THE EFFECT OF BETA-BLOCKADE ON THERMOREGULATION DURING EXERCISE (PROPRANOLOL, ESOPHAGEAL TEMPERATURE, HIGH-PRESSURE BARORECEPTORS)

To study the effect of nonselective beta-blockade (BB) on the thermoregulatory responses, six, fit men cycled at 60% VO(,2) max for 40 minutes (min) at ambient temperatures of 22(DEGREES)C and 32(DEGREES)C. Two hours prior to exercise, each subject ingested 80 mg of propranolol or placebo (Pl) capsules in single-blind fashion. Heart rate at 40 min of exercise was reduced (p \u3c 0.0001) from 125 (+OR-) 3 to 103 (+OR-) 1 beat (.) min(\u27-1) at 22(DEGREES)C and 137 (+OR-) 3 to 104 (+OR-) 1 beats (.) min(\u27-1) at 32(DEGREES)C demonstrating sufficient BB. Esophageal temperature (T(,es)) averaged 0.28(DEGREES)C higher with BB than with Pl (p (LESSTHEQ) 0.05). The elevated T(,es) was primarily the result of a reduced core-to-skin heat flux indicated by a reduction in: (1) the slope of the forearm blood flow (FBF):T(,es) rela- tionship, (2) the maximal FBF and (3) local sweating rate at any T(,es). Systolic blood pressure was decreased 20 mm Hg with BB (p (LESSTHEQ) 0.01) while diastolic blood pressure was unchanged, reducing arterial pulse pressure (PP) (p (LESSTHEQ) 0.01). Because PP was decreased and cardiac filling pressure was presumably not reduced (since cardiac stroke volume was elevated), the relative increase in peripheral vasomotor tone during BB was partially the consequence of high pressure baroreceptor stimulation.^ Hemoglobin, hematocrit, plasma sodium, plasma lactate (La), oxygen uptake, carbon dioxide production and minute ventilation were not altered by BB. The respiratory exchange ratio (p (LESSTHEQ) 0.05), plasma potassium (K(\u27+)) (p (LESSTHEQ) 0.001) and the Borg rating of perceived exertion (RPE) (p (LESSTHEQ) 0.01) were increased with BB compared to Pl. The elevated RPE during BB may have been the result of a relatively greater glucose depletion or an altered muscle cell excitability, due to the elevated plasma K(\u27+).