Triadic Ca2+ modulates charge movement in skeletal muscle

The effects of intracellular Ca2+ changes on charge movement in frog skeletal muscle were investigated using high concentrations (10-20 mmol/l) of buffers with different abilities to buffer Ca2+ at distances close to the SR Ca2+ release channels. In BAPTA compared with EGTA perfused fibers, charge movement was attenuated and lacked the characteristic kinetic features (I beta and I gamma) of E-C coupling charge movements. Qmax decreased by 9 nC/microF, Vmid was shifted 1-6 mV to more negative potentials, and the steepness factor increased by 3-5 mV. Results of varying the holding potential suggested that BAPTA decreases the amount of charge available to move upon depolarization. Raising intracellular Ca2+ to micromolar levels at a fixed BAPTA concentration prevented the decline in Qmax, suggesting that intracellular Ca2+ can modulate the amount of charge that is in the resting or available state. The different results obtained with BAPTA and EGTA can be explained by the greater ability of BAPTA to buffer dynamic Ca2+ changes at distances close to the release sites. These results are consistent with the proposals that an intracellular Ca2+ site on or near the dihydropyridine receptor, termed here the 'availability site', modulates the amount of charge available to move upon depolarization and is normally populated by Ca2+ released into the triad junction during activity.