Decreased insulin secretion in first-degree family members of NIDDM patients

Abstract

Young first-degree relatives of type 2 diabetic patients are insulin-resistant, with the insulin resistance mainly located in skeletal muscle due to decreased insulin-induced nonoxidative glucose metabolism and muscle glycogen synthase activation. We investigated whether the mechanism differs for dexamethasone (dex)-induced insulin resistance in first-degree relatives of type 2 diabetics versus healthy control subjects by quantifying intracellular glucose processing in muscle biopsies taken before and after 5 days of dex treatment (4 mg/d) in 20 normal glucose-tolerant relatives of type 2 diabetic patients and 20 matched controls (age, 29.4 ± 1.7 v 29.4 ± 1.6 years; body mass index, 25.1 ± 1.0 v 25.1 ± 0.9 kg/m2). In addition, an intravenous glucose tolerance test (IVGTT) combined with continuous indirect calorimetry was performed. Following 5 days of dex treatment, glucose tolerance deteriorated in both the relatives and the control subjects. Fasting dry-weight muscle glucose and fasting intracellular muscle glucose concentrations increased in response to dex only in the relatives (2.43 ± 0.21 v 2.97 ± 0.26 mmol/kg dry weight, P < .05; 0.28 ± 0.07 v 0.45 ± 0.08 mmol/L intracellular water, P < .05); no increases were observed in the control subjects. Fasting dry-weight muscle lactate also increased post-dex only in the relatives (7.37 ± 0.40 v 10.77 ± 1.22 mmol/kg dry weight, P < .001). Both basal muscle glucose and lactate concentrations from the IVGTT study correlated with the 2-hour post-dex glucose value obtained during the OGTT study in the relatives (R = .76 and R = .74, respectively, both P < .0001) but not in the control subjects. Basal intramuscular glycogen synthase activity decreased approximately 25% in both the relatives and control subjects post-dex; the decrement was significant (P < .01) only in control subjects. Indirect calorimetry during the post-dex IVGTT demonstrated increased glucose oxidation (P < .03) and reduced lipid oxidation (P < .03) in the relatives only. We postulate that the insulin resistance induced by dex in first-degree relatives of type 2 diabetic patients is associated with a preferential channeling of glucose into the glycolytic pathway (increased glucose oxidation and lactate production), probably associated with a preexisting downregulation of the glycosen synthase pathway.