Use of ion channel blockers in the exploration of possible mechanisms involved in the myopathy of diabetic mice

Summary

Changes in the muscle contractions of the phrenic nerve-diaphragm preparation from the diabetic mouse were investigated by means of K⁺- and Cl⁻-channel blockers and the Ca²⁺-mobilizing agent, selenite. The K⁺-channel blockers (UO₂ ²⁺ and 4-aminopyridine) cooperated synergistically with the Cl⁻-channel blockers (Cd²⁺ and 9-anthracenecarboxylic acid) in increasing normal muscle contraction as described previously, but failed to induce this effect in the diaphragm of the diabetic mouse. Treatment with a Cl⁻-channel blocker alone in 0.25 mmol/1 Ca²⁺ Krebs solution induced a myotonic activity accompanied by stimulus-bound repetitive action potential firings. This effect was also diminished in the diaphragm from diabetic mice. The membrane potential of the muscle cells in the diaphragm of the diabetic mouse was slightly but significantly decreased. The membrane input resistance was also increased and was refractory to being further increased by either a Cl⁻-channel blocker or a low Cl⁻-medium. Furthermore, the membrane chloride conductance was found to be decreased, but the membrane K+ conductance remained unchanged in the muscle from diabetic mice. These changes of membrane properties in the muscles from diabetic mice were shown to be similar to those induced by either Cl⁻-channel blockers or a low Cl⁻-medium. In addition, the combined treatment of the diaphragm from diabetic mice with Cd²⁺ Plus UO₂ ²⁺ in 0.25 mmol/l Ca²⁺ Krebs solution and then stepwise replenishment of Ca²⁺ led to a greater restoration of muscle contractions at a lower cumulative Ca²⁺ concentration than that was found with the normal diaphragm. The sustained muscle contracture of the mouse diaphragm induced by U0₂ ²⁺ plus selenite was partially inhibited in the diaphragm from diabetic mice, indicating that the Ca²⁺ mobilizing mechanism of the diaphragm of the diabetic mouse was also altered. All of these observations obtained with the diaphragm of the diabetic mouse can be attributed to the diabetic state, because most of them could be normalized by insulin administration in vivo. Therefore, it is concluded that diabetes-induced changes of sarcolemmal ion channels and ion transporters may cause inhibition of muscle contraction and eventually lead to diabetic myopathy.