Alpha adrenoceptor blockade with oral prazosin

α‐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‐µg/min infusion of phenylephrine at 3 hr after placebo or prazosin was 172.8 ± 6.2 and 108.7 ±4.4 mm Hg (p < 0.05). The phenylephrine effects were blocked from 1 hr to at least 7 hr after prazosin. The drug‐sensitivity ratio, an indicator of α‐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 α‐adrenoceptor blockade response did not correlate with prazosin plasma concentration. Clinical Pharmacology and Therapeutics (1981) 29, 143–148; doi: 10.1038/clpt.1981.23