Mechanisms of vasoactive intestinal peptide (VIP)-mediated vasodilation in human skin

Vasoactive intestinal peptide (VIP) is known to induce histamine release in human skin and known to include a 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 using laser-Doppler flowmetry (LDF) and cutaneous vascular conductance (CVC; LDF/mean arterial pressure) was calculated and normalized to maximal vasodilation. VIPmediated 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 to 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 which could not be explained by H1 and H2 receptor activation.