Sungchul Ji

Selective increase in pericentral oxygen gradient in perfused rat liver following ethanol treatment

The oxygen gradient across the liver lobule was measured in isolated, hemoglobin-free perfused livers from control and ethanol-treated rats using micro-light guides and miniature oxygen electrodes. Both techniques yielded similar values for the lobular, periportal and pericentral oxygen gradients. Oxygen uptake increased 33% in livers from rats treated with ethanol for 4 to 6 weeks. In addition, the pericentral oxygen gradient increased by 100%, but the periportal gradient was unchanged. These observations suggest that chronic ethanol treatment may increase the rate of cellular respiration in pericentral regions of the liver lobule selectively. One possible explanation for a region-specific effect of ethanol may be ethanol-induced inhibition of glycolysis which predominates in pericentral hepatocytes.

Alcohol-Induced Liver Injury

Alcoholism is a major health problem, and one of its primary manifestations is alcoholic liver disease. The mechanisms responsible for the various forms of alcoholic liver disease— fatty liver, alcoholic hepatitis, and cirrhosis—are at present poorly understood. Knowledge of these mechanisms is needed to provide a sound framework for the therapy and prevention of liver disease due to alcohol and for the identification of those individuals most susceptible to develop liver disease from alcohol abuse.

Hypoxic hepatocellular injury

Low flow hypoxia to the isolated, perfused rat liver produced stable, circumscribed zones of virtual anoxia which were confined to centrilobular regions of the liver lobule. As a result, centrilobular hepatocytes were injured while periportal tissue was spared. In hypoxic areas, blebs of hepatocyte plasma membrane cytoplasm protruded into sinusoids through fenestrations of the endothelium, evidently as a result of disruption of the cytoskeleton. Upon resumption of normal flow rates (reoxygenation), blebs disappeared and hepatocytes decreased markedly in volume. Concomitantly, sinusoids widened, endothelial fenestrations dilated, and lactate dehydrogenase activity appeared in the effluent. Filtration of perfusates following resumption of flow yielded cytoplasmic fragments, and it was concluded that blebs were released into the circulation. This shedding of cytoplasmic fragments may represent the cellular basis for the appearance of hepatic enzymes in the sera of patients with liver disease.

Lobular Oxygen Gradients: Possible Role in Alcohol-Induced Hepatotoxicity

Alcoholic liver disease is a major health problem, and specific therapies are lacking because we still do not understand how alcohol causes liver damage. The purpose of this chapter is to evaluate the evidence for and against the hypothesis that hypoxia is involved in this disease. We shall review hepatic oxygen uptake, ethanol metabolism and adaptations, and hypoxic tissue damage in general briefly.

Ethanol-induced changes in the intralobular oxygen gradient of perfused rat liver

A new tissue fluorometric method was developed to estimate the intralobular oxygen gradient in hemoglobin-free perfused rat liver. The method employs a two-branch micro-light guide with a tip diameter of 170 mu. With this light guide, it was possible to measure pyridine nucleotide fluorescence (366 nm leads to 450 nm) from periportal and pericentral regions of the liver lobule. By measuring inflow PO2 values at which pyridine nucleotide fluorescence increased in the pericentral regions of the liver lobule, the mean intralobular oxygen gradient was estimated. The measured gradient was approximately 180 torr in livers from sucrose-treated control rats. Chronic treatment with ethanol increased both the mean intralobular oxygen gradient and the rate of hepatic oxygen uptake by 30%. The antithyroid drug, 6-propyl-2-thiouracil, completely reversed the effects of ethanol on both the intralobular oxygen gradient and the rate of oxygen uptake. These data present direct physical evidence that the increased tissue respiration induced by chronic ethanol treatment indeed accentuates the intralobular oxygen gradient and thus support the hypothesis that selective depletion of oxygen in the pericentral region of the liver lobule may underlie ethanol-induced cellular injury confined to this site.

Metabolic heterogeneity in the perfused rat liver

New methods have been developed to monitor metabolic events non-invasively within periportal and pericentral regions of perfused rat livers. These techniques utilize two-fiber micro-light guides and miniature oxygen electrodes positioned on identified lobular regions of the perfused liver based on differential pigmentation of periportal and pericentral areas. Two-fiber micro-light guides detect the fluorescence of native and introduced fluors and are used to monitor redox changes of endogenous pyridine nucleotides and the generation of fluorescent products (e.g., 7-hydroxycoumarin) from exogenous substrates. Changes in fluorescence detected with two-fiber micro-light guides are correlated with changes measured with large, multi-fiber light guides and with whole organ rates of metabolism. Subsequently, local rates are estimated. With these techniques, we show that (a) rates of ethanol and acetaldehyde metabolism are similar in periportal and pericentral regions of the liver lobule; (b) mixed-function oxidation predominantes in pericentral regions in livers from phenobarbital-treated rats; (c) rates of sulfation of 7-hydroxycoumarin are greater in periportal than in pericentral hepatocytes; and (d) oxygen uptake is approximately 3-fold greater in periportal than in pericentral areas of the liver lobule.