Intracellular sodium and potassium activities of skeletal muscle fibres of hypothyroid rats.

In soleus muscle fibres of hypothyroid rats, the membrane potential (Vm) and the intracellular K+ activity (aKi) were significantly lower than in control muscles. These results are consistent with the well-documented decrease in the number of Na(+)-K+ pumps which occurs in hypothyroid muscles. aNai was unchanged in the hypothyroid muscles but this may reflect a change in passive Na+ fluxes which has been reported to occur in association with changes in the number of pump units.