Glycogen synthesis from pyruvate in the periportal and from glucose in the perivenous zone in perfused livers from fasted rats

Abstract

The isolated liver of 24 h fasted rats was perfused in a non-recirculating manner in the orthograde or retrograde direction with media containing glucose and/or gluconeogenic precursors. Glycogen formation was determined biochemically and demonstrated histochemically. With glucose as the only exogenous substrate glycogen was formed exclusively in the perivenous area during both orthograde and retrograde perfusion. With gluconeogenic precursors as the exogenous substrates glycogen was deposited in the periportal zone during orthograde perfusion and in the intermediate zone during retrograde perfusion. Supply of glucose and gluconeogenic substrates initiated glycogen synthesis only in the upstream region, i.e. in the periportal zone during orthograde and in the perivenous zone during retrograde perfusion. This localization of glycogen synthesis was probably due to an unavoidable, insufficient oxygen supply of the respective downstream area. In general, the results confirm the hypothesis that periportal and perivenous glycogen was synthesized from different substrates.

Mass and Positional Isotopomer Analysis of Glucose Metabolism in Periportal and Pericentral Hepatocytes

“…These results are in qualitative and quantitative agreement with those of our earlier study (i.e. direct flux was approximately the same in the two zones: periportal, 29%; pericentral, 35%) as determined from a comparison of [1][2][3][4][5][6][7][8][9][10][11][12][13] C]glycogen enrichment with plasma [1-…”

“…In vitro, liver perfusion studies showed that glycogen was deposited only in the periportal zone when gluconeogenic precursors were perfused and that glycogen was deposited only in the pericentral zone when glucose was perfused (2). Based on these observations, it has been proposed that hepatocytes in the periportal zone are gluconeogenic and that pericentral hepatocytes are glycolytic (3).…”

“…If so, this has important implications for interpretation of data obtained from virtually all in vivo based studies of liver metabolism. In a recent study, we administered [1][2][3][4][5][6][7][8][9][10][11][12][13] C]glucose to awake unstressed rats and selectively sampled periportal and pericentral glycogen by the dual-digitonin pulse technique (4,5), and we found no difference between the periportal and pericentral zones in the relative percentages of the direct (Glc3 Glc-6-P3 Glc-1-P3 UDP-glc3glycogen) and indirect (3-carbon units333glycogen) pathways of glycogen repletion (4). However, it is still possible that the source of the 3-carbon precursors for liver glycogen may differ between the zones.…”

“…This type of positional isotopomer analysis is unique and the most comprehensive way of addressing this question since it reflects the 13 C labeling in adjacent carbons within the same glucose molecule as opposed to examining the 13 C labeling distribution within a population of glucose molecules. This type of analysis was not possible in our previous study since only [1][2][3][4][5][6][7][8][9][10][11][12][13] C]glucose was used and does not lead to a comparable array of isotopomeric species, which provide a richer wealth of informational content.…”

“…However, there is a subtle distinction in the interpretation of the percent direct flux calculated by these two methods. Using [1][2][3][4][5][6][7][8][9][10][11][12][13] C]glucose as infusate, we calculate the fraction of glycogen synthesized directly from plasma glucose, the remainder of the glycogen synthesized by flux through the indirect pathway is inclusive of all carbon sources. In contrast, only glycogen that originated from plasma glucose is considered when a dilution factor of 1 is used in Equation 1 to calculate the percent direct flux from the mϩ1 3 mϩ6 mass isotopomer distribution of glycogen synthesized from [U- 13 C]glucose.…”