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We examined the role of thromboxane [A.sub.2] (TX[A.sub.2]) in LPS-induced hyperresponsiveness of hepatic portal circulation to endothelins (ETs) and whether Kupffer cells are the primary source of TX[A.sub.2] release in response to ET-1 in endotoxemia. After 6 h of LPS (1 mg/kg body wt ip) or saline (control), liver was isolated and perfused with recirculating Krebs-Henseleit bicarbonate buffer at a constant flow rate (100 ml x [min.sup.-1] x kg body [wt.sup.-1]). ET-1 (10 pmol/ min) was infused for 10 min. Portal pressure (PP) was continuously monitored during perfusion. Perfusate was sampled for enzyme immunoassay of thromboxane [B.sub.2] (TX[B.sub.2]; the stable metabolite of TX[A.sub.2]) and lactate dehydrogenase (LDH) assay. ET-1 infusion resulted in a significantly greater increase of PP in the LPS group than in controls. Both TX[A.sub.2] synthase inhibitor furegrelate (Fureg) and TX[A.sub.2] receptor antagonist SQ-29548 (SQ) substantially blocked enhanced increase of PP in the LPS group (4.9 [+ or -] 0.4 vs. 3.6 [+ or -] 0.5 vs. 2.6 [+ or -] 0.6 mmHg for LPS alone, LPS + Fureg, and LPS + SQ, respectively; P < 0.05) while having no significant effect on controls. Gd[Cl.sub.3] for inhibition of Kupffer cells had similar effects (4.9 [+ or -] 0.4 mmHg vs. 2.9 [+ or -] 0.4 mmHg for LPS alone and Gd[Cl.sub.3] + LPS, respectively; P < 0.05). In addition, the attenuated PP after ET-1 was found concomitantly with significantly decreased releases of TX[B.sub.2] and LDH in LPS rats treated with Fureg, SQ, and Gd[Cl.sub.3] (886.6 [+ or -] 73.4 vs. 110.8 [+ or -] 0.8 vs. 114.8 [+ or -] 54.7 vs. 135.2 [+ or -] 45.2 pg/ml, respectively; P < 0.05). After 6 h of LPS, Kupffer cells in isolated cell preparations released a significant amount of TX[A.sub.2] in response to ET-1. These results clearly indicate that hyperresponsiveness of hepatic portal circulation to ET-1 in endotoxemia is mediated at least in part by TX[A.sub.2]-induced receptor activation, and Kupffer cells are likely the primary source of increased TX[A.sub.2] release. isolated liver perfusion; furegrelate; SQ-29548

Although the mechanisms of cirrhosis-induced portal hypertension have been studied extensively, the role of thromboxane [A.sub.2] (TX[A.sub.2]) in the development of portal hypertension has never been explicitly explored. In the present study, we sought to determine the role of TX[A.sub.2] in bile duct ligation (BDL)-induced portal hypertension in Sprague-Dawley rats. After 1 wk of BDL or sham operation, the liver was isolated and perfused with Krebs-Henseleit bicarbonate buffer at a constant flow rate. After 30 min of nonrecirculating perfusion, the buffer was recirculated in a total volume of 100 ml. The perfusate was sampled for the enzyme immunoassay of thromboxane [B.sub.2] (TX[B.sub.2]), the stable metabolite of TX[A.sub.2]. Although recirculation of the buffer caused no significant change in sham-operated rats, it resulted in a marked increase in portal pressure in BDL rats. The increase in portal pressure was found concomitantly with a significant increase of TX[B.sub.2] in the perfusate (sham vs. BDL after 30 min of recirculating perfusion: 1,420 [+ or -] 803 vs. 10,210 [+ or -] 2,950 pg/ml; P < 0.05). Perfusion with a buffer containing indomethacin or gadolinium chloride for inhibition of cyclooxygenase (COX) or Kupffer cells, respectively, substantially blocked the recirculation-induced increases in both portal pressure and TX[B.sub.2] release in BDL group. Hepatic detection of COX gene expression by RT-PCR revealed that COX-2 but not COX-1 was upregulated following BDL, and this upregulation was confirmed at the protein level by Western blot analysis. In conclusion, these results clearly demonstrate that increased hepatic TX[A.sub.2] release into the portal circulation contributes to the increased portal resistance in BDL-induced liver injury, suggesting a role of TX[A.sub.2] in liver fibrosis-induced portal hypertension. Furthermore, the Kupffer cell is likely the source of increased TX[A.sub.2], which is associated with upregulation of the COX-2 enzyme. hepatic fibrosis; vasoconstriction; isolated liver perfusion; cyclooxygenase; portal pressure

The role of nitric oxide (NO) and endothelins (ETs) in the regulation of hepatic blood flow was analyzed in anesthetized rats during resuscitation from hemorrhagic shock (HS). Nitric oxide synthase inhibition by Nomega-nitro-L-arginine (L-NAME) in anesthetized rats elevated mean arterial pressure and systemic vascular resistance during resuscitation. However, NO- and HS-mediated vasodilation reduced SVR and preserved hepatic blood flow in anesthetized rats.

The role of endothelins in the regulation of portal resistance was investigated in isolated and perfused livers of rats given lipopolysaccharide (LPS) or saline treatment. Rats which received LPS showed an increase in total portal resistance, zero flow, incremental and sinusoid resistances. Subsequent exposure to with L-NAME enhanced the effects of LPS, but treatment with L-arginine reversed those effects. Since nitric oxide is a known vasodilator, it was suggested that the functional role endothelins is mediated by nitric oxide.

An investigation of hepatic portal circulation in anesthetized male Sprague-Dawley rats has provided evidence to support that host defense response after stressful conditions such as hemorrhage/resuscitation is accompanied by an increase in portal resistance. This increase in vascular control is necessary for the maintenance of low hepatic portal vascular resistance.

Endotoxin pretreatment increases the portal circulatory systems' contractile response to endothelin-1 (ET-1) at sinusoidal and presinusoidal levels. Rat livers injected with the bacterial endotoxin lipopolysaccharide (LPS) show increased total portal resistance and zero-flow pressures in response to ET-1. LPS decreases sinusoidal diameter, volumetric flow, and red blood cell velocity. Impairment of the hepatic microcirculation by LPS is associated with liver failure.

Endothelin (sub A) receptors on Ito cells are responsible for the effects of exogenous endothelin-1 (ET-1) on sinusoid constriction in normal livers. Endotheline (sub B) receptors in portal venules are responsible for presinusoidal effects. The contraction of Ito cells and Kupffer cells are responsible for Et-1-induced sinusoidal constriction. Isolated rat livers and high-power intravital microscope were used in the study.

Epifluorescence microscopy facilitates the detection of alterations in in vivo hepatic sinusoidal hemodynamics during sustained injection of endothelin-1 (ET-1) in pentobarbital anesthetized rats. Sinusoidal and extrasinusoidal domains exhibit the action of ET-1, which incites significant changes in hepatic microvascular blood flow. Lower concentrations of ET-1 are most effective in inciting heterogeneous sinusoidal constriction.

Hepatic sinusoidal constriction caused by endothelin (ET-1) distorts normal acinar flow methods and colocalizes with fat-storing Ito cells, which are also influenced by ET-1. Phenylephrine (PE) and ET-1 increase pressure inflow and decrease portal flow. The sinusoidal constrictions of PE and ET-1 differ, as PE functions through portal receptors. The Ito cells' involvement in constriction is suppressed by nitric oxide donor.

Hepatic neovascularization after partial portal vein ligation: novel mechanism of chronic regulation of blood flow. Am J Physiol Gastrointest Liver Physiol 280: G21-G31, 2001.--The present study was undertaken to investigate hepatic microcirculatory response following partial portal vein ligation (PPVL) in rats. Portal pressure was markedly increased 2-6 wk after PPVL, but no significant reduction in sinusoidal perfusion and hepatocellular injury were detected. However, marked neovascularization was observed in PPVL rats using intravital microscopy and scanning electron microscopy (SEM). Extremely high red blood cell velocity (2,000-4,900 [micro]m/s) was seen in these vessels. Injection of fluorescein sodium via the carotid artery revealed that the neovessels originated from the hepatic arterial vasculature. This was further confirmed by clamping the common hepatic artery and phenylephrine injection from the carotid artery. These vessels maintained sufficient flow after massive sinusoidal shutdown elicited by the portal infusion of endothelin receptor B agonist IRL-1620. SEM also showed extensive neovascularization at the hilum. Additionally, clamping the portal vein decreased sinusoidal perfusion only by 9.5% in PPVL, whereas a 71.2% decrease was observed in sham. These results strongly suggest that the liver maintains its microcirculatory flow by vascular remodeling from the hepatic arterial vasculature following PPVL. liver microcirculation; intravital microscopy; portal hypertension; hepatic vascular casts