CHANGES IN PITUITARY FUNCTION AND SKELETAL MUSCLE OXIDATIVE CAPACITY DEPEND ON EXERCISE TRAINING INTENSITY 1151

Abstract

We investigated effects of the different intensities of treadmill running training on the ACTH response to corticotropin-releasing factor (CRF) and the change in adrenal weight in rats, because it was reported that the increase in plasma ACTH level depended on the exercise intensity. We also measured the muscle oxidative enzyme activity to confirm the endurance training effect. Six-week-old male Wistar rats were divided into control group (CTL, n=6) and 3 trained groups which ran at 3 different treadmill running speeds (n=6 in each group). The running speeds for exercise training were 10 m/min (10R), 20 m/min(20R) and 30 m/min (30R). The duration, frequency and periods of exercise training were 60 min, 5 days/wks and 10wks, respectively. Blood samples were taken through a catheter implanted into right jugular vein at rest and 20 min after intravenous injection of 0.5 μg/kg of CRF. Plasma ACTH and corticosterone (Cort) levels were determined by radioimmunoassay. Succinate dehydrogenase (SDH) activity in plantaris muscle was measured by spectrophotometry. The plasma ACTH level after administration of 0.5 μg/kg of CRF was significantly (p<0.05) lower in the 30R (54.5±1.4 pg/ml, mean±SE) than in the CTL (67.7±3.3 pg/ml). The plasma Cort levels after injection of 0.5 μg/kg of CRF were not significantly different among groups. The basal plasma Cort level tended to be higher, but not significantly, in the 30R than in the CTL. The adrenal weight/body weight was significantly (p<0.01) heavier in the 30R (18.9±0.6 mg/100g body wt) than in the CTL (14.8±0.4 mg/100g body wt). The SDH activities in plantans muscle were significantly (p<0.05) higher in both the 20R and the 30R (20R:6.39±0.55, 30R:6.59±0.36 μmol/g wet wt/min) than in the CTL (4.82±0.42 μmol/g wet wt/min). These results suggest that the changes in ACTH response to CRF and adrenal weight could occur only with intensive running training (30m/min speed) and that moderate intensity of training (20m/min speed) could be effective on the increase in muscle oxidative capacity with no changes in the pituitary response to CRF and adrenal weight in rats.