Effects of growth hormone on growth and muscle Na+-K+ pump concentration in K+-deficient rats

K(+)-deficient rats and control rats were injected for 16 days with saline or human growth hormone (hGH, 200 micrograms/day). hGH treatment of K(+)-deficient rats resulted in increased weight gain and soleus muscle weight. Extensor digitorum longus (EDL) muscle weight and tail and tibia length were unchanged. In control rats, hGH induced an increase in all weight and length parameters. K+ deficiency was associated with reduced serum insulin-like growth factor I (IGF-I) and serum insulin but unchanged total 3,5,39-triiodothyronine (TT3) and total thyroxine. hGH treatment of the K(+)-deficient rats restored serum IGF-I, but not serum insulin, and decreased TT3. In saline-treated K(+)-deficient rats [3H]ouabain binding site concentration decreased by 44 and 39% in soleus and EDL muscle, respectively, as compared with the saline-treated controls. hGH had no effect on the [3H]ouabain binding site concentration in the K(+)-deficient group, but, in control rats, increases of 11 and 8% were observed in soleus and EDL muscle, respectively. When the increase in muscle weight was taken into account, this amounted to relative increases of 24 and 30%, respectively. Low circulating GH and IGF-I levels are not the sole explanation for the growth retardation in K+ deficiency. GH/IGF-I stimulate the synthesis of Na(+)-K+ pumps in rats with an otherwise normal hormonal status.