THE EFFECTS OF BETA-ADRENERGIC BLOCKADE ON EXERCISE CAPACITY AND THERMOREGULATION IN TRAINED AND UNTRAINED SUBJECTS.

Abstract

Two investigations were conducted to examine the influence of beta-adrenergic blockade on cardiovascular, respiratory, metabolic, and thermoregulatory responses to maximal and submaximal exercise in both highly trained and untrained subjects. In both studies, subjects received randomized and double-blind oral medication with atenolol (100 mg/day), propranolol (160 mg/day), and placebo. In the first study significant reductions in HR max and ‘VO₂ max resulted during the atenolol and propranolol treatments in both the trained and untrained subjects. However, the reductions in ‘VO₂ max were significantly greater in the trained subjects and both groups experienced their greatest reduction during the propranolol treatment. In all subjects, the magnitude of reduction in HR max was significantly greater than the concomitant decrease in ‘VO₂ max. It is concluded that untrained subjects have a greater compensatory reserve than do trained subjects during maximal exercise while under beta-adrenergic blockade. In addition, significant advantages were found with the use of a selective compared to a non-selective beta blocker. Thermoregulation during prolonged exercise in the heat with beta blockade was studied in fourteen subjects. Subjects performed 90-minute cycle ergometer rides at a workload equivalent to 40% of the subjects' unblocked ‘VO₂ max. Rectal temperature was slightly higher during the atenolol trial compared to the placebo but was not different during the propranolol trial compared to the placebo. Skin blood flow was significantly lower during the propranolol trial compared to both the atenolol and placebo trials, but it did not differ significantly between the atenolol and placebo trials. Maintenance of rectal temperatures appeared to be achieved through changes in sweat rate, skin blood flow, and a reduced heat production, i.e., lower ‘VO₂ during the propranolol trial. The decrease in cutaneous blood flow reported during the propranolol trial is likely associated with the associated increase in TPR. This increase in TPR would help to compensate for the lower ‘Q and, hence, help maintain mean arterial pressure. Changes in substrate utilization, i.e., decreased lipolysis, during the beta-blocked trials may also be indicated. Lastly, the inability of two subjects to complete the 90-minute ride, the elevated RPE values, and the additional side effects reported during the propranolol trial would indicate an advantage for the use of a selective blocker.