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 diminished 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.
Chen
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W
Department of Physiology and Biophysics, Indiana University Medical Center, Indianapolis 46223.
Hui
C S
CS
D000818:Animals; D004553:Electric Conductivity; D004594:Electrophysiology / Q000379:methods; D008564:Membrane Potentials; D009119:Muscle Contraction; D009132:Muscles / Q000502:physiology*; D011896:Rana temporaria