Portosystemic collateral circulation is a frequent complication of chronic liver disease [1,2]. The portacaval shunted rat is an experimental model of great interest for studying the metabolic alterations related to a portosystemic shunt [3]. Particularly, in this model it has been described that, portal blood flow deprivation (long-term ischemia) may make the atrophic liver more susceptible to oxidant-induced injury because the oxidant scavenging system of the liver decreases [4].

However, recent evidence has shown that the altered redox status in liver disease is not confined to the diseased liver, but that it is a systemic phenomenon involving extrahepatic tissues [5]. So, the determination of oxidant and antioxidant plasma levels in portacaval shunted rats could broaden the knowledge of the systemic pathophysiological mechanisms, which are activated by the systemic bypass of the portal blood flow.

This study has been carried out to determine serum advanced oxidation protein products (AOPP), lipid hydroperoxides (LOOH), total serum antioxidants (TAX), total thiols and albumin as markers of the plasma redox status.

The results reported in this study show a significant decrease of the TAX, associated with an increased AOPP plasmatic level of portacaval-shunted rats. The considerable decrease in TAX levels in long-term (4 weeks) portacaval shunted-rats suggest that a weakening of the antioxidative barrier of the body exists, perhaps as a consequence of the increased systemic oxidative stress produced by the portosystemic shunting in this experimental model.

Oxidative stress, in general, is the overpowering of the antioxidative defence system by the oxidative system [6]. A number of diseases, including liver disease [5], are associated with an imbalance between oxidant stress and antioxidative defence mechanisms that favour the former [5,7]. Oxidative stress is produced by free radicals, i.e., reactive oxygen species (ROS) and reactive nitroxy species (RNOS) and if they are not removed or neutralized, react with lipids, proteins, and nucleic acids, damaging the cellular functions and eventually causing cell death [5,6,8]. Both the excessive oxidative stress and the reduced antioxidant ability could participate in the imbalance between the oxidant stress and antioxidative defence mechanism, which is produced in the rats with portacaval shunt.

Chronic liver ischemia derived from the portal blood flow bypass in the rat impairs oxidant scavenging, but does not impair the oxidant generating systems of the liver [4]. However, the sources of ROS and RNOS in liver diseases can be subdivided into intrahepatic and extrahepatic. Particularly, the extrahepatic oxidative stress is considered a systemic phenomenon involving extrahepatic tissues [5] and mainly portal circulation [5,9]. In rats with portal vein stenosis and portosystemic collateral circulation, the existence of a causal relationship between oxidative stress and the hyperdynamic circulation developed has been accepted [9]. Since the hepatocellular injury is not a feature of this animal model it has been proposed that oxidative stress originates from the portal circulation and not the diseased liver [5,9]. Furthermore, portacaval shunted rats also develop a hyperdynamic splanchnic circulation related to portosystemic shunting [10-12]. Therefore, in this experimental model the hyperdynamic splanchnic circulation or mesenteric hyperemia could also be associated with intestinal oxidative stress. It has been proposed that the chronic hypoxemia of the intestinal mucosa related to vascular congestion could be an etiologic key factor in the production of bacterial translocation because the enterocytes would suffer injury by oxidative stress [13,14]. Moreover, NO-overproduction could represent an adaptive mechanism of the endothelium in response to chronic increases in flow-induced shear stress [15-18]. NO reacting with ROS, such as O₂^-., can also induce the peroxynitrite ion (ONOO^-) hyperproduction [19]. Intestinal oxidative stress could participate through this mechanism in the production of increased plasmatic levels of AOPP in rats with a portosystemic shunt.

Protein oxidation products have increasingly been used as markers instead of lipid peroxidation products in demonstrating oxidative stress [20]. A novel oxidative stress marker of protein, referred to as AOPP was developed in plasma [21]. Furthermore, AOPP oxidation of plasma thiol groups, termed "thiol stress," is quantitatively the major manifestation of protein oxidation [22]. Since AOPP is not only a marker of oxidative stress, but also acts as an inflammatory mediator [23-27] the knowledge of AOPP pathophysiology in this experimental model could provide valuable information with respect to the relationship between oxidative stress and the inflammatory response related to a portosystemic shunt. In this regard, since the liver and the spleen play important roles in the elimination of AOPP [28], the apoptosis and liver atrophy after portacaval shunting in the rat [29-31] could induce its decreased plasma clearance, thus favouring its increased plasmatic levels.

The marked plasmatic levels increase of thiostatin and the hypoalbuminemia in rats with a portosystemic shunt may be involved in the acute phase changes associated with a systemic inflammatory response [32,33]. The proteins acting as acute phase proteins differ from humans to animals and from one species to another. In the rat, thiostatin and α₁-acid glycoprotein (α₁-AGP) are among the major positive acute phase proteins while albumin reacts as a negative acute phase protein [34]. Thiostatin is a plasma proteinase inhibitor protecting against proteolytic auto-degradation [33]. Therefore, the synthesis of thiostatin benefits from the metabolic priority during decreased functional liver mass caused by the portosystemic shunt. However, α₁-AGP does not increase in these animals. Since it is considered that α₁-AGP prevents gram-negative infections [34] and has anti-inflammatory functions [35], rats with portacaval anastomoses would lose an essential component in nonspecific resistance to infection and inflammation.

Albumin plasma levels correlate with the TAX and with the hyperammonemia in portosystemic shunted rats. Albumin is a powerful extracellular antioxidant [36] and its decreased liver synthesis after portacaval shunt reduces its antioxidant functions. However, albumin synthesis increases when ammonia levels are higher. This could represent an attempt of compensating the deleterious metabolic effects caused by ammonium.

In rats with a portosystemic shunt, the acute-phase response could be associated with oxidative stress, as well as with inflammation. Particularly, IL-6, the major stimulator of most acute phase proteins, is primarily produced by Kupffer cells [37,38]. Upregulation of this cytokine may be related to the enhanced respiratory burst activity of Kupffer cells leading to the redox activation of NF-κB [39-41]. This compensatory response has already been described in order to re-establish homeostasis in the liver and extrahepatic tissues exhibiting oxidative stress [38].

Another metabolic feature that has been shown to be upregulated, though not always, due to a lack of oxygen or oxidative stress, is the antioxidant system [42]. It has been shown that the portosystemic bypass in the rat reduces the oxidant scavenging system of the liver with a significant reduction of superoxide dismutase and xanthine-dehydrogenase [4]. Furthermore, in the present study, the TAX (i.e., the fraction of antioxidant pool available for further anti-ROS activity) is significantly lower in portacaval shunted rats compared to control rats. These results may mean that a portosystemic shunt, including hyperdynamic circulatory syndrome and acute-phase response, has its own effect on lowering TAX. Since oxidative stress exhausts the antioxidative pool of the body, TAX could also decrease [8,42]. However, the ROS overproduction after portacaval shunting is not excessive, and indeed a plasmatic increase of lipid peroxidation is not produced, therefore it can be suspected that the novo antioxidant synthesis is reduced. If so, the reduction of the systemic antioxidant activity makes the organism susceptible to oxidant-induce multi-organ injury because a normal ROS production could be indeed a potential cause of oxidative stress when an antioxidative deficit coexists [6,42-44].

Since the existence of an anti-inflammatory redox-oxidant revolving axis has been suggested [43], in rats with portosystemic shunt, it could also be considered that the reduction of antioxidant ability would represent the mediator signal for the evolution and perpetuation of the inflammatory process that is often associated with the condition of oxidative stress, which involves gene regulation [43,45]. Thus, the altered redox homeostasis in this experimental model would be one of the hallmarks of the processes that regulate gene transcription in oxidative-stress-mediated inflammation [8,43,45]. If so, we could call it: "reduced antioxidative defence-mediated inflammation."

The decrease of the antioxidant protection in rats with portacaval shunt, evidenced by lower TAX and hypoalbuminemia, is noteworthy since it is correlated with hyperammonemia. This correlation suggests that, in this experimental model, the grade of insufficient antioxidant-mediated inflammation would be involved in a particular metabolic alteration related to the portosystemic shunt, as is the ammonia hyperproduction. Hyperammonemia is considered a key etiopathogenic factor in the development of hepatic encephalopathy [46-49]. Although, ammonia is believed to be responsible for the neurological abnormalities associated with hepatic encephalopathy, growing evidence supports the view that glutamine, synthesized from glutamic acid and ammonia, plays a major role in the deleterious effects of ammonia [49] and induce oxidative stress [46-48]. In turn, L-glutamic acid is also a precursor of the antioxidant glutathione [5,42,50,51]. Thus, hyperammonemia could be added as an etiopathogenic factor of the oxidative stress-mediated inflammation pathway that induces the portosystemic shunt [48,49].

The decreased liver antioxidant activity in portacaval shunted rats could potentiate the oxidative stress. In turn, the increased synthesis of acute phase proteins by the liver, since their anti-enzymatic ability, would attempt to balance the enzymatic stress in this experimental model.

AST: aspartate-aminotransferase ; ALT: alanine-aminotransferase; α₁-AGP: alpha-1 acid glycoprotein; AOPP: advanced oxidation protein products; BW: body weight; ch-: chloramine-T; DTNB: 5,5'-dithiobis-(2-nitrobenzoate); FBW: final body weight; FOX2: ferrous oxidation; GSH: reduced glutathione; IL: interleukin ; IVC: inferior vena cava ; LOOH: lipid hydroperoxides; LW: liver weight; LW/BW: liver weight to body weight ratio; NF-κB: nuclear factor kappa beta; NO: nitric oxide; ONOO^-: peroxynitrite ion; PBS: phosphate buffer solution; PCA: portacaval anastomoses; PCS: portacaval shunt; PV: portal vein; ROS: reactive oxygen species; RNOS: reactive nitroxy species; TAX: total antioxidant status/capacity of the serum; TNF-α: tumor necrosis factor alpha.

The author(s) declare that they have no competing interests.

MIGF, FSP, LS, JR, RA, MAA and JLA performed most of the experiments and provided assistance for the preparation of the manuscript. MAA, MIGF, JLA and JA participated in the design of the study and prepared the manuscript. All authors have read and approved the content of the manuscript.