Alpha adrenoceptor blockade with oral prazosin

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Abstract
.alpha.-Adrenoceptor blockade after placebo and 5 mg oral doses of prazosin [an antihypertensive] was assessed in 5 normal subjects over a range of phenylephrine infusion rates. Systolic blood pressure during a 400 .mu.g/min infusion of phenylephrine at 3 h 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 h to at least 7 h after prazosin. The drug-sensitivity ratio, an indicator of .alpha.-adrenoceptor blockade, before prazosin was 1.1, rising to 4 at 1 h, 4.9 at 3 h, 5.7 at 5 h, and 4 at 7 h after oral prazosin. The highest prazosin plasma concentration after a 5 mg oral dose was 29.3 .+-. 6.6 ng/ml occurring 1 h after prazosin. The elimination half-life was 2.9 .+-. 0.3 h. After oral prazosin the .alpha.-adrenoceptor blockade response did not correlate with prazosin plasma concentration.