GROWTH-HORMONE, IGF-I, AND EXERCISE EFFECTS ON NON-WEIGHT-BEARING FAST MUSCLES OF HYPOPHYSECTOMIZED RATS

The effects of growth hormone (GH) or insulin-like growth factor I (IC E-I) with or without exercise (ladder climbing) in countering the effe cts of unweighting on fast muscles of hypophysectomized rats during 10 days of hindlimb suspension were determined. Compared with untreated suspended rats, muscle weights were 16-29% larger in GH-treated and 5- 15% larger in IGF-I-treated suspended rats. Exercise alone had no effe ct on muscle weights. Compared with ambulatory control, the medial gas trocnemius weight in suspended, exercised rats was larger after GH tre atment and maintained with IGF-I treatment. The combination of GH or I GF-I plus exercise in suspended rats resulted in an increase in the si ze of each predominant fiber type, i.e., types I, I+IIa and IIa+IIx, i n the medial gastrocnemius compared with untreated suspended rats. Nor mal ambulation or exercise during suspension increased the proportion of fibers expressing embryonic myosin heavy chain in hypophysectomized rats. The phenotype of the medial gastrocnemius was minimally affecte d by GH, IGF-I, and/or exercise. These results show that there is an I GF-I, as well as a GH, and exercise interactive effect in maintaining medial gastrocnemius fiber size in suspended hypophysectomized rats.