Alterations in Postprandial Hepatic Glycogen Metabolism in Type 2 Diabetes

Abstract

Decreased skeletal muscle glucose disposal and increased endogenous glucose production (EGP) contribute to postprandial hyperglycemia in type 2 diabetes, but the contribution of hepatic glycogen metabolism remains uncertain. Hepatic glycogen metabolism and EGP were monitored in type 2 diabetic patients and nondiabetic volunteer control subjects (CON) after mixed meal ingestion and during hyperglycemic-hyperinsulinemic-somatostatin clamps applying ¹³C nuclear magnetic resonance spectroscopy (NMRS) and variable infusion dual-tracer technique. Hepatocellular lipid (HCL) content was quantified by ¹H NMRS. Before dinner, hepatic glycogen was lower in type 2 diabetic patients (227 ± 6 vs. CON: 275 ± 10 mmol/l liver, P < 0.001). After meal ingestion, net synthetic rates were 0.76 ± 0.16 (type 2 diabetic patients) and 1.36 ± 0.15 mg · kg⁻¹ · min⁻¹ (CON, P < 0.02), resulting in peak concentrations of 283 ± 15 and 360 ± 11 mmol/l liver. Postprandial rates of EGP were ∼0.3 mg · kg⁻¹ · min⁻¹ (30–170 min; P < 0.05 vs. CON) higher in type 2 diabetic patients. Under clamp conditions, type 2 diabetic patients featured ∼54% lower (P < 0.03) net hepatic glycogen synthesis and ∼0.5 mg · kg⁻¹ · min⁻¹ higher (P < 0.02) EGP. Hepatic glucose storage negatively correlated with HCL content (R = −0.602, P < 0.05). Type 2 diabetic patients exhibit 1) reduction of postprandial hepatic glycogen synthesis, 2) temporarily impaired suppression of EGP, and 3) no normalization of these defects by controlled hyperglycemic hyperinsulinemia. Thus, impaired insulin sensitivity and/or chronic glucolipotoxicity in addition to the effects of an altered insulin-to-glucagon ratio or increased free fatty acids accounts for defective hepatic glycogen metabolism in type 2 diabetic patients.