Effects of prazosin on blood pressure and plasma renin activity during onset and withdrawal of action in the anaesthetized rat.

1. The effects of single intramuscular injections of prazosin, 0.1 mg/kg, upon blood pressure, heart rate and plasma renin activity, were studied, over 24 h, in groups of anaesthetized rats, control rats receiving 0.9% saline. 2. Within 6 min of prazosin administration, heart rate fell by 50 beats/min (s.e.m = 19) and blood pressure by 40 mmHg (s.e.m. = 8). Starting levels were regained within 90 min and 16 h of injection, respectively, and thereafter both parameters remained unaltered. Plasma renin activity showed a slight though insignificant rise during the initial hypotension, but subsequently fell to 32% of the control level. 3. The hypotensive response to prazosin, unlike that to the centrally acting agents clonidine or guanfacine, was not followed by overshoots in blood pressure, heart rate or plasma renin activity. Instead, suppression of plasma renin activity was demonstrable after re-establishment of control blood pressure.