Non-invasive tracing of liver intermediary metabolism in normal subjects and in moderately hyperglycaemic NIDDM subjects. Evidence against increased gluconeogenesis and hepatic fatty acid oxidation in NIDDM

Abstract

To test whether gluconeogenesis is increased in non-insulin-dependent diabetic (NIDDM) patients we infused (post-absorptive state) healthy subjects and NIDDM patients with [6,6-2H2]glucose (150 min) and [3-13C]lactate (6 h). Liver glutamine was sampled with phenylacetate and its labelling pattern determined (mass spectrometry) after purification of the glutamine moiety of urinary phenylacetyl-glutamine. After correction for 13CO2 re- incorporation (control test with NaH13CO3 infusion) this pattern was used to calculate the dilution factor (F) in the hepatic oxaloacetate pool and fluxes through liver Krebs cycle. NIDDM patients had increased lactate turnover rates (16.18 ± 0.92 vs 12.14 ± 0.60 μmol · kg-1 · min-1, p < 0.01) and a moderate rise in glucose production (EGP) (15.39 ± 0.87 vs 12.52 ± 0.28 μmol · kg-1 · min-1, p = 0.047). Uncorrected contributions of gluconeogenesis to EGP were 31 ± 3% (control subjects) and 17 ± 2% (NIDDM patients). F was comparable (1.34 ± 0.02 and 1.39 ± 0.09, respectively) and the corrected percent and absolute contributions of gluconeogenesis were not increased in NIDDM (25 ± 3% and 3.8 ± 0.5 μmol · kg-1 · min-1) compared to control subjects (41 ± 3% and 5.1 ± 0.4 μmol · kg-1 · min-1). The calculated pyruvate carboxylase over pyruvate dehydrogenase activity ratio was comparable (12.1 ± 2.6 vs 11.2 ± 1.4). Lastly hepatic fatty oxidation, as estimated by the model, was not increased in NIDDM (1.8 ± 0.4 vs 1.6 ± 0.1 μmol · kg-1 · min-1). In conclusion, in the patients studied we found no evidence of increased hepatic fatty oxidation, or, despite the increased lactate turnover rate, an increased gluconeogenesis.