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Thermoregulation during prolonged exercise in heat: alterations with beta-adrenergic blockade
B J Freund1, M J Joyner, S M Jilka
1Department of Exercise and Sport Sciences, University of Arizona, Tucson 85721.
Journal of Applied Physiology (Bethesda, Md. : 1985)
|September 1, 1987
Summary
Beta-blockers like propranolol reduce blood flow during exercise in heat but do not significantly alter core body temperature. This suggests independent regulation of skin blood flow and sweating during exercise with beta-adrenergic blockade.
Area of Science:
- Exercise Physiology
- Cardiovascular Physiology
- Environmental Physiology
Background:
- Thermoregulation and cardiovascular drift are critical during prolonged exercise in warm environments.
- Beta-adrenergic blockade can influence cardiovascular responses and thermoregulation.
- Understanding these effects is important for athletes and individuals exercising in heat.
Purpose of the Study:
- To investigate the effects of beta-adrenergic blockade on thermoregulation and cardiovascular drift during prolonged exercise in a warm environment.
- To examine the relationship between forearm blood flow (FBF), sweat loss, and core body temperature (Tre) under beta-blockade.
- To determine the role of nonthermal factors and baroreflexes in controlling skin blood flow during exercise with beta-blockade.
Main Methods:
- Fourteen subjects completed 90-minute cycle ergometer rides at 40% maximal O2 uptake in a warm environment (31.7°C).
- Treatments included randomized, double-blind administration of atenolol, propranolol, or a placebo.
- Measurements included rectal temperature (Tre), forearm blood flow (FBF), forearm vascular resistance, sweat loss, and skin temperature.
Main Results:
- Propranolol significantly increased forearm vascular resistance and decreased FBF compared to placebo.
- Despite reduced FBF, rectal temperature (Tre) responses were not significantly altered by beta-blockade.
- Both beta-blockers showed lower FBF for a given Tre, indicating nonthermal influences and independent regulation of FBF and sweat loss.
- Reduced FBF during beta-blockade correlated with lower blood pressure, suggesting baroreflex involvement.
Conclusions:
- Beta-adrenergic blockade affects forearm blood flow through nonthermal mechanisms, independent of thermoregulatory demands.
- Enhanced sweat loss and lower mean skin temperature contribute to maintaining core body temperature despite reduced skin blood flow during beta-blockade.
- Baroreflexes play a significant role in regulating skin blood flow when challenged by both reduced blood pressure and thermal stress during exercise.