Effect of labetalol on plasma catecholamines at rest and during increased sympathetic activity.

Mean arterial blood pressure was decreased by intravenous labetalol in 14 hypertensive patients, resting in the supine position, when standing, during exercise and during psychic stress. Heart rate was unchanged during psychic stress and while resting in the supine position, whereas it fell in the other situations. Plasma noradrenaline increased in all situations after labetalol compared to control studies. Plasma adrenaline only increased when standing and during stress, and there was no correlation between changes in plasma noradrenaline and adrenaline. Blood glucose rose significantly after labetalol in the supine position, whereas it only tended to increase in the other experimental situations. The changes in plasma noradrenaline and blood glucose induced by labetalol are identical to those seen after pure alpha-adrenoceptor blockade. The beta-adrenoceptor blocking property of labetalol may contribute to the higher plasma concentration of noradrenaline when standing and during exercise as this has been found during pure beta-adrenoceptor blockade.