To investigate the temporal response of the liver to insulin and portal glucose delivery, somatostatin was infused into four groups of 42-h-fasted, conscious dogs ( n ϭ 6/group), basal insulin and glucagon were replaced intraportally, and hyperglycemia was created via a peripheral glucose infusion for 90 min (period 1). This was followed by a 240-min experimental period (period 2) in which hyperglycemia was matched to period 1 and either no changes were made (CON), a fourfold rise in insulin was created (

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 ...

To examine the relationship between net hepatic glucose uptake (NHGU) and the insulin level and to determine the effects of portal glucose delivery on that relationship, NHGU was evaluated at three different insulin levels in seven 42-h-fasted, conscious dogs during peripheral glucose delivery and during a combination of peripheral and portal glucose delivery. During peripheral glucose delivery, at arterial blood glucose levels of -175 mg/dl and insulin levels reaching the liver of 51±2, 92±6, and 191±6 ,uU/ml, respectively, NHGUs were 0.55±0.30, 1.52±0.44, and 3.04±0.79 mg/kg per min, respectively. At hepatic glucose loads comparable to those achieved during peripheral glucose delivery and inflowing insulin levels of 50±4, 96±5, and 170±8 gU per ml, respectively, NHGUs were 1.96±0.48, 3.67±0.68, and 5.52±0.92 mg/kg per min when a portion of the glucose load was delivered directly into the portal vein. The results of these studies thus indicate that net hepatic glucose uptake is dependent on both the plasma insulin level and the route of glucose delivery and that under physiological conditions the "portal" signal is at least as important as insulin in the determination of net hepatic glucose uptake. (J. Clin. Invest. 1991.87:930-939.)

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.

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Fructose activates glucokinase by releasing the enzyme from its inhibitory protein in liver. To examine the importance of acute activation of glucokinase in regulating hepatic glucose uptake, the effect of intraportal infusion of a small amount of fructose on net hepatic glucose uptake (NHGU) was examined in 42 h-fasted conscious dogs. Isotopic ([3-3H] and [U-14C]glucose) and arteriovenous difference methods were used. Each study consisted of an equilibration period (-90 to -30 min), a control period (-30 to 0 min), and a hyperglycemic/hyperinsulinemic period (0-390 min). During the latter period, somatostatin (489 pmol x kg(-1) x min(-1)) was given, along with intraportal insulin (7.2 pmol x kg(-1) x min(-1)) and glucagon (0.5 ng x kg(-1) x min(-1)). In this way, the liver sinusoidal insulin level was fixed at four times basal (456 +/- 60 pmol/l), and liver sinusoidal glucagon level was kept basal (46 +/- 6 ng/l). Glucose was infused through a peripheral vein to create hyperglycemia (12.5 mmol/l plasma). Hyperglycemic hyperinsulinemia (no fructose) switched net hepatic glucose balance (micromoles per kilogram per minute) from output (11.3 +/- 1.4) to uptake (14.7 +/- 1.7) and net lactate balance (micromoles per kilogram per minute) from uptake (6.5 +/- 2.1) to output (4.4 +/- 1.5). Fructose was infused intraportally at a rate of 1.7, 3.3, or 6.7 micromol x kg(-1) x min(-1), starting at 120, 210, or 300 min, respectively. In the three periods, portal blood fructose increased from <6 to 113 +/- 14, 209 +/- 29, and 426 +/- 62 micromol/l, and net hepatic fructose uptake increased from 0.03 +/- 0.01 to 1.3 +/- 0.4, 2.3 +/- 0.7, and 5.1 +/- 0.6 micromol x kg(-1) x min(-1), respectively. NHGU increased to 41 +/- 3, 54 +/- 5, and 69 +/- 8 micromol x kg(-1) x min(-1), respectively, and net hepatic lactate output increased to 11.0 +/- 3.2, 15.3 +/- 2.7, and 22.4 +/- 2.8 micromol x kg(-1) x min(-1) in the three fructose periods, respectively. The amount of [3H]glucose incorporated into glycogen was equivalent to 69 +/- 3% of [3H]glucose taken up by the liver. These data suggest that glucokinase translocation within the hepatocyte is a major determinant of hepatic glucose uptake by the dog in vivo.

The role of the brain in directing counterregulation during hypoglycemia induced by insulin infusion was assessed in overnight-fasted conscious dogs. Concomitant brain and peripheral hypoglycemia was induced in one group of dogs (n = 5) by infusing insulin peripherally at a rate of 3.5 mU.kg-1.min-1. In another group (n = 4), insulin was infused as described above to induce peripheral hypoglycemia, and brain hypoglycemia was minimized by infusing glucose bilaterally into the carotid and vertebral arteries to maintain the brain glucose level at a calculated concentration of 85 mg/dl. Glucose was also infused peripherally as needed so that the peripheral glucose levels in both of the protocols were similar (45 +/- 2 mg/dl with and 48 +/- 3 mg/dl without brain glucose infusion, both P less than .05). The responses (in terms of change of area under the curve) of epinephrine, norepinephrine, cortisol, and pancreatic polypeptide when brain glycemia was controlled during insulin infusion were only 14 +/- 6, 39 +/- 12, 17 +/- 8, and 9 +/- 4%, respectively, of those present during insulin infusion without concomitant brain glucose infusion (all P less than .05). Of particular interest was the glucagon response that occurred when head hypoglycemia was minimized; the glucagon level was only 21 +/- 8% of that present when marked brain hypoglycemia accompanied insulin infusion (P less than .05). During hypoglycemia resulting from insulin infusion, endogenous glucose production (EGP), as assessed with [3-3H]glucose, rose from 2.6 +/- 0.1 to 4.4 +/- 0.5 mg.kg-1.min-1 (P less than .05). In contrast, EGP decreased from 2.7 +/- 0.2 to 2.0 +/- 0.3 mg.kg-1.min-1 when brain hypoglycemia was minimized. In an additional set of studies, when insulin was infused at 3.5 mU.kg-1.min-1 and glucose was infused peripherally to maintain both the head and peripheral glucose concentrations at 88 +/- 6 mg/dl, EGP decreased from 2.6 +/- 0.1 to 1.2 +/- 0.2 mg.kg-1.min-1. These results suggest that under marked hyperinsulinemic conditions the brain is the primary director of glucagon release and that it is responsible for approximately 75% of the life-sustaining glucose production.

The ability of portal vein insulin to control hepatic glucose production (HGP) is debated. The aim of the present study was to determine, therefore, if the liver can respond to a selective decrease in portal vein insulin. Isotopic ([3H]glucose) and arteriovenous difference methods were used to measure HGP in conscious overnight fasted dogs. A pancreatic clamp (somatostatin plus basal portal insulin and glucagon) was used to control the endocrine pancreas. A 40-min control period was followed by a 180-min test period. During the latter, the portal vein insulin level was selectively decreased while the arterial insulin level was not changed. This was accomplished by stopping the portal insulin infusion and giving insulin peripherally at half the basal portal rate (PID, n=5). In a control group (n=5), the portal insulin infusion was not changed and glucose was infused to match the hyperglycemia that occurred in the PID group. A selective decrease of 120 pmol/l in portal vein insulin was achieved (basal, 150+/-36 to last 30 min, 30+/-12 pmol/l) in the absence of a change in the arterial insulin level (basal, 30+/-3 to last 30 min, 36+/-4 pmol/l). Neither arterial nor portal insulin levels changed in the control group (30+/-6 and 126+/-30 pmol/l, respectively). In response to the selective decrease in portal vein insulin, net hepatic glucose output (NHGO) increased significantly, from 8+/-1 (basal) to 30+/-6 and 14+/-2 micromol x kg(-1) x min(-1) by 15 min and the last 30 min (P < 0.05) of the experimental period, respectively. Arterial plasma glucose increased from 5.9+/-0.2 (basal) to 10.5+/-0.4 micromol/l (last 30 min). Three-carbon gluconeogenic precursor uptake fell from 11.2+/-2.9 (basal) to 5.9+/-0.7 micromol x kg(-1) x min(-1) (last 30 min), and thus a change in gluconeogenesis could not account for any of the increase in NHGO. With matched hyperglycemia (basal, 5.5+/-0.3 to last 30 min, 10.5+/-0.8 micromol/l) but no change in insulin, NHGO decreased from 12+/-1 (basal) to 0 (-1+/-6 micromol x kg(-1) x min(-1), last 30 min, P < 0.05) and hepatic gluconeogenic precursor uptake did not change (basal, 8.0+/-1.7 to last 30 min, 8.9+/-2.2 micromol x kg[-1] x min[-1]). Thus, the liver responds rapidly to a selective decrease in portal vein insulin by markedly increasing HGP as a result of increased glycogenolysis. These studies indicate that after an overnight fast, basal HGP (glycogenolysis) is highly sensitive to the hepatic sinusoidal insulin level.