Angiotensin-converting enzyme inhibitors have beneficial effects that are presumably mediated by decreased angiotensin II (ANG II) production. In this study, we measure for the first time ANG I and ANG II levels in the interstitial fluid (ISF) space of the heart. ISF and aortic plasma ANG I and II levels were obtained at baseline, during intravenous infusion of ANG I (5 M, 0.1 ml/min, 60 min), and during ANG I ϩ the angiotensin-converting enzyme inhibitor captopril (

Background-We have previously demonstrated that angiotensin II (Ang II) levels in the interstitial fluid (ISF) space of the heart are higher than in the blood plasma and do not change after systemic infusion of Ang I. In this study, we assess the enzymatic mechanisms (chymase versus ACE) by which Ang II is generated in the ISF space of the dog heart in vivo. Methods and Results-Cardiac microdialysis probes were implanted in the left ventricular (LV) myocardium (3 to 4 probes per dog) of 12 anesthetized open-chest normal dogs. ISF Ang I and II levels were measured at baseline and during ISF infusion of Ang I (15 mol/L, nϭ12), Ang Iϩthe ACE inhibitor captopril (cap) (2.5 mmol/L, nϭ4), Ang Iϩthe chymase inhibitor chymostatin (chy) (1 mmol/L, nϭ4), and Ang Iϩcapϩchy (nϭ4). ISF infusion of Ang I increased ISF Ang II levels 100-fold (PϽ0.01), whereas aortic and coronary sinus plasma Ang I and II levels were unaffected and were 100-fold lower than ISF levels. Compared with ISF infusion of Ang I alone, Ang Iϩcap (nϭ4) produced a greater reduction in ISF Ang II levels than did Ang Iϩchy (nϭ4) (71% versus 43%, PϽ0.01), whereas Ang Iϩcapϩchy produced a 100% decrease in ISF Ang II levels. Conclusions-This study demonstrates for the first time a very high capacity for conversion of Ang I to Ang II mediated by both ACE and chymase in the ISF space of the dog heart in vivo. (Circulation. 1999;99:2583-2589.)

Angiotensin-converting enzyme (ACE) inhibitors (I) have beneficial effects that are presumably mediated by decreased angiotensin II (ANG II) production. However, in vitro assays in human heart extracts have demonstrated that > 75% of ANG II-forming enzyme activity was not inhibited by captopril (Cap) and therefore did not appear to be related to ACE but was inhibited by chymostatin, suggesting that it was predominantly chymase-like activity. Previous work in our laboratory has demonstrated a similar relative contribution of ACE and chymase-like activity toward ANG II formation in vitro in dog heart tissue extracts. Accordingly, we compared Cap-inhibitable ANG II formation in vitro in heart tissue of five adult mongrel dogs to the in vivo Cap-inhibitable, ANG II-forming activity across the myocardial bed in four openchest, adult mongrel dogs. In vitro studies demonstrated that only 6 +/- 2% of ANG II formation was inhibited by Cap from heart tissue extracts of the left ventricular midwall. In in vivo studies, ANG I (0.5 nmol/min) followed by ANG I plus the ACE inhibitor Cap (0.1 mumol/min) was infused into the left anterior descending artery, and ANG II was assayed in the proximal aorta and coronary sinus. The arterial-venous (A-V) difference of ANG II across the myocardial circulation increased significantly during ANG I infusion (-13.4 +/- 23.5 to 142.8 +/- 71.4 pg/ml; P < 0.03). Subsequent coinfusion of Cap with ANG I significantly decreased the myocardial A-V difference of ANG II by 60 +/- 18% (P < 0.05). Thus, in contrast to the in vitro situation, ANG II formation in vivo is inhibited significantly by Cap in the normal dog heart. This comparison of in vivo and in vitro conversion of ANG I to ANG II by ACE and chymase-like activity suggests that in vitro assays may underestimate the functional contribution of ACE to intracardiac ANG II formation.