Extreme Insulin Resistance of the Central Adipose Depot In Vivo

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Abstract
Despite the well-described association between obesity and insulin resistance, the physiologic mechanisms that link these two states are poorly understood. The present study was performed to elucidate the role of visceral adipose tissue in whole-body glucose homeostasis. Dogs made abdominally obese with a moderately elevated fat diet had catheters placed into the superior mesenteric artery so that the visceral adipose bed could be insulinized discretely. Omental insulin infusion was extracted at ∼27%, such that systemic insulin levels were lower than in control (portal vein) insulin infusions. Omental infusion did not lower systemic free fatty acid levels further than control infusion, likely because of the resistance of the omental adipose tissue to insulin suppression and the confounding lower systemic insulin levels. The arteriovenous difference technique showed that local infusion of insulin did suppress omental lipolysis, but only at extremely high insulin concentrations. The median effective dose for suppression of lipolysis was almost fourfold higher in the visceral adipose bed than for whole-body suppression of lipolysis. Thus, the omental adipose bed represents a highly insulin-resistant depot that drains directly into the portal vein. Increased free fatty acid flux to the liver may account for hepatic insulin resistance in the moderately obese state.