Perchlorate enhances transmission in skeletal muscle excitation-contraction coupling.

Abstract

A B S T R A C T The effects of the anion perchlorate (present extracellularly at 8 raM)were studied on functional skeletal muscle fibers from Rana pipiens, voltage-clamped in a Vaseline gap chamber. Established methods were used to monitor intramembranous charge movement and flux of Ca release from the sarcoplasmic reticulum (SR) during pulse depolarization. Saponin permeabilization of the end portions of the fiber segment (Irving, M., J. Maylie, N. L. Sizto, and W. K. Chandler. 1987. Journal of General Physiology. 89:1--41) substantially reduced the amount of charge moving during conventional control pulses, thus minimizing a technical error that plagued our previous studies. Perchlorate prolonged the ON time course of charge movement, especially at low and intermediate voltages. The OFFs were also made slower, the time constant increasing twofold. The hump kinetic component was exaggerated by CIO 4 or was made to appear in fibers that did not have it in reference conditions. CIO 4 had essentially no kinetic ON effects at high voltages (>_ 10 mV). C10~ changed the voltage distribution of mobile charge. In single Boltzmann fits, the midpoint potential P was shifted -20 mV and the steepness parameter K was reduced by 4.7 mV (or 1.78-fold), but the maximum charge was unchanged (n = 9). Total Ca content in the SR, estimated using the method of Schneider et al. (Schneider, M. F., B. J. Simon, and G. Szucs. 1987. Journal of Physiology. 392:167-192) for correcting for depletion, stayed constant over tens of minutes in reference conditions but decayed in CIO 4 at an average rate of 0.3 ~mol/liter myoplasmic water per s. C10 4 changed the kinetics of release flux, reducing the fractional inactivation of release after the peak. CIO 4 shifted the voltage dependence of Ca release flux. In particular, the threshold voltage for Ca release was shifted by about -20 mV, and the activation of the steady component of release flux was shifted by > 20 mV in the negative direction. The shift of release activation was greater than th]~t of mobile charge. Thus the threshold charge, defined as the minimum charge moved for eliciting a detectable Ca transient, was reduced from 6 nC/I~F (0.55, n = 7) to 3.4 (0.53). The average of the paired differences was 2.8 (0.33, P < 0.01). The effects ofC10 4 were then studied in fibers in modified functional situations. Depletion of Ca in the SR, achieved by high frequency pulsing in the presence of intracellular BAPTA and EGTA, simplified but did not eliminate the effects of CIO 4. ON humps were not observed in the depleted fibers but the slowing effect of CIO~, both in ON and OFF, was still present. The shift in P was reduced to -15 mV, the change in steepness was reduced to ~ 15%, and Qmax was unchanged. The threshold charge was reduced by CIO 4 regardless of depletion. In fibers inactivated by prolonged depolarization CIO~ did not change the kinetics of charge movement (charge 2) but changed its voltage distribution, shifting Vby -14 mV (n = 6). The CI channel blockers A9C, SITS, and DIDS shif...

Sarcoplasmic reticulum Ca 2+ release flux underlying Ca 2+ sparks in cardiac muscle

“…3g was fitted with a different set of parameter values, including a fast buffer compartment of two times greater concentration, the peak value of release flux increased 50%, but the triplet structure was not affected. Therefore, and as was the case in skeletal muscle (19), the determination of kinetic and spatial properties of release was robust; it did not depend steeply on the parameters of removal, provided that the fit of the transients was good.…”

“…Among the parameters of removal, we chose to change those of the SR pump. Varying the diffusion coefficients of the Ca 2ϩ buffers and assuming, as in previous work (19), a transport rate proportional to the second power of the pump site occupancy, resulted in additional improvements to the fit.…”

A retrograde signal from RyR1 alters DHP receptor inactivation and limits window Ca 2+ release in muscle fibers of Y522S RyR1 knock-in mice

“…Assuming a constant residual release flux during the plateau component, the total flux trace can be corrected for the putative depletion (dotted trace in Fig. 3A) as shown by Gonzalez and Rios (13). The analysis leads to an estimate of the initial SR Ca 2ϩ content (before the pulse) using a least-squares optimization procedure.…”

Mechano‐sensitive linkage in excitation‐contraction coupling in frog skeletal muscle.

“…4). In addition, exposure to 1 mM perchlorate for 5 min, which is known to augment EC coupling (Fill & Best, 1990;Gonzalez & Rios, 1993), slightly potentiated tetanic tension from 2 to 4 % (see Fig. 4).…”