ANTI-HYPERTENSIVE ACTION OF LABETALOL - A DETAILED PROFILE

Continuous ambulatory blood pressure (BP) and self measured home recordings were used to study the response of 10 mild to moderate hypertensives taking a twice daily labetalol dosage. Testing of cardiovascular reflexes was carried out 12 h after a dose of drug. The mean 24 h BP was reduced from 145/80 .+-. 14/7 (SD) mm Hg to 130/71 .+-. 11/6 (P < 0.001) and home BP from 147/96 .+-. 15/9 to 134/84 .+-. 10/9 (P < 0.01). The predominant effect of labetalol was evident during the waking hours. A residual 9.5% BP reduction was seen 12 h after a dose compared to an 11% lowering effect over the entire 24 h (NS). No symptomatic hypotensive episodes were noted. Significant inhibition of heart rate and BP rise during dynamic exercise (P < 0.01) and reduction of post-release BP overshoot (P < 0.01) in Valsalva''s maneuver indicated a .beta.-adrenoceptor blocking effect. There was a reduction in the rate of rise of BP during static handgrip (P < 0.01) indicating a small .alpha.-adrenoceptor blocking component. This study has demonstrated the efficacy of a twice daily regimen of labetalol in continuously lowering BP. The mode of action of the drug during exercise (static and dynamic) and Valsalva''s maneuver would indicate a predominant .beta.-adrenoceptor blocking action.