Increased intrahepatic resistance in severe steatosis: endothelial dysfunction, vasoconstrictor overproduction and altered microvascular architecture

Non-alcoholic fatty liver disease can progress to steatohepatitis and fibrosis, and is also associated with impaired liver regeneration. The pathophysiology remains elusive. We recently showed that severe steatosis is associated with an increase in portal pressure, suggesting liver flow impairment. The objective of this study is to directly assess total intrahepatic resistance and its potential functional and structural determinants in an in situ perfusion model. Male Wistar rats fed a control (n=30) or a methionine choline-deficient (MCD) diet (n=30) for 4 weeks were compared. Liver tissue and serum analysis, in vivo haemodynamic measurements, in situ perfusion experiments and vascular corrosion casts were performed. The MCD group showed severe steatosis without inflammation or fibrosis on histology. Serum levels and liver tissue gene expression of interleukin (IL)-6, tumour necrosis factor-alpha IL-1 beta and interferon-gamma, liver tissue myeloperoxidase activity and liver immunohistochemistry with anti-CD68 and anti-alpha smooth muscle actin were comparable between groups, excluding significant inflammation. Flow-pressure curves were significantly different between groups for all flows (slope values: 0.1636 +/- 0.0605 mm Hg/ml/min in controls vs 0.7270 +/- 0.0408 mm Hg/ml/min in MCD-fed rats, P<0.001), indicating an increased intrahepatic resistance, which was haemodynamically significant (portocaval pressure gradient 2.2 +/- 1.1 vs 8.2 +/- 1.3 mm Hg in controls vs MCD, P<0.001). Dose-response curves to acetylcholine were significantly reduced in MCD-fed rats (P<0.001) as was the responsiveness to methoxamine (P<0.001). Vascular corrosion casts showed a replacement of the regular sinusoidal anatomy by a disorganized pattern with multiple interconnections and vascular extensions. Liver phosphorylated endothelial NO synthase (eNOS)/eNOS and serum nitrite/nitrate were not increased in severe steatosis, whereas liver thromboxane synthase expression, liver endothelin-1 (ET-1) expression and serum andothelin-1 concentration were significantly increased. Severe steatosis induces a haemodynamically significant increase in intrahepatic resistance, which precedes inflammation and fibrogenesis. Both functional (endothelial dysfunction and increased thromboxane and ET-1 synthesis) and structural factors are involved. This phenomenon might significantly contribute to steatosis-related disease. Laboratory Investigation (2012) 92, 1428-1439; doi:10.1038/labinvest.2012.103; published online 13 August 2012