Increase in electrically-stimulated Ca2+ release and suppression of caffeine response in diaphragm muscle of alloxan-diabetic mice compared with the denervation effect.

Changes in intracellular Ca2+ release in the diaphragm muscle of alloxan-diabetic mice were compared with changes in normal muscles and non-diabetic denervated muscles. We measured Ca2+ transient aequorin luminescence by direct electrical stimulation of these muscles. External Ca2(+)-free solution readily decreased the Ca2+ transient in normal muscles but had less of an effect in diabetic muscles. Only when the muscles were pre-injected with EGTA (reducing intracellular levels of free Ca2+) did the Ca2+ transients decrease significantly in diabetic muscles, however, there was no effect in denervated muscles. The caffeine-induced increase in Ca2+ transients, however, was delayed in both diabetic muscles and non-diabetic denervated muscles. The caffeine response was observed in normal muscles under the external Ca2(+)-free conditions even after EGTA-pretreatment, whereas it was suppressed, after a brief increase, in both diabetic and non-diabetic denervated muscles. These results demonstrate (1) the insensitivity of intracellular Ca2+ mobilization to external Ca2+ levels and the ready accumulation of intracellular Ca2+ in the cytosol in the diabetic state, (2) increased permeability to Ca2+ in the denervated state and (3) impairment of the Ca2+ pool which responds to caffeine in both diabetes and the non-diabetic denervated state. Diabetic neuromyopathy thus appears to be a state of abnormal Ca2(+)-mobilization caused secondarily by high levels of blood glucose.