The effect of tri-iodothyronine on the membrane potential and intracellular sodium and potassium activities of rat soleus and extensor digitorum longus muscles.

Tri-iodothyronine (T3) induces a considerable increase in the number of Na/K pump units in rat skeletal muscles, but this effect is more pronounced in red muscles, such as soleus, than in white muscles such as extensor digitorum longus (EDL). In the present study, the effect of T3 on the membrane potential (Em) and the intracellular activities of K+ and Na+ (aKi and aNai) in the two muscle types has been studied using microelectrodes. Hyperthyroidism was induced by seven subcutaneous injections of 50 micrograms T3 per 100 g body weight on alternate days. Following treatment of rats with T3, the mean T3 concentration in the serum was 8.4 +/- 0.46 nmol/l (S.D.) compared with 1.1 +/- 0.16 nmol/l in the serum of untreated control rats, and the heart weight/body weight ratio of the treated rats had increased from 0.29 +/- 0.01 in control rats to 0.39 +/- 0.02. In soleus muscle fibres from the T3-treated rats, the Em, aKi and aNai were unchanged, whereas, in EDL muscle fibres from these rats, the Em had depolarized by 2 mV (P < 0.01) and aNai had increased by 2 mmol/l (P < 0.05). Since T3 induces not only an increase in the number of pump units and active movement of Na+ and K+ but also an increase in the passive fluxes of these ions, the results are discussed in terms of these two opposing ionic movements.