Tetrahydrobiopterin increases NO-dependent vasodilation in hypercholesterolemic human skin through eNOS-coupling mechanisms

Abstract

Localized exogenous R-tetrahydrobiopterin (R-BH(4)) corrects the deficit in local heat-induced vasodilation (VD) in hypercholesterolemic (HC) human skin through one of two plausible mechanisms: by serving as an essential cofactor to stabilizing endothelial nitric oxide (NO) synthase (eNOS) or through generalized antioxidant effects. We used the stereoisomer S-BH(4), which has the same antioxidant properties but does not function as an essential NOS cofactor, to elucidate the mechanism by which R-BH(4) restores cutaneous VD in HC humans. Intradermal microdialysis fibers were placed in 20 normocholesterolemic (NC), 13 midrange cholesterolemic (MC), and 18 HC (LDL: 94 ± 3, 124 ± 3 and 179 ± 6 mg/dl, respectively) men and women to perfuse Ringer (control site) and R-BH(4). In 10 NC, 13 MC, and 9 HC subjects (LDL: 94 ± 3, 124 ± 3, 180 ± 10 mg/dl), S-BH(4) was perfused at a third microdialysis site. Skin blood flow was measured during a standardized local heating protocol to elicit eNOS-dependent VD. After cutaneous vascular conductance (CVC = LDF/MAP) plateaued, NO-dependent VD was quantified by perfusing N(G)-nitro-l-arginine methyl ester (l-NAME). Data were normalized as %CVC(max). Fully expressed VD (NC: 97.9 ± 2.3 vs. MC: 85.4 ± 5.4, HC: 79.9 ± 4.2%CVC(max)) and the NO-dependent portion (NC: 62.1 ± 3 vs. MC: 45.8 ± 3.9, HC: 35.7 ± 2.8%CVC(max)) were reduced in HC (both P < 0.01 vs. NC), but only the fully expressed VD was reduced in MC (P < 0.01 vs. NC). R-BH(4) increased the fully expressed (93.9 ± 3.4%CVC(max); P < 0.01) and NO-dependent VD (52.1 ± 5.1%CVC(max); P < 0.01) in HC but not in NC or MC. S-BH(4) increased full-expressed VD in HC (P < 0.01) but did not affect NO-dependent VD in HC or MC. In contrast S-BH(4) attenuated NO-dependent VD in NC (control: 62.1 ± 3 vs. S-BH(4): 41.6 ± 7%CVC(max); P < 0.001). Exogenous R-BH(4) restores NO-dependent VD in HC human skin predominantly through NOS coupling mechanisms but increases full expression of the local heating response through generalized antioxidant properties.

Fig. 1.

Representative tracing. The time course of cutaneous vascular conductance (%CVC_max) throughout the local heating response in a hypercholesterolemic subject for the R-tetrahydrobiopterin (R-BH₄) and S-BH₄ sites. The nitric oxide (NO)-dependent plateau and the post-N^G-nitro-l-arginine methyl ester (l-NAME) plateau are labeled. The difference between the plateau and the post-l-NAME plateau is the decrease with NO synthase (NOS) inhibition respresented by the arrows for each site. The control site was omited for clarity.

Fig. 2.

Mean skin blood flow responses for the plateau and the post-l-NAME plateau. CVC represented at percent maximum at the plateau in skin blood flow as a result of local heating and after NOS inhibition with l-NAME in normocholesterolemic (NC) control subjects, midrange cholesterolemic (MC) subjects, and hypercholesterolemic (HC) subjects. Each panel illustrates a different localized microdialysis treatment site: control site (A), R-BH₄ site (B), S-BH₄ site (C), and l-NAME (D) throughout local heating. *P < 0.01 different vs. NC group, †P < 0.05 vs. NC control site.δP < 0.001 vs. HC control site.

Fig. 3.

Within-site NO-dependent vasodilation during local heating. The decrease in CVC with NOS inhibition in NC control subjects, MC subjects, and HC subjects. Each panel illustrates a different localized microdialysis treatment site: control site (A), R-BH₄ site (B), and S-BH₄ site (C). *P < 0.001 vs. NC control site, †P = 0.005 vs. HC control site.