Richards D A, Tuckman J, Prichard B N
Br J Clin Pharmacol. 1976 Oct;3(5):849-55. doi: 10.1111/j.1365-2125.1976.tb00637.x.
Isoprenaline dose-response curves plotting increases in heart rate before and after labetalol are suggestive of competitive antagonism at beta-adrenoceptor sites. Phenylephrine dose-response curves using increases in systolic pressure before and after labetalol are suggestive of competitive antagonism at alpha-adrenoceptor sites. The ratio of alpha:beta-adrenoceptor antagonism induced by labetalol is approximately 1:3. Peak pharmacological responses after a single oral dose of labetalol (400 mg) occurred between 90-120 min after administration.