Abstract
Hepatic glucose uptake (HGU) is critical for maintaining normal postprandial glucose metabolism. Insulin is clearly a key regulator of HGU, but the physiologic mechanisms by which it acts have yet to be established. This study sought to determine the mechanisms by which insulin regulates liver glucose uptake under postprandial-like conditions (hyperinsulinemia, hyperglycemia, and a positive portal vein-to-arterial glucose gradient). Portal vein insulin infusion increased hepatic insulin levels fivefold in healthy dogs. In one group (n = 7), the physiologic response was allowed to fully occur, while in another (n = 7), insulin's indirect hepatic effects, occurring secondary to its actions on adipose tissue, pancreas, and brain, were blocked. This was accomplished by infusing triglyceride (intravenous), glucagon (portal vein), and inhibitors of brain insulin action (intracerebroventricular) to prevent decreases in plasma free fatty acids or glucagon, while blocking increased hypothalamic insulin signaling for 4 h. In contrast to the indirect hepatic effects of insulin, which were previously shown capable of independently generating a half-maximal stimulation of HGU, direct hepatic insulin action was by itself able to fully stimulate HGU. This suggests that under hyperinsulinemic/hyperglycemic conditions insulin's indirect effects are redundant to direct engagement of hepatocyte insulin receptors.