Volume regulation and plasma membrane injury in aerobic, anaerobic, and ischemic myocardium in vitro. Effects of osmotic cell swelling on plasma membrane integrity.
The relationship between cell swelling and plasma membrane disruption has been evaluated in thin myocardial slices incubated in oxygenated or anoxic Krebs-Ringer phosphate media. Electron microscopy and measurements of inulin-diffusible space were used to monitor plasma membrane integrity. Inulin is excluded from the intracellular space of intact cells; therefore, an increase in tissue inulin content is an excellent marker of loss of plasma membrane integrity. Cell volume was increased during exposure of aerobic slices to hypotonic media, but the inulin-diffusible space was not increased and electron micrographs showed no detectable plasma membrane alterations. Likewise, during prolonged anoxic isotonic incubation, no evidence of plasma membrane damage was observed. Incubation in anoxic hypotonic media for 60 minutes resulted in a larger increase in cell volume than under aerobic conditions, but plasma membrane integrity was maintained. Extended anoxic hypotonic incubation (300 minutes) produced no further change in tissue water, but the inulin-diffusible space was increased and electron micrographs revealed breaks in the plasma membranes primarily in association with large subsarcolemmal blebs. Likewise, myocardial slices incubated in isotonic anoxic media for 240 minutes and hypotonic anoxic media for 60 minutes had an increased inulin-diffusible space and the ultrastructural appearance was similar. This ultrastructural appearance is indistinguishable from that observed in myocytes lethally injured by ischemia. Measurements of tissue osmolarity during total ischemia showed that osmotically induced cell swelling could occur in ischemic myocardium prior to the onset of plasma membrane disruption. Our results indicate that cell swelling per se is incapable of rupturing plasma membranes; however, after prolonged periods of energy deficiency, the plasma membrane or its cytoskeletal scaffold become injured, which allows the membrane to rupture if the cell is swollen, as might occur during ischemia or reperfusion.