injury, as manifested by osmotic fragility. Additionally the appearance of this band was significantly reduced by the endogenous cardioprotective mechanism, in vitro ischemic preconditioning, which delays the onset of osmotic fragility. In addition to dystrophin, betaDG binds caveolin-3 and Grb-2 at its C-terminus. The presence of Grb-2 and caveolin-3 in the membrane fractions of oxygenated and ischemic cardiomyocytes was determined by Western blotting. An increase in the level of membrane Grb-2 and caveolin-3 was observed following ischemic preconditioning as compared to control cells. The formation of this 30 Kd-betaDG degradation product is potentially related to the transition from the reversible to the irreversible phase of myocardial ischemic cell injury and a decrease in 30 Kd-betaDG might mediate the cardioprotection provided by ischemic preconditioning.