New Insights Into the Regulation of Hepatic Blood Flow After Ischemia and Reperfusion

Ischemia and subsequent reperfusion (I/R) of the liver may occur under different clinical conditions. These include the complete temporary occlusion of the supplying blood vessels during liver surgery, especially when dealing with extensive hepatic trauma or resection of large intrahepatic lesions such as cysts or tumors (“Pringle’s maneuver”) (1). The use of extracorporeal circulation in cardiac or vascular surgery is associated with low-flow I/R of the liver. Moreover, hepatic low-flow I/R often occurs during hemorrhagic, cardiogenic, or septic shock states followed by resuscitation. Besides these different types of warm I/R, cold I/R is a central element of the liver transplantation procedure. There is ample evidence that the sequence of hepatic I/R may cause liver injury. In addition to postoperative bleeding, the development of hepatic failure is the main cause of death in the hospital after warm I/R of the liver (2,3). Moreover, in patients undergoing hepatectomy, the degree of the resulting impairment of hepatic perfusion and oxygenation determines the extent of liver injury, as well as the incidence of liver failure and mortality (4,5). Similar results have also been reported in patients after cold I/R associated with liver transplantation. For example, Klar et al. (6) observed an inverse correlation between the intraoperatively measured hepatic microvascular blood flow rate and the maximum postoperative enzyme release from the liver. In addition, Nakatani et al. (7) reported that the degree of impairment of the hepatic mitochondrial redox state determines the survival rate of patients after hemorrhagic shock and resuscitation. The extent of liver injury caused by I/R depends primarily on the presence of preexisting liver diseases, such as cirrhosis (3,8), and on the duration of ischemia (9). If the injury is severe enough to cause liver dysfunction, this is associated with a profound deterioration of the prognosis (10). Because the liver plays a central role in the metabolic and immunologic response to stress (11), this increased mortality is most likely due to the subsequent progression to multiple organ failure (12,13). Although the development of bioartificial liver devices is an area of extensive research, none of these systems is readily available for routine clinical use (14). Therefore, identification of the underlying pathophysiological mechanisms is of major importance, because this may form the basis for the development of new strategies aimed at limiting IR-induced liver injury. Mechanisms of Liver Injury During I/R In addition to the direct ischemic insult, hepatic injury is in large part the result of processes that occur during reperfusion. During the first hours of reperfusion, the local macrophages residing within the liver, i.e., the Kupffer cells, get stimulated by complement factors such as C5a (15). Kupffer cells induce inflammation by producing and releasing proinflammatory cytokines such as tumor necrosis factor-α or interleukin-1β. In addition, these cells seem to be the major source of reactive oxygen species generated during early reperfusion (<6 h), leading to a primarily intravascular oxidative stress. The sum of these processes may either directly or indirectly damage nonparenchymal cells of the liver, such as sinusoidal endothelial cells and parenchymal cells. As a result, circulating neutrophils accumulate in the postischemic liver. During this later phase of reperfusion (>6 h), the recruited neutrophils may also induce hepatocellular injury by releasing oxidants and proteases. The contribution of this inflammatory response and the resulting oxidative stress to hepatic reperfusion injury has long been recognized and is reviewed in detail elsewhere (16–18). Accumulating evidence suggests that alterations of hepatic perfusion may also play a major role in the pathogenesis of I/R-mediated liver injury. For example, it has been reported that hepatosplanchnic blood flow was still reduced in humans at 60 min of reperfusion after 55 min of hypovolemia, even though arterial blood pressure, cardiac output, and blood flow to the muscles, skin, and kidneys were already fully restored (19). In addition, perturbations of the hepatic microcirculation have been observed in different animal models after I/R, including decreases in sinusoidal diameters and flow, increases in the heterogeneity of hepatic microvascular perfusion, and even complete cessation of blood flow within individual sinusoids (20). The impaired restoration of hepatic microvascular flow correlates with the extent of liver injury after hemorrhagic shock and resuscitation (21) and during reperfusion after portal triad cross-clamping (22). Physical prevention of microvascular shutdown by use of a flow-controlled reperfusion mode largely prevents parenchymal cell necrosis after I/R of the liver, suggesting that the degree of microvascular failure determines the extent of lethal hepatocyte injury (23). Similar relationships between liver blood flow and injury could be observed in humans (5,24). Over the past decade, our knowledge about the pathogenesis of hepatic perfusion failure after I/R increased substantially. This review summarizes the most important findings that led to these new insights. If not specified otherwise, the results described were obtained in animal studies. Levels of Regulation of Hepatic Blood Flow Hepatic blood flow is regulated at different levels. Changes at each of these levels can ultimately cause alterations in nutritive blood flow through the hepatic sinusoids and may thus contribute to the subsequent development of hepatocellular injury described previously. The first level of regulation is at the systemic circulation. Decreases in cardiac output, reductions in the arterial inflow resistance to other vascular beds, and increases in right atrial pressure above a level of approximately 3–4 mm Hg (i.e., above the portal venous critical closing pressure) may all reduce blood flow through the liver (25,26). The second level of regulation is at the regional macrocirculation. Blood is supplied to the liver via the hepatic artery and the portal vein. The amount of blood flowing through the hepatic artery is determined by its vascular resistance. Because the hepatic artery has only a small autoregulatory capacity, the ability to maintain the hepatic arterial flow rate constant in the presence of a decrease in arterial perfusion pressure is limited (27,28). Hepatic arterial resistance may change so as to buffer the effect of portal flow alterations on total hepatic blood flow, thus tending to maintain total hepatic flow at a constant level. This mechanism has been called the “hepatic arterial buffer response”(29). Ezzat and Lautt (27) have provided evidence that both the hepatic arterial pressure-flow autoregulation and buffer response are mediated through changes in the washout of locally produced adenosine. The total resistance to flow from the portal vein to the inferior vena cava is low, and the relative contribution of individual anatomical sites, such as the hepatic veins, may vary significantly depending on the conditions and the species studied (30–33). The portal vein passively drains most of the venous blood from the splanchnic compartment. Therefore, portal venous flow is primarily determined by the arterial inflow resistance to these organs. However, splanchnic resistance may increase linearly with increases in portal venous pressure (34). Consequently, in the presence of such a “portal-splanchnic response,” alterations in portal flow may well be accomplished by changes in portal venous resistance. The third level of flow regulation is at the hepatic microcirculation. Changes in hepatic microvascular flow have long been exclusively interpreted as the result of systemic and regional macrohemodynamic changes and of obstructions of the sinusoidal lumen by swollen perisinusoidal cells, trapped blood cells, or thrombotic material. However, the findings described below show that sinusoidal flow can also be actively regulated and redistributed at the level of the microcirculation. Local Control of Vascular Tone Changes in vascular resistance have been traditionally ascribed to alterations in autonomic nervous activity and to the influence of circulating hormones. However, the discovery that endothelial cells can actively control various functions of the vasculature, including local control of vascular tone, had a profound effect on our understanding of how organ blood flow is regulated. In 1980, Furchgott and Zawadzki (35) reported that the vascular relaxation by acetylcholine depends on the presence of endothelial cells. In 1987, Palmer et al. (36) suggested that the gas nitric oxide (NO) accounts for the biological activity of the “endothelium-derived relaxing factor.” However, Hickey et al. (37) reported in 1985 about the existence of an “endothelium-derived contracting factor.” Shortly thereafter, Yanagisawa et al. (38) identified a peptide responsible for this vasoconstrictive action, named “endothelin” (ET). Meanwhile, there are indications that the endogenous generation of another gaseous monoxide, i.e., carbon monoxide (CO), may also exert local vasodilatory effects (39). During the past decade, evidence accumulated that these “endothelial” mediators may control hepatic blood flow under physiologic and, in particular, under pathologic conditions, such as I/R of the liver. Although all these mediators have also been shown to exert numerous other effects that could affect the extent of liver injury, this review focuses on their vasoregulatory properties. The major findings described within the following paragraphs are summarized in Figure 1.Figure 1: Inducing factors, sources, targets, hemodynamic effects, and interactions of endothelin-1 (ET-1), nitric oxide (NO), and carbon monoxide (CO). See text for details and references. Please note that arrows pointing at the outer cell membrane indicate an effect on the expression of the respective target gene, whereas those pointing directly at an enzyme indicate an effect on its activity. ROI = reactive oxygen intermediates; LPS = bacterial lipopolysaccharides; (S)EC = (sinusoidal) endothelial cells; KC = Kupffer cells; HSC = hepatic stellate cells; VSMC = vascular smooth muscle cells; HC = hepatocytes; AP-1 = activator protein-1; HIF-1 = hypoxia-inducible factor-1; NF-1 = nuclear factor-1; NF-κB = nuclear factor-κB; STAT = signal transducers and activators of transcription; Nrf2 = nuclear factor-erythroid 2 related factor; eNOS = endothelial “constitutive” nitric oxide synthase (Type III); iNOS = inducible nitric oxide synthase (Type II); HO-1 = inducible heme oyxgenase-1; HO-2 = constitutive heme oxygenase-2; ETA/B1/B2 = endothelin receptor A/B1/B2; sGC = soluble guanylate cyclase.The Role of ETs ETs are 21-amino acid peptides that share structural and functional characteristics with sarafotoxins, a class of snake venoms. Three isopeptides of the ET family have been identified, named ET-1, ET-2, and ET-3. Their actions are mediated through different ET receptors. The ETA receptor binds ET-1 and ET-2 with a much higher affinity than ET-3. In contrast, the ETB receptor binds all three isopeptides with the same affinity. The common response to ETA receptor stimulation is a contraction of the effector cell. However, pharmacological studies have provided evidence for the existence of different ETB receptor subtypes. Whereas stimulation of the ETB1 receptor on endothelial cells results in a relaxation of adjacent vascular smooth muscle cells that seems to involve the release of NO, stimulation of ETB2 receptors may induce contractions (40). Hemodynamic Effects of ETs in the Normal Liver Binding for ET-1 in liver are exclusively to nonparenchymal and vascular cells of the portal the hepatic and the central vein et al. reported that ET receptor expression was on hepatic stellate cells in the perisinusoidal of followed by sinusoidal endothelial cells and Kupffer cells. ET a increase in portal venous vascular resistance This seems to be mediated not only via such as portal and hepatic also via sinusoidal This is on the that of ET-1 decreases the and increases the heterogeneity of sinusoidal perfusion of these changes caused by ETs in the liver are of alterations of hepatic blood This the of how hepatic blood flow is regulated at the level of the hepatic microcirculation in the of vascular smooth muscle cells within this of the hepatic In that the decrease in is most at that with the of that are in the perisinusoidal with long stellate that the sinusoids for the that these may as a target for ETs from studies that stellate cells in are of in a and in response to these Although the contribution of generated ETs to total portal resistance is limited in the liver, this seems to be different under conditions ET and I/R oxygen bacterial proinflammatory and increases in vascular stress can induce ET expression Because most of these factors play an role during I/R of the liver, increased generation of ETs has been observed after hemorrhagic and after or complete ischemia of the liver of the of ETs have also been reported in patients after in response to blood during major surgery and during hepatic resection for ET in the liver are (sinusoidal) endothelial cells, Kupffer cells, and stellate cells. suggests that the factors activator hypoxia-inducible and nuclear may play a role in the control of ET in in response to Besides the increase in the generation of changes in the of ET receptor expression have also been In I/R ETB receptor expression is whereas expression of the ETA receptor is reduced Hepatic Hemodynamic Effects of ETs I/R The response to ET-1 is in models of hepatic stress including hemorrhagic shock and hepatic I/R with the increased generation of this may contribute to the impairment of hepatic perfusion and under these conditions. hemorrhagic of the ETA receptor increases cardiac and portal venous flow In addition, the of or ET receptor after hepatic ischemia in or in hepatic perfusion failure Moreover, this in hepatic blood flow is associated with reduced liver or injury However, ETs not only induce under these conditions, may also exert vasodilatory For example, pharmacological studies with different ET receptor have provided evidence that ETB1 receptor stimulation can blood flow through the liver and may the vasoconstrictive effects mediated via ETA and ETB2 receptors suggests that in the of these vasodilatory effects, the extent of IR-induced liver injury be even The Role of is a gaseous generated from the acid by Three different have been constitutive inducible and endothelial “constitutive” The activity of and eNOS is and In contrast, it has been iNOS much of in a The vasodilatory effect of depends on its with the in the heme of the soluble guanylate that increases the of within the respective target thus its relaxation Hemodynamic Effects of in the Normal Liver can exert vasodilatory effects within the liver. The addition of either or of increases portal flow in the liver Moreover, may not only the macrohemodynamic effects of ET-1, also its For example, of the to sinusoidal endothelial cells, and in the contraction of stellate cells in response to ET-1 The of a vasodilatory effect within the hepatic arterial vascular whereas this effect was only small in the portal vein This is with the that a vasodilatory effect in than in under physiologic conditions. The source for in the liver is most likely the is not only by endothelial cells of the large resistance vessels also by sinusoidal endothelial cells activity increases in response to increases of vascular thus the hemodynamic effects of vasoconstrictive in both vascular of the liver of I/R The generation of the presence of and oxygen Although the of both is limited during the ischemic the increases reperfusion, to an increase in eNOS activity Besides this of may induce an increase in of this “constitutive” in endothelial cells Consequently, an increased generation of has been observed during early reperfusion after hemorrhagic shock and after low-flow I/R of the liver between studies the amount of during early reperfusion most likely in the level of vascular a of eNOS activity. and proinflammatory cytokines can induce iNOS expression in parenchymal and nonparenchymal cells of the liver within to via signal that involve the of the nuclear a central of the response of and of HIF-1 iNOS large of the is In patients undergoing with an increased iNOS expression can be in the liver after reperfusion, associated with an increased of the amount of generated during later reperfusion depends primarily on the of the inflammatory response Hepatic Hemodynamic Effects of I/R There is evidence that the increased endogenous generation of hepatic blood flow during early reperfusion. these conditions, reduce hepatic arterial and portal venous flow and increase the degree of sinusoidal perfusion failure This is associated with impaired hepatic oxygenation and an increase in lethal hepatocyte injury could be by the effect of on increases in total hepatic and sinusoidal resistance described previously. The small increase in the amount of generated and the of iNOS expression that eNOS is the source for during early reperfusion. This is by a that in either eNOS or Whereas liver injury was significantly in to I/R with postischemic this was not the for at later of reperfusion, studies to a role of generated by iNOS in the regulation of hepatic perfusion and the subsequent development of liver injury However, the of the inflammatory response that I/R is large may well contribute to the changes in vascular and that to maintain liver blood flow during the later reperfusion The Role of During the past decade, evidence has accumulated suggesting that another generated gaseous monoxide, i.e., may as a second gas The of the heme by the enzyme heme of and three different have been Whereas the HO-1 is inducible by a of HO-2 is In contrast, is a heme and may be in heme NO, may induce as the result of an increased generation of via of the soluble guanylate by to the heme of this enzyme (39). However, evidence suggests that may also vascular resistance by a mechanism the of Hemodynamic Effects of in the Normal Liver of activity in decreases the amount of and increases portal resistance The of either or of prevents this increase in resistance. These of seem to involve because the major of the associated with to sinusoidal with the of these cells suggests that the vasodilatory effect of a physiologic mechanism that is also in the in to portal resistance and pressure low, whereas such can be in the hepatic artery the liver, HO-2 is primarily by whereas HO-1 can be in Kupffer cells Although both could a source for the responsible for the vasodilatory effects in the circulation under physiologic conditions seems to be primarily from HO-2 by parenchymal cells of the liver of I/R factors that play a role during I/R of the liver are of HO-1 shock These include the increased generation of reactive oxygen impaired the release of heme from cells, and increases in as well as bacterial and proinflammatory cytokines Consequently, increases in total hepatic HO-1 expression have been observed after ischemia of the liver and after hemorrhagic shock is primarily the result of HO-1 expression in sinusoidal cells and evidence suggests that this HO-1 results primarily from an of the AP-1 by reactive oxygen from Kupffer cells and that it is associated with an increase in the activity of However, HO-1 may also on other factors such as signal transducers and activators of and or nuclear factor-erythroid 2 related Hepatic Hemodynamic Effects of I/R of HO-1 in increases hepatic venous portal and the vasoconstrictive effects of ET-1 This suggests that this effect may not be to of HO-1 could also be a result of the of HO-1 after I/R of the liver. This is by the following The increase in portal resistance after is much after hemorrhagic shock as with in and results in a profound decrease in portal flow these conditions, generated has been shown to hepatic sinusoidal perfusion via a relaxing mechanism Moreover, this to perturbations of hepatic mitochondrial redox and the three systems can on different levels. and may exert vasodilatory effects, because both are to soluble guanylate Although the relative of to this enzyme is can the effect of Because may with that are generated in increased reperfusion, the resulting of may its and in increase the relative of under these conditions, with these Because the are also an increased hepatic activity after I/R could the and of Moreover, because is generated in increased under these conditions, it could to the heme of this enzyme However, may either or hepatic HO-1 depending on the amount of available Although the vasodilatory effects of ETs are at in part mediated by both gaseous can the vasoconstrictive effects of ETs in the liver In addition, smooth muscle can the expression of ET-1 by endothelial cells in response to has been shown to the release of ET-1 from endothelial cells by a mechanism Moreover, may ET by of ET-1 from its receptor and by with its signal that an of for the of of the results summarized have been obtained in animal studies. Although the systems of have been during and of the findings from animal studies have been in is are to be as a basis for the development of new clinical strategies aimed at limiting of hepatic perfusion and This that all from these in animal and studies to are to not only nutritive hepatic blood flow, also the of the liver. Moreover, I/R injury of the liver may to organ failure of the the and the systemic circulation Consequently, it be of major to these may also the after I/R of the liver. The of a physiologic conditions, the influence of and on hepatic blood flow is only and their effects are I/R of the liver, there is an of and However, because this is not in and it may result in an of these at all levels of blood flow regulation that can cause severe perturbations of hepatic For example, the increase in portal resistance after shock at in due to a profound increase in sinusoidal and vasoconstrictive that is only and by increases in Moreover, the profound alterations of nutritive sinusoidal flow described and the subsequent functional may even in the of macrohemodynamic as a result of an heterogeneity of hepatic microvascular flow This has important for the development of new it be the of to a new complete of a is to be this has been primarily on the basis of the interactions between ETs and gaseous it can and be to other mediators in the pathogenesis of of hepatic perfusion after may to a of a new after I/R, i.e., either an of or a of could be to these These include the use of receptor of of of pharmacological that an source of and of of target as well as aimed at the expression of of of the most important changes in the generation and in the hemodynamic effects of nitric oxide (NO), and carbon monoxide after ischemia of the liver during early (<6 and (>6 right reperfusion as described in detail within the Please note that the changes are by in and of and eNOS = endothelial “constitutive” nitric oxide synthase (Type III); iNOS = inducible nitric oxide synthase (Type II); HO-1 = inducible heme oyxgenase-1; HO-2 = constitutive heme for The following results that the of the of a could to hepatic perfusion and under clinical conditions associated with I/R of the liver. of by receptor has been shown to to the release of from the liver, and to reduce postoperative portal pressure by to as well as to increase the survival rate in a of liver transplantation from in the control to in the with the ET of may have a effect under these conditions. For example, with HO-1 significantly portal venous blood flow, increases decreases hepatocyte injury, and the survival rate in a liver effect could be observed after hepatic of HO-1 in a of liver transplantation after cold Moreover, these results that this could also be to other conditions of impaired hepatic For example, it has been shown that the of an increase of portal flow in patients with cirrhosis that is much than the increase in cardiac output, a of blood to the liver by the basis of the results of these it is likely that the new the regulation of hepatic blood flow after I/R may ultimately to the development of new strategies aimed at limiting the subsequent development of liver injury. The to and for the