Sodium withdrawal contractures in rat slow twitch skeletal muscle.

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.