No Functional Involvement of 5-Hydroxytryptamine1A Receptors in Nitric Oxide-Dependent Dilatation Caused by Serotonin in the Human Forearm Vascular Bed

The vascular effects of serotonin (5-hydroxytryptamine, 5-HT) are complex and heterogeneous. In human forearm, we showed that low doses of 5-HT cause marked but transient vasodilatation followed by a persistent vasodilator response. In in vitro and in animal experiments, 5-HT induced release of nitric oxide (NO) through stimulation of endothelial 5-HT1-like receptors. In the present study, we investigated involvement of the "NO pathway" and possible involvement of the 5-HT1A receptor subtype in 5-HT-induced persistent vasodilator response. In 8 healthy volunteers, we infused 5-HT (0.1, 0.3, and 1 ng/kg/min) and the selective 5-HT1A receptor agonist flesinoxan (15, 45, and 150 ng/kg/min) intraarterially (i.a.) with NG-monomethyl-L-arginine (L-NMMA 30 micrograms/kg/min) or saline. Forearm blood flow (FBF) was measured by automated R-wave-triggered venous occlusion plethysmography. Forearm vascular resistance (FVR) was derived from simultaneously recorded i.a. blood pressure (BP) and FBF. 5-HT dose-dependently decreased FVR (p < 0.001). The persistent vasodilator response to 5-HT appears to be mediated by NO release, as suggested by its complete abolition by L-NMMA (p < 0.001). Flesinoxan decreased FVR slightly, but only at high doses (p < 0.05). The present findings indicate that 5-HT1A receptors are not functionally involved in 5-HT-mediated vasodilatation in human forearm.