USE OF ION-CHANNEL BLOCKERS IN THE EXPLORATION OF POSSIBLE MECHANISMSINVOLVED IN THE MYOPATHY OF DIABETIC MICE

Changes in the muscle contractions of the phrenic nerve-diaphragm prep aration from the diabetic mouse were investigated by means of K+- and Cl--channel blockers and the Ca2+-mobilizing agent, selenite. The K+-c hannel blockers (UO22+ and 4-aminopyridine) cooperated synergistically with the Cl--channel blockers (Cd2+ and 9-anthracenecarboxylic acid) in increasing normal muscle contraction as described previously, but f ailed to induce this effect in the diaphragm of the diabetic mouse. Tr eatment with a Cl--channel blocker alone in 0.25 mmol/l Ca2+ Krebs sol ution induced a myotonic activity accompanied by stimulus-bound repeti tive action potential firings. This effect was also diminished in the diaphragm from diabetic mice. The membrane potential of the muscle cel ls in the diaphragm of the diabetic mouse was slightly but significant ly 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 w as found to be decreased, but the membrane K+ conductance remained unc hanged in the muscle from diabetic mice. These changes of membrane pro perties 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 a ddition, the combined treatment of the diaphragm from diabetic mice wi th Cd2+ plus UO22+ in 0.25 mmol/l Ca2+ Krebs solution and then stepwis e replenishment of Ca2+ led to a greater restoration of muscle contrac tions at a lower cumulative Ca2+ concentration than that was found wit h the normal diaphragm. The sustained muscle contracture of the mouse diaphragm induced by UO22+ plus selenite was partially inhibited in th e diaphragm from diabetic mice, indicating that the Ca2+ mobilizing me chanism of the diaphragm of the diabetic mouse was also altered. All o f these observations obtained with the diaphragm of the diabetic mouse can be attributed to the diabetic state, because most of them could b e normalized by insulin administration in vivo. Therefore, it is concl uded that diabetes-induced changes of sarcolemmal ion channels and ion transporters may cause inhibition of muscle contraction and eventuall y lead to diabetic myopathy.