Glucosamine-induced inhibition of liver glucokinase impairs the ability of hyperglycemia to suppress endogenous glucose production.

Abstract

Although the kinetic characteristics of hepatic glucokinase (GK) suggest its potential role as the hepatic "glucose sensor," its impact on the regulation of in vivo hepatic glucose production (HGP) is still controversial. Since decreased GK activity has been linked to experimental and human diabetes, we examined whether a moderate and transient inhibition of GK activity diminishes the ability of hyperglycemia to suppress HGP. We first determined the concentration of the competitive inhibitor, glucosamine (GlcN), which decreases hepatic GK activity by approximately 60% in vitro. GlcN was then infused into conscious rats to achieve a similar inhibition of the in vivo GK activity (plasma GlcN levels = approximately 2 mmol/l; rats infused with saline served as control, n = 20). To maintain equal plasma insulin and glucagon concentrations throughout the studies, somatostatin and insulin (basal replacement) were infused for 4 h. [3-(3H)]-glucose and [U-(14C)]-lactate were infused to measure HGP, gluconeogenesis, and glucose cycling (GC) during 2 h of euglycemia (glucose approximately 8 mmol/l) followed by 2 h of hyperglycemia (glucose approximately 18 mmol/l). Our results support the notion that hepatic GK activity is indeed decreased by GlcN in vivo. In fact, in response to hyperglycemia the "direct" pathway of hepatic glucose-6-phosphate (G-6-P) formation was approximately 40% lower with GlcN compared with saline infusion (37 +/- 3 vs. 63 +/- 3%; P < 0.001). Furthermore, while hyperglycemia stimulated GC by approximately 2.5-fold during saline infusion (from 3.0 +/- 0.6 to 7.7 +/- 1.4 mg.kg-1.min-1, P < 0.001, euglycemia vs. hyperglycemia), this increase was blunted in the presence of GlcN (4.6 +/- 0.6 mg.kg-1.min-1, P = NS). Finally, in the presence of GlcN, the hepatic concentration of G-6-P was decreased by approximately 40% compared with saline (234 +/- 38 and 390 +/- 24 nmol/g, P < 0.01). During the euglycemic studies, HGP was similar (12.6 +/- 0.6 and 11.3 +/- 0.2 mg .kg-1.min-1 with GlcN or saline infusion, respectively). However, while hyperglycemia per se suppressed HGP by approximately 65%, HGP was inhibited by approximately 38% and it was approximately twofold higher than in the saline-infused rats (7.8 +/- 0.8 and 4.0 +/- 0.3 mg.kg-1.min-1, P < 0.01) in the presence of GlcN-induced inhibition of hepatic GK. This increase in HGP was largely accounted for by the decreased inhibition of hepatic net glycogenolysis by hyperglycemia (3.3 +/- 0.8 and 1.1 +/- 0.3 mg.kg-1.min-1 with GlcN or saline infusion, respectively, P < 0.01). We conclude that intact GK activity is required for the normal suppression of HGP by hyperglycemia and its impairment may contribute to increased HGP in experimental and human diabetes.