Effect of β-blockade on the drift in O₂ consumption during prolonged exercise

The effect of β-adrenergic blockade on the drift in O₂ consumption (V̇O₂ drift) typically observed during prolonged constant-rate exercise was studied in 14 healthy males in moderate heat at 40% of maximal O₂ consumption (V̇O(2 max)). After an initial maximum cycle ergometer test to determine the subject's control V̇O(2 max), subjects were administered each of three medications: placebo, atenolol (100 mg once daily), and propranolol (80 mg twice daily), in a randomized double-blind fashion. Each medication period was 5 days in length and was followed by a 4-day washout period. On the 3rd day of each medication period, subjects performed a maximal cycle ergometer test. On the final day of each medication period, subjects exercised at 40% of their control V̇O(2 max) for 90 min on a cycle ergometer in a warm (31.7±0.3°C) moderately humid (44.7±4.7%) environment. β-Blockade caused significant (P<0.05) reductions in V̇O(2 max), maximal minute ventilation (V̇E(max)), maximal heart rate (HR(max)), and maximal exercise time. Significantly greater decreases in V̇O₂(max), V̇E(max), and HR(max) were associated with the propranolol compared with the atenolol treatment. During the 90-min submaximal rides, β-blockade significantly reduced heart rate. Substantially lower values for O₂ consumption (V̇O₂) and minute ventilation (V̇E) were observed with propranolol compared with atenolol or placebo. Furthermore, V̇O₂ drift and HR drift were observed under atenolol and placebo conditions but not with propranolol. Respiratory exchange ratio decreased significantly over time during the placebo and atenolol trials but did not change during the propranolol trial. These results suggest that the lower submaximal V̇O₂ and the absence of V̇O₂ drift with propranolol were due primarily to its effects on the β₂-receptors, resulting in a greater reliance on carbohydrates for substrate, a lower V̇E, and by blocking the actions of circulating catecholamines.