Hemodynamics in the microvasculature of thioacetamide-induced cirrhotic rat livers

BACKGROUND/AIMS: To evaluate the hepatic microcirculatory changes in liver cirrhosis, in vivo microscopic findings were assessed quantitatively in cirrhotic rats. METHODOLOGY: Using in vivo microscopy, the blood flow velocity through terminal portal venules and terminal hepatic venules, and their diameters were measured. The rats were classified into a normal group, fibrosis group, and cirrhosis group, histopathologically. To estimate intrahepatic blood flow of the liver surface, laser-Doppler flowmeter was used for the three groups, and portal venous pressures were measured. RESULTS: Blood flow velocity through terminal portal venules increased significantly in cirrhosis rats. However, among the three groups, there were no significant differences with blood flow velocity through terminal portal venules, diameters of terminal portal venules and terminal hepatic venules. Portal venous pressure and intrahepatic blood flow of the liver surface increased significantly. CONCLUSIONS: These data indicate that pre-sinusoidal alterations to hemodynamics become manifest in the liver cirrhosis, which might be related to intrahepatic shunt formation.     

Effect of notoginsenoside R1 on hepatic microcirculation disturbance induced by gut ischemia and reperfusion

AIM： To assess the effect of notoginsenoside R1 on hepatic microcirculatory disturbance induced by gut ischemia/reperfusion （I/R） in mice. METHODS： The superior mesenteric artery （SMA） of C57/BL mice was ligated for 15 min to induce gut ischemia followed by 30-rain reperfusion. In another set of experiments, R1 was continuously infused （10 mg/kg per hour） from 10 min before I/R until the end of the investigation to study the influence of R1 on hepatic microcirculatory disturbance induced by gut I/R. Hepatic microcirculation was observed by inverted microscopy, and the vascular diameter, red blood cell （RBC） velocity and sinusoid perfusion were estimated. Leukocyte rolling and adhesion were observed under a laser confocal microscope. Thirty and 60 min after reperfusion, lactate dehydrogenase （LDH）, alanine aminotransferase （ALl＇） and aspartate transaminase （AST） in peripheral blood were determined. The expression of adhesion molecules CD11b/CD18 in neutrophils and tumor necrosis factor- alpha （TNF-α）, interleukin-6 （IL-6） and monocyte chemotactic protein-1 （MCP-1） in plasma were evaluated by flow Oltometry. E-selectin and intercellular adhesion molecule-1 （ICAM-1） in hepatic tissue were examined by immunofluorescence.RESULTS： After gut I/R, the diameters of terminal portal venules and central veins, RBC velocity and the number of perfused sinusoids were decreased, while the leukocyte rolling and adhesion, the expression of E-selectin in hepatic vessels and CD18 in neutrophils, IL-6, MCP-1, LDH, ALT and AST were increased. R1 treatment attenuated these alterations except for IL-6 and MCP-1. CONCLUSION： R1 prevents I/R-induced hepatic microcirculation disturbance and hepatocyte injury, The effect of R1 is related to its inhibition of leukocyte rolling and adhesion by inhibiting the expression of E-selectin in endothelium and CD18 in neutrophils.     

The microcirculatory hepatic unit

The hepatic microcirculation, the result of the total merging of hepatic arterial and portal stream in the sinusoidal delta, is organized in well-defined microcirculatory hepatic units.The unit consists of a terminal portal venule (TPV) and a glomus of sinusoids branching off from it. The associated hepatic arterioles form a periductular plexus wherefrom efferent arterial capillaries join the sinusoids close to TPV; other arterioles bypass the periductular plexus and empty directly into the sinusoids.The microcirculatory unit provides the vascular framework for the structural and functional hepatic unit, the liver acinus. Simple and complex acini are microscopic masses of parenchyma surrounding like berries their supplying terminal and preterminal vessels, respectively, along with the associated ductules, lymph vessels, and nerve fibres. The structure and dimensions of the hepatic microvasculature and their hemodynamic relevance is discussed.Hydrostatic pressure in the TPV is low. The impressive drop of pressure in the sinusoidal bed down to hepatic venular level is tentatively explained.In the sinusoids, the site of A-V shunting, the energy of arteriolar pressure is transformed into velocity; the fast flow of arterial blood exerts a symphoning effect on interconnected sinusoids.Intermittent flow in the microcirculatory units is regulated by the smooth muscles in the arteriolar wall and by precapillary sphincters responding to nervous stimuli, hormones, metabolites, bile salts, and vasoactive substances.Portal flow is controlled by the mesenteric and splenic microvessels.The definition of the microcirculatory unit within the acinus offers a hemodynamic approach to the understanding of pathologic circulatory and structural changes in the liver.     

Blood flow in hepatic sinusoids in experimental hemorrhagic shock in the rat

Sequential changes of blood flow in the hepatic sinusoids were measured in anesthetized rats which were subjected to the Wiggers method of hemorrhagic shock. The ventral margin of the liver was transilluminated by a fiber-optic light-guide and observed microscopically by a television method. Systemic arterial and portal venous pressures and several direct quantitative microvascular measurements in the sinusoids were recorded, including diameter, erythrocyte velocity, and erythrocyte flux, from which blood flow, hematocrit index, and flow resistance were indirectly calculated. In the initial phase of hemorrhage, hepatic sinusoids constricted with reduced blood flow but showed increased flow resistance and progressive hemoconcentration. When hemorrhage was severe, flow in about two-thirds of the sinusoids observed became stagnant; some of them also dilated while others remained constricted. The possible mechanism for these responses in the hepatic sinusoids to hemorrhagic shock is also discussed.     

Collaterals through hepatic sinusoids after embolization of terminal portal venules: an in vivo study on mice

The changes of intrahepatic microcirculation during the very early stage of portal venule obstruction are not well known. The aim of this study was to clarify the immediate alterations of intrahepatic microcirculation after selective obstruction of the terminal portal venules in vivo. After selective embolization of terminal portal venules with plastic beads whose diameter was 30-35 μm, the hepatic microcirculation of 136 embolized areas on 15 mice was observed with epi-illuminated in vivo fluorescent microscopy. There was reverse blood flow in sinusoids resulting in three types of collaterals after embolization of the terminal portal venules. These collaterals were derived from the proximal trunk of the embolized terminal portal venules, adjacent branches and neighboring terminal portal venules, and they could appear simultaneously with a frequency of 59/136 (43.4%), 77/136 (56.6%) and 131/136 (96.3%), respectively. Collateral formation was strongly correlated with the state of blood flow in the surrounding area (P < 0.0001), but not with the extent of the embolized areas. The results indicated that after embolization of terminal portal venules, reverse blood flow occurred in sinusoids to form collaterals, and these collaterals could maintain the microcirculation of the embolized area.     

Role of hepatic artery in haemodynamics in normal and cirrhotic livers in rats

To examine the degree of influence of the hepatic artery on microcirculation in the liver, microscopic observation of blood flow in the hepatic minute blood vessels and the sinusoids and pressure measurements at key points in hepatic vascular pathways in vivo were performed before and after hepatic artery ligation in normal and cirrhotic rats. In normal rats, portal vein pressure (109 mmH2O) fell 10 mmH2O after hepatic artery ligation, but the pressures of the terminal portal venule, the terminal hepatic venule and the inferior vena cava did not change. In cirrhotic rats, portal vein pressure (206 mmH2O) and terminal portal venule pressure (106 mmH2O) fell 23 and 10 mmH2O after hepatic artery ligation respectively: the pressures in the terminal hepatic venule and the inferior vena cava did not change. These results suggests that the pressure transmitted from the hepatic artery was mostly supplied to the intrahepatic portal vein in normal rats and both to the intrahepatic portal vein and to the sinusoids in cirrhotic rats. In both normal and cirrhotic rats, however, the pressure transmitted from the hepatic artery was about 10 per cent of the initial portal vein pressure, and the blood flow in minute vessels and sinusoids did not change after hepatic artery ligation. Accordingly, it is believed that the hepatic artery plays only a small role in the haemodynamics of the liver in both normal and cirrhotic rats, irrespective of the distribution and manner of the hepatic arterial termination.     

Regulatory mechanisms of hepatic microcirculation

Hepatic microvasculature receives blood from two types of afferent vessels: the terminal portal venule (TPVn) and the terminal hepatic arteriole (THAo). The TPVns directly connect with the capillary bed in the liver parenchyma, which is referred to as sinusoids. Hepatic arterial blood pours into the hepatic sinusoids not only indirectly via the anastomosis between the THAo and the portal venule (PVn), but also directly through the THAo or the capillaries derived from the arterial capillary network around the bile duct. From a regulatory point of view, the hepatic arterial system is considered to be supplementary, but hepatic arterial flow is essential for supplying oxygen to sinusoidal blood flow as well as to the bile ducts, portal venules and nerves in the portal tract. The main regulators of hepatic sinusoidal blood flow are present in the portal venous system. By intravital and scanning electron microscopy, it is evident that a potent vasoconstrictor endothelin (ET)-1 causes a contraction of the SEF via the ET_B receptors, as well as a significant contraction of the PVn and TPVn, resulting in an increase in sinusoidal and pre-sinusoidal microvascular resistance. This phenomenon implies that the TPVn, particularly the transitional part to the sinusoid, would provide an essential regulatory site for hepatic sinusoidal blood flow as an inlet sphincter-like function. The endothelial cell linings along the hepatic sinusoids are characterized by the presence of a large number of sieve plate-like pores, 100 nm in diameter, i.e. the sinusoidal endothelial fenestrae (SEF). The SEF are dynamic structures, forming the racemose invaginations of the endothelial plasma membrane across the endothelium, and regulating not only the permeability of hepatic sinusoids, but also the sinusoidal blood flow by the Ca++ -actomyosin-mediated contraction and dilatation of the SEF. Our recent immunoelectron microscopic and Western blot studies have revealed that caveolin-1, i.e. the principal structural protein of caveolae, and endothelial nitric oxide synthase (eNOS) co-exist in the plasma membrane of the SEF, implying that the SEF may correspond to a permanent (stationary) type of fused and interconnected caveolae, thus contributing to the local control of hepatic sinusoidal blood flow by the regulation of NO synthesis.     

In vivo and electron microscopic observations of the hepatic microvasculature in the rat following portacaval anastomosis

The livers of rats subjected to end-to-side portacaval anastomoses were studied 3 to 5 months postoperatively by in vivo and electron microscopy. Compared with sham-operated controls, the livers of portacaval anastomoses animals contained dilated, tortuous networks of sinusoids. The velocity of blood flow in these vessels tended to be slower and more variable than controls, but always progressed toward the hepatic venules. Blood entered the sinusoids from portal venules and from arteriosinus twigs which terminated in the initial segments of some of the sinusoids at the periphery of the lobule. Together, the arteriosinus twigs and the short, initial segments of these sinusoids formed functional arterioportal anastomoses. These, in combination with the lack of portal venous flow, resulted in retrograde blood flow in portal venules. Nevertheless, blood still flowed from these portal venules into the sinusoids unless the sinusoid was fed by an arteriosinus twig. In addition to these microcirculatory alterations, the number of Kupffer cells that phagocytized latex particles was less in the animals with portacaval anastomoses, as was the number of particles ingested by these cells. Scanning and transmission electron microscopy confirmed the paucity of Kupffer cells. Those seen appeared inactive since they were flattened, exhibited few microplicae and filopodia and contained few latex particles. The endothelial cells of the sinusoid lining were perforated by increased numbers of large fenestrate which may be a reflection of elevated intrasinusoid pressures generated by the expanded arterialization of the sinusoid bed. The observed dilated sinusoid network interspersed by narrowed plates of hepatocytes is also consistent with this hypothesis. Finally, scattered nodular foci were observed which contained enlarged hepatocytes, narrow sinusoids, active Kupffer cells, and more normal rates of blood flow. Such sites may represent attempts by the liver to regenerate its normal architecture.     

Liver microvascular architecture: an insight into the pathophysiology of portal hypertension

Structural adaptations in the liver to constantly receive and release a large volume of circulating blood at low pressure are present at many levels; alteration of these structures can modify flow and perturb pressure gradients. Liver growth multiplies the lobule number by a factor of 4-5 after birth. Lobule configuration conforms with observations in space division, each unit being bordered by planes; curvature will impede expansibility and retractability among units. Lobular organization with hepatocytic plates and sinusoids, being radial centrally and reticular peripherally, maximizes its reversible distensibility. Resistance sites in the portal, sinusoidal, and hepatic system are subject to species variations; real portal sphincters are photographed in the frog. Small venules are demonstrably resistive. In endothelin-1-induced rat portal hypertension, the distal segment of preterminal portal venules constricts most intensely, whereas the terminal portal venules and sinusoids are flaccid. Their pericytes and arachnocytes (stellate cells, Ito cells, retinol-storing cells), respectively, possess no effective contractile machinery. In the dog, the initial sublobular veins react with venoconstriction to many stimulations. Well-developed musculature in hepatic veins, as in man and pig, can regulate flow by junctional constriction. These histoarchitectonics provide hepatic hemodynamics with high capacitance and high compliance properties. The hepatic artery supplies oxygenated blood to five stromal compartments: peribiliary vascular plexus, portal tract interstitium, portal vein vasa vasorum, hepatic capsule, and central-sublobular-hepatic vein vasa vasorum. Its role as the nutrient vessel to the veins is established, but what influence it may have in the pathophysiology of portal hypertension awaits clarification.     

Differential response of the microvasculature in the liver during bacteremia

To determine the initial hepatic microvascular responses to bacteremia, male Sprague-Dawley rats (n = 19) were decerebrated and the left liver lobe from each animal exteriorized and suffused with environmentally controlled Krebs solution. Direct in vivo videomicroscopy was used to measure diameter changes in at least four portal venules (PV) and four proximal periportal sinusoids (PS) at the inlet of hepatic lobules in each of seven livers or four terminal centrilobular sinusoids (CS) and four collecting central venules (CV) at the outlet in each of 12 livers during a baseline period and for 2 hr after intravenous (i.v.) infusion of 1 X 10(9) live Escherichia coli or saline (control). Cardiac output, systemic arterial blood pressure, and body temperature were monitored continuously during the experiments. These data indicate that E. coli bacteremia causes a redistribution of hepatic microvascular blood flow within the liver lobule at both the inlet and outlet regions with increased perfusion of certain microvascular segments and decreased perfusion of others. In the areas observed, a 2:1 dilated/constricted microvessel ratio suggests an initial increased overall liver blood flow within the first 2 hr of experimentally induced bacteremia.     

急性重症酒精性肝炎肝动脉缓冲效应研究进展*

目的 急性重症酒精性肝炎患者肝窦存在致密胶原沉积,阻力增加阻碍了血液流经肝窦,窦性压力增加,门静脉血流不畅,门静脉向肝窦的灌注显著减少,此时就启动了肝动脉缓冲效应,后者可以抵消肝脏灌注的两个主要血管肝动脉或门静脉中任何一个的流量减少,维持肝脏总血流量在一个生理范围内,使肝脏灌注(肝动脉和门静脉血流之和)恢复正常。双功能多普勒超声可以无创评估肝脏血流动力学和定量肝动脉缓冲效应。因此,肝动脉缓冲效应可能成为诊断急性重症酒精性肝炎的重要检测方法之一。     

Intraportal perfusion of prostaglandin E1 attenuates hepatic postischaemic microcirculatory impairments in rats

BACKGROUND: The efficacy of intraportal perfusion with prostaglandin E1(PGE1) in decreasing postischaemic hepatic microcirculatory damage was studied in rats. METHODS: An extrahepatic portosystemic shunt was created by attaching the spleen to a subcutaneous site on the left lateral wall of the abdomen in male Wistar rats weighing between 200 and 350 g. Four weeks later, when the shunt was mature, the portal vein and hepatic artery were occluded for 60 min. The animals were divided into the following three groups according to the type of intraportal perfusion during the ischaemic phase: group 1 consisted of untreated animals; group 2, animals perfused with lactated Ringer's solution; and group 3, animals perfused with PGE1 (0.1 microg/kg per min). The hepatic microcirculation was observed under an inverted intravital microscope after the injection of fluorescent dyes to label leucocytes and damaged cells 30 and 60 min after reperfusion. The liver was removed 60 min after reperfusion and stained immunohistochemically using 1A29, an anti-rat intercellular adhesion molecule-1 (ICAM-1) antibody. RESULTS: The leucocyte velocity during reperfusion was lowest in group 1 and highest in group 3. Of the three groups, group 3 showed the least leucocyte adhesion to the sinusoidal walls and terminal venules, the lowest damaged cell count and the lowest ICAM-1 expression on the sinusoidal walls. CONCLUSION: The results of this study suggest that hepatic perfusion with PGE1 markedly alleviates microcirculatory damage associated with ischaemia and reperfusion through the inhibition of leucocyte-endothelium interactions.     

Anti-ICAM-1 blockade reduces postsinusoidal WBC adherence following cold ischemia and reperfusion, but does not improve early graft function in rat liver transplantation

BACKGROUND/AIM: The present in vivo study investigated the impact of a monoclonal antibody directed against the intercellular adhesion molecule-1 (ICAM-1) on initial microvascular reperfusion injury after liver transplantation. METHODS: Orthotopic, syngeneic liver transplantation including arterial reconstruction was performed in male Lewis rats after 24 h graft storage in University of Wisconsin (UW) solution at 4 degrees C. Animals received either an anti-ICAM-1 antibody (n=7), an IgG1 control antibody (n=8) or saline only (n=7). Hepatic microvascular alterations during the initial 90 min of reperfusion were assessed using intravital fluorescence microscopy. Early graft dysfunction was determined by analysis of bile flow. RESULTS: After treatment with anti-ICAM-1 mAb, hepatic microvascular perfusion was found improved when compared with that of IgG1- and saline-treated controls. In addition, anti-ICAM-1 mAb effectively reduced the number of permanently adherent white blood cells in postsinusoidal venules (284.4+/-59.1 mm(-2) vs IgG1: 371.9+/-26.7 mm(-2) and saline: 431.8+/-46.4 mm(-2); p<0.05). In contrast, the number of stagnant white blood cells in sinusoids was higher (p<0.05) in liver grafts with blocked ICAM-1 (320.6+/-17.2 mm(-2)) compared with that of IgG1- (215.2+/-11.1 mm(-2)) and saline-treated controls (226.4+/-14.0 mm(-2)). Measurement of hepatic uptake of fluorescent-labeled latex particles did not reveal significant differences in phagocytic activity. Finally, bile flow also did not differ between the three groups studied. CONCLUSION: Together these results indicate that ICAM-1 is involved in the process that mediates white blood cells adherence in postsinusoidal venules, whereas in hepatic sinusoids other mechanisms apart from ICAM-1-mediated white blood cells adherence seem to be fundamental for posttransplant white blood cells accumulation. Our data further suggest that white blood cells adherence in postsinusoidal venules via ICAM-1 does not make a major contribution to the pathogenesis of early cold ischemia/reperfusion injury after liver transplantation.     

Hepatic arterial flow becomes the primary supply of sinusoids following partial portal vein ligation in rats

BACKGROUND AND AIM: Partial portal vein ligation (PPVL) is a commonly used procedure to induce prehepatic portal hypertension in animal models. The aim of this study was to test the hypothesis that the hepatic arterial flow becomes the primary source feeding the sinusoids in the liver after PPVL. METHODS: Sprague-Dawley rats underwent either sham operation or partial portal vein ligation (PPVL). The number of vessels in the liver at 2 weeks postoperatively was determined by factor VIII immunolocalization and the gene expression of angiogenic factors was assessed by RT-PCR. The total hepatic arterial supply to the liver was measured using the fluorescent microsphere injection technique. To further test the hypothesis, two additional groups of rats underwent hepatic artery ligation (HAL) or PPVL plus HAL (PPHAL). The integrity of hepatic microcirculation was then evaluated in all four groups by intravital microscopy. RESULTS: At 2 weeks after operation, the number of vessels detected by factor VIII staining was significantly higher in PPVL compared to sham. Densitometric analysis of RT-PCR bands revealed a significant increase of vascular endothelial growth factor gene expression in PPVL compared to sham. Arterial flow to the liver measured by fluorescent microspheres was increased by 190% in PPVL compared to sham. When all four groups were compared, no prominent histological abnormality was observed in sham, HAL, and PPVL groups; however, PPHAL livers showed focal necrosis and inflammatory cell infiltration around the portal triads. Additionally, only the PPHAL livers showed a decreased sinusoidal diameter and significantly lower perfusion index (PPHAL 42.9+/-6.1; sham 85.7+/-7.0, PPVL 80.2+/-6.5, HAL 70.9+/-4.5). CONCLUSIONS: These results suggest that the hepatic artery flow becomes the primary source for the blood supply of sinusoids and the compensatory change in the hepatic arterial system plays a critical role in maintaining microcirculatory perfusion following the restriction of the portal vein flow by PPVL.     

Role of endothelin in endotoxin-induced hepatic microvascular dysfunction in rats fed chronically with ethanol

BACKGROUND: We examined the role of endothelin in endotoxin-induced hepatic microcirculatory disturbance in pair-fed rats given a liquid diet containing ethanol or isocaloric control. METHODS AND RESULTS: One lobe of the liver was observed with the use of an intravital microscope. Erythrocytes (RBCs) labeled with fluorescein isothiocyanate (FITC) were injected, and the flow velocity of the FITC-RBCs in the sinusoids was measured with an off-line velocimeter. The flow velocity decreased 30 min after 1 mg/kg of lipopolysaccharide (LPS) was administered to the controls, and portal pressure (PP) was increased at 60 min. In ethanol-fed rats, however, both the flow velocity and PP increased in the early phase (at 10 min), and in the late phase, flow velocity decreased and PP increased more than in the controls. The LPS-induced decrease in flow velocity was blunted, when BQ-123, an antagonist of endothelin receptor subtype A, was infused into ethanol-fed rats, and BQ-123 also attenuated the change in PP. The plasma endothelin levels in both systemic and portal blood of the ethanol-fed rats were higher than in the controls. CONCLUSIONS: These results suggest that endothelin plays a role in the LPS-induced hepatic microcirculatory disturbance, especially in alcohol-fed animals.     

Hepatic microvascular regulatory mechanisms. I. Adrenergic mechanisms

The neural and pharmacologic responses of the hepatic microvasculature were evaluated in Sprague-Dawley rats anesthetized with urethane or pentobarbital. Various concentrations of several potential vasoactive substances alone or in combination with appropriate blocking agents were administered topically to the livers of these rats while changes in the microvasculature were measured using in vivo microscopic methods. The results provided logarithmic dose-response data for these substances in order to establish the relative sensitivity of various segments of the microvasculature and demonstrated adrenergic receptors in the microvasculature. Alpha receptors were demonstrated on all segments of the microvasculature while beta receptors (beta₂) were isolated on portal venules and sinusoids. In the sinusoids, the lining cells were the site responsive to adrenergic substances. These cells appear to be the primary site for locally regulating flow through the sinusoids. Electrical stimulation of the celiac ganglion and the nerves associated with the celiac artery elicited alpha-mediated constriction of portal venules, hepatic arterioles, and sinusoids while stimulation of the nerves associated with the portal vein elicited responses of a lesser magnitude in these vessels. The constriction of hepatic arterioles was of a much larger magnitude than that of portal venules. All of the microvascular responses to neural stimulation were antagonized by alpha-receptor blockade.     

Hepatic sinusoidal vasodilators improve transplanted cell engraftment and ameliorate microcirculatory perturbations in the liver

After transplantation, hepatocytes entering liver sinusoids are engrafted, whereas cells entrapped in portal spaces are cleared. We studied whether hepatic sinusoidal dilatation will increase the entry of transplanted cells in the liver lobule, improve cell engraftment, and decrease microcirculatory perturbations. F344 rat hepatocytes were transplanted intrasplenically into syngeneic dipeptidyl peptidase IV (DPPIV)-deficient rats. Animals were treated with adrenergic receptor blockers (phentolamine, labetalol), a calcium channel blocker (nifedipine), and splanchnic vasodilators (nitroglycerine, calcitonin gene-related peptide [CGRP], glucagon). Transplanted cells were localized by histochemistry. The hepatic microcirculation was studied with in vivo videomicroscopy. Changes in cell translocations were analyzed by injection of (99m)Tc-labeled hepatocytes. Pretreatment with phentolamine and nitroglycerine increased transplanted cell entry in liver sinusoids, whereas labetalol, nifedipine, CGRP, and glucagon were ineffective. Increased deposition of transplanted cells in sinusoids resulted in greater cell engraftment. In vivo microscopy showed disruption of sinusoidal blood flow immediately after cell transplantation with circulatory restoration requiring more than 12 to 24 hours after cell transplantation. However, in nitroglycerine-treated animals, sinusoidal blood flow was perturbed less. Nitroglycerine did not meaningfully increase intrapulmonary cell translocations. In conclusion, these findings indicate that hepatic sinusoidal capacitance is regulated by alpha-adrenergic- and nitroglycerine-responsive elements. Sinusoidal vasodilatation benefited intrahepatic distribution of transplanted cells and restored hepatic microcirculation after cell transplantation. This shall facilitate optimization of clinical cell transplantation and offers novel ways to investigate vascular mechanisms regulating hepatic sinusoidal reactivity.     

Local regulators of hepatic sinusoidal microcirculation: recent advances

This article reviews our recent studies on the local regulation of hepatic microcirculation with special reference to the inlet sphincter-like structures, the roles of sinusoidal endothelial cells and the mechanism of dynamic changes in the sinusoidal endothelial fenestrae (SEF) as well as in the terminal portal venules and the terminal hepatic arterioles induced by the potent vasoconstrictor endothelin (ET)-1. There are two types of sphincter-like structures at the entering sites of hepatic sinusoids. One is located at the junction between the terminal portal venule and the sinusoid, and is characterized by the large endothelial cells surrounded with Ito cells (hepatic stellate cells: HSCs). The other is located at the junction between the terminal hepatic arteriole and the sinusoid, and corresponds to the precapillary sphincter since our enzymohistochemical demonstration of arterial capillaries in close association with the sinusoids combined with intravital microscopy has revealed that the terminal hepatic arteriole directly terminates in the sinusoid. It is essential for the local control of hepatic sinusoidal blood flow that the dynamic contracting and relaxing changes not only in these inlet sphincter-like structures but also in the SEF correspond with those of the HSCs, both of which are mediated by the sinusoidal endothelium-derived vasoconstrictor endothelins (ETs) and vasodilator nitric oxide (NO). The contractility of the SEF and HSCs depends on the intracellular Ca++-calmodulin-actomyosin system.     

Morphological mechanisms for regulating blood flow through hepatic sinusoids

This review summarizes what is known about the various morphological sites that regulate the distribution of blood flow to and from the sinusoids in the hepatic microvascular system. These sites potentially include the various segments of the afferent portal venules and hepatic arterioles, the sinusoids themselves, and central and hepatic venules. Given the paucity of smooth muscle in the walls of these vessels, various sinusoidal lining cells have been suggested to play a role in regulating the diameters of sinusoids and influencing the distribution and velocity of blood flow in these vessels. While sinusoidal endothelial cells have been demonstrated to be contractile and to exhibit sphincter function, attention has recently focused on the perisinusoidal stellate cell as the cell responsible for controlling the sinusoidal diameter. A very recent study, however, suggested that the principal site of vasoconstriction elicited by ET-1 was the pre-terminal portal venule. This raised the question of whether or not the diameters of sinusoids might decrease due to passive recoil when inflow is reduced or eliminated and intra-sinusoidal pressure falls. In more recent in vivo microscopic studies, clamping of the portal vein dramatically reduced sinusoidal blood flow as well as the diameters of sinusoids. The sinusoidal lumens rapidly returned to their initial diameters upon restoration of portal blood flow suggesting that sinusoidal blood pressure normally distends the sinusoidal wall which can recoil when the pressure drops. Stellate cells may be responsible for this reaction given the nature of their attachment to parenchymal cells by obliquely oriented microprojections from the lateral edges of their subendothelial processes. This suggests that care must be exercised when interpreting the mechanism for the reduction of sinusoidal diameters following drug administration without knowledge of changes occurring to the portal venous and hepatic inflow.     

Shunting of the microcirculation after mesenteric ischemia and reperfusion is a function of ischemia time and increases mortality

OBJECTIVE: Shunting of the microcirculation contributes to the pathology of sepsis and septic shock. The authors address the hypothesis that shunting of the microcirculation occurs after superior mesenteric artery occlusion (SMAO) and reperfusion, and explore functional consequences. METHODS: Spontaneously breathing animals (rats) (n = 30) underwent SMAO for 0 (controls), 30 (SMAO_30) or 60 min (SMAO_60) followed by reperfusion (4 h) with normal saline. Leukocyte-endothelial interactions in mesenteric venules were quantified in an exteriorized ileal loop using intravital microscopy. Abdominal blood flow was recorded continuously, and arterial blood gases were analyzed at intervals. The above groups were matched by comparable groups with continuous superior mesenteric artery blood flow measurements and without exteriorizing an ileal loop (controls*, SMAO_30*, SMAO_60*). RESULTS: Adherent leukocytes increased shortly after reperfusion in ischemia groups, and plateaued in these groups. Centerline velocity in the recorded venules was significantly reduced after reperfusion down to low-flow/no-flow in SMAO_60 as compared to SMAO_30 and controls, whereas perfusion of the SMA and ileal vessels persisted. The microcirculatory changes in SMAO_60 were accompanied by progressive metabolic acidosis, substantially larger volumes of intravenous fluids needed to support arterial blood pressure and significantly reduced survival (30%). SMA blood flow increased in relation to abdominal blood flow after reperfusion in SMAO_60*, and remained constant in SMAO_30* and controls*. Survival was 80% in SMAO_60*. CONCLUSION: Shunting of the microcirculation can be observed after SMAO for 60 min and reperfusion, and contributes significantly to the pathology of mesenteric ischemia and poor outcome.