1. The converting-enzyme inhibitor captopril (25–50 mg orally every 6 h for 66 h) was used to dissociate the circulating levels of angiotensin II (ANG II) from changes in sodium balance in 11 patients with normal renin essential hypertension on 10 mmol of sodium/day intake. Pressor, renal vascular and adrenal responses to graded infusions of ANG II (0.3, 1 and 3 pmol kg−1 min−1) were measured before and after captopril administration. Systemic vascular responses were assessed by measuring diastolic blood presusre (DBP), renovascular responses by measuring p-aminohippurate (PAH) clearance and adrenal responses by measuring plasma aldosterone.

2. After receiving captopril for 66 h the hypertensive subjects showed a significantly (P<0.004) enhanced blood pressure response to the infused ANG II but not to noradrenaline when compared with the response before captopril. ANG II (3 pmol kg−1 min−1) also produced a significantly (P<0.03) greater reduction in PAH clearance after (−194 ± 40 ml/min) compared with before (−104 ± 15 ml/min) captopril. These results suggest that the responsiveness to ANG II in these two target tissues is determined by the circulating ANG II level.

3. In the adrenal gland the aldosterone responses to ANG II also were significantly greater after (P < 0.01) than before captopril (increment at 3 pmol kg−1 min−1: 660 ± 88 vs 381 ± 94 pmol/l). These results are in distinct contrast with the responses previously reported for normotensive subjects and support the hypothesis that the regulation of aldosterone secretion is altered in subjects with essential hypertension.

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