Role of Glucose-6 Phosphatase, Glucokinase, and Glucose-6 Phosphate in Liver Insulin Resistance and Its Correction by Metformin

Abstract

We investigated the role of glucose-6 phosphatase (Glc6Pase), glucokinase (GK), and glucose-6 phosphate (Glc6P) in liver insulin resistance, an early characteristic of type 2 diabetes, and its correction by metformin. We determined hepatic glucose production (HGP) by tracer dilution, and enzyme activities and substrate concentrations after saline or insulin perfusions during euglycemic clamps in rats fed: 1) a standard hyperglucidic diet (S); 2) a high-fat diet (HF); and 3) a high-fat diet and treated with the oral antidiabetic metformin (HF/Met). Basal HGP was similar in the 3 groups: 75 ± 8, 65 ± 9.5 and 71 ± 3 μmol · kg −1 · min −1 (means ± SEM, N = 5) in S, HF and HF/Met rats, respectively. Upon insulin perfusion at 240 pmol/hr, HGP was decreased by 35% in S rats (49 ± 4.5 μmol · kg −1 · min −1, P < 0.01 vs. basal) and 65% in HF/Met rats (23 ± 10 μmol · kg −1 · min −1, P < 0.01 vs basal), whereas it was not decreased in HF rats (60 ± 12 μmol · kg −1 · min −1), revealing insulin resistance. GK activity was lower (by 65%, P < 0.01) in HF and HF/Met rats (0.8 ± 0.1 and 0.9 ± 0.1 U/g liver, respectively) than in S rats (2.4 ± 0.3 U/g). Microsomal Glc6Pase activity was lower (by 35%, P < 0.01) in HF and HF/Met rats (0.25 ± 0.01 and 0.27 ± 0.02 μmol · min −1 · mg prot · −1, respectively) than in S rats (0.39 ± 0.03 μmol · min −1 · mg prot · −1). Glc6P concentration was decreased by insulin perfusion at 480 pmol/hr in S and HF/Met rats ( P < 0.05 vs. saline), but not in HF rats, in agreement with insulin resistance in the latter group. However, the differential inhibitions of HGP by insulin could not be ascribed to the variations in Glc6P concentrations. Metformin was present in the liver at a concentration of 27 ± 2 nmol/g wet tissue and was not detected in the plasma. These results strongly suggest that the regulation of HGP by insulin additionally involves short-term regulatory mechanism(s) of Glc6Pase, occurring in vivo, and lost under in vitro conditions. These might be impaired in HF rats, in keeping with insulin resistance of HGP, and restored by metformin.