title: Sodium withdrawal contractures in rat slow twitch skeletal muscle.
abstract: Contractile responses due to alterations in [Na+]o have been investigated in fast (iliacus) and slow (soleus) twitch muscles of the rat. On exposure to a Na-free solution, the soleus in contrast to the iliacus cells, generated contractile responses without depolarizing the surface membrane. Following glycerol treatment, the twitch and a part of the Na-withdrawal contracture were abolished. The amplitude of the remaining contracture was between 5 and 50% of the original response and the time to peak was 0.4 to 2 times longer. In intact and detubulated preparations, the amplitude of the zero-Na contracture was modified by changes in [Ca2+]o and a linear relationship was found if the reciprocal of tension was plotted against 1/square root[Ca2+]o. In intact and detubulated fibres, a steep dependence of the Na-withdrawal contracture on [Na+]o was found and [Na+]o which induced the half maximal response at each [Ca2+]o was the same, the responses were inhibited by Mg-ions in a competitive way. It is assumed that the activator Ca is triggered at the tubular and sarcolemmal membrane level by lowering [Na+]o and that a calcium-induced calcium release mechanism at the S.R. level may also be involved.
mesh_terms: D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000187:drug effects*; D009132:Muscles / Q000502:physiology; D051381:Rats; D012964:Sodium / Q000494:pharmacology*
chemical_list: D002110:Caffeine; D012964:Sodium; D008274:Magnesium; D002118:Calcium