Regulation of urea synthesis in sublobular regions of the liver lobule by oxygen

Takei, Yoshiyuki; Kauffman, Frederick C.; Misra, U. K.; Yamanaka, Hirofumi; Thurman, Ronald G.

doi:10.1016/0304-4165(90)90041-t

Cited by 7 publications

(1 citation statement)

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“…First, kinetics of changes in local 02 concentrations correlate with glycolysis in the data in figure 6 under steady-state conditions, where stopped flow was not used and diffusion cannot occur, and agree very well with data from stoppedflow experiments. Moreover, hormones in creased oxygen uptake in downstream re gions, hardly what one would expect if a diffusion artifact exists [73], Further, the oxygen dependence of urea synthesis calcu lated from stopped flow agrees well with data from isolated cylinders of periportal and perivenous regions where there was no stopped flow [74]. Finally, rates calculated in vivo, where oxygen tension is nearly an order of magnitude lower than in perfused liver (i.e.…”

Section: Differences Of Opinion In the Interpretation Of Studies Withsupporting

confidence: 72%

Hepatocyte Heterogeneity in the Metabolism of Carbohydrates

Jungermann

Thurman²

1992

Enzyme

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109

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Periportal and perivenous hepatocytes possess different amounts and activities of the rate-generating enzymes of carbohydrate and oxidative energy metabolism and thus different metabolic capacities. This is the basis of the model of metabolic zonation, according to which periportal cells catalyze predominantly the oxidative catabolism of fatty and amino acids as well as glucose release and glycogen formation via gluconeogenesis, and perivenous cells carry out preferentially glucose uptake for glycogen synthesis and glycolysis coupled to liponeogenesis. The input of humoral and nervous signals into the periportal and perivenous zones is different; gradients of oxygen, substrates and products, hormones and mediators and nerve densities exist which are important not only for the short-term regulation of carbohydrate metabolism but also for the long-term regulation of zonal gene expression. The specialization of periportal and perivenous hepatocytes in carbohydrate metabolism has been well characterized. In vivo evidence is provided by the complex metabolic situation termed the ‘glucose paradox’ and by zonal flux differences calculated on the basis of the distribution of enzymes and metabolites. In vitro evidence is given by the different flux rates determined with classical invasive techniques, e.g. in periportal-like and perivenouslike hepatocytes in cell culture, in periportal- and perivenous-enriched hepatocyte populations and in perfused livers during orthograde and retrograde flow, as well as with noninvasive techniques using miniature oxygen electrodes, e.g. in livers perfused in either direction. Differences of opinion in the interpretation of studies with invasive and noninvasive techniques by the authors are discussed. The declining gradient in oxygen concentrations, the decreasing glucagon/insulin ratio and the different innervation could be important factors in the zonal expression of the genes of carbohydrate-metabolizing enzymes. While it is clear that the hepatocytes sense the glucagon/insulin gradients via the respective hormone receptors, it is not known how they sense different oxygen tensions; the O(2) sensor may be an oxygen-binding heme protein. The zonal separation of glucose release and uptake appears to be important for the liver to operate as a ‘glucostat’. Thus, zonation of carbohydrate metabolism develops gradually during the first weeks of life, in part before and in part with weaning, when (in rat and mouse) the fat- and protein-rich but carbohydrate-poor nutrition via milk is replaced by carbohydrate-rich food. Similarly, zonation of carbohydrate metabolism adapts to longer lasting alterations in the need of a ‘glucostat’, such as starvation, diabetes, portocaval anastomoses or partial hepatectomy.

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Section: Differences Of Opinion In the Interpretation Of Studies Withsupporting

confidence: 72%