Chronic high blood flow and the hyperdynamic circulatory syndrome in portal hypertension are associated with endothelial constitutive nitric oxide (NO) synthase (eNOS) upregulation and increased NO release. In portal vein-ligated (PVL) rats the splanchnic circulation is not yet hyperdynamic on day 3 postoperatively. In vitro perfused superior mesenteric arteries (SMAs) of day 3 PVL and sham rats were challenged with increasing flow rates or the alpha-adrenoreceptor agonist methoxamine (30 and 100 microM) before and after incubation with the NO inhibitor, Nomega-nitro-L-arginine (L-NNA, 10(-4) M). Perfusate NO metabolite (NOx) concentrations were measured by chemiluminescence. PVL rats expressed a significant hyporesponsiveness to increases in flow rate or methoxamine that was overcome by incubation with L-NNA. The PVL vasculature showed significantly higher slopes of NOx production vs. flow-induced shear stress, higher increases in perfusate NOx concentration in response to methoxamine, and higher eNOS protein levels (Western blot) compared with sham rats. In conclusion, eNOS-upregulation and increased NO release by the SMA endothelium occur before the development of the hyperdynamic splanchnic circulation, suggesting a primary role of NO in the pathogenesis of arterial vasodilatation.