Na(+)-K+ pump in rat muscle: effects of hypophysectomy, growth hormone, and thyroid hormone

We investigated the effects of growth hormone and thyroid hormone on the synthesis of Na(+)-K+ pumps in rat soleus muscle. Hypophysectomized rats were treated for 11 days with saline, human growth hormone (hGH; 140 micrograms/day), thyroxine (T4; 3 micrograms/day), or hGH plus T4. Age-matched nonhypophysectomized control rats received no treatment. The concentration of Na(+)-K+ pumps was reduced by 75% in the hypophysectomized rats. Treatment with hGH alone or combined with T4 restored the growth rate, whereas T4 alone did not stimulate growth. In contrast, the synthesis of Na(+)-K+ pumps was only responsive to T4 treatment when given alone or in combination with hGH. The concentration of Na(+)-K+ pumps increased around threefold in the T4-treated groups and showed full normalization to the control level after 11 days of treatment. It is concluded that growth hormone does not play any major role in the de novo synthesis of Na(+)-K+ pumps in skeletal muscle. More important, thyroid hormone, also at physiological doses, seems to be the major endocrine factor determining the concentration of Na(+)-K+ pumps in skeletal muscle.