Evidence for the non-existence of a negative phase in the hump charge movement component (I gamma) in Rana temporaria.

1. Charge movement was studied in cut twitch fibres of Rana temporaria with a double Vaseline-gap voltage-clamp technique. 2. When charge movement was measured from stretched fibres (3.5 microns sarcomere length) bathed in a TEA-Cl Ringer solution, the ON transients in the appropriate potential range showed an early I beta component followed by an I gamma hump. The late I gamma hump generally decayed monotonically towards the maintained current level. 3. On some rare occasions, the ON transient showed an undershoot immediately following the I gamma hump before reaching the steady-state level. This dip in current, when it occurred, could only be observed in a very narrow potential range and might not persist until the end of the experiment. 4. A replacement of the Cl- in the external solution by CH3SO3- reduced the magnitude of the dip, suggesting that the dip is ionic in origin. 5. When charge movement was measured in slack fibres (2.2 microns sarcomere length, the probability of observing the dip in current was increased. 6. In some experiments in which the dip in current was very stable, the signal was studied by a sequence of TEST pulses to the same potential but with different durations. It was found that, if the dip in current was included as a negative phase of the I gamma hump, then the amount of ON charge was smaller than that of OFF charge. Also, as the pulse duration was increased progressively so that a longer portion of the dip was recorded, the OFF charge remained constant instead of being decreased.(ABSTRACT TRUNCATED AT 250 WORDS)