EXERCISE METABOLISM DURING 1 HOUR OF TREADMILL WALKING WHILE TAKING HIGH AND LOW-DOSES OF PROPRANOLOL, METOPROLOL, OR PLACEBO

Aerobic exercise and beta-blocking drugs are regularly prescribed as t reatment for hypertension and as a prophylactic for patients at risk f rom coronary heart disease and for those recovering from an infarct. S ome beta blockers, particularly non-beta1-selective drugs, may make ex ercise more difficult, possibly by interfering with substrate metaboli sm during exercise. This study examined the effects of low and high do ses of a beta1-selective blocker, metoprolol, and a non-selective beta blocker, propranolol, on exercise metabolism. The study involved 20 h ealthy subjects (10 men, 10 women) who walked on a treadmill at 50% of their maximal oxygen uptake for 1 h on five occasions, separated by 7 days. On each of the five occasions they received one of the followin g treatments, given in random order: placebo, metoprolol 50 mg, metopr olol 100 mg, propranolol 40 mg, or propranolol 80 mg, all taken twice daily. Fat oxidation, expressed as a percentage of total energy expend iture, was significantly lower than with placebo for all of the active treatments except metoprolol 50 mg (placebo: 42.7 +/- 11.6%; metoprol ol 50 mg: 38.7 +/- 14.1%, p = NS; metoprolol 100 mg: 36.3 +/- 13.7%, p = 0.05; propranolol 40 mg: 31.2 +/- 9.3%, p = 0.01; propranolol 80 mg : 29.5 +/- 10.9%, p = 0.01); and significantly lower with propranolol than with metoprolol (propranolol 40 mg: p = 0.0036; propranolol 80 mg : p = 0.01). Plasma ammonia concentration was significantly higher tha n with placebo with propranolol 40 mg, propranolol 80 mg, and metoprol ol 100 mg (p = 0.01 for all); with metoprolol 50 mg, there was no diff erence from placebo (p = NS). Both beta blockers in this study reduced fat metabolism and increased perceived exertion to some degree. Addit ional inhibition of fat oxidation occurred with the nonselective drug, probably in intramuscular rather than adipose lipolysis, and was prob ably beta2 mediated. The results of this study suggest that a selectiv e beta blocker has less of an adverse effect on substrate metabolism t han does a nonselective beta blocker. Beta1-selective drugs may offer advantages in patients who undertake regular aerobic exercise.