Charge Movements Measured During Transverse-tubular Uncoupling in Frog Skeletal Muscle

Capacity transients and slow asymmetric charge-movements are measured in frog skeletal muscle using the Vaseline-gap voltage-clamp technique. Capacity transients show a rapid phase lasting 10-30 microseconds, due to the charging of the surface membrane capacitance, and a slower phase lasting several milliseconds, consistent with the charging of the transverse tubular system (T-system). Exposure to isotonic CsF caused the ratio of the slowly-charging capacitance (Cslow) to the fast-charging capacitance to decline by 88 +/- 9% (n = 16). Electron micrographs of four fibers treated with CsF show disruption and disorganization of the T-system and sarcoplasmic reticulum membranes and a greater than 90% decrease in the number of dyads and triads. The role of CsF was investigated: Fibers exposed to CsF internally or externally, exhibit slower and less complete loss of Cslow than fibers exposed both internally and externally. Little loss of Cslow occurs during the external exposure to CsF. The bulk of loss occurs only after the fiber is returned to Ca++-containing solution. Elevated external Ca++ causes more rapid and more complete loss of Cslow. The time-course of Cslow loss is gradual, occurring over a period of 10 min to 2 h. The progressive loss of Cslow is accompanied by a progressive decline in the peak of the slow asymmetric charge-movement and a progressive slowing of charge movement kinetics. These effects are qualitatively accounted for by including gradual tubular uncoupling in a distributed model of charge movement proposed by B. Simon and K. G. Beam (1985, J. Gen. Physiol., 85:21-42).