Attenuation of exercise conditioning by beta-adrenergic blockade

Abstract

High levels of β-adrenergic stimulation accompany strenuous exercise, but the possibility that β-adrenergic blockade might prevent exercise conditioning has not been adequately investigated. We studied normal, sedentary men, ages 21-35 years, before and after 5 weeks of intensive aerobic conditioning. On a double-blind protocol, eight received placebo and nine propranolol throughout the conditioning period. A high level of β-adrenergic blockade was documented in all subjects receiving propranolol; individual mean plasma propranolol concentrations were 100-292 ng/ml. Both groups trained at comparable intensities. Graded maximal treadmill tests were performed before starting drugs or training, and were repeated 3-5 days after completing the conditioning period, when β-adrenergic blockade was no longer present. In subjects who received placebo, training increased exercise duration (16.4 1.3 to 21.2 ± 1.5 minutes [± SEM], p < 0.01) and maximal oxygen uptake (43.6 ± 2.9 to 52.7 ± 3.2 ml/kg/min, p <0.05). Subjects who received propranolol had only modest improvement in exercise duration (16.0 ± 0.6 to 17.3 ± 0.9 minutes, p <0.05), and no significant change in maximal oxygen uptake (40.4 ± 1.4 to 40.9 ± 0.9 ml/kg/min). With training, diastolic pressure at maximal exercise decreased in subjects who received placebo (63 ± 3 to 48 ± 3 mm Hg, p <0.05) but was unchanged in subjects who received propranolol. Training did not alter maximal heart rate in either group. Thus, high levels of β-adrenergic blockade marked by attenuated aerobic conditioning in these normal subjects. We conclude that β-adrenergic stimulation is essential in exercise conditioning.