Urocortin 1 (Ucn1) is a neuropeptide that regulates vascular tone and is implicated in both the vascular and immune cell-mediated responses to inflammation. The role of Ucn1 in regulating microvascular permeability has not been determined. We hypothesized that local Ucn1 release promotes microvascular permeability and that this effect augments the local gastrointestinal vascular response to lipopolysaccharide (LPS)-induced systemic inflammation.Wemeasured hydraulic (Lp) and macromolecule permeability in mesenteric venules. We show that a continuous infusion of 10-7 M Ucn1 in a postcapillary venule increased Lp 2-fold over baseline, as did LPS-induced inflammation. However, simultaneous infusion of Ucn1 and LPS markedly increased Lp by 7-fold. After local knockdown of Ucn1 using RNAinterference, infusion of Ucn1 with LPS resulted in return to 2-fold increase, confirming that Ucn1 synergistically augments hydraulic permeability during inflammation. LPS and Ucn1 treatment also resulted in increased numbers of interstitial microspheres, which colocalized with CD31+ immunecells. Ucn1 activity is mediated through two receptor subtypes, CRH-R1 and CRH-R 2. CRH-R1 receptor blockade exacerbated, whereas CRH-R2 receptor blockade decreased the LPS-induced increase in L p. Finally, treatment with the c-JUN N-terminal kinase (JNK) antagonist SP600125 during infusion of LPS, but not Ucn1, decreased L p. These findings suggest that Ucn1 increases microvascular permeability and acts synergistically with LPS to increase fluid and macromolecule losses during inflammation. Knockdown of endogenous Ucn1 during inflammation attenuates synergistic increases in Lp. Ucn1's effect on Lp is partiallymediated by the CRH-R2 receptor and acts independently of thec-JUNN-terminal kinase signal transduction pathway.