Factors affecting the appearance of the hump charge movement component in frog cut twitch fibers.

Abstract

Charge movement was measured in frog cut twitch fibers with the double Vaseline gap technique. Five manipulations listed below were applied to investigate their effects on the hump component (Iv) in the ON segments of TEST minus CONTROL current traces. When external CI-was replaced by MeSO~ to eliminate CI current, I v peaked earlier due to a few millivolts shift of the voltage dependence of I, kinetics in the negative direction. The Q-V plots in the TEA.CI and TEA.MeSO~ solutions were well fitted by a sum of two Boltzmann distribution functions. The more steeply voltage-dependent component (Q,) had a F ~ 6 mV more negative in the TEA.MeSO s solution than in the TEA.CI solution. These voltage shifts were partially reversible. When creatine phosphate in the end pool solution was removed, the I v hump disappeared slowly over the course of 20-30 min, partly due to a suppression of Qv' The hump reappeared when creatine phosphate was restored. When 0.2-1.0 mM Cd 2+ was added to the center pool solution to block inward Ca current, the I v hump became less prominent due to a prolongation in the time course of I v but not to a suppression of Qv" When the holding potential was changed from -90 to -120 mV, the amplitude of Ia was increased, thereby obscuring the I v hump. Finally, when a cut fiber was stimulated repetitively, I, lost its hump appearance because its time course was prolonged. In an extreme case, a 5-rain resting interval was insufficient for a complete recovery of the waveform. In general, a stimulation rate of once per minute had a negligible effect on the shape of I v. Of the five manipulations, MeSO~ has the least perturbation on the appearance of I v and is potentially a better substitute for C1-than SO~-in eliminating CI current if the appearance of the I v hump is to be preserved.

Comparison of charge movement components in intact and cut twitch fibers of the frog. Effects of stretch and temperature.

“…The effect of temperature studied in this paper is another possibility. Other possible factors affecting the appearance of I~ will be described in the following paper (Hui, 1991). This paper shows that, under the conditions of the present experiments, I~ in cut fibers appeared as a prominent hump at temperatures above 10°C.…”

“…On the other hand, the ability of a fiber to release Ca in the absence of I v hump (Melzer et al, 1986) could rule out the association of Qv with Ca release, whether it is the cause or consequence. Results in this and the following paper (Hui, 1991) broaden our perception of I v. I v is generally manifested as a hump in the ON current in intact fibers, but under a variety of conditions (e.g., low temperature) I v might have a very broad waveform in cut fibers such that it cannot be visualized as a hump but still contributes to the current transient. Whether it is manifested as a hump or not, its charge constitutes the steeply voltage-dependent component in the Q-V plot.…”

“…This reveals another intrinsic difference between charge movement in intact and cut fibers. Other factors that might affect the appearance of I v will be studied in the following paper (Hui, 1991).…”

Intramembrane charge movements in frog skeletal muscle in strongly hypertonic solutions.

“…Delayed charging phases lost their distinct "hump" appearance with temperature reductions below 6°C in cut but not intact fibers. Yet steady-state charge distributions still fulfilled expectations for two distinguishable charge species (Hui, 1991). Additionally, cut fiber preparations where current flow beneath the Vaseline seals was monitored and corrected for yielded kinetic and steady-state results that strongly suggested distinct charge species (Hui and Chandler, 1990).…”

Charge conservation in intact frog skeletal muscle fibres in gluconate‐containing solutions.

“…Only then could their dielectric ;ributions be determined unambiguously for integration Aive the steady-state capacitative charge (Adrian & ers, 1974). This separation was precluded in earlier C -a studies by the slow outward currents, attributed to the delayed rectifier, that followed the q7 decays (Adrian & Peres, 1979 system resulting from the interposed experimental manoeuvres (see Hui, 1991;Hui & Chen, 1992a, b;. At all events, the ionic current templates in such difference traces would be inherently uncertain.…”