Electrical models of excitation-contraction coupling and charge movement in skeletal muscle.

Abstract

A B ST R A C T The consequences of ionic current flow from the T system to the sarcoplasmic reticulum (SR) of skeletal muscle are examined. The Appendix analyzes a simple model in which the conductance gx, linking T system and SR, is in series with a parallel resistor and capacitor having fixed values. The conductance gx is supposed to increase rapidly with depolarization and to decrease slowly with repolarization. Nonlinear transient currents computed from this model have some of the properties of gating currents produced by intramembrane charge movement. In particular, the integral of the transient current upon depolarization approximates that upon repolarization. Thus, equality of nonlinear charge movement can occur without intramembrane charge movement. A more complicated model is used in the text to fit the structure of skeletal muscle and other properties of its charge movement. Rectification is introduced into gx and the membrane conductance of the terminal cisternae to give asymmetry in the time-course of the transient currents and saturation in the curve relating charge movement to depolarization, respectively. The more complex model fits experimental data quite well if the longitudinal tubules of the sarcoplasmic reticulum are isolated from the terminal cisternae by a substantial resistance and if calcium release from the terminal cisternae is, for the most part, electrically silent. Specific experimental tests of the model are proposed, and the implications for excitation-contraction coupling are discussed.

Paralysis of frog skeletal muscle fibres by the calcium antagonist D‐600.

“…The electrical model of T-s.r. coupling (Mathias, Rae & Eisenberg, 1979;Mathias, Levis & Eisenberg, 1980) includes a conductive connexion between T system and s.r., as well as a Ca2+ release site in the t.c. membrane, which might be subject to D-600 blockade.…”

Perchlorate‐induced alterations in electrical and mechanical parameters of frog skeletal muscle fibres.

“…If some idea about the specific action of C104-emerges it may become feasible to distinguish between the two competing models about the spread of activation from the T-system to the s.r., i.e. the charge movement concept (Schneider & Chandler, 1973), which we applied to interpret our results, or the hypothesis (Mathias, Levis & Eisenberg, 1980) that suggests the existence of activatable pores between the two membranal systems.…”

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study.

“…It is possible, ofcourse, for charge movement to release a diffusible transmitter and any of the above mechanisms may be further coupled to Ca release through depolarization of the SR (11,25,71,101). The first of these hypotheses envisions direct depolarization of the TC membrane by current flow from the T-tubules through the opening of nonspecific ionic channels that are closed at rest, but open during activation (22,64) . We found no significant change in either the Na or Cl content of the TIC during activation, but the expected changes are below the limits of detectable difference.…”

“…We found no significant change in either the Na or Cl content of the TIC during activation, but the expected changes are below the limits of detectable difference. To be consistent with the measured passive electrical properties of striated muscle (16,80), this hypothesis also requires that the TC be electrically insulated, perhaps at the intermediate cisternae (99,100) from the longitudinal reticulum (64). The absence of Ca accumulation in the longitudinal tubules of the SR during tetanus fails to support this notion of ionic isolation .…”