EFFECTS OF GROWTH-HORMONE ON RAT SKELETAL-MUSCLE AFTER HINDLIMB SUSPENSION

To examine the effects of growth hormone (GH) on the preferential atro phy of the soleus muscle (SOL) occurring after hindlimb suspension (HS ), two groups of male rats received daily injections of 2 IU.kg(-1) bo dy mass of recombinant human growth hormone (rhGH). Rats were either s uspended by the tail for 21 days (HS-GH, n = 5) or nonsuspended (C-GH, n = 5). The effects of rhGH treatment on SOL and extensor digitorum l ongus muscles (EDL) were compared in two groups of animals receiving d aily injections of saline, either suspended by the tail (HS-SA, n = 5) or nonsuspended (C-SA, n = 5). The results showed that the SOL hypert rophy in response to rhGH administration was mostly observed in C rats (+ 33%, P < 0.01). This increase in muscle mass was correlated with a concomitant increase in the size of type I fibres (+ 21%, P < 0.05). Although SOL mass decreased during HS in rhGH treated animals (- 44%, P < 0.001), the mean normalized mass of this muscle did not significan tly differ between C-SA and HS-GH groups. A statistically significant increase in the absolute mass of EDL occurred with rhGH treatment in C GH (+ 12%, P < 0.05). The HS-induced decrease in the percentage distri bution of type I fibres in SOL was unaffected by the rhGH treatment. I n addition, a decrease in the citrate synthase activity in the whole S OL was observed in the two groups of tail-suspended rats (- 31%, P < 0 .05; - 21%, P < 0.05 in SA and GH animals, respectively). The activity of 3-hydroxyacyl coenzyme A dehydrogenase was enhanced by the rhGH tr eatment (P < 0.05) with similar magnitude in both C (+ 25%) and HS rat s (+ 24%). Therefore, GH prevented only slightly the atrophy of SOL, o ccurring after 21 days of HS. The effects of rhGH treatment appeared m ost effective in C rats, suggesting that HS impaired the growth-promot ing effects of this hormone on skeletal muscle.