Alpha adrenoceptor blockade with oral prazosin.

alpha-Adrenoceptor blockade after placebo and 5-mg oral doses of prazosin was assessed in five normal subjects over a range of phenylephrine infusion rates. Systolic blood pressure during a 400-microgram/min infusion of phenylephrine at 3 hr after placebo or prazosin was 172.8 +/- 6.2 and 108.7 +/- 4.4 mm Hg (p less than 0.05). The phenylephrine effects were blocked from 1 hr to at least 7 hr after prazosin. The drug-sensitivity ratio, an indicator of alpha-adrenoceptor blockade, before prazosin was 1.1, rising to 4 at 1 hr, 4.9 at 3 hr, 5.7 at 5 hr, and 4 at 7 hr after oral prazosin. The highest prazosin plasma concentration after a 5-mg oral dose was 29.3 +/- 6.6 ng/ml. It occurred 1 hr after prazosin. The elimination half-life was 2.9 +/- 0.3 hr. After oral prazosin the alpha-adrenoceptor blockade response did not correlate with prazosin plasma concentration.