Staircase-like potentiation of calcium release in mouse myotubes during repetitive short-term application of threshold caffeine

Abstract.

The release of Ca²⁺ in response to caffeine at threshold concentration (5 mM) was studied in mouse skeletal myotubes. Repeated 5-s applications of caffeine, each followed by a 30-s washout, caused Ca²⁺ releases of consecutively growing amplitude (staircase phenomenon). Each response declined rapidly and had a slow tail. Repeated applications of threshold caffeine lowered the threshold concentration. The interval between threshold applications could be increased to 30 min without loss of potentiation. When threshold caffeine was applied continuously for up to 10 min, the increase in Ca²⁺ concentration as seen with staircase potentiation did not occur. Depolarization by elevated [K⁺] or by voltage-clamp steps potentiated caffeine-induced Ca²⁺ release rapidly as compared to the slow exponential growth of staircase-like potentiation. Gd³⁺ prevented the depolarization-induced potentiation, but not the staircase phenomenon. Staircase-like potentiation of Ca²⁺ release was evident even when the voltage sensors were clamped in their resting state; in contrast, potentiated Ca²⁺ release and its rapid termination apparently require conversion of the voltage sensors to an activated state. Staircase potentiation was blocked when Ca²⁺ was omitted from the bath, thus pinpointing the source of Ca²⁺. We suggest that staircase-like potentiation is conditioned by a caffeine-dependent Ca²⁺ influx across the plasma membrane.