Imaging elementary events of calcium release in skeletal muscle cells.

In skeletal muscle cells, calcium release to trigger contraction occurs at triads, specialized junctions where sarcoplasmic reticulum channels are opened by voltage sensors in the transverse tubule. Scanning confocal microscopy was used in cells under voltage clamp to measure the concentration of intracellular calcium, [Ca2+]i, at individual triads and [Ca2+]i gradients that were proportional to calcium release. In cells stimulated with small depolarizations, the [Ca2+]i gradients broke down into elementary events, corresponding to single-channel currents of about 0.1 picoampere. Because these events were one-tenth to one-fifth the size of calcium sparks (elementary release events of cardiac muscle), skeletal muscle control mechanisms appear to be fundamentally different.