Beta blockade by oral propranolol and labetalol.

beta-Adrenergic blockade after single, oral doses of labetalol or propranolol was evaluated in a double-blind, placebo-controlled study by an isoproterenol sensitivity test and handgrip exercise in 10 healthy men. The tests were performed with subjects resting in a supine position. At the doses used, there was no effect on heart rate and blood pressure in either the resting position or in the isometric exercise phase. It is possible that exercise-induced changes in blood pressure and heart rate were reduced by higher vagal tone in this young group tested in the supine position. Isoproterenol increased heart rate and reduced diastolic blood pressure in a dose-dependent manner. The dose of isoproterenol at which heart rate increased 25 bpm above the resting rate (CD25; 1.36 +/- 0.18 microgram) was of the order of that at which diastolic blood pressure fell 25 mm Hg from baseline (HD25; 1.07 +/- 0.07 microgram). There was a significant positive correlation between CD25 and HD25 in the 10 subjects. Propranolol and labetalol induced a dose-dependent, parallel shift to the right in the dose-response curves of isoproterenol effects on heart rate and diastolic blood pressure, indicating that both drugs are nonselective, competitive antagonists of beta-adrenergic receptors. On the average, propranolol was 17 and 19 times more potent than labetalol in antagonizing the chronotropic and hypotensive actions of isoproterenol, respectively.