Ischemic loss of sarcolemmal dystrophin and spectrin: correlation with myocardial injury.

Sarcolemmal blebbing and rupture are prominent features of irreversible ischemic myocardial injury. Dystrophin and spectrin are sarcolemmal structural proteins. Dystrophin links the transmembrane dystroglycan complex and extracellular laminin receptors to intracellular F-actin. Spectrin forms the backbone of the membrane skeleton conferring an elastic modulus to the sarcolemmal membrane. An ischemic loss of membrane dystrophin and spectrin, in ischemically pelleted rabbit cardiomyocytes or in vivo 30--45 min permanently ischemic, LAD-ligated hearts, was detected by immunofluorescence with monoclonal antibodies. Western blots of light and heavy microsomal vesicles and Triton-extracted membrane fractions from ischemic myocytes demonstrated a rapid loss of dystrophin coincident with sub-sarcolemmal bleb formation, subsequent to a hypotonic challenge. The loss of spectrin from purified sarcolemma of autolysed rabbit heart, and both isolated membrane vesicles and Triton solubilized membrane fractions of ischemic cardiomyocytes correlated linearly with the onset of osmotic fragility as assessed by membrane rupture, subsequent to a hypotonic challenge. In contrast to the ischemic loss of dystrophin and spectrin from the membrane, the dystrophin-associated proteins, alpha-sarcoglycan and beta-dystroglycan and the integral membrane protein, sodium-calcium exchanger, were maintained in the membrane fraction of ischemic cells as compared to oxygenated cells. Preconditioning protected cells, but did not significantly alter ischemic dystrophin or spectrin translocation. This previously unrecognized loss of sarcolemmal dystrophin and spectrin may be the molecular basis for sub-sarcolemmal bleb formation and membrane fragility during the transition from reversible to irreversible ischemic myocardial injury.