[Significance of angiotensin converting enzyme (ACE) in coronary circulation]

It has been suggested that angiotensin II can be synthesized by other enzymatic pathways besides angiotensin converting enzyme. We evaluated the importance of angiotensin converting enzyme in the coronary circulation during the development of hypertension. Hearts obtained from normotensive (n = 4) and hypertensive rats (n = 4) as well as from hypertensive rats treated with ramipril (n = 4) were stimulated with either angiotensin II or angiotensin I. In a Langendorff perfusion system, angiotensin II induced a greater dose-dependent coronary vasoconstriction in the hearts of hypertensive rats than in normotensive rats (p < 0.05). Furthermore, angiotensin I also induced coronary vasoconstriction, which was greater in the hearts of hypertensive rats than in normotensive rats (p < 0.05). Acute angiotensin converting enzyme inhibition reduced angiotensin I-induced vasoconstriction by 78% in the hearts of normotensive rats and by 82% in the hypertensive rats (p < 0.05), whereas in vivo angiotensin converting enzyme inhibition potentiated angiotensin I-induced vasoconstriction in the hearts of normotensive and hipertensive rats (p < 0.05). Bradykinin receptor's blockade decreased ramiprilat's inhibitory effect on angiotensin I-induced vasoconstriction (p < 0.05). Thus, the present study suggests that, in coronary circulation, angiotensin II synthesis is mainly angiotensin converting enzyme dependent. However, chronic in vivo inhibition could favor induction of other enzymes involved in angiotensin II synthesis. Evenmore, it is possible that the effect of angiotensin converting enzyme inhibition in coronary circulation depends on bradykinin activity.