Effects of opioid antagonism on thermoregulation during prolonged exercise in the heat

Five adult male volunteers were studied to investigate the effect of opiate receptor blockade on the physiological response to a maximum of 60 minutes of stationary cycling at 70% V02peak in a hot (33 0 C/65% RH) environment. Exercise bouts were conducted following the administration of naloxone (4mg IV) 5 minutes prior to exercise with a follow-up 4mg dose at 25 minutes of exercise. In the placebo trial, volume-matched doses of saline were administered at the same points. No significant drug effect was observed on rectal or mean skin temperature during exercise. Post-exercise skin temperature was significantly (P<.001) higher on naloxone versus saline. Forearm blood flow (FBF) was consistently higher from minute 25 of exercise until test termination, although only the minute 25 and minute 55 data points were significantly elevated (P<.05, P<.005, respectively) . The rectal temperature threshold at which FBF plateaued was higher on naloxone (P=.054), and the FBF: rectal temperature slope was higher on naloxone throughout the trial. No significant changes were observed in heart rate or estimated mean arterial pressures, although both were consistently lower on naloxone. Gross sweat response was not altered by the drug. Plasma Beta-Endorphin was significantly (P<.Ol) higher on naloxone versus saline, and Beta-Endorphin was significantly elevated in the naloxone trial only. The observation that FBF was significantly higher on naloxone without inducing compensatory heart rate or blood pressure changes suggests that the opioids may be involved in the blood volume shifts that occur during prolonged exercise in the heat.