Response of the systemic and pulmonary circulation to alpha- and beta-receptor blockade (labetalol) at rest and during exercise in hypertensive patients.

Labetalol (L), a drug with both a-and fl-adrenoceptor blocking properties, was administered to 18 hypertensive patients for an average duration of 2.44 weeks, with an average final daily dose of 1.65 g.L decreased resting heart rate (HR) by 16% and maximal exercise HR by 21%; the phenylephrine-induced rise of brachial artery pressure (BAP) was reduced by 30-40%, and the rise of systemic vascular resistance (SVR) by 50%.L lowered BAP by 29/15 mm Hg in the recumbent position (RR), by 41/23 mm Hg at rest sitting (RS), and by 53/23 mm Hg at maximal exercise; SVR was not significantly affected at RR but was reduced at RS and at exercise; cardiac output (CO) decreased in all conditions.L reduced mean pulmonary artery and capillary wedge pressures only at RS.These hemodynamic observations suggest that the antihypertensive ac- tion of L is based mainly on its f-receptor blocking properties at RR, and on both its a-and f-receptor blocking effects sitting and at exercise.Finally pulmonary vascular resistance (PVR) was not influenced by L, and the phenylephrine-induced increase of PVR was unaffected; the pulmonary arterioles thus seem to react differently to L than the systemic arterioles.