Abstract
Apart from impaired beta-oxidation, Pparalpha-deficient (Pparalpha(-/-)) mice suffer from hypoglycemia during prolonged fasting, suggesting alterations in hepatic glucose metabolism. We compared hepatic glucose metabolism in vivo in wild type (WT) and Pparalpha(-/-) mice after a short term fast, applying novel isotopic methods. After a 9-h fast, mice were infused with [U-(13)C]glucose, [2-(13)C]glycerol, [1-(2)H]galactose, and paracetamol for 6 h, and blood and urine was collected in timed intervals. Plasma glucose concentrations remained constant and were not different between the groups. Hepatic glycogen content was 69 +/- 11 and 90 +/- 31 microM/g liver after 15 h of fasting in WT and Pparalpha(-/-) mice, respectively. The gluconeogenic flux toward glucose 6-phosphate was not different between the groups (i.e. 157 +/- 9 and 153 +/- 9 microM/kg/min in WT and Pparalpha(-/-) mice, respectively). The gluconeogenic flux toward plasma glucose, however, was decreased in PPARalpha(-/-) mice (i.e. 142 +/- 9 versus 124 +/- 13 microM/kg/min) (p < 0.05), accounting for the observed decrease (-15%) in hepatic glucose production in Pparalpha(-/-) mice. Expression of the gene encoding glucose-6-phosphate hydrolase (G6ph) was lower in the PPARalpha(-/-) mice compared with WT mice. In conclusion, Pparalpha(-/-) mice were able to maintain a normal total gluconeogenic flux to glucose 6-phosphate during moderate fasting, despite their inability to up-regulate beta-oxidation. However, this gluconeogenic flux was directed more toward glycogen, leading to a decreased hepatic glucose output. This was associated with a down-regulation of the expression of G6ph in PPARalpha-deficient mice.
Highlights
Fuel selection to meet the body’s energy demand is of crucial importance during feeding-fasting transitions
Glucose concentration tended to be lower at 15 h of fasting but became significantly lower after 24 h of fasting in Ppar␣Ϫ/Ϫ mice compared with wild type (WT) mice
The expressions of glucose-6-phosphate hydrolase (G6ph) and Gs were significantly affected at 15 h of fasting in Ppar␣Ϫ/Ϫ mice compared with WT mice
Summary
Animals—Male Ppar␣Ϫ/Ϫ mice and WT mice on a SV129 background were housed in a temperature-controlled (21 °C) room on a 10-h dark, 14-h light cycle. Mice were fasted up to 24 h, and blood samples were taken at t ϭ 0 (fed), 15, and 24 h by tail bleeding, and livers were removed at t ϭ 0, 15, and 24 h for lipid analysis and RNA isolation. A large blood sample was taken by heart puncture under halothane anesthesia at the end of the experiment, and the liver was quickly excised and frozen immediately in liquid N2 for lipid analysis and RNA isolation. Hepatic ATP levels were measured using a bioluminescence assay kit (Roche Applied Science). Measurement of Mass Isotope Distribution by GC-MS—Analytical procedures for extraction of glucose and paracetamol-glucuronide from bloodspot and urine filter paper strips, respectively, derivatization of the extracted compounds, and GC-MS measurements of derivatives were essentially according to Van Dijk et al [5, 7]. For each derivative, the fractional contribution of m1, m2, and m6 to total ion Glucose metabolism
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