2002. Sustained release of calcium elicited by membrane depolarization in ryanodine-injected mouse skeletal muscle fibers

ABSTRACT The effect of micromolar intracellular levels of ryanodine was tested on the myoplasmic free calcium concen-tration ([Ca2]i) measured from a portion of isolated mouse skeletal muscle fibers voltage-clamped at 80 mV. When ryanodine-injected fibers were transiently depolarized to 0 mV, the early decay phase of [Ca2]i upon membrane repolar-ization was followed by a steady elevated [Ca2]i level. This effect could be qualitatively well simulated, assuming that ryanodine binds to release channels that open during depolarization and that ryanodine-bound channels do not close upon repolarization. The amplitude of the postpulse [Ca2]i elevation depended on the duration of the depolarization, being hardly detectable for pulses shorter than 100 ms, and very prominent for duration pulses of seconds. Within a series of consecutive pulses of the same duration, the effect of ryanodine produced a staircase increase in resting [Ca2]i, the slope of which was approximately twice larger for depolarizations to 0 or 10 mV than to 30 or 20 mV. Overall results are consistent with the “open-locked ” state because of ryanodine binding to calcium release channels that open during depolarization. Within the voltage-sensitive range of calcium release, increasing either the amplitude or the duration of the depolarization seems to enhance the fraction of release channels accessible to ryanodine