SUPPRESSION OF CALCIUM-RELEASE BY CALCIUM OR PROCAINE IN VOLTAGE-CLAMPED RAT SKELETAL-MUSCLE FIBERS

1. Calcium transients were measured in fast-twitch rat skeletal muscle fibres stretched to 3.7-4.0 mu m per sarcomere, and voltage clamped at a holding potential of -80 mV using the double-seal Vaseline gap technique. Resting calcium was monitored with fura-2 and the calcium transients were measured with antipyrylazo III. The rate of release of calcium from the sarcoplasmic reticulum was calculated from the calcium transient records. The temperature was 14-17 degrees C. 2. The steady-state calcium dependence of inactivation of release was studied with a two-pulse protocol in which 200 ms prepulses of different amplitudes elevated the internal calcium concentration to various levels. The inactivation of release was then measured in the test pulse that followed the prepulses. The calcium concentration at which the inactivation of release was half-maximal was similar to 0.22 mu M, the average number of pound calcium ions needed to cause inactivation was about three per release channel and the amount of release that could be inactivated was, on average, 2.48 times the steady level of release during the test pulses. 3. Procaine (0.3 mM) reversibly decreased the amplitude and the rate of rise of the calcium transient. Both the peak and the steady level of release were decreased by about 50 %. The shape of the release waveform was not modified.