Penetration of horseradish peroxidase into the terminal cisternae of frog skeletal muscle fibers and blockade of caffeine contracture by Ca++ depletion

Components of nonlinear capacitance, or charge movement, were localized in the membranes of frog skeletal muscle fibers by studying the effect of 'detubulation' resulting from sudden withdrawal of glycerol from a glycerol-hypertonic solution in which the muscles had been immersed. Linear capacitance was evaluated from the integral of the transient current elicited by imposed voltage clamp steps near the holding potential using bathing solutions that minimized tubular voltage attenuation. The dependence of linear membrane capacitance on fiber diameter in intact fibers was consistent with surface and tubular capacitances and a term attributable to the capacitance of the fiber end. A reduction in this dependence in detubulated fibers suggested that sudden glycerol withdrawal isolated between 75 and 100% of the transverse tubules from the fiber surface. Glycerol withdrawal in two stages did not cause appreciable detubulation. Such glycerol-treated but not detubulated fibers were used as controls. Detubulation reduced delayed (q gamma) charging currents to an extent not explicable simply in terms of tubular conduction delays. Nonlinear membrane capacitance measured at different voltages was expressed normalized to accessible linear fiber membrane capacitance. In control fibers it was strongly voltage dependent. Both the magnitude and steepness of the function were markedly reduced by adding tetracaine, which removed a component in agreement with earlier reports for q gamma charge. In contrast, detubulated fibers had nonlinear capacitances resembling those of q beta charge, and were not affected by adding tetracaine. These findings are discussed in terms of a preferential localization of tetracaine-sensitive (q gamma) charge in transverse tubule membrane, in contrast to a more even distribution of the tetracaine-resistant (q beta) charge in both transverse tubule and surface membranes. These results suggest that q beta and q gamma are due to different molecules and that the movement of q gamma in the transverse tubule membrane is the voltage-sensing step in excitation-contraction coupling.

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Chapter 2 Hyperpolarization of Frog Skeletal Muscle Fibers and of Canine Cardiac Purkinje Fibers during Enhanced Na+-K+ Exchange: Extracellular K+ Depletion or Increased Pump Current?

Current Topics in Membranes and Transport - Electrogenic Ion Pumps ◽

10.1016/s0070-2161(08)60692-9 ◽

1982 ◽

pp. 17-34 ◽

Cited By ~ 7

Author(s):

David C. Gadsby

Keyword(s):

Skeletal Muscle ◽

Muscle Fibers ◽

Pump Current ◽

Frog Skeletal Muscle ◽

Purkinje Fibers ◽

Skeletal Muscle Fibers ◽

Cardiac Purkinje Fibers ◽

K Depletion

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Determinants of relaxation rate in skinned frog skeletal muscle fibers

AJP Cell Physiology ◽

10.1152/ajpcell.1998.274.6.c1608 ◽

1998 ◽

Vol 274 (6) ◽

pp. C1608-C1615 ◽

Cited By ~ 17

Author(s):

Philip A. Wahr ◽

J. David Johnson ◽

Jack. A. Rall

Keyword(s):

Skeletal Muscle ◽

Muscle Fibers ◽

Slow Phase ◽

Frog Skeletal Muscle ◽

Force Of Contraction ◽

Thin Filaments ◽

Fast Phase ◽

Skeletal Muscle Fibers ◽

Kinetics Of ◽

Overall Kinetics

The influences of sarcomere uniformity and Ca2+ concentration on the kinetics of relaxation were examined in skinned frog skeletal muscle fibers induced to relax by rapid sequestration of Ca2+ by the photolysis of the Ca2+ chelator, diazo-2, at 10°C. Compared with an intact fiber, diazo-2-induced relaxation exhibited a faster and shorter initial slow phase and a fast phase with a longer tail. Stabilization of the sarcomeres by repeated releases and restretches during force development increased the duration of the slow phase and slowed its kinetics. When force of contraction was decreased by lowering the Ca2+concentration, the overall kinetics of relaxation was accelerated, with the slow phase being the most sensitive to Ca2+ concentration. Twitchlike contractions were induced by photorelease of Ca2+ from a caged Ca2+ (DM-Nitrophen), with subsequent Ca2+ sequestration by intact sarcoplasmic reticulum or Ca2+ rebinding to caged Ca2+. These twitchlike responses exhibited relaxation kinetics that were about twofold slower than those observed in intact fibers. Results suggest that the slow phase of relaxation is influenced by the degree of sarcomere homogeneity and rate of Ca2+ dissociation from thin filaments. The fast phase of relaxation is in part determined by the level of Ca2+ activation.

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