GLUCONEOGENESIS AND INTRAHEPATIC TRIOSE PHOSPHATE FLUX IN RESPONSE TOFASTING OR SUBSTRATE LOADS - APPLICATION OF THE MASS ISOTOPOMER DISTRIBUTION ANALYSIS TECHNIQUE WITH TESTING OF ASSUMPTIONS AND POTENTIAL PROBLEMS

We measured gluconeogenesis (GNG) in rats by mass isotopomer distribut ion analysis, which allows enrichment of the true biosynthetic precurs or pool (hepatic cytosolic triose phosphates) to be determined. Fracti onal GNG from infused [3-C-13]lactate, [1-C-13]lactate, and [2-C-13]gl ycerol was 88 +/- 2, 89 +/- 3, and 87 +/- 2%, respectively, after 48 h of fasting, [2-C-13]Glycerol was the most efficient label and allowed measurement of rate of ap appearance of intrahepatic triose phosphate (Ra triose-P), by dilution. IV fructose (10-15 mg/kg/min) increased a bsolute GNG by 81-147%, Ra triose-P increased proportionately, but end ogenous Ra triose-P was almost completely suppressed, suggesting feedb ack control, Interestingly, 15-17% of fructose was directly converted to glucose without entering hepatic triose-P, IV glucose reduced GNG a nd Ra triose-P, 24-h fasting reduced hepatic glucose production by hal f, but absolute GNG was unchanged due to increased fractional GNG (51- 87%). Reduced hepatic glucose production was entirely due to decreased glycogen input, from 7.3 +/- 1.8 to 1.1 +/- 0.2 mg/kg/min, Ra triose- P fell during fasting, but efficiency of triose-P disposal into GNG in creased, maintaining GNG constant, Secreted glucuronyl conjugates and plasma glucose results correlated closely, In summary, GNG and intrahe patic triose-P flux can be measured by mass isotopomer distribution an alysis with [2-C-13]glycerol.