Blood glucose concentration dependent ACTH and cortisol responses to prolonged exercise

The purpose of this study is to investigate responses of serum ACTH and cortisol concentration to low intensity prolonged exercise. In experiment 1, 10 subjects fasted for 12 h and performed bicycle exercise at 49.3% VO2max (+/- 4.3%) until exhaustion or up to 3 h. During the early part of the exercise, serum ACTH and cortisol concentrations did not increase from the pre-exercise values (ACTH: 44 +/- 5 micrograms/l, cortisol: 139 +/- 52 micrograms/l). Whilst the time to serum ACTH concentration increasing varied among the subjects (60-180 min), the increases of this hormone occurred for all subjects (175 +/- 85 ng/l, P less than 0.05) when blood glucose concentration decreased to a critical level of 3.3 mmol/l. At the end of the exercise, blood glucose concentration decreased to 2.60 +/- 0.21 mmol/l, and serum ACTH and cortisol concentrations increased to 313 +/- 159 ng/l and 371 +/- 151 micrograms/l, respectively. In experiment 2, four subjects performed the same intensity exercise until exhaustion, and were then given 600 ml of 20 g glucose solution, and immediately afterwards, they were asked to repeat the same exercise. The subjects continued the exercise for between 30 to 90 min until again reaching exhaustion. During the second exercise, blood glucose concentration increased to the pre-exercise value (2.72 +/- 0.58 to 4.00 +/- 0.22 mmol/l, P less than 0.05) and simultaneously, serum ACTH concentration decreased considerably (354 +/- 22 to 119 +/- 54 ng/l, P less than 0.05). The results of the present study suggest that serum ACTH and cortisol concentration during low intensity prolonged exercise may be dependent on blood glucose concentration.