MEASUREMENT OF ANGIOTENSIN-I CONVERTING-ENZYME INHIBITION IN THE HEART

Angiotensin (Ang) I converting enzyme (ACE) inhibitors represent a maj or advance in the treatment of congestive heart failure, and tissue, r ather than circulating ACE, may be their major site of action. However , assessments of tissue ACE inhibition in treated patients has not alw ays supported this contention. In these studies, ACE activity was meas ured in homogenates of sampled tissue by biochemical methods. In the p resent study, using a model system, we have examined the validity of t hese tissue-sampling methods. Functional ACE activity was determined b y comparing positive inotropic responses to [Pro10]Ang I in either veh icle-pretreated or ACE inhibitor-pretreated papillary muscles. [Pro10] Ang I elicits a response, which is entirely dependent on ACE-mediated conversion to Ang II. The ACE inhibitors studied were captopril, enala prilat, lisinopril, and quinaprilat. In a parallel study, papillary mu scle ACE activity was also measured in homogenates using [I-125]MK-351 A (a radiolabeled ACE inhibitor) binding. The studies indicate that th e tissue-sampling method significantly underestimated functional ACE i nhibition in hamster papillary muscles (p < 0.001). Kinetic studies in dicated that the half-time for the dissociation of [H-3]enalaprilat an d [H-3]lisinopril from hamster ventricular ACE was 4.5 and 6.2 minutes , respectively. The dissociation of [H-3]quinaprilat was biphasic (hal f-time, 47 and 90 minutes), indicating that the two active sites of so matic ACE differ in their ability to bind to this inhibitor. The rapid rate of ACE inhibitor dissociation suggests that, during the time tak en to assay ACE activity biochemically, the enzyme becomes ''disinhibi ted,'' leading to an underestimation of functional ACE inhibition. ACE inhibitor dissociation rates were partially predictive of the duratio n of functional ACE inhibition in papillary muscles; other factors tha t appeared to contribute were ''tissue trapping'' of the inhibitor and de novo synthesis of ACE in papillary muscles. Quantification of tiss ue ACE inhibition and its relation to drug efficacy must, therefore, i nvolve a careful consideration of these factors to avoid artifacts in clinical decision making and in assessments of pathogenic mechanisms i nvolved in congestive heart failure.