Folic Acid Prevents Nitroglycerin-Induced Nitric Oxide Synthase Dysfunction and Nitrate Tolerance

Abstract

Background-In healthy humans, continuous treatment with nitroglycerin (GTN) causes nitric oxide synthase dysfunction, probably through the reduced bioavailability of tetrahydrobiopterin. Recent studies proposed that folic acid is involved in the regeneration of tetrahydrobiopterin in different disease states. Therefore, we investigated whether folic acid administration would prevent this phenomenon. We also sought to determine if folic acid supplementation could prevent the development of tolerance to GTN. Methods and Results-On the first visit, 18 healthy male volunteers (aged 19 to 32 years) were randomized to receive either oral folic acid (10 mg once a day) or placebo for 1 week in a double-blind designed study. All subjects also received continuous transdermal GTN (0.6 mg/h). On the second visit, forearm blood flow was measured with venous occlusion strain gauge plethysmography in response to incremental infusions of acetylcholine (7.5, 15, and 30 g/min), N-monomethyl-L-arginine (1, 2, and 4 mol/min), and GTN (11 and 22 nmol/min). Folic acid prevented GTN-induced endothelial dysfunction, as assessed by responses to intraarterial acetylcholine and N-monomethyl-L-arginine (PϽ0.01). Moreover, in the subjects treated with folic acid plus transdermal GTN, responses to intraarterial GTN were significantly greater than those observed after transdermal GTN plus placebo (PϽ0.05). Conclusion-Our data demonstrate that supplemental folic acid prevents both nitric oxide synthase dysfunction induced by continuous GTN and nitrate tolerance in the arterial circulation of healthy volunteers. We hypothesize that the reduced bioavailability of tetrahydrobiopterin is involved in the pathogenesis of both phenomena. Our results confirm the view that oxidative stress contributes to nitrate tolerance.

Roles of Superoxide, Peroxynitrite, and Protein Kinase C in the Development of Tolerance to Nitroglycerin

“…Therefore, antioxidants and L-arginine, by reducing ONOO Ϫ levels or formation, can be expected to promote normal coupling of eNOS by the further action of maintaining BH 4 levels. This may partially explain a recent observation that folic acid prevents tolerance to GTN (Gori et al, 2001). In addition to being a precursor for BH 4 synthesis and a necessary cofactor for proper NOS function, folic acid and BH 4 are also antioxidants (Nakamura et al, 2001;Verhaar et al, 2002).…”

The Puzzle of Nitrate Tolerance

“…We documented that supplemental folic acid is able to prevent the development of both NOS dysfunction and nitrate tolerance as assessed by forearm plethysmographic measures of arterial blood flow. 28 The mechanism of this effect remains unknown, but a number of possibilities can be proposed, including: (1) a folate-mediated increase in tetrahydrobiopterin regeneration with subsequent NOS recoupling 29,30 ; (2) inhibition of the other sources of ·O 2 Ϫ , such as xanthine and/or NADPH membrane oxidases 29,30 ; (3) depletion of NAD(P)H reserves 31 ; (4) direct antioxidant effect 32 ; and, finally (5) direct substitution of tetrahydrobiopterin as a cofactor for NOS. 33 Whatever the mechanism, it appears that, during GTN treatment, NOS uncoupling might be both cause and effect (via tetrahydrobiopterin oxidation) of the described increase in vascular ·O 2 Ϫ generation.…”

Insulin resistance and endothelial function are improved after folate and vitamin B12 therapy in patients with metabolic syndrome: relationship between homocysteine levels and hyperinsulinemia

“…A direct effect of folic acid in keeping BH4 functionally intact to maintain a coupled eNOS reaction has been demonstrated (10). In addition, folic acid was able to prevent NO synthase dysfunction induced by nitrate tolerance in the arterial circulation of healthy subjects (11) and pretreatment with oral folic acid prevented the endothelial dysfunction induced by acute hyperlipidemia following an oral fat load (12).…”