Growth hormone induces muscle fibre type transformation in growth hormone‐deficient rats

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Abstract
The effect of recombinant human growth hormone (rhGH) on growth and composition of muscle was studied in growth hormone‐deficient rats (dw/dw) treated for 10 days with either rhGH (GH) or with placebo (PLA). Age‐matched control rats (DW/dw) (AGE) were treated as PLA. Growth rate increased (P < 0.05) when rats were treated with rhGH and plasma insulin‐like growth factor‐1 concentration was higher (P < 0.05) in GH and AGE than in PLA. The wet weight of the soleus (SOL) and the extensor digitorum longus muscles (EDL) was less in PLA compared to GH and AGE (P < 0.05). In the SOL, the amount of myosin heavy chain (MHC) I was lower (69.1 ± 1.7%) (Mean ± SEM) in PLA compared to both GH (85.3 ± 2.3%) and AGE (76.4 ± 1.6%) (P < 0.05). At the same time the amount of MHC IIA/IIX was higher (30.9 ± 2.2%) in PLA compared to GH (14.7 ± 2.3%) and AGE (23.6 ± 1.6% (P < 0.05)). In EDL, treatment with rhGH did not significantly affect MHC‐isoforms or the fibre type composition, but 11% more MHC IIB and 11% less MHC IIA/IIX was observed in PLA compared to AGE (P < 0.05) suggesting a long‐term effect of growth hormone. MHC‐isoform data were confirmed using histochemistry. In addition, in the SOL, the maximal activity of 3‐hydroxyacyl‐CoA dehydrogenase (HAD) in GH and AGE was higher (22 and 27%, respectively) than in PLA (P < 0.05). In the EDL, no differences were observed in maximal activity of HAD. In conclusion, the data support a role for growth hormone in muscle fibre growth and differentiation.