Effects of type 2 diabetes on the ability of insulin and glucose to regulate splanchnic and muscle glucose metabolism: evidence for a defect in hepatic glucokinase activity.

Abstract

Insulin-induced stimulation of muscle glucose uptake (MGU) is impaired in people with type 2 diabetes. To determine whether insulin-induced stimulation of splanchnic glucose uptake (SGU) is also impaired, we simultaneously measured leg glucose uptake (LGU) and SGU in 14 nondiabetic subjects and 16 subjects with type 2 diabetes using a combined organ catheterization-tracer infusion technique. Glucose was clamped at ~9.3 mmol/l, while insulin concentrations were maintained at ~72 pmol/l (low) and ~150 pmol/l (high) for 3 h each. Endogenous hormone secretion was inhibited with somatostatin. Total body glucose disappearance was lower (P < 0.01) and glucose production higher (P < 0.01) during both insulin infusions in the diabetic compared with the nondiabetic subjects, indicating insulin resistance. Splanchnic glucose production was higher (P < 0.05) in the diabetic subjects during the low but not the high insulin infusion. SGU was slightly lower in the diabetic than in the nondiabetic subjects during the low insulin infusion and 50-60% lower (P < 0.05) during the high insulin infusion. LGU (P < 0.001), but not SGU, was inversely correlated with the degree of visceral adiposity. The contribution of the indirect pathway to hepatic glycogen synthesis did not differ in the diabetic and nondiabetic subjects. In contrast, both flux through the UDP-glucose pool (P < 0.05) and the contribution of the direct pathway to glycogen synthesis (P < 0.01) were lower in the diabetic than in the nondiabetic subjects, indicating decreased uptake and/or phosphorylation of extracellular glucose. On the other hand, glycogenolysis was equally suppressed in both groups. In summary, type 2 diabetes impairs the ability of insulin to stimulate both MGU and SGU. The defect appears to reside at a proximal (e.g., glucokinase) metabolic step and is not related to the degree of visceral adiposity. These data suggest that impaired hepatic glucose uptake as well as MGU contribute to hyperglycemia in people with type 2 diabetes. D i a b e t e s 4 9 :2 7 2-283, 2000 T ype 2 diabetes is associated with a decrease in insulin-induced stimulation of glucose uptake (1-9). The ability of glucose to enhance its own uptake (commonly referred to as glucose effectiveness) is also abnormal (10-14). The sites of these defects remain an area of active investigation. Although a large number of studies have shown that the response of muscle to glucose and insulin is impaired in people with type 2 diabetes (1-9), the effects of diabetes on splanchnic glucose metabolism are less clear (2,15,16). This distinction is important because while insulin and glucose stimulate glucose uptake in both tissues, the mechanism by which they do so differs.In nondiabetic humans, glucose transport appears to be rate limiting in muscle, whereas phosphorylation appears to be rate limiting in the liver (6,(17)(18)(19)(20)(21). Insulin increases glycogen synthase activity in both muscle and liver but inhibits glycogen phosphorylase activity only in liver (5,(22)(23)(24)(25)(26). Perhaps ...

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