Beta adrenoceptor antagonism and pressor response to phenylephrine.

Patients on beta-blockers may experience a potentially harmful blood pressure rise after topical alpha-adrenoceptors such as phenylephrine. The presumed mechanism for this enhanced pressor responsiveness is unopposed alpha-adrenoceptor activity. This possibility has been investigated in 12 patients with hypertension by infusing increasing intravenous doses of phenylephrine after 2 wk on propranolol, metoprolol, or placebo within a randomized, double-blind, crossover study design. Mean (+/- SE) phenylephrine doses required to increase systolic blood pressure by 25 mm Hg were 4.8 +/- 0.7, 4.7 +/- 0.8, and 5.3 +/- 0.9 microgram/kg on propranolol, metoprolol, and placebo; these differences are not significant. Baroreceptor-mediated decreases in heart rate during phenylephrine infusion were in the same range, 15.8, 15.6, and 17.4 mm Hg on propranolol, metoprolol, and placebo, over baseline heart rate values. In these 12 patients with hypertension, beta-blockade did not induce any appreciable enhancement of alpha-adrenoceptor activity, suggesting that patients with hypertension who receive beta-blockers do not have enhanced pressor responsiveness.