Depolarization and contraction of skeletal muscle induced by intracellular stimulation -role of T-tubules in electro-chemical coupling.

Response of skeletal muscle fibers upon intracellular stimulation was investigated to elucidate the role of the T-system in the electro-chemical coupling process. Upon stimulation, the fiber generates twitch and tonic tension which is independent of the excitation of the membrane (not inhibited by TTX). The fiber placed in low Ca2+ medium (10(-7) M, controlled by EGTA-Ca) for more than ten minutes, exhibited explosive contraction; this phenonomenon was not observed in Na-free choline Ringer or glycerol-treated (T-tubule disrupted) fibers. Injection of a small amount of Mn2+ into the muscle cells produced strong, long lasting contraction without excitation of the membrane. Glycerol-treated fibers failed to respond to the stimulus using a Mn2+ electrode. These facts suggest that Ca2+ release or entry which probably caused by depolarization of T-tubule membrane in the presence of Na+, may be essential for the initiation of Ca2+ release from sarcoplasmic reticulum.