Interacting effects of Ca2+ and hypoxia in the induction of sarcolemmal damage in mouse diaphragm in vitro

Experiments have been undertaken to investigate the basis for the selective damage of centrally placed fibres in mouse diaphragms exposed to Ca2+ loading in vitro. Incubation under hypoxic conditions (non-aerated saline) for 30 min had no discernible effect on the muscle. Incubation for 120 min led to permeabilisation of the sarcolemma (assayed by penetration of Procion Yellow) in 54% of cells. Sarcolemmal permeabilisation was almost completely restricted to centrally placed cells, as has previously been described for the effects of the Ca2+ channel agonist Bay K 8644. Ultrastructural damage to the myofibrils and mitochondrial swelling were also widespread amongst centrally placed cells. Permeabilisation was inhibited when hypoxic incubations were carried out in Ca(2+)-free saline. Incubation in hypoxic Ca(2+)-containing saline for 30 min followed by further incubation in Ca(2+)-free, hypoxic saline, up to a total of 120 min, resulted in permeabilisation similar to that seen in muscles incubated for 120 min in Ca(2+)-containing, hypoxic saline. It is suggested that selective damage to central fibres induced by Ca2+ loading resulting from treatment with Bay K 8644 is related to increased oxygen demand.