Contraction threshold and the "hump" component of charge movement in frog skeletal muscle.

Abstract

The delayed component of intramembranous charge movement (hump, Iv) was studied around the contraction threshold in cut skeletal muscle fibers of the frog (Rana esculenta) in a single Vaseline-gap voltage clamp. Charges (Q) were computed as 50-ms integrals of the ON (QoN) and OFF (QoFr) of the asymmetric currents after subtracting a baseline. The hump appeared in parallel with an excess of QoN over QoFF by ~2.5 nC/I~F. Caffeine (0.75 mM) not only shifted the contraction threshold but moved both the hump and the difference between the ON and OFF charges to more negative membrane potentials. When using 10-mV voltage steps on top of different prepulse levels, the delayed component, if present, was more readily observable. The voltage dependences of the ON and OFF charges measured with these pulses were clearly different: QoN had a maximum at or slightly above the contraction threshold, while QoFv increased monotonically in the voltage range examined. Caffeine (0.75 mM) shifted this voltage dependence of QoN toward more negative membrane potentials, while that of QoFF was hardly influenced. These results show that the delayed component of intramembranous charge movement either is much slower during the OFF than during the ON, or returns to the OFF position during the pulse. Tetracaine (25 ~M) had similar effects on the charge movement currents, shifting the voltage dependence of the ON charge in parallel with the contraction threshold, but to more positive membrane potentials, and leaving QorF essentially unchanged. The direct difference between the charge movement measured in the presence of caffeine and in control solution was either biphasic or resembled the component isolated by tetracaine, suggesting a common site of caffeine and tetracaine action. The results can be understood if the released Ca plays a direct role in the generation of the hump, as proposed in the first paper of this series (Csernoch et al. 1991.J. Gen. Physiol. 97:845-884)

Kinetic separation of charge movement components in intact frog skeletal muscle.

“…This also contrasted with the earlier reports (Szucs et al 1991). Running integrals of qr charge thus tended asymptotically towards steady values which increased with depolarization and returned to the baseline with the end of the step (Fig.…”

“…Sixth, the running integrals representing qy charge with time confirmed an 'on'-'off' charge equality under all these conditions whether test or prepulse voltage was varied. This contrasts with the relative deficit in 'off' qy charge reported in cut fibres (Szucs et al 1991). All these features are consistent with a qy transition mediated by intramembrane capacitative charge as opposed to a hypothesis in which qy results from Ca2+ release.…”

“…They isolated a delayed 'on' current followed by an inward charging phase which showed biphasic 'off' responses and resulted in an 'on'-'off' charge inequality. These traces were identified with a q, transient secondary to intracellular Ca2+ release Szucs et al 1991). However, they predicted a far smaller quantity of qy charge than reported in intact fibres (Huang, 1982;Hui, 1983;Hui & Chandler, 1991).…”

Kinetic isoforms of intramembrane charge in intact amphibian striated muscle.

“…Such schemes were prompted by the observations of late inward-going phases that followed "humps" isolated from ON charge movement waveforms, as these could reflect the subsequent diffusion of Ca 2+ away from that space. There were also the complexities observed in the biphasic OFF charge movements and their smaller integrals compared with the ON transients Pizarro et al, 1991;Szucs et al, 1991;see also Hui, 1994). However, the experimental conditions adopted here resulted in complete decays to steady DC levels without intervening inward-going phases or other time-dependent currents.…”

A slow component of intramembranous charge movement during sarcoplasmic reticulum calcium release in frog cut muscle fibers.

“…The first demonstration of such an effect was reported in a series of articles on voltage-clamped frog cut muscle fibers by Csernoch et al (1991), Garcfa et al (1991), Szfics et al (1991), and Pizarro et al (1991). These authors found that, during a step depolarization, I~m consisted of two components, I~ and I~, as first described by Peres (1977, 1979) in intact fibers and subsequently studied in several other laboratories.…”