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 pericent ral zones, we studied seven 24-h-fasted conscious rats given a constan t 2-h intraduodenal infusion of glucose, 40% labeled with [U-C-13]gluc ose (99% C-13 enriched), to raise and maintain plasma glucose concentr ation at similar to 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 C-13 NMR for positional isotopomer distribution and by gas chromatography-m ass spectrometry for mass isotopomer distribution. Plasma glucose mass isotopomer distribution was determined from gas chromatography-mass s pectrometry. The isotopomer distribution indicates that there was no s ignificant difference between the zones with respect to 1) percent dir ect flux of glucose into the glycogen (periportal, 34 +/- 4; pericentr al, 38 +/- 4), 2) extent of oxaloacetate/fumarate equilibration (perip ortal, 0.54 +/- .01;, pericentral, 0.53 +/- 0.01), 3) dilution of trac er 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 iso topomer populations, determined from the C-13-C-13 splitting in C3 and C4 of periportal and pericentral glycogen, were indistinguishable, in dicating no significant differences in the source of the 3-carbon prec ursors for hepatic glycogen synthesis by the indirect pathway, In conc lusion, glucose metabolism is the same in the periportal and pericentr al 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-carb on precursors through the tricarboxylic acid cycle.