Distribution pattern of alanine aminotransferase activity in rat liver

Role of BAF60a/BAF60c in chromatin remodeling and hepatic lipid metabolism

The switching defective/sucrose non-fermenting (SWI/SNF) complexes play an important role in hepatic lipid metabolism regulating both transcriptional activation and repression. BAF60a is a core subunit of the SWI/SNF chromatin-remodeling complexes that activates the transcription of fatty acid oxidation genes during fasting/glucagon. BAF60c, another subunit of SWI/SNF complexes, is recruited to form the lipoBAF complex that activates lipogenic genes, promoting lipogenesis and increasing the triglyceride level in response to feeding/insulin. Interestingly, hepatocytes located in the periportal and perivenous zones of the liver display a remarkable heterogeneity in the activity of various enzymes, metabolic functions and gene expression. Especially, fatty-acid oxidation was shown to be mostly periportal, whereas lipogenesis was mostly perivenous. Therefore, the present review highlights the role of of SWI/SNF regulating lipid metabolism under nutritional and hormonal control, which may be associated with hepatocyte heterogeneity.

Stereological measurement of porto-central gradients in gene expression in mouse liver

The liver is thought to consist of lobules, numerous repeating, randomly oriented units. Within these lobules, genes are expressed in gradients along the porto-central axis, which spans the distance between portal and central veins. We have developed a robust stereological method to map all points in an image to their position on this porto-central axis. This approach is based on the distribution of well-characterized periportal and pericentral enzymes, which are visualized on sections preceding and following the section of interest. Because expression of the model genes phosphoenolpyruvate carboxykinase and ornithine aminotransferase declines gradually with increasing distance from the portal vein and central vein, respectively, these genes can be used to prepare images with topographical information without any assumption about the shape of the hepatic unit, or about the direction or shape of the gradient to be determined. The "relative distance" image is a 2-dimensional image that accurately maps the relative position of hepatocytes on the porto-central axis in 3-dimensional space. It is superimposed on the serial section under investigation to relate local staining density to position on the porto-central axis and obtain the gene expression gradient. The method was used to determine the expression gradient of 2 periportal and 2 pericentral enzymes and their response to fasting. The "total distance" image was used to measure the length of the porto-central axis, which was approximately 210 microm in mice and found to decrease 13% after 1 day of starvation. The method can be applied to any tissue component that can be stained quantitatively.

Isocitrate dehydrogenase as a marker of centrilobular hepatic necrosis in the experimental model of rats

BACKGROUND AND AIMS

Serum alanine aminotransferase (ALT) and aspartate aminotransferase may not detect centrilobular hepatic necrosis (CLN) of a mild degree because these enzymes are known to be located predominantly in the periportal area. The aim of this study was to evaluate the usefulness of plasma isocitrate dehydrogenase (ICDH), which is located predominantly in the centrilobular zone, as a marker of CLN.

METHODS

In 56 adult male rats, centrilobular (n = 21) and periportal hepatic necrosis (PPN; n = 21) were induced experimentally by the intraperitoneal injection of bromobenzene and allylalcohol, respectively. Seven rats were used as solvent controls in both groups. Isocitrate dehydrogenase and ALT activities were measured in the plasma of rats with mild to moderate hepatic necrosis (17 CLN and 19 PPN). Isocitrate dehydrogenase and ALT were compared according to the sampling time (12, 24 and 48 h) and the location of hepatic necrosis. Ratios of ICDH/ALT were also calculated and compared between CLN and PPN groups at any time points.

RESULTS

Plasma ICDH activities were higher in rats with CLN than in those with PPN. In contrast, plasma ALT levels were higher in rats with PPN than in those with CLN at 12 h and were similar in both groups after 12 h. The ICDH/ALT ratios were much higher in rats with CLN compared to those with PPN (P< 0.001). The ratios were above 1.0 in 13 of 17 rats (77%) with CLN in contrast to none of the 19 rats with PPN.

CONCLUSIONS

Our data suggested that the plasma ICDH/ALT ratio might be useful to differentiate between mild to moderate degrees of CLN from PPN, at least in the experimental model of rats.

Lobular distribution pattern of lactate dehydrogenase and 6-phosphogluconate dehydrogenase activity in rat liver

Lactate dehydrogenase (LDH) and 6-phosphogluconate dehydrogenase (6-PGDH) activities were measured in lobular areas expanding between 3 portal tracts and an efferent central vein in the livers of male Wistar rats, using a Lowry technique. The maximum of LDH activity was found in a nearly uniform broad area in the lobular periphery. From that area values decreased along periportal/septal-->perivenous gradients, but only slightly within that area along the periportal-->septal axis of the vascular septum. Maximum values of 6-PGDH activity were present in an intermediate area close to the central vein demonstrating a rather inhomogeneous distribution pattern without a clear definition of zonal limits. Our data on the distribution pattern of LDH are in agreement with the concept of the metabolic lobulus and are supported by a recent evaluation of the vascular architecture in rat liver. The lobular distribution pattern of 6-PGDH cannot be interpreted without doubt in accordance with that concept.

Three-dimensional reconstruction of parenchymal units in the liver of the rat

To investigate the parenchymal units in the liver of the rat three-dimensionally, 15 micrometer cryosections were used for the demonstration of glucose-6-phosphatase (G6Pase) activity to visualize the borders of the individual units. Together with the supplying and draining vessels, they were traced through a sequence of 146 sections and reconstructed. A cone-shaped secondary unit with a height of 2.1 mm and a volume of 3.3 mm3 was reconstructed. It was "covered" by a continuous vascular surface, consisting of portal tracts and vascular septa, connecting the portal venular branches. The secondary unit was subdivided by portal tracts and vascular septa, and by branches of a draining central venular tree into 14 primary units. Most of them were tri- to heptahedral in shape. The height varied between 330 and 840 micrometer, and the volume varied between 0.094 and 0.621 mm3. The branches of the portal venular tree, with diameters from 28 +/- 5 micrometer to 61 +/- 14 micrometer, were oriented preferentially along the vertical axis of the units. Most of the primary units were drained by single branches of the central venular tree, located in the center and oriented along the vertical axis of the units. Vessel diameters ranged from 62 +/- 14 micrometer to 216 +/- 9 micrometer. The average length of the sinusoids was 355 +/- 3 micrometer. From the results of this reconstruction study, it was concluded that the concept of the liver acinus cannot be applied to the liver of the rat.

Effect of pretreatment with some mixed-function oxidase enzyme inducers on the acute hepatotoxicity of coumarin in the rat

Male Sprague-Dawley rats were pretreated with saline, corn oil, sodium phenobarbitone (PB) (100 mg/kg body weight/day), 20-methylcholanthrene (20 MC) (20 mg/kg body weight/day) or Aroclor 1254 (ARO) (100 mg/kg body weight/day) by daily ip injections for 5 days. Animals were then given single oral doses of either 250 or 500 mg coumarin/kg body weight and hepatotoxicity was assessed after 24 hr. Coumarin produced hepatotoxicity, which comprised hepatocyte necrosis and elevation of plasma alanine aminotransferase and aspartate aminotransferase activities, in all pretreated groups. Hepatic microsomal cytochrome P-450 levels were reduced after coumarin administration. In rats pretreated with saline, corn oil or PB, coumarin produced centrilobular hepatic necrosis, whereas in rats pretreated with 20 MC or ARO, coumarin produced periportal hepatic necrosis. These results demonstrate that mixed-function oxidase enzyme inducers can modulate acute coumarin-induced hepatotoxicity in the rat. As coumarin is known to be bioactivated by cytochrome P-450-dependent enzymes, the change in the lobular distribution of toxicity after pretreatment with 20 MC or ARO is presumably due to the induction of particular cytochrome P-450 isoenzymes in periportal hepatocytes.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.