title: Gluconeogenesis and intrahepatic triose phosphate flux in response to fasting or substrate loads. Application of the mass isotopomer distribution analysis technique with testing of assumptions and potential problems.
abstract: We measured gluconeogenesis (GNG) in rats by mass isotopomer distribution analysis, which allows enrichment of the true biosynthetic precursor pool (hepatic cytosolic triose phosphates) to be determined. Fractional GNG from infused [3-13C]lactate, [1-13C]lactate, and [2-13C]glycerol was 88 +/- 2, 89 +/- 3, and 87 +/- 2%, respectively, after 48 h of fasting. [2-13C]Glycerol was the most efficient label and allowed measurement of rate of appearance of intrahepatic triose phosphate (Ra triose-P), by dilution. IV fructose (10-15 mg/kg/min) increased absolute GNG by 81-147%. Ra triose-P increased proportionately, but endogenous Ra triose-P was almost completely suppressed, suggesting feedback control. Interestingly, 15-17% of fructose was directly converted to glucose without entering hepatic triose-P. IV glucose reduced GNG and Ra triose-P. 24-h fasting reduced hepatic glucose production by half, 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 increased, maintaining GNG constant. Secreted glucuronyl conjugates and plasma glucose results correlated closely. In summary, GNG and intrahepatic triose-P flux can be measured by mass isotopomer distribution analysis with [2-13C]glycerol.
authors.affiliation: Department of Nutritional Sciences, University of California, Berkeley 94720, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Neese
authors.forename: J M
authors.initials: JM
authors.lastname: Schwarz
authors.forename: D
authors.initials: D
authors.lastname: Faix
authors.forename: S
authors.initials: S
authors.lastname: Turner
authors.forename: A
authors.initials: A
authors.lastname: Letscher
authors.forename: D
authors.initials: D
authors.lastname: Vu
authors.forename: M K
authors.initials: MK
authors.lastname: Hellerstein
mesh_terms: D000818:Animals; D001692:Biological Transport; D002247:Carbon Isotopes; D002849:Chromatography, Gas; D005508:Food Deprivation; D005632:Fructose / Q000008:administration & dosage* / Q000378:metabolism; D005943:Gluconeogenesis / Q000502:physiology*; D005947:Glucose / Q000008:administration & dosage* / Q000032:analysis / Q000378:metabolism; D005990:Glycerol / Q000378:metabolism; D007773:Lactates / Q000378:metabolism; D019344:Lactic Acid; D008099:Liver / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D013403:Sugar Phosphates / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D007773:Lactates; D013403:Sugar Phosphates; D005632:Fructose; D019344:Lactic Acid; D005947:Glucose; D005990:Glycerol