EXISTENCE OF Q-GAMMA IN FROG CUT TWITCH FIBERS WITH LITTLE Q-BETA

Charge movements were measured in frog cut twitch fibers with the double Vaseline-gap voltage-clamp technique. In most fibers, when a depolarizing pulse to -60 to -40 mV was applied at 13-14-degrees-C, the ON segment of a charge movement trace showed an early I-beta component and a late I-gamma hump component. An ongoing controversy is whether the I-gamma hump component triggers calcium release from the sarcoplasmic reticulum or arises as a consequence of the release. Interestingly, a number of cut fibers showed normal I-gamma components but greatly diminished, or unresolvable, I-beta components. When the amount of charge associated with the current transient was plotted against the membrane potential, the steeply voltage-dependent Q-gamma component appeared normal whereas the less steeply voltage-dependent Q-beta component was also greatly diminshed or unresolvable. These results suggest that I-gamma can flow in the absence of I-beta, thereby ruling out the possibility that Q-beta triggers calcium release which, in turn, causes Q-gamma to move. The results, however, do not rule out the positive feedback of calcium release to activate Q-gamma, if calcium release is not triggered by Q-beta but by Q-gamma itself or by some other signal.