Local tetrahydrobiopterin administration augments reflex cutaneous vasodilation through nitric oxide-dependent mechanisms in aged human skin

Functional constitutive nitric oxide synthase (NOS) is required for full expression of reflex cutaneous vasodilation that is attenuated in aged skin. Both the essential cofactor tetrahydrobiopterin (BH ₄) and adequate substrate concentrations are necessary for the functional synthesis of nitric oxide (NO) through NOS, both of which are reduced in aged vasculature through increased oxidant stress and upregulated arginase, respectively. We hypothesized that acute local BH ₄ administration or arginase inhibition would similarly augment reflex vasodilation in aged skin during passive whole body heat stress. Four intradermal microdialysis fibers were placed in the forearm skin of 11 young (22 ± 1 yr) and 11 older (73 ± 2 yr) men and women for local infusion of 1) lactated Ringer, 2) 10 mM BH4, 3) 5 mM (S)-(2- boronoethyl)-L-cysteine +5 mM N ^ωy-nor-L-arginine to inhibit arginase, and 4) 20 mM N ^G-nitro-L-arginine methyl ester (L-NAME) to inhibit NOS. Red cell flux was measured at each site by laser-Doppler flowmetry (LDF) as reflex vasodilation was induced. After a 1.0° in oral temperature (T _or), mean body temperature was clamped and 20 mM L-NAME was perfused at each site. Cutaneous vascular conductance was calculated (CVC =LDF/mean arterial pressure) and expressed as a percentage of maximum (%CVC _max; 28 mM sodium nitroprusside and local heat, 43°asodilation was attenuated at the control site of the older subjects compared with young beginning at a 0.3° in Tor. BH ₄ and arginase inhibition both increased vasodilation in older (BH ₄: 55 ± 5%; arginaseinhibited: 47 ± 5% vs. control: 37 ± 3%, both P <1) but not young subjects compared with control (BH ₄: 51 ± 4%CVC _max; arginase-inhibited: 55 ± 4%CVC _max vs. control: 56 ± 6%CVC _max, both P <5) at a 1°e in Tor. With a 1°e in Tor, local BH ₄ increased NO-dependent vasodilation in the older (BH ₄: 31.8 ± 2.4%CVC _max vs. control: 11.7 ± 2.0%CVC _max, P <1) but not the young (BH ₄: 23 ± 4%CVC _max vs. control: 21 ± 4%CVC _max, P =0.718) subject group. Together these data suggest that reduced BH ₄ contributes to attenuated vasodilation in aged human skin and that BH ₄ NOS coupling mechanisms may be a potential therapeutic target for increasing skin blood flow during hyperthermia in older humans.