Overproduction of nitric oxide inhibits vascular reactivity in portal hypertensive rats

AIM: To evaluate the relationship between nitric oxide (NO) and hyperdynamic circulatory status in portal hypertension.METHODS: Twenty male Sprague Dawley rats (weighing 200 ± 20 g) were randomized into two groups: portal hypertension group (n = 12) and sham-operated control group (n = 8). The portal hypertensive model was established by means of graded constriction of the portal vein. The concentrations of nitrite (NO₂^-) in the portal vein and peripheral blood were measured by fluorometric assay to reflect NO levels. The reactivity of isolated abdominal aortic rings from rats with partial portal vein constriction and controls was determined by assessing response to administration of potassium chloride (KCl) (10–80 mmol/L) and phenylephrine (10^-9-10^-4 mol/L) with or without preincubation with NO synthase inhibitor Nω-nitro-L-arginine (L-NNA).RESULTS: Serum concentrations of NO₂^- in the portal vein blood (0.766 ± 0.097 μmol/L) and peripheral blood (0.687 ± 0.092 μmol/L) were elevated in portal hypertensive rats, as compared with the concentrations in controls (0.613 ± 0.084 μmol/L and 0.591 ± 0.045 μmol/L respectively, both P < 0.01). In addition, the rates of NO₂^- in portal vein blood were markedly higher than those in peripheral blood (P < 0.05) in the portal hypertensive rats. Abdominal aortic rings from rats with portal vein constriction exhibited significantly impaired contractility to phenylephrine and KCl, as compared with the control rats. The EC₅₀ values of KCl were markedly higher in the portal hypertensive rings (26.5 ± 0.9 mmol/L) than in the control rings (22.3 ± 1.7 mmol/L, P < 0.01), as were the EC₅₀ values of phenylephrine (37.2 ± 0.4 nmol/L vs control rings: 28.1 ± 0.2 nmol/L, P < 0.01). After preincubation of rings with L-NNA, the difference in EC₅₀ values between portal hypertensive and control rings was no longer statistically significant for either KCl (20.18 ± 0.8 mmol/L vs 19.4 ± 1.2 mmol/L, P > 0.05) or phenylephrine (22.4 ± 1.8 nmol/L vs 21.8 ± 1.4 nmol/L, P > 0.05). However, the maximal concentrations of KCl and phenylephrine for inducing contractions were still significantly lower in the portal hypertensive rings (1.08 ± 0.1 g and 1.43 ± 0.14 g) than in the control rings (1.21 ± 0.11 g and 1.72 ± 0.11 g respectively, both P < 0.05). Thus, addition of the NO synthase inhibitor L-NNA could partially restore contractile responses to KCl and phenylephrine in portal hypertensive rings.CONCLUSION: NO overproduction inhibits the vascular reactivity to vasoconstrictors, and it might be one of the main causes of vasodilatation and hyperdynamic circulatory status in portal hypertension.