Effects of membrane polarization on sarcoplasmic calcium release in skeletal muscle

Calcium release from the sarcoplasmic reticulum was investigated in voltage-clamped, tetrodotoxin-treated frog skeletal muscle fibres injected with arsenazo III. Short (5 ms) depolarizing pulses (test pulses) produced a transient change in arsenazo III absorption, signalling an increase in intracellular calcium ion concentration (calcium transient). Conditioning subthreshold depolarizations, which preceded the test pulse, potentiated the calcium transient triggered by the test pulse. Conditioning hyper-polarizations, applied either before or after the test pulse, inhibited the calcium transient. These effects of conditioning polarizations on the calcium transient may explain similar effects of subthreshold polarizations on muscle contraction that have previously been reported. The potentiating effect of subthreshold depolarizations was observed only when the test pulse was short (5 ms). The potentiating effect develops at -48 mV with a time constant of about 7 ms at 6.5°C; this seems to be slower than that predicted by the potential spread from the surface along the tubular system. Thus, part of the effect could arise from the coupling process between tubular depolarization and calcium release.