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 Ca2+-free saline. Incubation in hypoxic Ca2+-containing saline for 30 min followed by further incubation in Ca2+-free, hypoxic saline, up to a total of 120 min, resulted in permeabilisation similar to that seen in muscles incubated for 120 min in Ca2+-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.