Effects of naloxone on maximal stress testing in females

To examine the effect of naloxone (an opiate antagonist) on exercise performance, six normal females (20-28 yr) walked on a treadmill (93.8 m/min) with the grade increasing 2.5% every 4 min until exhaustion. Each subject completed three trials: naloxone infusion (0.4 mg/ml saline), saline infusion (1 ml), and an infusion control. The order of trials was randomized and double blind. Each subject responded similarly at each submaximal stage for all three trials. The results during maximal exertion were different. Time to exhaustion was similar for all conditions, averaging 32.6 +/- 3.0 min with a maximal O2 uptake of 38.8 +/- 2.8 ml X kg-1 X min-1 and a lactate of 9.1 +/- 1.1 mmol/l. Maximal ventilation (BTPS) was 7.9 l/min less during the naloxone treatment when compared with either control or placebo trials (P less than 0.05). This difference was attributed to a 4-breath/min reduction in respiratory frequency (P less than 0.05). End-tidal CO2 partial pressure was approximately 4 Torr higher at maximum after naloxone injection. Maximal heart rates averaged 190.8 +/- 3.8 beats/min for both control and placebo trials. This was significantly greater (P less than 0.05) than the 185 +/- 2.4 beats/min recorded for the naloxone trials. Ratings of perceived exertion were 17.0 +/- 1.3 units at maximum, similar for all three trials. Despite these statistically significant cardiorespiratory differences, naloxone produced no effect on maximum O2 uptake or exercise duration.