DISTRIBUTION AND FUNCTIONAL-SIGNIFICANCE OF CARDIAC ANGIOTENSIN-CONVERTING ENZYME IN HYPERTROPHIED RAT HEARTS

Background. The intracardiac conversion rate of angiotensin (Ang) I to Ang II and the expression of angiotensin converting enzyme (ACE) mRNA are amplified in rat hearts with left ventricular hypertrophy (LVH). To examine whether the accelerated intracardiac Ang II generation in LVH is related to an induction of cardiac ACE, we studied localization and function of cardiac ACE in hypertrophied rat hearts using specific ACE inhibitors. Methods and Results. Cardiac ACE was localized and quantified in hearts from male Wistar rats with LVH due to chronic experimental aortic stenosis and from control rats. With the ACE inhibitor I-125-351A, a derivative of lisinopril, as a radioligand on coronal sections of LVH and control hearts, in vitro autoradiography demonstrated ACE binding in aorta, coronary arteries, atria, and ventricles of both groups. Quantitative analyses revealed that ACE density (counts per minute per cross-sectional area of tissue) was twofold higher within the myocardium of hypertrophied left ventricles compared with controls (p<0.005). Quantitative morphometry demonstrated a modest increase in the fractional volume of myocytes as well as capillary volume without an increase in the fractional volume of endothelial cells in left ventricular tissue from aortic stenosis rats. These data suggest that an increase in endothelial cell volume per se cannot alone account for the observed doubling of ACE density and support an upregulation of ACE production in hypertrophied tissue. The role of cardiac ACE in intracardiac conversion of Ang I to Ang II and its specific inhibition was studied in isolated, isovolumic beating, buffer-perfused LVH and control hearts. Biochemical conversion rates as well as functional changes in response to 3 x 10(-7) M Ang I were examined in the absence or presence of the ACE inhibitor enalaprilat (4 x 10(-6) M). After a brief stabilization period, groups of LVH and control hearts were subjected to the following infusion protocols: 15 minutes of vehicle followed by 30 minutes of Ang I plus vehicle, 15 minutes of enalaprilat followed by 30 minutes of Ang I plus enalaprilat (enal/Ang I), or 45 minutes of vehicle only to allow comparison with a time control. Intracardiac Ang I-to-Ang II conversion rate was fourfold higher in LVH than in control hearts (p<0.05). Infusion of enalaprilat reduced the intracardiac Ang I-to-Ang II conversion rate in LVH hearts by 70% (p < 0.05 versus Ang I). At similar levels of constant coronary flow per gram, Ang I increased coronary perfusion pressure by 23+/-5 mm Hg (p<0.01 versus vehicle) in LVH hearts and by 36+/-10 mm Hg (p<0.005 versus vehicle) in control hearts. When enalaprilat was infused with Ang I, the increase in perfusion pressure was limited to 5+/-5 mm Hg (NS versus vehicle) in LVH hearts and 12+/-3 mm Hg (p<0.05 versus vehicle) in control hearts and was significantly lower than in hearts infused with Ang I only (p<0.05 in LVH and p<0.05 in control hearts, respectively). Systolic function was not affected by either infusion protocol. In contrast, Ang I infusion was associated with diastolic dysfunction. In LVH hearts, left ventricular end-diastolic pressure (LVEDP) increased from 10+/-1 mm Hg at baseline to 25+/-2 mm Hg at the end of the Ang I infusion (p<0.001 versus vehicle), which was inhibited by infusion of enalaprilat. In control hearts, there was a lesser increase in LVEDP from 10+/-I mm Hg to 15+/-l mm Hg in response to Ang I (p <0.05 versus LVH). Control hearts treated with enalaprilat with Ang I displayed no increase in LVEDP. Conclusions. These observations indicate that ACE protein is increased within the myocardium of LVH hearts, extending recent findings of increased cardiac ACE activity and mRNA levels in this model of pressure-overload LVH in the rat. Blockade of the enzyme by an ACE inhibitor decreases intracardiac Ang I-to-Ang II conversion rate and prevents the functional changes of Ang I-to-Ang II activation otherwise seen in LVH hearts. Taken together, these data suggest that ACE is a key enzyme involved in the enhanced conversion of Ang I to Ang II in the intact hypertrophied rat heart.