Absence of nitrate tolerance after long-term treatment with ramipril: An endothelium-dependent mechanism

To determine whether nitrate tolerance is attenuated on aortas isolated fro m rats treated in the long term with an angiotensin-converting enzyme (ACE) inhibitor, five groups of rats were studied in parallel. Group 1 received ramipril, 1 mg/kg/day, p.o., for 6 weeks; group 2 received ramipril at the same dose for 4 weeks, and the last 2 weeks, a cotreatment with ramipril pl us HOE 140 (a bradykinin B-2, antagonist, 500 mu g/kg/day, s.c. injections) ; group 3 received losartan, 2 mg/kg/day, p.o,, for 6 weeks; group 4 receiv ed losartan at the same dose, and the last 2 weeks, a cotreatment with losa rtan plus HOE 140; and group 5 served as control. Rings of thoracic aorta f rom these groups were studied in organ baths. After nitroglycerin preincuba tion (10 mu M for 30 min) in vitro, the dose-response curves to nitroglycer in were significantly shifted to the right in the control group but not in group 1. This protective effect was partially present in group 3; it was co mpletely abolished in groups 2 and 4. In groups 1 and 3, it also was abolis hed after nitric oxide synthase (cNOS) inhibition (L-NMMA incubation) or re moval of the endothelium. Superoxide anion accumulation (assessed by lucige nin chemiluminescence) was increased by nitroglycerin incubation in the con trol group but not in groups 1 and 3. After in vivo exposure to nitroglycer in (50 mg/kg subcutaneously twice daily for 4 days), this protection agains t nitrate tolerance also was observed in groups 1 and 3. Thus long-term ACE inhibition prevents nitrate tolerance by an endothelium-dependent mechanis m involving mainly an enhanced NO availability via B-2-kinin receptor. This effect on the cNOS pathway seems to attenuate the superoxide anion accumul ation induced by nitroglycerin exposure (probably via a downregulation of o xidative enzyme).