326
Opioid Antagonism, Perceived Exertion and Tolerance to Exercise-Thermal Stress M. S. Hickey, W. D. Franke, W. G. Herbert, J. Walberg-Rankin, J. C. Lee Laborataory for Exercise, Sport, and Work Physiology, Department of Health, Physical Education and Recreation, and the School of Veterinary Medicine, VPI & SU, Blacksburg, VA 24061
Introduction M. S. Hickey, W. D. Franke, W. G. Herbert, J.
Walberg-Rankin and J. C. Lee, Opioid Antagonism, Perceived Exertion and Tolerance to Exercise-Thermal Stress. IntJ Sports Med, Vol 1 3,No 4, pp 326—331, 1992.
Accepted: December 10, 1991
The endogenous opioid peptide (EOP) system has stimulated considerable interest among exercise physiologists since the discovery of these stress-related peptides in the mid 1970's. Because of their pharmacological similarity to the opiate alkaloid morphine, attempts have been made to link the EOPs to exercise-induced alterations in mood state and pain tolerance (3, 7, 11, 13, 17, 23, 31). While evidence in support of
EOP-induced alterations in post-exercise mood state is In an attempt to investigate the physiological
responses to opioid receptor blockade during exercise in the heat, live male volunteers completed two bouts of stationary cycling at 70% VO2max 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 sa-
line. No significant changes were observed in estimated mean arterial pressure or gross sweat responses to exercise. Plasma immunoreactive El-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 ex-
ercise 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, exer-
equivocal, results from investigations using adequate doses of the opiate antagonist naloxone suggest a role for the EOPs in perception of effort during exercise (3, 6, 7, 12, 27, 30). It has further been suggested that a "feed forward" relationship may exist between RPE and EOP release, wherein perceived exertion may act as a signal for EOP release during exercise (27).
Previous research in this laboratory (20) has demonstrated that exercise-thermal stress induces a significantly greater rise in plasma immunoreactive 3-endorphin (3En — a specific EOP) than the same level of exercise in a ther-
moneutral environment. Given the stress-dependent nature of these peptides, this observation is not surprising. Nevertheless, the physiological significance of this heat-induced elevation has not been elucidated. Recent evidence suggests that naloxone does not influence temperature regulation during acute exercise in a thermoneutral environment (28, 29), but the lack of concomitant heat-stress makes it difficult to apply the results of these investigations to exercise in hyperthermic environments. While it must be considered that EOPs act as part of the overall stress-response system during a work-heat challenge, we hypothesized that these peptides may be involved in mediating an individual's perceptual thermal tolerance, i. e., the perceived effort related to a work-heat challenge may be influenced by the EOPs. Thus, the purpose of this investigation was to assess the role of FOPs in perception of effort and thermal homeostasis during a combined exercise-thermal stress.
cise tolerance
Methods
Five well-trained, competitive male cyclists participated in this investigation. Mean (± SE) age, height, weight, VO2max and % fat were: 22.6± 1.3 yrs, 180.2± 1.1 cm, 72.6± 2.4 kg, 4.8±0.5 l-min1and 7.9±0.7%, respectively. After orientation to the experimental protocol, written consent was obtained. The approval of the human subjects committee of this institution was secured prior to the initiation of any data collection. Int.J. Sports Med. 13(1992)326—331 GeorgThieme Verlag Stuttgart New York
Downloaded by: National University of Singapore. Copyrighted material.
Abstract
Int.J.SportsMed. 13 (1992) 327
Opioid Antagonism and Exercise Tolerance
140 130 120 110
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E
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exercise time (mm)
exercise time (mm)
Fig. 1 RPE response to opioid antagonism during exercise in the heat. A significant (p
Opioid Antagonism, Perceived Exertion and Tolerance to Exercise-Thermal Stress M. S. Hickey, W. D. Franke, W. G. Herbert, J. Walberg-Rankin, J. C. Lee Laborataory for Exercise, Sport, and Work Physiology, Department of Health, Physical Education and Recreation, and the School of Veterinary Medicine, VPI & SU, Blacksburg, VA 24061
Introduction M. S. Hickey, W. D. Franke, W. G. Herbert, J.
Walberg-Rankin and J. C. Lee, Opioid Antagonism, Perceived Exertion and Tolerance to Exercise-Thermal Stress. IntJ Sports Med, Vol 1 3,No 4, pp 326—331, 1992.
Accepted: December 10, 1991
The endogenous opioid peptide (EOP) system has stimulated considerable interest among exercise physiologists since the discovery of these stress-related peptides in the mid 1970's. Because of their pharmacological similarity to the opiate alkaloid morphine, attempts have been made to link the EOPs to exercise-induced alterations in mood state and pain tolerance (3, 7, 11, 13, 17, 23, 31). While evidence in support of
EOP-induced alterations in post-exercise mood state is In an attempt to investigate the physiological
responses to opioid receptor blockade during exercise in the heat, live male volunteers completed two bouts of stationary cycling at 70% VO2max 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 sa-
line. No significant changes were observed in estimated mean arterial pressure or gross sweat responses to exercise. Plasma immunoreactive El-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 ex-
ercise 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, exer-
equivocal, results from investigations using adequate doses of the opiate antagonist naloxone suggest a role for the EOPs in perception of effort during exercise (3, 6, 7, 12, 27, 30). It has further been suggested that a "feed forward" relationship may exist between RPE and EOP release, wherein perceived exertion may act as a signal for EOP release during exercise (27).
Previous research in this laboratory (20) has demonstrated that exercise-thermal stress induces a significantly greater rise in plasma immunoreactive 3-endorphin (3En — a specific EOP) than the same level of exercise in a ther-
moneutral environment. Given the stress-dependent nature of these peptides, this observation is not surprising. Nevertheless, the physiological significance of this heat-induced elevation has not been elucidated. Recent evidence suggests that naloxone does not influence temperature regulation during acute exercise in a thermoneutral environment (28, 29), but the lack of concomitant heat-stress makes it difficult to apply the results of these investigations to exercise in hyperthermic environments. While it must be considered that EOPs act as part of the overall stress-response system during a work-heat challenge, we hypothesized that these peptides may be involved in mediating an individual's perceptual thermal tolerance, i. e., the perceived effort related to a work-heat challenge may be influenced by the EOPs. Thus, the purpose of this investigation was to assess the role of FOPs in perception of effort and thermal homeostasis during a combined exercise-thermal stress.
cise tolerance
Methods
Five well-trained, competitive male cyclists participated in this investigation. Mean (± SE) age, height, weight, VO2max and % fat were: 22.6± 1.3 yrs, 180.2± 1.1 cm, 72.6± 2.4 kg, 4.8±0.5 l-min1and 7.9±0.7%, respectively. After orientation to the experimental protocol, written consent was obtained. The approval of the human subjects committee of this institution was secured prior to the initiation of any data collection. Int.J. Sports Med. 13(1992)326—331 GeorgThieme Verlag Stuttgart New York
Downloaded by: National University of Singapore. Copyrighted material.
Abstract
Int.J.SportsMed. 13 (1992) 327
Opioid Antagonism and Exercise Tolerance
140 130 120 110
w
E
0 cc
0
100
1j 90 80 70
—30
0
E15
E30
E45
+30
exercise time (mm)
exercise time (mm)
Fig. 1 RPE response to opioid antagonism during exercise in the heat. A significant (p
