Altered fluxes responsible for reduced hepatic glucose production and gluconeogenesis by exogenous glucose in rats

The net release of glucose from the liver, or hepatic glucose production (HGP), and apparent gluconeogenesis (GNG) are reduced by exogenous glucose. We investigated the changes in metabolic fluxes responsible. Flux through the hepatic GNG pathway was quantified by mass isotopomer distribution analysis (MIDA) from [2-13C]glycerol. Unidirectional flux across hepatic glucose-6-phosphatase (G-6-Pase), or total hepatic glucose output (THGO), and hepatic glucose cycling (HGC) were also measured by using glucuronate (GlcUA) to correct for glucose 6-phosphate (G-6-P) labeling. Infusion of glucose (15-30 mg.kg-1.min-1 iv) to 24 h-fasted rats caused two important metabolic alterations. First was a significant increase in hepatic glucose uptake and HGC: > 60% of THGO was from HGC. Second, although flux through hepatic G-6-P increased (from 15.7 to 17.7-22.7 mg.kg-1.min-1), the partitioning of G-6-P flux changed markedly [from 30-35% to 55-60% entering UDP-glucose (UDP-Glc), P < 0.01]. Total flux through the GNG pathway remained active during intravenous glucose, but increased partitioning into UDP-Glc lowered GNG flux plasma glucose by 50%. In summary, the suppression of HGP and GNG flux into glucose is not primarily due to reduced carbon flow through hepatic G-6-Pase or the hepatic GNG pathway. THGO persists, but hepatic G-6-P is derived increasingly from plasma glucose, and flow through GNG persists, but the partitioning coefficient of G-6-P into UDP-Glc doubles. These adjustments permit net HGP to fall despite increased total production of hepatic G-6-P during administration of glucose.