Ascorbic acid and tetrahydrobiopterin: looking beyond nitric oxide bioavailability.

Endothelial dysfunction, manifested as reduced vasodilatory capacity, occurs in arteries exposed chronically to cardiovascular risk factors. Hyperglycaemia, hypercholesterolaemia, hypertension, ageing, and smoking have all been associated with endothelial dysfunction. Although the molecular basis of endothelial dysfunction remains incompletely understood, numerous studies support a loss of bioavailable nitric oxide (NO) as a key mechanism underlying the dysfunction. Both tetrahydrobiopterin (BH4) and ascorbic acid play a role in sustaining NO bioavailabilty, and supplementation with either BH4 or ascorbic acid has been shown to improve endothelial function in humans and in animal models. As a result, investigations of the cardiovascular implications of treatment with exogenous BH4 and ascorbic acid have focused on the endothelial effects of these compounds and their contribution to chemical stabilization of the enzyme endothelial nitric oxide synthase (eNOS) and protection of NO from scavenging by free radicals, respectively. Both BH4 and ascorbic acid may contribute to promoting NO-dependent vasodilation; however, both compounds possess redox capabilities that may alter vascular reactivity independent of their effects on the bioavailability of NO. In a study in this issue of Cardiovascular Research, Garry et al. investigated the possibility that pro-oxidant effects of BH4 and ascorbic acid potentiate NO-independent vasodilatory responses of arterial rings to both the G-protein-coupled agonist, acetylcholine (ACh), and the endoplasmic reticulum Ca2þ-ATP-ase (SERCA) inhibitor, cyclopiazonic acid (CPA). These authors reported that in the presence of molecular oxygen, both BH4 and ascorbic acid increased extracellular hydrogen peroxide (H2O2), thereby potentiating endothelium-dependent dilation. Vasodilatory responses to ACh and CPA were assessed in the presence of NOS and cyclooxygenase inhibitors to eliminate NO and vasodilatory prostanoids. Scavenging of BH4and ascorbic acid-generated H2O2 with catalase inhibited the potentiation of the vasodilatory responses to CPA and ACh, and treatment with exogenous H2O2 mimicked the potentiation of vasodilation that occurred in response to treatment with BH4 and ascorbic acid. These authors have previously reported that H2O2 enhances vasodilatory responses to CPA through a mechanism that involves an increase in Ca2þ store depletion, with a subsequent increase in Ca2þ mobilization promoting the opening of hyperpolarizing calcium-activated potassium channels (KCa) (see Figure 1). 12 In this most recent manuscript, Garry et al. describe a novel mechanism whereby H2O2 generated upon oxidation of supplemental BH4 or ascorbic acid enhances endothelium-dependent vasodilation through a mechanism that is NO-independent and which appears to involve hyperpolarization. In models of endothelial dysfunction, endothelium-derived hyperpolarizing factor (EDHF)-induced relaxations increase in a compensatory manner in arteries in which NO-dependent relaxations are impaired. The results of Garry et al. suggest that supplemental BH4 and/or ascorbic acid could reverse endothelial dysfunction through potentiation of dilation occurring in response to EDHF as well as through stabilization of eNOS function and preservation of NO. This report from Garry et al. also suggests that ascorbic acid and BH4, through generation of H2O2 in the interstitial space, could modulate vascular smooth muscle tone independent of plasma changes in BH4 and ascorbic acid. These findings suggest that although supplementation has been primarily attributed to endothelial effects of these compounds, attention should be given to the interstitial levels of these compounds that occur with supplementation, and of which little is currently known. Although these investigators did discern the effects of luminal vs. abluminal application of H2O2, they did not directly assess the possible pro-oxidant effects of H2O2 on relaxation to CPA and ACh in arteries denuded of endothelium. Thus, the direct effects of BH4 or ascorbic acid-induced increases in interstitial H2O2 on vascular smooth muscle tone were not determined in this study, and the possibility remains that pro-oxidant effects of BH4 and ascorbic acid occur through direct effects on vascular smooth muscle tone. The endothelium and vascular smooth muscle are electrotonically coupled via gap junctions, and Garry et al. found that blockade of gap junctions with connexin-mimetic The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology. * Corresponding author. Tel: þ1 352 392 3791; fax: þ1 352 846 0270. E-mail address: [email protected]