Why use angiotensin converting enzyme inhibitors to lower blood pressure?

Captopril given for 5 days to normotensive subjects caused a significant fall in blood pressure. The fall in blood pressure was greater on a low sodium diet (19.6%) and less on a high sodium diet (11%) whereas on a normal sodium intake it was 16.5%. This fall in blood pressure was related to the initial level of angiotensin II and the fall in angiotensin II and was also associated with large falls in plasma aldosterone and, on the normal sodium intake, a loss of sodium that amounted to approximately 140 mmol/normal subject studied over the 5 days. Patients with essential hypertension who were studied on their normal diets had a similar fall in blood pressure for a given plasma renin activity. Angiotensin converting enzyme inhibitors are therefore used in hypertension not because they are specific for patients with high blood pressure but because they are effective in lowering blood pressure in both normotensive and hypertensive subjects and cause few side effects. They are also particularly effective when used in conjunction with sodium restriction, diuretics, or calcium entry antagonists but less effective when combined with a beta-blocker.