Abstract
Plasma propranolol levels associated with reductions in endogenous and exogenous cardiac beta-stimulation were determined in normal people. The levels associated with a given degree of blockade of exercise-induced tachycardia were about three times greater after intravenous administration than after oral administration. This shows that an active metabolite of propranolol is formed only after the drug is taken by mouth. The greatest reduction in the tachycardia of strenuous exercise was associated with plasma levels of 40 ng./ml. with oral administration and 100 ng./ml. with intravenously administered propranolol.The effect on isoprenaline-induced tachycardia following intravenously administered propranolol showed that the dose ratio for isoprenaline was about 30 with plasma levels of 100 ng./ml. and 10 with levels of 10-20 ng./ml. These plasma levels give 100% and 20-30% blockade of exercise-induced tachycardia. These findings suggest that some of the therapeutic effects of propranolol may be unrelated to beta-adrenergic blockade.