Does the sarcoplasmic reticulum achieve chemiosmotic equilibrium in relaxed muscle?
Measurements of 45Ca2+ efflux from bovine skeletal muscle fibres preloaded with this isotope indicate that the sarcoplasmic free Ca2+ concentration rises and falls under anaerobic (or uncoupled) and aerobic conditions, respectively. Dantrolene has only a small effect on these responses which are larger in predominantly slow-twitch fibre preparations. Dantrolene slightly reduces the resting free Ca2+; this is much more pronounced in rectus abdominis (mainly fast-twitch) than in sternomandibularis (mainly slow-twitch). It is proposed that the sarcoplasmic reticulum in the resting muscle fibre achieves a state close to chemiosmotic equilibrium and that aerobic metabolism generates a higher phosphorylation potential, permitting a larger Ca2+ electrochemical potential difference to develop across the sarcoplasmic reticulum membrane under aerobic conditions.