1. Carbohydrate and lipid metabolism and the capacity to perform prolonged submaximal physical exercise were studied in six young healthy subjects treated in a randomized double-blind fashion for 2 days with either placebo, the non-selective β-adrenoceptor antagonist propranolol (80 mg b.i.d.) or the cardioselective agent metoprolol (100 mg b.i.d.). On day 3, 1 h after the last dose, the subjects exercised for 30 min periods followed up 10 min rest up to the point of exhaustion.
2. The capacity to perform exercise was decreased with both β-adrenoceptor antagonists. However, at an equal degree of β1-adrenoceptor blockade, all subjects could exercise for a longer period of time on the cardioselective agent as compared with the non-selective drug.
3. Blood glucose levels decreased during exercise irrespective of the type of treatment, but the attenuation occurred most rapidly on propranolol. At exhaustion the average non-esterified fatty acid levels had increased 256% on placebo, 148% on metoprolol and 65% on propranolol. A significant positive correlation was found between changes in non-esterified fatty acid levels during exercise and total working time. It is concluded that β-adrenoceptor blockade diminishes the capacity for prolonged submaximal exercise at least in part by reducing the availability of substrates to the working muscles.