Role of nitric oxide in the vascular effects of local warming of the skin in humans

Local warming of skin induces vasodilation by unknown mechanisms. To test whether nitric oxide (NO) is involved, we examined effects of NO synthase (NOS) inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) on vasodilation induced by local warming of skin in six subjects. Two adjacent sites on the forearm were instrumented with intradermal microdialysis probes for delivery of L-NAME and sodium nitroprusside. Skin blood flow was monitored by laser-Doppler flowmetry (LDF) at microdialysis sites. Local temperature (T(loc)) of the skin at both sites was controlled with special LDF probe holders. Mean arterial pressure (MAP; Finapres) was measured and cutaneous vascular conductance calculated (CVC = LDF/MAP = mV/mmHg). Data collection began with a control period (T(loc) at both sites = 34°C). One site was then warmed to 41°C while the second was maintained at 34°C. Local warming increased CVC from 1.44 ± 0.41 to 4.28 ± 0.60 mV/mmHg (P < 0.05). Subsequent L-NAME administration reduced CVC to 2.28 ± 0.47 mV/mmHg (P < 0.05 vs. heating), despite the continued elevation of T(loc). At a T(loc) of 34°C, L-NAME reduced CVC from 1.17 ± 0.23 to 0.75 ± 0.11 mV/mmHg (P < 0.05). Administration of sodium nitroprusside increased CVC to levels no different from those induced by local warming. Thus NOS inhibition attenuated, and sodium nitroprusside restored, the cutaneous vasodilation induced by elevation of T(loc); therefore, the mechanism of cutaneous vasodilation by local warming requires NOS generation of NO.