CHANGES IN THE GROWTH-HORMONE AXIS DUE TO EXERCISE TRAINING IN MALE AND FEMALE RATS - SECRETORY AND MOLECULAR RESPONSES

GH secretion is altered by exercise in humans. In an attempt to invest igate the underlying mechanisms, we developed a rodent model. GH secre tion was assayed in male and female rats that were sedentary (not exer cised), acutely exercised, and chronically exercised. Sedentary males showed typical pulsatile GH secretion. The acutely exercised males had low GH concentrations during the exercise bout, but showed partial re covery of GH pulses during the 5.5-h posterercise period. GH secretion in the chronically exercised males was low during both the exercise a nd postexercise periods. Sedentary females displayed the typical patte rn of GH secretion for this sex. The acutely exercised females had low GH concentrations during the exercise period; the pulsatile pattern o f GH secretion did not return during the postexercise period. In contr ast, the chronically exercising females had suppressed GH secretion du ring the exercise bout, but concentrations immediately returned to nor mal during the postexercise bout. The effects of exercise on GH, GH-re leasing hormone (GHRH), and somatostatin messenger RNA (mRNA) levels u sing Northern and slot blot analyses were also determined. Acutely and chronically exercised male rats had decreased levels of GH mRNA compa red to sedentary male rats. The acutely exercised female rats had incr eased levels of GH mRNA compared to the sedentary females, whereas the chronically exercised females had decreased levels. GHRH mRNA levels in acutely exercising male rats was decreased and in chronically exerc ising male rats was increased compared to those in the sedentary contr ols. The pattern of GHRH mRNA in female rats was the opposite of this. Somatostatin mRNA levels decreased in acutely exercised male rats and were not affected in chronically exercised male rats. This signal inc reased in both acute and chronically exercised female rats. These stud ies suggest that GH secretion is suppressed in response to exercise in the rat. This contrasts with the increase observed after exercise in humans.