Mechanisms of vasoactive intestinal peptide-mediated vasodilation in human skin

Abstract

Vasoactive intestinal peptide (VIP) is known to induce histamine release in human skin and to include a nitric oxide (NO)-dependent dilation in several other vascular beds. However, the relative contribution of histamine and NO to VIP-mediated vasodilation in human skin is unknown. Forty-three subjects volunteered to participate in two studies designed to examine the mechanism of VIP-mediated vasodilation in human skin. Study 1 examined the contribution of NO in the skin blood flow response to eight doses of VIP ranging from 25 to 800 pmol. In addition, study 1 examined a specific role for NO in VIP-mediated dilation. Study 2 examined the relative contribution of NO and histamine to VIP-mediated dilation via H₁ and H₂ histamine receptors. Infusions were administered to skin sites via intradermal microdialysis. Red blood cell flux was measured by using laser-Doppler flowmetry (LDF), and cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was calculated and normalized to maximal vasodilation. VIP-mediated vasodilation includes a NO-dependent component at doses above 100 pmol, where NO synthase inhibition significantly attenuates CVC ( P < 0.05). Inhibition of H₁ receptors attenuates the rise in CVC to exogenous VIP ( P < 0.05); however, combined H₁-receptor inhibition and NO synthase inhibition further reduced VIP-mediated vasodilation compared with either H₁ inhibition or NO synthase inhibition alone ( P < 0.05). In contrast to H₁-receptor inhibition, H₂-receptor inhibition did not affect vasodilation to exogenous VIP. Thus, in human skin, VIP-mediated vasodilation includes a NO-dependent component that could not be explained by H₁- and H₂-receptor activation.