Insulin inhibits glucose production through both direct and indirect effects on the liver; however, considerable controversy exists regarding the relative importance of these effects. The first aim of this study was to determine which of these processes dominates the acute control of hepatic glucose production (HGP). Somatostatin and portal vein infusions of insulin and glucagon were used to clamp the pancreatic hormones at basal levels in the nondiabetic dog. After a basal sampling period, insulin infusion was switched from the portal vein to a peripheral vein. As a result, the arterial insulin level doubled and the hepatic sinusoidal insulin level was reduced by half. While the arterial plasma FFA level and net hepatic FFA uptake fell by 40-50%, net hepatic glucose output increased more than 2-fold and remained elevated compared with that in the control group. The second aim of this study was to determine the effect of a 4-fold rise in head insulin on HGP during peripheral hyperinsulinemia and hepatic insulin deficiency. Sensitivity of the liver was not enhanced by increased insulin delivery to the head. Thus, this study demonstrates that the direct effects of insulin dominate the acute regulation of HGP in the normal dog. IntroductionHepatic glucose production (HGP) accounts for the majority of whole-body glucose production and is tightly regulated by insulin in the healthy individual. Since hepatic insulin resistance in diabetic patients results in excess HGP and fasting hyperglycemia (1), it is critical to understand the mechanisms by which insulin regulates this process. Insulin reduces HGP by acting both directly and indirectly on the liver (2); however, there is considerable controversy regarding the relative importance of insulin's direct versus indirect effects under physiological conditions. Insulin acts directly by binding to hepatic insulin receptors and thereby activating insulin signaling pathways in the liver. These effects have been demonstrated in various models. In isolated rat hepatocytes, insulin inhibits glucose production through inhibition of gluconeogenesis (3) and glycogenolysis (4). In the dog, an acute selective increase (5) or decrease (6) in hepatic insulin level (so that the arterial insulin level was kept constant) resulted in very rapid suppression or stimulation, respectively, of HGP. In addition, liver-specific insulin receptor knockout (LIRKO) mice, which lack hepatic insulin receptors from birth, demonstrate severe hepatic insulin resistance (7). These studies, and others, demonstrate that insulin acts directly on the liver to regulate HGP.Insulin's indirect effects include reduction of glucagon secretion at the pancreas (8), inhibition of lipolysis in fat (which reduces circulating lipids and glycerol availability for gluconeogenesis) (9), and decreased protein catabolism in muscle (which further reduces gluconeogenic precursor availability) (10), and in addition, recent studies in the mouse and rat suggest that hypothalamic insulin signaling may also play an important role ...

In rodents, acute brain insulin action reduces blood glucose levels by suppressing the expression of enzymes in the hepatic gluconeogenic pathway, thereby reducing gluconeogenesis and endogenous glucose production (EGP). Whether a similar mechanism is functional in large animals, including humans, is unknown. Here, we demonstrated that in canines, physiologic brain hyperinsulinemia brought about by infusion of insulin into the head arteries (during a pancreatic clamp to maintain basal hepatic insulin and glucagon levels) activated hypothalamic Akt, altered STAT3 signaling in the liver, and suppressed hepatic gluconeogenic gene expression without altering EGP or gluconeogenesis. Rather, brain hyperinsulinemia slowly caused a modest reduction in net hepatic glucose output (NHGO) that was attributable to increased net hepatic glucose uptake and glycogen synthesis. This was associated with decreased levels of glycogen synthase kinase 3β (GSK3β) protein and mRNA and with decreased glycogen synthase phosphorylation, changes that were blocked by hypothalamic PI3K inhibition. Therefore, we conclude that the canine brain senses physiologic elevations in plasma insulin, and that this in turn regulates genetic events in the liver. In the context of basal insulin and glucagon levels at the liver, this input augments hepatic glucose uptake and glycogen synthesis, reducing NHGO without altering EGP.

OBJECTIVEInsulin-mediated suppression of hepatic glucose production (HGP) is associated with sensitive intracellular signaling and molecular inhibition of gluconeogenic (GNG) enzyme mRNA expression. We determined, for the first time, the time course and relevance (to metabolic flux) of these molecular events during physiological hyperinsulinemia in vivo in a large animal model.RESEARCH DESIGN AND METHODS24 h fasted dogs were infused with somatostatin, while insulin (basal or 8× basal) and glucagon (basal) were replaced intraportally. Euglycemia was maintained and glucose metabolism was assessed using tracer, 2H2O, and arterio-venous difference techniques. Studies were terminated at different time points to evaluate insulin signaling and enzyme regulation in the liver.RESULTSHyperinsulinemia reduced HGP due to a rapid transition from net glycogen breakdown to synthesis, which was associated with an increase in glycogen synthase and a decrease in glycogen phosphorylase activity. Thirty minutes of hyperinsulinemia resulted in an increase in phospho-FOXO1, a decrease in GNG enzyme mRNA expression, an increase in F2,6P2, a decrease in fat oxidation, and a transient decrease in net GNG flux. Net GNG flux was restored to basal by 4 h, despite a substantial reduction in PEPCK protein, as gluconeogenically-derived carbon was redirected from lactate efflux to glycogen deposition.CONCLUSIONSIn response to acute physiologic hyperinsulinemia, 1) HGP is suppressed primarily through modulation of glycogen metabolism; 2) a transient reduction in net GNG flux occurs and is explained by increased glycolysis resulting from increased F2,6P2 and decreased fat oxidation; and 3) net GNG flux is not ultimately inhibited by the rise in insulin, despite eventual reduction in PEPCK protein, supporting the concept that PEPCK has poor control strength over the gluconeogenic pathway in vivo.

OBJECTIVEInsulin represses the expression of gluconeogenic genes at the mRNA level, but the hormone appears to have only weak inhibitory effects in vivo. The aims of this study were 1) to determine the maximal physiologic effect of insulin, 2) to determine the relative importance of its effects on gluconeogenic regulatory sites, and 3) to correlate those changes with alterations at the cellular level.RESEARCH DESIGN AND METHODSConscious 60-h fasted canines were studied at three insulin levels (near basal, 4×, or 16×) during a 5-h euglycemic clamp. Pancreatic hormones were controlled using somatostatin with portal insulin and glucagon infusions. Glucose metabolism was assessed using the arteriovenous difference technique, and molecular signals were assessed.RESULTSInsulin reduced gluconeogenic flux to glucose-6-phosphate (G6P) but only at the near-maximal physiological level (16× basal). The effect was modest compared with its inhibitory effect on net hepatic glycogenolysis, occurred within 30 min, and was associated with a marked decrease in hepatic fat oxidation, increased liver fructose 2,6-bisphosphate level, and reductions in lactate, glycerol, and amino acid extraction. No further diminution in gluconeogenic flux to G6P occurred over the remaining 4.5 h of the study, despite a marked decrease in PEPCK content, suggesting poor control strength for this enzyme in gluconeogenic regulation in canines.CONCLUSIONSGluconeogenic flux can be rapidly inhibited by high insulin levels in canines. Initially decreased hepatic lactate extraction is important, and later reduced gluconeogenic precursor availability plays a role. Changes in PEPCK appear to have little or no acute effect on gluconeogenic flux.