1. The haemodynamic effects of 100 mg of captopril in renovascular hypertension (n = 11), essential hypertension (n = 12) and the anephric state (n = 7) were compared. Brachial artery pressure was measured in all patients, and changes in right atrial pressure, pulmonary artery pressure, pulmonary capillary wedge pressure and cardiac output were followed in renovascular and essential hypertension. Nephrectomized patients were studied before and after fluid withdrawal by ultrafiltration.

2. The pretreatment concentration of active renin in plasma was 100 ± 24 μ-units/ml (mean ± sem) in renovascular hypertension, and 24 ± 4 μ-units/ml in essential hypertension. In nephrectomized patients pretreatment renin was 1.7 ± 0.3 μ-units/ml, and renin was unresponsive to withdrawal of 1.8 ± 0.2 1 of body fluid.

3. The effects of captopril were maximal after 60–90 min. Mean arterial pressure after 90 min was lowered by 19 ± 4% in renovascular hypertension, by 17 ± 4% in essential hypertension and by 16 ± 3% in fluid-depleted nephrectomized patients. These changes were not significantly different despite the marked differences in renin. Captopril had no effect on arterial pressure in the fluid-replete anephric state.

4. The effects of captopril on cardiac filling pressures and cardiac output in renovascular and essential hypertension were also not different.

5. It is concluded that the antihypertensive action of captopril may be largely independent of circulating renin.

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