To examine the effect of chronic administration of angiotensin I-converting enzyme (ACE) inhibitor on circulating angiotensin II (AII) concentration, 20 mg of lisinopril was administered once daily for 7 consecutive days to eight healthy volunteers. 2. Plasma ACE activity was inhibited to less than approximately 30% of the pretreatment level during the repeated administration. 3. Mean arterial pressure (MAP) was slightly but significantly reduced during the administration period. Plasma AII concentration measured by an established method using high performance liquid chromatography combined with a radioimmunoassay, however, was maintained at approximately the pretreatment level when it was measured at 24 h intervals after each administration of lisinopril. 4. With the gradual recovery of ACE activity following discontinuation of administration, the plasma AII concentration correlated with AI concentration (r = 0.46), and also with the product of AI and ACE activity (AI x ACE; r = 0.80), corresponding to the formula obtained from the kinetics of ACE activity. No correlation was observed between MAP and AII levels throughout the study period. 5. We conclude that in normal subjects repeatedly administered with ACE inhibitor, the AII level in the circulation is still determined by an elevated level of AI and any remaining ACE activity, thus maintaining AII at pretreatment levels. We confirmed that it is not necessary to achieve a decrease in plasma AII concentration through the chronic administration of ACE inhibitor in order to effectively lower blood pressure.
