A chloride channel blocker prevents the suppression by inorganic phosphate of the cytosolic calcium signals that control muscle contraction

Abstract

Accumulation of inorganic phosphate (P i) may contribute to muscle fatigue by precipitating calcium salts inside the sarcoplasmic reticulum (SR). Neither direct demonstration of this process nor definition of the entry pathway of P i into SR are fully established. r We showed that P i promoted Ca 2+ release at concentrations below 10 mM and decreased it at higher concentrations. This decrease correlated well with that of [Ca 2+ ] SR. r Pre-treatment of permeabilized myofibres with 2 mM Cl − channel blocker 9-anthracenecarboxylic acid (9AC) inhibited both effects of P i. r The biphasic dependence of Ca 2+ release on [P i ] is explained by a direct effect of P i acting on the SR Ca 2+ release channel, combined with the intra-SR precipitation of Ca 2+ salts. The effects of 9AC demonstrate that P i enters the SR via a Cl − pathway of an as-yet-undefined molecular nature.

Can a chloride channel blocker mitigate muscle fatigue?

“…Although Ferreira et al . (2021) present limited data for [Ca 2+ ] SR at lower [P i ], the ∼150% increase in spark frequency reported at 5 m m is substantially greater than the 30% increase in [Ca 2+ ] SR in fibre 011504c seen in Table 2. This departure from the proportionality of spark frequency and [Ca 2+ ] SR is reconcilable with the P i ‐mediated activation of the RyR complex (Allen et al .…”

“…The experiments in Ferreira et al clearly indicate that 9AC prevented P i -induced declines in free [Ca 2+ ] SR and CaP i precipitation in the SR. However, 9AC has effects distinct from inhibition of the ClC; this fact was carefully considered and acknowledged by Ferreira et al (2021). Thus, channels or transporters aside from the ClC could be important conduits for P i into the SR. As P i -activation of the RyR did not prevent P i -mediated inhibition of spark frequency or declines in free [Ca 2+ ] SR at high [P i ], the putative activation of RyR by ClC blockers other than 9AC should not preclude the notion that these ClC blockers could mitigate the P i -mediated inhibition of spark frequency or declines in [Ca 2+ ] SR when [P i ] is high.…”

“…Thus, channels or transporters aside from the ClC could be important conduits for P i into the SR. As P i -activation of the RyR did not prevent P i -mediated inhibition of spark frequency or declines in free [Ca 2+ ] SR at high [P i ], the putative activation of RyR by ClC blockers other than 9AC should not preclude the notion that these ClC blockers could mitigate the P i -mediated inhibition of spark frequency or declines in [Ca 2+ ] SR when [P i ] is high. As different ClC blockers could vary in effectiveness against different types of transporters and anion channels, an exploration of this variability could help determine the specific access point(s) for P i into the SR. Ferreira et al (2021) have provided compelling evidence that Ca 2+ and P i precipitate in the SR and that the passage of P i into the SR is mitigated by the ClC blocker 9AC. However, since 9AC is not specific to the ClC, P i could conceivably enter the SR via another 9AC-sensitive pathway.…”

“…This could be accomplished by impairing Ca 2+ reuptake by the sarco-endoplasmic reticulum Ca 2+ -ATPase (SERCA), and/or by forming a precipitate with Ca 2+ (CaP i ) within the SR (Fryer et al 1995). The focus of the paper by Ferreira et al (2021,) recently published in The Journal of Physiology, aimed to elucidate the mechanism by which P i enters the SR.…”

“…Chloride channels as the putative pathway for P i entry into the SR Anion channels are considered the entry point for P i into the SR and, based on prior speculation, Ferreira et al (2021) ] SR were made using the fluorescent dyes Mag-Indo-1 and Fluo-5N. They found that spark frequency was increased above baseline (0 mm P i ) when P i concentrations ([P i ]) were below 10 mm, but as expected, further increases in [P i ] greatly reduced Ca 2+ spark frequency and [Ca 2+ ] SR .…”