Key result
Inorganic phosphate reduced force production in dogfish muscle fibres activated from the relaxed state, with greater sensitivity at lower temperatures, but had no effect on fibres activated from rigor.
The study demonstrates that the sensitivity of muscle force to inorganic phosphate depends on temperature and the activation state (relaxed vs. rigor), providing insights into the mechanisms of muscle fatigue.
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Phosphate effects on force are activation-state and temperature dependent in this model; extends basic fatigue mechanisms but leaves open mammalian cardiac translation.
Effects of Pi (inorganic phosphate) are relevant to the in vivo function of muscle because Pi is one of the products of ATP hydrolysis by actomyosin and by the sarcoplasmic reticulum Ca(2+) pump. We have measured the Pi sensitivity of force produced by permeabilized muscle fibres from dogfish (Scyliorhinus canicula) and rabbit. The activation conditions for dogfish fibres were crucial: fibres activated from the relaxed state at 5, 12, and 20 degrees C were sensitive to Pi, whereas fibres activated from rigor at 12 degrees C were insensitive to Pi in the range 5-25 mmol l(-1). Rabbit fibres activated from rigor were sensitive to Pi. Pi sensitivity of force produced by dogfish fibres activated from the relaxed state was greater below normal body temperature (12 degrees C for dogfish) in agreement with what is known for other species. The force-temperature relationship for dogfish fibres (intact and permeabilized fibres activated from relaxed) showed that at 12 degrees C, normal body temperature, the force was near to its maximum value.
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Park‐Holohan et al. (2010) studied this question. Inorganic phosphate and temperature variations vs. No added inorganic phosphate; physiological temperatures was evaluated on Force production by muscle fibres. Inorganic phosphate reduced force production in dogfish muscle fibres activated from the relaxed state, with greater sensitivity at lower temperatures, but had no effect on fibres activated from rigor.
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