Mass and positional isotopomer analysis of glucose metabolism in periportal and pericentral hepatocytes.

To determine whether the source of carbon for the indirect pathway of hepatic glycogen synthesis differs between the periportal and pericentral zones, we studied seven 24-h-fasted conscious rats given a constant 2-h intraduodenal infusion of glucose, 40% labeled with [U-13C]glucose (99% 13C enriched), to raise and maintain plasma glucose concentration at approximately 10 mM. Glycogen, glutamate, aspartate, and alanine were selectively sampled from the periportal and pericentral zones of the liver by the dual-digitonin pulse technique and analyzed by 13C-NMR for positional isotopomer distribution and by gas chromatography-mass spectrometry for mass isotopomer distribution. Plasma glucose mass isotopomer distribution was determined from gas chromatography-mass spectrometry. The isotopomer distribution indicates that there was no significant difference between the zones with respect to 1) percent direct flux of glucose into the glycogen (periportal, 34 +/- 4; pericentral, 38 +/- 4), 2) extent of oxaloacetate/fumarate equilibration (periportal, 0.54 +/- .01;, pericentral, 0.53 +/- 0.01), 3) dilution of tracer in oxaloacetate (periportal, 0.64 +/- 0.07;, pericentral, 0.64 +/- 0.07), or 4) inflow of pyruvate versus tricarboxylic acid cycle flux (periportal, 0.70 +/- 0.20; pericentral, 0.68 +/- 0.16). Positional isotopomer populations, determined from the 13C-13C splitting in C3 and C4 of periportal and pericentral glycogen, were indistinguishable, indicating no significant differences in the source of the 3-carbon precursors for hepatic glycogen synthesis by the indirect pathway. In conclusion, glucose metabolism is the same in the periportal and pericentral zones with regard to 1) the relative flux of carbon via the direct/indirect pathways, 2) the source of the 3-carbon precursor used in the indirect pathway of glycogen synthesis, and 3) the flux of the 3-carbon precursors through the tricarboxylic acid cycle.