Ca2+ uptake and cellular integrity in rat EDL muscle exposed to electrostimulation, electroporation, or A23187

We tested the hypothesis that increased Ca2+ uptake in rat extensor digitorum longus (EDL) muscle elicits cell membrane damage as assessed from release of the intracellular enzyme lactate dehydrogenase (LDH). This was done by using 1) electrostimulation, 2) electroporation, and 3) the Ca2+ ionophore A23187. Stimulation at 1 Hz for 120-240 min caused an increase in 45Ca uptake that was closely correlated to LDH release. This LDH release increased markedly with temperature. After 120 min of stimulation at 1 Hz, resting 45Ca uptake was increased 5.6-fold compared with unstimulated muscles. This was associated with an eightfold increase in LDH release, and this effect was halved by lowering extracellular Ca2+ concentration ([Ca2+]o). The poststimulatory increase in resting 45Ca uptake persisted for at least 120 min. An acute increase in sarcolemma leakiness induced by electroporation markedly increased 45Ca uptake and LDH leakage. Both effects depended on [Ca2+]o. A23187 increased 45Ca uptake. Concomitantly, LDH leakage increased 18-fold within 30 min, and this effect was abolished by omitting Ca2+ from the buffer. We conclude that increased Ca2+ influx may be an important cause of cell membrane damage that arises during and after exercise or electrical shocks. Because membrane damage allows further influx of Ca2+, this results in positive feedback that may further increase membrane degeneration.