Discrepancy between plasma angiotensin converting enzyme activity and in vivo extent of angiotensin I conversion in hypertensive rats

RESUMO

The relationship of plasma angiotensin converting enzyme (ACE) activity to changes in theextent of angiotensin I (ANG I) conversion in vivo in rats with short-term and chronic (8 weeks) hypertension was examined. Plasma ACE activity was measured by a fluorimetric assay and the extent of ANG I conversion was calculated from the equipressor doses of ANG I and ANGII in conscious rats. The extent of ANG I conversion was higher in chronic on-kidney, one clip hypertensive rats than in the normotensive age-matched control rats (64.1 ñ 3.6% vs 34.8 ñ 5.2%), while no difference was found in the ACE activity measured in plasma from both groups. In rats with acute hypertension, produced upon unclamping the renal pedicle occluded for 5hours,the extent of ANG i conversion was increased when compared to the period prior to renal pedicle occlusion (from 38.8ñ4.1% to 68.6ñ8.5%),and plasma ACE activity remained unchanged. These results indicate that circulating ACE cannot be used as an index of ANG I conversionin vivo and support the proposal that tissue ACE is responsible for the augmented ANG I conversion observed in vivo in both acute renal hypertension and chronic one-kidney, one clip hypertension