Developmentally Delayed Sensitivity of Acetylcholine Receptor in Myotubes of Nerve-Muscle Cocul-Tures from Genetically Diabetic Mouse Embryos

The neuromuscular junctions of genetically diabetic KK-CA^y mice are reported to be hypersensitive to succinylcholine (SuCh) but not to d-tubocurarine (d-TC). Spinal cord-muscle cocultures from normal ddY and diabetic KK-CA^y mouse embryos were studied to examine the involvement of genetic factors in this hypersensitivity to SuCh. KK-CA^y myotubes were morphologically normal, as determined by light microscopy. KK-CA^y myotubes showed a progressive increase in the resting membrane potentials and acetylcholine (ACh) sensitivity with development, but this development was delayed when compared with ddY myotubes. The ACh receptor clusters, fluorescently labeled by fluorescein isothiocyanate conjugated α-bungarotoxin (FITC- αBuTX), were formed on the surface membrane of KK-CA^y myotubes. The developmental increase of the total amount of fluorescence within ACh receptor clusters on KK-CA^y myotubes was also slower than that of ddY myotubes. Depolarization by SuCh was sustained at a higher level in KK-CA^y myotubes. In regards to the inhibition of ACh potentials, KK-CA^y myotubes were not hypersensitive to both SuCh and d-TC when compared with ddY myotubes. These results suggest that the hypersensitivity to SuCh is not dependent on the genetic difference between ddY and KK-CA^y mice, and is probably due to the developmentally diabetic state of the neuromuscular junction.