IGF-I, GROWTH-HORMONE, AND OR EXERCISE EFFECTS ON NON-WEIGHT-BEARING SOLEUS OF HYPOPHYSECTOMIZED RATS/

The effects of insulin-like growth factor (IGF-I) or growth hormone (G H) with and without exercise on predominantly slow muscles of hypophys ectomized hindlimb-suspended (HS) rats were determined. IIS resulted i n a 21, 23, and 30% decrease in soleus, adductor longus, and vastus in termedius masses, respectively, compared with ambulatory rats. Compare d with values in IIS rats, IGF-I increased the vastus intermedius mass and GH or Exercise alone increased both tile soleus and vastus interm edius masses. There was a strong interactive effect between GI-I, but not IGF-I, and exercise in all three muscles of HS rats. The soleus fi ber type distribution of IIS rats was not affected by any treatment. H S resulted in a 24, 18 (P > 0.05), 32, and 20% (P > 0.05) decrease in the size of soleus fibers containing type I, IIa, I+IIa, and IIa+IIx m yosin heavy chains, respectively, compared with ambulatory hypophysect omized rats. Hormone or exercise alone had no effect on fiber size in HS rats. However, all fiber sizes (except for type IIa+IIx in ICF-I wi th exercise rats were larger In IIS rats treated with GH or IGF-I and exercise than those in HS rats. These data indicate an interactive eff ect of both GH and IGF-I with exercise in maintaining fiber size of ch ronically nonweight-bearing predominantly slow muscles. Further more, the results suggest that the myosin heavy-chain phenotype in rats defi cient in all pituitary factors is unresponsive to short-term administr ation of either GH or IGF-I or to exercise or HS.