Long-term Angiotensin-Converting Enzyme Inhibition With High-Dose Enalapril Retards Nitrate Tolerance in Large Epicardial Arteries and Prevents Rebound Coronary Vasoconstriction In Vivo

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Abstract
Background Rebound myocardial ischemia develops in patients with unstable or stable angina pectoris after sudden cessation of nitroglycerin therapy. Long-term nitroglycerin infusion is associated with increases in plasma renin activity and catecholamine release rates, both of which may lead to excess angiotensin II and α-adrenergic–mediated vasoconstriction, particularly on withdrawal of nitroglycerin. Methods and Results Chronically instrumented dogs were treated for 5 days with nitroglycerin (1.5 μg·kg −1 ·min −1 IV) alone or in combination with the angiotensin-converting enzyme (ACE) inhibitor enalapril (0.1 mg/kg two times daily or 1 mg/kg). With long-term nitroglycerin therapy, the left anterior circumflex artery was maximally dilated 4 hours after the start of nitroglycerin infusion (9.5±0.6%) and returned to baseline levels within the third day of treatment (baseline, 2.52±0.07 mm; day 3, 2.55±0.07 mm; P =NS), indicating a complete loss of nitroglycerin-induced coronary vasodilatation. Nitroglycerin infusion also was accompanied by a transient increase in plasma renin activity. Sudden withdrawal of nitroglycerin infusion caused a progressive constriction of the left anterior circumflex artery, which peaked 4 hours after nitroglycerin infusion cessation (−7.8±0.2%). This occurred in the absence of elevated plasma renin activity. Concomitant treatment with high-dose enalapril (1 mg·kg −1 ·d −1 ) markedly reduced the degree of tolerance and prevented the rebound constriction on cessation of nitroglycerin therapy. Conclusions Long-term ACE inhibition with high-dose enalapril reduces nitroglycerin tolerance and prevents rebound vasoconstriction in coronary arteries. These phenomena were not associated with an activated circulating renin-angiotensin system. This observation suggests that during long-term nitroglycerin treatment, intrinsic abnormalities of the vascular smooth muscle may have developed that are suppressed by concomitant ACE inhibitor therapy. The present study also favors a combination of nitroglycerin and ACE inhibitors to maintain nitrate sensitivity of the vasculature during long-term nitroglycerin treatment.

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