Role of glucokinase and glucose-6-phosphatase in the acute and chronic regulation of hepatic glucose fluxes by insulin

Increased hepatic glucose production (HGP) is the major cause of fasting hyperglycemia in all forms of diabetes. Glucokinase (GK) and glucose-6-phosphatase (Glc-6-Pase) are the proximal and the distal enzymatic steps, respectively, in the regulation of HGP. We examined the impact of changes in GK and Glc-6-Pase activities on in vivo hepatic glucose fluxes in diabetic (D) and control (C) rats. In particular, the acute regulation by insulin was investigated using the euglycemic hyperinsulinemic clamp technique in conscious rats. In experimental diabetes (6 weeks): (a) GK mRNA was decreased by ≈40%; (b) the V_max of GK was markedly decreased (≈4 versus 9 pmol/g wet weight/min) and that of Glc-6-Pase was 2-fold increased (≈30 versus 15 umol/g wet weight/min, D versus C), while (c) the K_m of GK (≈10 mM) and Glc-6-Pase (≈1.5 mM) were unchanged. HGP was increased by 65% in diabetes and correlated highly with the ratio of Glc-6-Pase/GK (r = 0.81, p < 0.01). Following acute hyperinsulinemia (2 h): (a) GK mRNA increased by ∼2-fold in both C and D; (b) GK V_max did not change in C, but doubled to near-normal in D; (c) Glc-6-Pase V_max decreased by 23% in C and by 34% in D; (d) the K_m of GK decreased by ≈40% (p < 0.01) in C. Acute hyperinsulinemia almost completely inhibited HGP in both C and D, and no correlation was demonstrated between HGP and the ratio of Glc-6-Pase/GK in these groups. Our data suggest that GK and Glc-6-Pase are important determinants of fasting HGP in diabetes. However, acute changes in Glc-6-Pase and GK activities can account for only a small portion of the in vivo inhibition of hepatic glucose flux by insulin, suggesting additional mechanisms for the short-term regulation of HGP.