Thermoregulation during prolonged exercise in heat: Alterations with β-adrenergic blockade

Thermoregulation and cardiovascular drift were studied under conditions of prolonged exercise in a warm environment (dry bulb temperature 31.7 ± 0.3°C, rh 44.7 ± 4.7%) during β-adrenergic blockade. Fourteen subjects performed 90-min rides on a cycle ergometer at a work rate equivalent to 40% of their control maximal O₂ uptake under each of three treatments provided in a randomized double-blind manner: atenolol (100 mg/day), propranolol (160 mg/day), and a placebo. Exercise during the propranolol trial resulted in significantly higher forearm vascular resistance values and significantly lower forearm blood flows (FBF) compared with the placebo trial. However, the significantly lower FBF during propranolol did not significantly alter the rectal temperature (T(re)) response to prolonged exercise. In addition, both β-blockers produced lower FBF for any given T(re), suggesting that β-adrenergic blockade affects FBF through nonthermal factors. The slight differences in T(re), despite the large differences in FBF between the various treatments, are apparently the result of an enhanced sweat loss and a lower mean skin temperature during exercise with β-blockade. The uncoupling of FBF and sweat loss provides evidence of independent regulation. The reduction in FBF at any given T(re) was concomitant to lower blood pressure values during β-blockade and suggests that baroreflexes provide significant input to the control of skin blood flow when both pressure and temperature maintenance are simultaneously challenged.