Abstract
The liver is the major source of glucose production during fasting under normal physiological conditions. However, the kidney may also contribute to maintaining glucose homeostasis in certain circumstances. To test the ability of the kidney to compensate for impaired hepatic glucose production in vivo, we developed a stable isotope approach to simultaneously quantify gluconeogenic and oxidative metabolic fluxes in the liver and kidney. Hepatic gluconeogenesis from phosphoenolpyruvate was disrupted via liver-specific knockout of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C; KO). 2H/13C isotopes were infused in fasted KO and WT littermate mice, and fluxes were estimated from isotopic measurements of tissue and plasma metabolites using a multicompartment metabolic model. Hepatic gluconeogenesis and glucose production were reduced in KO mice, yet whole-body glucose production and arterial glucose were unaffected. Glucose homeostasis was maintained by a compensatory rise in renal glucose production and gluconeogenesis. Renal oxidative metabolic fluxes of KO mice increased to sustain the energetic and metabolic demands of elevated gluconeogenesis. These results show the reciprocity of the liver and kidney in maintaining glucose homeostasis by coordinated regulation of gluconeogenic flux through PEPCK-C. Combining stable isotopes with mathematical modeling provides a versatile platform to assess multitissue metabolism in various genetic, pathophysiological, physiological, and pharmacological settings.
Highlights
Biochemical methods to quantify gene transcript, enzyme, and metabolite levels are widely used to assess metabolic pathway regulation
To better understand the renal contribution to gluconeogenesis in the absence of hepatic PEPCK-C, a range of metabolites was isolated from the plasma, liver, and kidney of WT and KO mice obtained at the end of an infusion of 2H2O, [6,62H2]glucose, and [13C3]propionate
The mass isotopomer distribution (MID) of liver glutamate showed significantly higher enrichment in KO mice compared with WT mice, indicating that the livers of KO mice were able to extract the administered 2H/13C isotopes from plasma (Figure 1E)
Summary
Biochemical methods to quantify gene transcript, enzyme, and metabolite levels are widely used to assess metabolic pathway regulation. Burgess et al have shown that PEPCK-C expression can vary widely while exerting limited control over gluconeogenic flux in perfused livers [1]. Isotopic tracer techniques have been developed to address the limitations of static metabolite and enzyme measurements in order to more accurately quantify metabolic flux. These methods introduce a stable isotope to a live biological system; metabolic fluxes are determined by analyzing the isotopic enrichment of metabolites in that system using mathematical models [4, 5]. Several groups have focused on quantifying liver gluconeogenic and oxidative metabolism using stable isotopes, including our own prior contributions to assess in vivo fluxes in conscious, catheterized mice and rats [6, 11,12,13,14,15]
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