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

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.