Effects of various doses of captopril on plasma aldosterone concentrations in patients with essential hypertension

Various doses (5 mg, 12.5 mg and 25 mg) of angiotensin I converting enzyme inhibitor (SQ 14,255, captopril) were administered to 8 patients with essential hypertension on a three-crossover study design, and the time course of mean blood pressure (MBP), plasma renin activity (PRA), plasma angiotensin converting enzyme activity (ACE-A), plasma cortisol (PC) and plasma aldosterone (PA) were determined following administration of the drug. MBP fell in a dose dependent manner, and PRA showed a minor but significant increases in cases receiving 5 and 12.5 mg of the drug. A large and significant increase in PRA was observed following 25 mg of captopril. ACE-A was also reduced in a dose dependent manner. There was no difference between changes in PC at any of the three dose levels. The serum potassium concentration was determined before and 3 hr after 25 mg of captopril treatment and no significant change was observed. In spite of the dose dependent and theoretical changes in the above parameters, lowered responses of PA to each dose of the drug were shown in reverse order against an increasing dose. That is to say, the grade of fall in PA following 25 of captopril was smaller than that following the other doses of the drug, and 5 mg induced a greater decrease in PA than 12.5 mg. Based on these findings, the relatively high dose of captopril in the present study was apparently more effective in increasing some factors which suppressed reduction of PA by a fall in angiotensin II than a low dose of the drug.