Reduced nitric oxide production by endothelial cells in cirrhotic rat liver: endothelial dysfunction in portal...

Intrahepatic vascular resistance is regulated in part by the vasoconstricting and relaxing effects of compounds such as endothelin and nitric oxide (NO). Although the hepatic endothelium is likely to be a prominent source of NO via production by endothelial cell NO synthase (ecNOS), this isoform has not been described in sinusoidal endothelial cells (SECs). The aim of this study was to determine whether ecNOS is produced by SECs, and, if so, to determine whether ecNOS- (mRNA/protein) or SEC-dependent production of NO varies after liver injury.

A cDNA encoding a 279-bp fragment homologous to bovine ecNOS was isolated and used to detect rat ecNOS mRNA. Liver injury and/or cirrhosis was induced by bile duct ligation or administration of CCl4. NOS activity was quantitated and the presence of NO was detected with highly sensitive nitrite and cGMP assays.

ecNOS mRNA and protein (detected by immunoblot) were present only in SECs. Neither ecNOS mRNA abundance nor protein levels varied after liver injury induced by bile duct ligation or repeated CCl4 administration, including in animals with cirrhosis. Because inducible NOS is induced after bile duct ligation but not after CCl4-induced cirrhosis, we examined NOS activity and nitrite- and NO-dependent cGMP production after the latter form of injury. NOS activity and nitrite and cGMP production were significantly reduced in SECs from cirrhotic animals compared with controls (P < 0.05, n = 4-6).

SECs are an important source of ecNOS in the liver; although ecNOS mRNA and protein levels in SECs do not seem to vary with liver injury, ecNOS function is altered, resulting in diminished NO release in SECs from cirrhotic livers. These data have important implications for the pathogenesis of intrahepatic portal hypertension as well as other disorders in which endothelia may be damaged.