Abstract
In an attempt to investigate the physiological responses to opioid receptor blockade
during exercise in the heat, five male volunteers completed two bouts of stationary
cycling at 70% V̇O2 max in a hot (33 °C765% RH) environment. Exercise was conducted following the administration
of either naloxone or saline (4 mg i. v.) five minutes prior to exercise. A second
4 mg dose was administered at 25 minutes of exercise. Performance time was 11% shorter
(p = 0.06), and RPE response was significantly higher at test termination on naloxone.
No drug effect was observed on rectal or mean skin temperature during exercise. Forearm
blood flow (FBF) was higher on naloxone, while exercise heart rates were lower on
the drug versus saline. No significant changes were observed in estimated mean arterial
pressure or gross sweat responses to exercise. Plasma immunoreactive β-endorphin was
significantly elevated in the naloxone trial only. Thus, while opioids may play some
hemodynamic role during exercise in the heat, it appears that opioid mediation of
the perceived stress of exercise contributes more to an individual's thermal tolerance.
Additionally, the results suggest that perceptual and hemodynamic/cardiovascular responses
that may be mediated by these peptides are dissociable phenomena.
Key words
Beta-Endorphin - forearm blood flow - exercise tolerance
