Flux control in the rat gastrocnemius glycogen synthesis pathway by in vivo13C/31P NMR spectroscopy

Abstract

To determine the relative contributions of glucose transport/hexokinase, glycogen synthase (GSase), and glycolysis to the control of insulin-stimulated muscle glycogen synthesis, we combined13C and 31P NMR to quantitate the glycogen synthesis rate and glucose 6-phosphate (G-6- P) levels in rat (Sprague-Dawley) gastrocnemius muscle during hyperinsulinemia at euglycemic (E) and hyperglycemic (H) glucose concentrations under thiopental anesthesia. Flux control was calculated using metabolic control analysis. The combined control coefficient of glucose transport/hexokinase (GT/Hk) for glycogen synthesis was 1.1 ± 0.03 (direct measure) and 1.14–1.16 (calculated for a range of glycolytic fluxes), whereas the control coefficient for GSase was much lower (0.011–0.448). We also observed that the increase in in vivo [G-6- P] from E to H (0.22 ± 0.03 to 0.40 ± 0.03 mM) effects a supralinear increase in the in vitro velocity of GSase, from 14.6 to 26.1 mU · kg−1 · min−1 (1.8-fold). All measurements suggest that the majority of the flux control of muscle glycogen synthesis is at the GT/Hk step.