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

The release of Ca2+ in response to caffeine at threshold concentration (5 m M) was studied in mouse skeletal myotubes. Repeated 5-s applications of caf feine, each followed by a 30-s washout, caused Ca2+ releases of consecutive ly growing amplitude (staircase phenomenon). Each response declined rapidly and had a slow tail. Repeated applications of threshold caffeine lowered t he threshold concentration. The interval between threshold applications cou ld be increased to 30 min without loss of potentiation. When threshold caff eine was applied continuously for up to 10 min, the increase in Ca2+ concen tration as seen with staircase potentiation did not occur. Depolarization b y elevated [K+] or by voltage-clamp steps potentiated caffeine-induced Ca2 release rapidly as compared to the slow exponential growth of staircase-li ke potentiation. Gd3+ prevented the depolarization-induced potentiation, bu t not the staircase phenomenon. Staircase-like potentiation of Ca2+ release was evident even when the voltage sensors were clamped in their resting st ate; in contrast, potentiated Ca2+ release and its rapid termination appare ntly require conversion of the voltage sensors to an activated state. Stair case potentiation was blocked when Ca2+ was omitted from the bath, thus pin pointing the source of Ca2+. We suggest that staircase-like potentiation is conditioned by a caffeine-dependent Ca2+ influx across the plasma membrane .