Potassium contractures and asymmetric charge movement in extensor digitorum longus and soleus muscles from thyrotoxic rats

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Abstract
Potassium contractures and asymmetric charge movement were recorded from extensor digitorum longus (EDL) and soleus muscle from normal rats and rats that had been made thyrotoxic by daily intraperitoneal injections of triiodothyronine (150 μg kg−1) for two to three weeks. Potassium contracture tension (relative to tetanic tension) in thyrotoxic rats was greater in EDL muscles and smaller in soleus muscles than in normal rats. As the relationship between membrane potential and potassium concentration was unaltered by thyroid treatment, it was concluded that the changes in potassium contracture tension were due to changes in excitation-contraction coupling. In thyrotoxic rats there was an average negative shift of −5 mV in the voltage sensitivity of tension in EDL fibres and a positive shift of 5 mV in soleus. As a result, the tension-membrane potential curves for EDL and soleus fibres essentially coincided. There was a corresponding average negative shift of −4 mV in the voltage sensitivity of asymmetric charge in EDL fibres, and a positive shift of 4 mV in soleus fibres from thyrotoxic rats. The dependence of asymmetric charge movement on membrane potential became essentially the same in EDL and soleus fibres from thyrotoxic rats. The maximum asymmetric charge in soleus fibres increased from an average value of 6.5 nC μF−1 in normal rats (n=33) to 8.9 nC μF−1 in thyrotoxic rats (n=32;p<0.005). The increase in charge movement in EDL fibres in thyrotoxic rats was not statistically significant. The real increase was masked by an increase in linear capacity due to proliferation of T-system membrane: the density of triads where charge movement probably occurs did not increase.