Calcium current-dependent staircase in rat myotubes and myoballs developing in culture

Calcium current and contraction were simultaneously recorded in whole-cell patch-clamped rat skeletal muscle cells grown in primary culture. Repetitive depolarizations at low frequency, which elicited calcium currents, led to a staircase response, characterized by the progressive increase of both twitch amplitude and activation rate. It was sometimes possible to elicit a staircase response in 2 or 3 day old postfusion myotubes which did not or weakly contract initially. The staircase response was dependent on calcium entry through calcium channels, since it was reversed when calcium current was depressed by means of inorganic calcium blockers or depolarization to large positive potential. The entry of calcium was also necessary to allow the development of a staircase response following caffeine-induced contractures which partly emptied the intracellular stores of calcium. These features are consistent with the idea that calcium currents allow the initial loading of intracellular calcium stores and, in later stages, serve to replenish and maintain them constant.