Excitation-contraction coupling and charge movement in denervated rat extensor digitorum longus and soleus muscles.

K contractures and asymmetrical charge movement were recorded in extensor digitorum longus (e.d.l.) and soleus muscles that had been denervated for 2-68 days. The relationship between maximum tension during a K contracture and membrane potential shifted to more negative potentials in denervated e.d.l. muscles (by -25 mV on average) and to a lesser extent in soleus (by -8 mV on average), and became steeper, more so in e.d.l. than soleus. Apart from an early negative shift of -11 mV in the voltage dependence of tension in e.d.l. muscles during the first week, the other changes in K contractures following denervation occurred progressively during the first 3 weeks and then stabilized. There was a clear difference in charge movement in denervated e.d.l. fibres but little change in denervated soleus fibres, so that the characteristics of charge movement in e.d.l. and soleus became very similar. The maximum amount of charge movement fell from an average normal value of 23 nC/microF to 6 nC/microF in e.d.l. within the first 2 weeks. The voltage sensitivity shifted to more negative potentials (by about -12 mV on average) within the first week. There was no significant change in the slope of the relationship between charge and membrane potential. The effects of denervation on charge movement could only partly explain the changes in K contractures. The only obvious parallels were the early negative shift in the voltage dependence of charge movement and tension in denervated e.d.l. fibres. The other changes in K contractures in denervated fibres could be due to a change in the relationship between charge movement and Ca concentration in the myoplasm or an increase in the Ca affinity of the myofilaments. Although charge movement fell to about a quarter of normal in denervated e.d.l. fibres, membrane capacity increased approximately 3-fold. A similar increase in capacity in soleus fibres was not associated with a change in charge movement. Fewer indentations were seen in denervated than in normal e.d.l. fibres. The decrease paralleled the fall in charge movement.