Excitation-contraction coupling and contractile properties in denervated rat EDL and soleus muscles.

The long-term (up to ten weeks) effects of denervation on isometric tension and potassium (K) contractures were studied in isolated bundles of fibres from rat extensor digitorum longus (EDL) and soleus muscles, at 21 degrees C, bathed in solutions with low concentrations of chloride ions (to reduce the effects of high membrane chloride conductance). The usual increases in twitch time course and twitch for tetanus ratio were attributed to changes in excitation-contraction coupling because both developed between one to three weeks after denervation. Transient changes during the first week in the time course of the twitch and twitch to tetanus ratio in EDL, and post-tetanic twitch size and specific tension in both muscles, were attributed to the surface membrane electrical properties which are maximally altered within three days after denervation. In contrast to results obtained in solutions of normal chloride concentration, the resting membrane potentials of chronically denervated fibres were hyperpolarized and spontaneous action potentials and fibrillations were seen. The time to the peak of the K-contracture was faster than normal and very slow inactivation kinetics appeared in the decay phase. Mechanical repriming was normally slower in EDL than in soleus but similar rates were seen in the two muscles after denervation. These changes in the K-contracture were not due to a change in the relationship between membrane potential and potassium ion concentration. It is concluded that denervation caused changes in the kinetics of mechanical activation, inactivation and repriming and that these aspects of excitation-contraction coupling are normally controlled by an influence of the motor nerve.