Perchlorate potentiation of excitation-contraction coupling in mammalian skeletal muscles

The action of perchlorate (ClO₄/^-), an agonist of the voltage sensor in excitation-contraction (EC) coupling, has been examined using bundles of intact muscle cells, isolated membrane vesicles [sarcoplasmic reticulum (SR) and transverse tubule (TT)], and cultured myotubes. The effect of ClO₄/^- on mechanical parameters was investigated in isolated murine limb muscles. The presence of ClO₄/^- (5 or 10 mM) greatly increased twitch tension (>250%), slightly enhanced tetanic tension, and increased K contracture tension. K contracture thresholds of extensor digitorum longus (EDL, 40 mM K⁺) and soleus (30 mM K⁺) muscles were not altered by ClO₄/^-. However, in whole cell patch-clamp studies of mouse myotubes, contractile activation was shifted by approximately -10 mV by 10 mM ClO₄/^-. To further define the site of alteration of EC coupling by ClO₄/^-, studies were conducted with isolated porcine SR and TT vesicles and with cultured mouse myotubes. The rate constant of Ca-induced ⁴⁵Ca release from SR vesicles was significantly increased by ClO₄/^-. However, neither the affinity nor level of [³H]PN200- 110 binding to TT vesicles was significantly affected by ClO₄/^- concentrations that increased twitch tension. Furthermore, slow plasmalemmal Ca currents of myotubes recorded in the whole cell patch-clamp mode were enhanced by 10 mM ClO₄/^-, and the current-voltage relationship was shifted approximately -7mV. Thus, in enhancing EC coupling in mammalian muscle, ClO₄/^- may act at multiple sites including the SR Ca release channel and the TT Ca channel-voltage sensor.