Hepatocellular dysfunction–basic considerations1

Jarrar D, Wang P, Chaudry IH.

Publication Details

Introduction

Accidental or intentional physical injuries remain the leading cause of death during the first three decades of life and rank the fourth overall in causes of mortality in the United States. Severe hemorrhage, which often occurs with trauma, is known to produce many life-threatening sequelae. Subsequent sepsis, septic shock, and the ensuing multiple organ failure (MOF) continue to be the most common causes of mortality in surgical intensive care units. Despite advances in the management of the critically ill patient, the incidence of sepsis has increased significantly over the past decade. It is encouraging, however, that the complex pathophysiology and pathobiology of trauma and sepsis are been better understood as more studies have been reported. Through such investigations, novel information may be forthcoming which will lead to a better management of trauma victims.

Although dysfunction of organ systems such as the cardiovascular system, liver, intestines and adrenals occurs early after trauma and hemorrhage, and remains depressed for a prolonged period of time (15), the liver's central role in metabolism and homeostasis makes this organ the critical one for survival of the host following severe injury. The liver lies in the right upper quadrant of the abdominal cavity and is attached to the diaphragm. It is the largest gland in the human body, and also a vital blood reservoir by virtue of its large vascular capacity. Under normal resting conditions in humans, total hepatic blood flow is 1200 to 1400 ml/min (~ 100 ml/min/100 g), which represents about 25% of cardiac output. Blood supply to the liver originates from two sources, the portal vein and hepatic artery. The hepatic artery is a branch of the celiac axis and accounts for 25–30% of total hepatic blood flow and 40–50% of the oxygen supply. The portal vein carries blood from the stomach, spleen, pancreas, and intestines, and supplies 70–75% of total hepatic blood flow but only 50–60% of the oxygen supply, since this blood is partially deoxygenated in the preportal tissues. In addition, the liver is the major site of amino acid, carbohydrate, and fat metabolism. Synthesis of important proteins such as albumin and the coagulation factors also takes place in the liver parenchyma. Moreover, secretion of bilirubin from the destruction of senescent erythrocytes and biotransformation of xenobiotics by the cytochrome P-450 system are also vital functions of the liver. Thus, any disturbance of liver function may adversely affect host defense mechanisms.

Hepatic damage versus hepatocellular dysfunction

In order to understand the subtle alterations in hepatocellular function, it is important to distinguish the difference between active hepatocellular dysfunction, [e.g., energy-dependent transport processes of indocyanine green (ICG)] and hepatic damage which indicates irreversibility of the insult on the hepatocyte. Clinically, the most common method of determining “liver function” is to assess the release of liver enzymes such as aspartate and alanine aminotransferase (AST, ALT) into the circulation. It should be noted that the elevated levels of circulating liver enzymes is a reliable indicator of liver injury, but does not provide any indication of the depression in active hepatocellular function which occurs prior to hepatocyte damage. Furthermore, release of cellular enzymes into the circulation can be misleading, since the most severely injured areas are most poorly perfused, resulting in retention of the enzymes in the interstitial spaces of necrotic portions of the liver. Assessment of hepatocellular function is an important issue for the clinician so that she/he can alter therapeutic interventions in order to prevent hepatocyte damage and also to evaluate the efficiency of the treatment modality. In addition, the long half-life time of ALT and AST makes it difficult to monitor the efficiency of any intervention. Moreover, a variety of organs other than the liver, for example the heart, skeletal muscle, adipose tissue, brain, and kidney, contain aminotransferases, and the lack of tissue specificity limits the clinical usefulness of liver enzymes as an indicator of hepatic injury. Because the elevated serum levels of liver enzymes do not accurately reflect hepatocellular dysfunction but rather liver damage, an alternative method to assess active hepatocellular function following trauma-hemorrhage has been developed, as described below.

Hepatocellular function assessment

The unique capacity of the liver to remove substances from the circulation makes clearance techniques well suited for measurement of active hepatocellular function. Studies have shown that negligible extra-hepatic elimination of ICG makes it well suited for estimation of hepatocellular function (6). ICG is cleared exclusively by the liver through an energy-dependent membrane transport process and low doses of ICG have been used extensively to measure hepatocyte functions in various human diseases. In 1970 Paumgartner and colleagues suggested that the capacity of the liver to remove ICG from the circulation has a maximal limit and that the classic Michaelis-Menten kinetics (with Lineweaver-Burk plot) could be applied to the initial ICG uptake in the rat and human liver. The maximal velocity (Vmax) of ICG clearance can be determined from three or more submaximal doses of ICG, and appears to be a reliable measure of active hepatocellular function. In this regard, we have developed a small animal model in which repeated measurements of active hepatocellular function using the ICG clearance technique can be performed (6, 7). To avoid the need for repeated blood sampling, a technique for in vivo ICG measurement using a fiber-optic catheter and an in vivo hemoreflectometer was utilized (6, 8). In addition to determination of active hepatocellular functions, this technique allows one to measure cardiac output, effective hepatic blood flow, circulating blood volume, and oxygen saturation (2, 7, 9). Three different doses of ICG (0.167, 0.333, and 0.833 mg/kg body weight) were administered intravenously, and ICG concentration was measured in vivo with a fiber-optic catheter and an in vivo hemoreflectometer. Vmax of ICG clearance (the number of functional ICG receptors/carriers on hepatocytes) and the kinetic constant (Km: the efficiency of the active transport processes) were determined from the Lineweaver-Burk plot (8).

Alterations in hepatocellular function following trauma and hemorrhage

A non-heparinized model of trauma and severe hemorrhagic shock in the rat was used in our studies. In brief, a 5-cm midline laparotomy (representing trauma) was performed in adult male rats, and the animals were then bled to and maintained at a mean arterial pressure of 40 mmHg until the animal could not maintain this pressure. Ringer's lactate (RL) was given to maintain a blood pressure of 40 mmHg until 40% of the maximal shed blood volume was returned in the form of RL. The rats were then resuscitated with 4 times the shed blood volume in the form of RL over 60 min. This model, in which about 60% of the total circulating blood volume is removed, has a mortality of ~ 40–50% over 48 h, and allows one to study the consequences of trauma-hemorrhage on various organ systems, and also the potential beneficial effects of different pharmacological agents and resuscitation regimens. Our findings have indicated that both Vmax and Km values decreased significantly at the time of maximal bleedout (~ 45 min after the onset of hemorrhage) (8). Resuscitation with 2–3 times the volume of maximal bleedout in the form of Ringer's lactate did not restore the depressed Vmax of ICG clearance at 4 h after the completion of resuscitation (8). Similarly, hepatocellular function remained depressed when the fluid resuscitation volume increased from 2–3 to 4–5 times the volume of maximal bleedout with Ringer's lactate following hemorrhagic shock (1). Thus, active hepatocellular dysfunction occurred early during hemorrhage and persisted despite large volume of crystalloid resuscitation. In addition, hepatic microsomal enzyme activity was significantly compromised following an adverse circulatory event, leading to increased circulating levels of glucocorticoids and thereby depressing adrenal function (5). The diminished hepatic energy status following trauma and hemorrhage has been shown to be an important contributor of hepatocellular dysfunction (10). Since the increase of the resuscitation volume had minimal effects on active hepatocellular function following trauma and hemorrhage, we performed additional studies to determine whether administration of pharmacological agents has any salutary effects on hepatocellular function. The results indicated that administration of a non-anticoagulant heparin (GM 1892) during resuscitation significantly improved the depressed Vmax of ICG clearance in addition to producing beneficial effects on cardiac output and hepatic blood flow (11). In addition, infusion of ATP-MgCl2 during and following fluid resuscitation in hemorrhaged animals restored the depressed hepatocellular function and hepatic microcirculation. It appears that the beneficial effects of ATP-MgCl2 were associated with down-regulation of proinflammatory cytokines such as TNF and IL-6 (12). Similarly, administration of diltiazem, a calcium antagonist, after hemorrhage and resuscitation significantly improved hepatocellular function (13). Thus, administration of pharmacological agents such as ATP-MgCl2, diltiazem or GM 1892 during resuscitation attenuated hepatocellular dysfunction following trauma-hemorrhage even in the absence of blood transfusion.

Effects of sepsis on hepatocellular function

Sepsis, which is a common complication following trauma and hemorrhage, can be clinically and experimentally divided into two stages: an early, hyperdynamic phase (characterized by an increased cardiac output, tissue perfusion, and decreased vascular resistance) followed by a late, hypodynamic phase (characterized by a reduced cardiac output and tissue perfusion) (6). To study the effects of infection on hepatocellular function, a single hit model of cecal ligation and puncture (CLP), which produces polymicrobial sepsis, and mimics many features of clinical peritonitis, was used (14). The results indicated that hepatocellular dysfunction occurred earlier than the onset of hyperdynamic circulation (6, 15), and persisted despite increased volume of crystalloid resuscitation. The fact that both Vmax and Km of ICG clearance decreased significantly as early as 1.5 h after CLP (6), and that circulating levels of liver enzymes (AST, ALT) did not increase until 10 h after the onset of sepsis, suggests that in vivo ICG clearance is an extremely sensitive measure of hepatocellular function under pathophysiological conditions. This technique, therefore, should be used not only experimentally but also clinically for detecting early hepatocellular abnormality.

The role of proinflammatory cytokines in producing hepatocellular dysfunction

The release of proinflammatory cytokines such as tumor necrosis factor alpha (TNF-α) may play an important role in producing hepatocellular dysfunction following trauma-hemorrhage and sepsis. In this regard, it has been shown that trauma-hemorrhage leads to increased circulating levels of TNF-α as early as 45 min after the onset of blood loss (16). In addition, our studies have shown that infusion of exogenous TNF-α, in a concentration that does not compromise cardiac output and hepatic microcirculation, produces hepatocellular dysfunction to an extent similar to that seen after trauma-hemorrhage (17). Kupffer cells, which represent the largest pool of sessile macrophages, are an important source of proinflammatory cytokine release. Inhibition of TNF-α production and/or release by pharmacological agents such as pentoxifylline or ATP-MgCl2 has been shown to be an effective approach to improve hepatocellular function following trauma and hemorrhage and also to improve the survival rate from subsequent sepsis (12, 18).

Summary

In vivo ICG clearance is an extremely sensitive and reliable technique for assessing early changes in hepatocellular function following trauma and hemorrhage as well as during polymicrobial sepsis. Our results indicate that active hepatocellular function is compromised early after hemorrhage or sepsis, and persists despite fluid resuscitation. Although early treatment of trauma victims includes control of bleeding and rapid restoration of intravascular volume to improve tissue perfusion, cell and organ dysfunction can not be prevented by simply increasing the volume of crystalloid fluids. Thus, pharmacologic agents in addition to fluid resuscitation are necessary to prevent hepatocellular dysfunction and ultimately liver failure. In this regard, adjuncts such as pentoxifylline or short term use of testosterone receptor antagonism have been shown to be effective in restoring the depressed hepatocellular and cardiovascular functions, as well as the cell-mediated immune responses following trauma-hemorrhage (1820).

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Footnotes

This investigation was supported by the National Institute of Health grant RO1 GM 39519 (I.H.C.) and R29 GM 53008 (P.W.). P. Wang is the recipient of the NIH Independent Scientist Award KO2 AI 01461.