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

Summary 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.