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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 (E_m) and the intracellular activities of K⁺ and Na⁺ (aK_i and aNa_i) in the two muscle types has been studied using microelectrodes. Hyperthyroidism was induced by seven subcutaneous injections of 50 µg 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 E_m, aK_i and aNa_i were unchanged, whereas, in EDL muscle fibres from these rats, the E_m had depolarized by 2 mV (P<0·01) and aNa_i 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.

Journal of Endocrinology (1993) 138, 503–507

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