Continuous low-dose fructose infusion does not reverse glucagon-mediated decrease in hepatic glucose utilization.

An adaptation to continuous total parenteral nutrition (TPN; 75% of nonprotein calories as glucose) is the liver becomes a major consumer of glucose with lactate release as a by-product. The liver is able to further increase liver glucose uptake when a small dose of fructose is acutely infused via the portal system. Glucagon, commonly elevated during inflammatory stress, is a potent inhibitor of glucose uptake by the liver during TPN. The aim was to determine if continuous fructose infusion could overcome the glucagon-mediated decrease in hepatic glucose uptake. Studies were performed in conscious, insulin-treated, chronically catheterized, pancreatectomized dogs that adapted to TPN for 33 hours. They were then assigned to 1 of 4 groups: TPN (C), TPN + fructose (4.4 µmol kg(-1) min(-1); F), TPN + glucagon (0.2 pmol kg(-1) min(-1); GGN), or TPN + fructose and glucagon (F + GGN) for an additional 63 hours (33-96 hours). Insulin, fructose, and glucagon were infused into the portal vein. During that period, all animals received a fixed insulin infusion of 0.4 mU·kg(-1)·min(-1) (33-96 hours); and the glucose infusion rates were adjusted to maintain euglycemia (6.6 mmol/L). Continuous fructose infusion was unable to further enhance net hepatic glucose uptake (in micromoles per kilogram per minute) (31.1 ± 2.8 vs 36.1 ± 5.0; C vs F), nor was it able to overcome glucagon-mediated decrease in net hepatic glucose uptake (10.0 ± 4.4 vs 12.2 ± 3.9; GGN vs F + GGN). In summary, continuous fructose infusion cannot augment liver glucose uptake during TPN; nor can it overcome the inhibitory effects of glucagon.

Glucagon-mediated impairments in hepatic and peripheral tissue nutrient disposal are not aggravated by increased lipid availability.

Glucose, fat, and glucagon availability are increased in diabetes. The normal response of the liver to chronic increases in glucose availability is to adapt to become a marked consumer of glucose. Yet this fails to occur in diabetes. The aim was to determine whether increased glucagon and lipid interact to impair the adaptation to increased glucose availability. Chronically catheterized well controlled depancreatized conscious dogs (n = 21) received 3 days of continuous parenteral nutrition (TPN) that was either high in glucose [C; 75% nonprotein calories (NPC)] or in lipid (HL; 75% NPC) in the presence or absence of a low dose (one-third basal) chronic intraportal infusion of glucagon (GN; 0.25 ng.kg(-1).min(-1)). During the 3 days of TPN, all groups received the same insulin algorithm; the total amount of glucose infused (GIR) was varied to maintain isoglycemia ( approximately 120 mg/dl). On day 3 of TPN, hepatic metabolism was assessed. Glucose and insulin levels were similar in all groups. GIR was decreased in HL and C + GN ( approximately 30%) and was further decreased in HL + GN (55%). Net hepatic glucose uptake was decreased approximately 15% in C + GN, and HL and was decreased approximately 50% in HL + GN. Lipid alone or combined with glucagon decreased glucose uptake by peripheral tissues. Despite impairing whole body glucose utilization, HL did not limit whole body energy disposal. In contrast, glucagon suppressed whole body energy disposal irrespective of the diet composition. In summary, failure to appropriately suppress glucagon secretion adds to the dietary fat-induced impairment in both hepatic and peripheral glucose disposal. In addition, unlike increasing the percentage of calories as fat, inappropriate glucagon secretion in the absence of compensatory hyperinsulinemia limits whole body nutrient disposition.

Hepatic and muscle insulin action during late pregnancy in the dog.

We evaluated the effects of physiologic increases in insulin on hepatic and peripheral glucose metabolism in nonpregnant (NP) and pregnant (P; 3rd trimester) conscious dogs (n = 9 each) using tracer and arteriovenous difference techniques during a hyperinsulinemic euglycemic clamp. Insulin was initially (-150 to 0 min) infused intraportally at a basal rate. During 0-120 min (Low Insulin), the rate was increased by 0.2 mU x kg(-1) x min(-1), and from 120 to 240 min (High Insulin) insulin was infused at 1.5 mU x kg(-1) x min(-1). Insulin concentrations were significantly higher in NP than P during all periods. Matched subsets (n = 5 NP and 6 P) were identified. In the subsets, insulin was 7 +/- 1, 9 +/- 1, and 28 +/- 3 microU/ml (basal, Low Insulin, and High Insulin, respectively) in NP, and 5 +/- 1, 7 +/- 1, and 27 +/- 3 microU/ml in P. Net hepatic glucose output was suppressed similarly in both subsets (> or =50% with Low Insulin, 100% with High Insulin), as was endogenous glucose rate of appearance. During High Insulin, NP dogs required more glucose (10.8 +/- 1.5 vs. 6.2 +/- 1.0 mg x kg(-1) x min(-1), P < 0.05), and hindlimb (primarily skeletal muscle) glucose uptake tended to be greater in NP than P (18.6 +/- 2.5 mg/min vs. 13.6 +/- 2.0 mg/min, P = 0.06). The normal canine liver remains insulin sensitive during late pregnancy. Differing insulin concentrations in pregnant and nonpregnant women and excessive insulin infusion rates may explain previous findings of hepatic insulin resistance in healthy pregnant women.

Glucagon chronically impairs hepatic and muscle glucose disposal.

Defects in insulin secretion and/or action contribute to the hyperglycemia of stressed and diabetic patients, and we hypothesize that failure to suppress glucagon also plays a role. We examined the chronic impact of glucagon on glucose uptake in chronically catheterized conscious depancreatized dogs placed on 5 days of nutritional support (NS). For 3 days of NS, a variable intraportal infusion of insulin was given to maintain isoglycemia (approximately 120 mg/dl). On day 3 of NS, animals received a constant low infusion of insulin (0.4 mU.kg-1.min-1) and either no glucagon (CONT), basal glucagon (0.7 ng.kg-1.min-1; BasG), or elevated glucagon (2.4 ng.kg-1.min-1; HiG) for the remaining 2 days. Glucose in NS was varied to maintain isoglycemia. An additional group (HiG+I) received elevated insulin (1 mU.kg-1.min-1) to maintain glucose requirements in the presence of elevated glucagon. On day 5 of NS, hepatic substrate balance was assessed. Insulin and glucagon levels were 10+/-2, 9+/-1, 7+/-1, and 24+/-4 microU/ml, and 24+/-5, 39+/-3, 80+/-11, and 79+/-5 pg/ml, CONT, BasG, HiG, and HiG+I, respectively. Glucagon infusion decreased the glucose requirements (9.3+/-0.1, 4.6+/-1.2, 0.9+/-0.4, and 11.3+/-1.0 mg.kg-1.min-1). Glucose uptake by both hepatic (5.1+/-0.4, 1.7+/-0.9, -1.0+/-0.4, and 1.2+/-0.4 mg.kg-1.min-1) and nonhepatic (4.2+/-0.3, 2.9+/-0.7, 1.9+/-0.3, and 10.2+/-1.0 mg.kg-1.min-1) tissues decreased. Additional insulin augmented nonhepatic glucose uptake and only partially improved hepatic glucose uptake. Thus, glucagon impaired glucose uptake by hepatic and nonhepatic tissues. Compensatory hyperinsulinemia restored nonhepatic glucose uptake and partially corrected hepatic metabolism. Thus, persistent inappropriate secretion of glucagon likely contributes to the insulin resistance and glucose intolerance observed in obese and diabetic individuals.

Glucagon secretion and autonomic signaling during hypoglycemia in late pregnancy.

We examined net pancreatic norepinephrine (NE) spillover, pancreatic polypeptide (PP) release, and the decrement in C-peptide to identify factors involved in the blunted counterregulatory glucagon response in pregnancy. Conscious pregnant [pregnant hypoglycemic (Ph); 3rd trimester; n = 8] and nonpregnant [nonpregnant hypoglycemic (NPh); n = 6] dogs were studied during insulin-induced (approximately 12-fold basal insulin concentrations) hypoglycemia (plasma glucose 3.1 mM). Additional dogs were studied during hyperinsulinemic euglycemia [nonpregnant euglycemic (NPe), n = 4; pregnant euglycemic (Pe), n = 5; plasma glucose 6 mM]. Arterial glucagon concentrations declined similarly in NPe and Pe. Areas under the curve (AUCs) of the changes in glucagon and epinephrine were seven- and threefold greater in NPh than Ph (P < 0.05 between groups for both). Glucagon secretion fell below basal in NPe, Pe, and Ph but rose significantly in NPh. C-peptide declined 0.25 +/- 0.06, 0.12 +/- 0.11, 0.28 +/- 0.05, and 0.13 +/- 0.02 ng/ml in NPe, Pe, NPh, and Ph, respectively (P < 0.05, NPh vs. Ph). AUCs of NE spillover were 516 +/- 274, 265 +/- 303, 506 +/- 94, and -63 +/- 79 ng, respectively (P < 0.05, NPh vs. Ph). The AUC of PP release was approximately threefold greater in NPh than Ph (P < 0.05) but not different between euglycemic groups. The current evidence strongly suggests that the blunting of glucagon secretion during insulin-induced hypoglycemia in pregnancy is related to generalized impairment of a number of different signals, including parasympathetic and sympathoadrenal stimuli and altered sensing of circulating and/or intraislet insulin.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside renders glucose output by the liver of the dog insensitive to a pharmacological increment in insulin.

This study aimed to test whether stimulation of net hepatic glucose output (NHGO) by increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-beta-d-ribosyl-5-monophosphate, can be suppressed by pharmacological insulin levels. Dogs had sampling (artery, portal vein, hepatic vein) and infusion (vena cava, portal vein) catheters and flow probes (hepatic artery, portal vein) implanted >16 days before study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and hyperinsulinemic-euglycemic (0-150 min) periods. At time (t) = 0 min, somatostatin was infused, and basal glucagon was replaced via the portal vein. Insulin was infused in the portal vein at either 2 (INS2) or 5 (INS5) mU.kg(-1).min(-1). At t = 60 min, 1 mg.kg(-1).min(-1) portal venous 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) infusion was initiated. Arterial insulin rose approximately 9- and approximately 27-fold in INS2 and INS5, respectively. Glucagon, catecholamines, and cortisol did not change throughout the study. NHGO was completely suppressed before t = 60 min. Intraportal AICAR stimulated NHGO by 1.9 +/- 0.5 and 2.0 +/- 0.5 mg.kg(-1).min(-1) in INS2 and INS5, respectively. AICAR stimulated tracer-determined endogenous glucose production similarly in both groups. Intraportal AICAR infusion significantly increased hepatic acetyl-CoA carboxylase (ACC, Ser(79)) phosphorylation in INS2. Hepatic ACC (Ser(79)) phosphorylation, however, was not increased in INS5. Thus intraportal AICAR infusion renders hepatic glucose output insensitive to pharmacological insulin. The effectiveness of AICAR in countering the suppressive effect of pharmacological insulin on NHGO occurs even though AICAR-stimulated ACC phosphorylation is completely blocked.

Route-dependent effect of nutritional support on liver glucose uptake.

The liver is a major site of glucose disposal during chronic (5 day) total parenteral (TPN) and enteral (TEN) nutrition. Net hepatic glucose uptake (NHGU) is dependent on the route of delivery when only glucose is delivered acutely; however, the hepatic response to chronic TPN and TEN is very similar. We aimed to determine whether the route of nutrient delivery altered the acute (first 8 h) response of the liver and whether chronic enteral delivery of glucose alone could augment the adaptive response to TPN. Chronically catheterized conscious dogs received either TPN or TEN containing glucose, Intralipid, and Travasol for either 8 h or 5 days. Another group received TPN for 5 days, but approximately 50% of the glucose in the nutrition was given via the enteral route (TPN+EG). Hepatic metabolism was assessed with tracer and arteriovenous difference techniques. In the presence of similar arterial plasma glucose levels (approximately 6 mM), NHGU and net hepatic lactate release increased approximately twofold between 8 h and 5 days in TPN and TEN. NHGU (26 +/- 1 vs. 23 +/- 3 micromol.kg(-1).min(-1)) and net hepatic lactate release (44 +/- 1 vs. 34 +/- 6 micromol.kg(-1).min(-1)) in TPN+EG were similar to results for TPN, despite lower insulin levels (96 +/- 6 vs. 58 +/- 16 pM, TPN vs. TPN+EG). TEN does not acutely enhance NHGU or disposition above that seen with TPN. However, partial delivery of enteral glucose is effective in decreasing the insulin requirement during chronic TPN.

Chronic estradiol and progesterone treatment in conscious dogs: effects on insulin sensitivity and response to hypoglycemia.

We evaluated the effect of chronic (3 wk) subcutaneous treatment with progesterone and estradiol (PE; producing serum levels observed in the 3rd trimester of pregnancy) or placebo (C) on hepatic and whole body insulin sensitivity and response to hypoglycemia in conscious, overnight-fasted nonpregnant female dogs, using tracer and arteriovenous difference techniques. Insulin was infused peripherally for 3 h at 1.8 mU x kg(-1) x min(-1). Glucose was allowed to fall to 3 mM (Hypo) or maintained at 6 mM (Eugly) by peripheral glucose infusion. Insulin concentrations were significantly higher in Eugly-PE (n = 7) and Hypo-PE (n = 7) than in Eugly-C (n = 6) and Hypo-C groups (n = 7), but there were no significant differences in hepatic insulin extraction. Concentrations of glucagon, cortisol, epinephrine, and norepinephrine did not differ significantly between Eugly groups or between Hypo groups. Whole body glucose disposal, adjusted for the differences in insulin between groups, was 35% higher in Eugly-C vs. Eugly-PE groups (P < 0.05). Eugly-C and Eugly-PE groups exhibited similar rates of net hepatic glucose uptake, but the rate of glucose appearance was greater in Eugly-PE in the last hour (P < 0.05). Net hepatic glucose output was greater (P < 0.05) in Hypo-PE than in Hypo-C groups, and the glucose infusion rate required to maintain equivalent hypoglycemia was less (P < 0.05). The rate of gluconeogenic flux did not differ between Hypo groups. Chronic progesterone and estradiol exposure caused whole body (primarily skeletal muscle) insulin resistance and enhanced the liver's response to hypoglycemia without altering counterregulatory hormone concentrations.

Impact of continuous and pulsatile insulin delivery on net hepatic glucose uptake.

The pancreas releases insulin in a pulsatile manner; however, studies assessing the liver's response to insulin have used constant infusion rates. Our aims were to determine whether the secretion pattern of insulin [continuous (CON) vs. pulsatile] in the presence of hyperglycemia 1) influences net hepatic glucose uptake (NHGU) and 2) entrains NHGU. Chronically catheterized conscious dogs fasted for 42 h received infusions including peripheral somatostatin, portal insulin (0.25 mU x kg(-1) x min(-1)), peripheral glucagon (0.9 ng x kg(-1) x min(-1)), and peripheral glucose at a rate double the glucose load to the liver. After the basal period, insulin was infused for 210 min at either four times the basal rate (1 mU x kg(-1) x min(-1)) or an identical amount in pulses of 1 and 4 min duration, followed by intervals of 11 and 8 min (CON, 1/11, and 4/8, respectively) in which insulin was not infused. A variable peripheral glucose infusion containing [3H]glucose clamped glucose levels at twice the basal level ( approximately 200 mg/dl) throughout each study. Hepatic metabolism was assessed by combining tracer and arteriovenous difference techniques. Arterial plasma insulin (microU/ml) either increased from basal levels of 6 +/- 1 to a constant level of 22 +/- 4 in CON or oscillated from 5 +/- 1 to 416 +/- 79 and from 6 +/- 1 to 123 +/- 43 in 1/11 and 4/8, respectively. NHGU (-0.8 +/- 0.3, 0.4 +/- 0.2, and -0.9 +/- 0.4 mg x kg(-1) x min(-1)) and net hepatic fractional extraction of glucose (0.04 +/- 0.01, 0.04 +/- 0.01, and 0.05 +/- 0.01 mg x kg(-1) x min(-1)) were similar during the experimental period. Spectral analysis was performed to assess whether a correlation existed between the insulin secretion pattern and NHGU. NHGU was not augmented by pulsatile insulin delivery, and there is no evidence of entrainment in hepatic glucose metabolism. Thus the loss of insulin pulsatility per se likely has little or no impact on the effectiveness of insulin in regulating liver glucose uptake.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside causes acute hepatic insulin resistance in vivo.

The infusion of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) causes a rise in tissue concentrations of the AMP analog 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranotide (ZMP), which mimics an elevation of cellular AMP levels. The purpose of this work was to determine the effect of raising hepatic ZMP levels on hepatic insulin action in vivo. Dogs had sampling and infusion catheters as well as flow probes implanted 16 days before an experiment. After an 18-h fast, blood glucose was 82 +/- 1 mg/dl and basal net hepatic glucose output 1.5 +/- 0.2 mg . kg(-1) . min(-1). Dogs received portal venous glucose (3.2 mg . kg(-1) . min(-1)), peripheral venous somatostatin, and basal portal venous glucagon infusions from -90 to 60 min. Physiological hyperinsulinemia was established with a portal insulin infusion (1.2 mU . kg(-1) . min(-1)). Peripheral venous glucose infusion was used to clamp arterial blood glucose at 150 mg/dl. Starting at t = 0 min, dogs received portal venous AICAR infusions of 0, 1, or 2 mg . kg(-1) . min(-1). Net hepatic glucose uptake was 2.4 +/- 0.5 mg . kg(-1) . min(-1) (mean of all groups) before t = 0 min. In the absence of AICAR, net hepatic glucose uptake was 1.9 +/- 0.4 mg . kg(-1) . min(-1) at t = 60 min. The lower-dose AICAR infusion caused a complete suppression of net hepatic glucose uptake (-1.0 +/- 1.7 mg . kg(-1) . min(-1) at t = 60 min). The higher AICAR dose resulted in a profound shift in hepatic glucose balance from net uptake to a marked net output (-6.1 +/- 1.9 mg . kg(-1) . min(-1) at t = 60 min), even in the face of hyperglycemia and hyperinsulinemia. These data show that elevations in hepatic ZMP concentrations, induced by portal venous AICAR infusion, cause acute hepatic insulin resistance. These findings have important implications for the targeting of AMP kinase for the treatment of insulin resistance, using AMP analogs.

Portal venous 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion overcomes hyperinsulinemic suppression of endogenous glucose output.

AMP-activated protein kinase (AMPK) plays a key role in regulating metabolism, serving as a metabolic master switch. The aim of this study was to assess whether increased concentrations of the AMP analog, 5-aminoimidazole-4-carboxamide-1-beta-D-ribosyl-5-monophosphate, in the liver would create a metabolic response consistent with an increase in whole-body metabolic need. Dogs had sampling (artery, portal vein, hepatic vein) and infusion (vena cava, portal vein) catheters and flow probes (hepatic artery, portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and hyperinsulinemic-euglycemic or -hypoglycemic clamp periods (0-150 min). At t = 0 min, somatostatin was infused and glucagon was replaced in the portal vein at basal rates. An intraportal hyperinsulinemic (2 mU . kg(-1) . min(-1)) infusion was also initiated at this time. Glucose was clamped at hypoglycemic or euglycemic levels in the presence (H-AIC, n = 6; E-AIC, n = 6) or absence (H-SAL, n = 6; E-SAL, n = 6) of a portal venous 5-aminoimidazole-4-carboxamide-ribofuranoside (AICAR) infusion (1 mg . kg(-1) . min(-1)) initiated at t = 60 min. In the presence of intraportal saline, glucose was infused into the vena cava to match glucose levels seen with intraportal AICAR. Glucagon remained fixed at basal levels, whereas insulin rose similarly in all groups. Glucose fell to 50 +/- 2 mg/dl by t = 60 min in hypoglycemic groups and remained at 105 +/- 3 mg/dl in euglycemic groups. Endogenous glucose production (R(a)) was similarly suppressed among groups in the presence of euglycemia or hypoglycemia before t = 60 min and remained suppressed in the H-SAL and E-SAL groups. However, intraportal AICAR infusion stimulated R(a) to increase by 2.5 +/- 1.0 and 3.4 +/- 0.4 mg . kg(-1) . min(-1) in the E-AIC and H-AIC groups, respectively. Arteriovenous measurement of net hepatic glucose output showed similar results. AICAR stimulated hepatic glycogen to decrease by 5 +/- 3 and 19 +/- 5 mg/g tissue (P < 0.05) in the presence of euglycemia and hypoglycemia, respectively. AICAR significantly increased net hepatic lactate output in the presence of hypoglycemia. Thus, intraportal AICAR infusion caused marked stimulation of both hepatic glucose output and net hepatic glycogenolysis, even in the presence of high levels of physiological insulin. This stimulation of glucose output by AICAR was equally marked in the presence of both euglycemia and hypoglycemia. However, hypoglycemia amplified the net hepatic glycogenolytic response to AICAR by approximately fourfold.

Time course of the hepatic adaptation to TPN: interaction with glycogen depletion.

In response to chronic (5 days) TPN, the liver becomes a major site of glucose disposal, removing approximately 45% (4.5 mg.kg(-1).min(-1)) of exogenous glucose. Moreover, approximately 70% of glucose is not stored but released as lactate. We aimed to determine in chronically catheterized conscious dogs the time course of adaptation to TPN and the glycogen depletion impact on early time course. After an 18-h (n = 5) fast, TPN was infused into the inferior vena cava for 8 (n = 5) or 24 h (n = 6). A third group, of 42-h-fasted animals (n = 6), was infused with TPN for 8 h. TPN was infused at a rate designed to match the dog's calculated basal energy and nitrogen requirements. NHGU (-2.3 +/- 0.1 to 2.2 +/- 0.7 to 3.9 +/- 0.6 vs. -1.7 +/- 0.3 to 1.1 +/- 0.5 to 2.9 +/- 0.4 mg.kg(-1).min(-1), basal to 4 to 8 h, 18 vs. 42 h) and net hepatic lactate release (0.7 +/- 0.3 to 0.6 +/- 0.1 to 1.4 +/- 0.2 vs. -0.6 +/- 0.1 to 0.1 +/- 0.1 to 0.8 +/- 0.1 mg.kg(-1).min(-1), basal to 4 to 8 h) increased progressively. Net hepatic glycogen repletion and tracer determined that glycogen syntheses were similar. After 24 h of TPN, NHGU (5.4 +/- 0.6 mg.kg(-1).min(-1)) and net hepatic lactate release (2.6 +/- 0.4 mg.kg(-1).min(-1)) increased further. In summary, 1) most hepatic adaptation to TPN occurs within 24 h after initiation of TPN, and 2) prior glycogen depletion does not augment hepatic adaptation rate.

Exercise-induced changes in insulin and glucagon are not required for enhanced hepatic glucose uptake after exercise but influence the fate of glucose within the liver.

To test whether pancreatic hormonal changes that occur during exercise are necessary for the postexercise enhancement of insulin-stimulated net hepatic glucose uptake, chronically catheterized dogs were exercised on a treadmill or rested for 150 min. At the onset of exercise, somatostatin was infused into a peripheral vein, and insulin and glucagon were infused in the portal vein to maintain basal levels (EX-Basal) or simulate the response to exercise (EX-Sim). Glucose was infused as needed to maintain euglycemia during exercise. After exercise or rest, somatostatin infusion was continued in exercised dogs and initiated in dogs that had remained sedentary. In addition, basal glucagon, glucose, and insulin were infused in the portal vein for 150 min to create a hyperinsulinemic-hyperglycemic clamp in EX-Basal, EX-Sim, and sedentary dogs. Steady-state measurements were made during the final 50 min of the clamp. During exercise, net hepatic glucose output (mg x kg(-1) x min(-1)) rose in EX-Sim (7.6 +/- 2.8) but not EX-Basal (1.9 +/- 0.3) dogs. During the hyperinsulinemic-hyperglycemic clamp that followed either exercise or rest, net hepatic glucose uptake (mg x kg(-1) x min(-1)) was elevated in both EX-Basal (4.0 +/- 0.7) and EX-Sim (4.6 +/- 0.5) dogs compared with sedentary dogs (2.0 +/- 0.3). Despite this elevation in net hepatic glucose uptake after exercise, glucose incorporation into hepatic glycogen, determined using [3-3H]glucose, was not different in EX-Basal and sedentary dogs, but was 50 +/- 30% greater in EX-Sim dogs. Exercise-induced changes in insulin and glucagon, and consequent glycogen depletion, are not required for the increase in net hepatic glucose uptake after exercise but result in a greater fraction of the glucose consumed by the liver being directed to glycogen.

Portal vein caffeine infusion enhances net hepatic glucose uptake during a glucose load in conscious dogs.

We determined whether intraportal caffeine infusion, at rates designed to create concentrations similar to that seen with normal dietary intake, would enhance net hepatic glucose uptake (NHGU) during a glucose load. Dogs (n = 15) were implanted with sampling and infusion catheters as well as flow probes >16 d before the studies. After a basal sampling period, dogs were administered a somatostatin infusion (0-150 min) as well as intraportal infusions of glucose [18 micromol/(kg . min)], basal glucagon [0.5 ng/(kg . min)], and insulin [8.3 pmol/(kg . min)] to establish mild hyperinsulinemia. Arterial glucose was clamped at 10 mmol/L with a peripheral glucose infusion. At 80 min, either saline (Control; n = 7) or caffeine [1.5 micromol/(kg . min); n = 8] was infused into the portal vein. Arterial insulin, glucagon, norepinephrine, and glucose did not differ between groups. In dogs infused with caffeine, NHGU was significantly higher than in controls [21.2 +/- 4.3 vs. 11.2 +/- 1.6 micromol/(kg . min)]. Caffeine increased net hepatic lactate output compared with controls [12.5 +/- 3.8 vs. 5.5 +/- 1.5 micromol/(kg . min)]. These findings indicate that physiologic circulating levels of caffeine can enhance NHGU during a glucose load, and the added glucose consumed by the liver is in part converted to lactate.

Portal venous hyperinsulinemia does not stimulate gut glucose absorption in the conscious dog.

The purpose of the present study was to assess whether physiological portal vein hyperinsulinemia stimulates gut glucose absorption in vivo. Chronically catheterized (femoral artery, portal vein, inferior vena cava, and proximal and distal duodenum) and instrumented (Doppler flow probe on portal vein) insulin (INS, 2 mU.kg(-1).min(-1), n = 6) or saline (SAL, n = 5) infused dogs were studied during basal (30 minutes) and experimental (90 minutes) periods. Arterial and portal vein plasma insulin were 3.3- and 3.2-fold higher, respectively, throughout the study in INS compared to SAL. An intraduodenal glucose infusion of 8 mg.kg(-1).min(-1) was initiated at t = 0 minutes. At t = 20 and 80 minutes, a bolus of 3-O-[3H]methylglucose (MG) and L-[14C]glucose (L-GLC) was injected intraduodenally. Phloridzin, an inhibitor of the Na+ -dependent glucose transporter (SGLT1), was infused from t = 60 to 90 minutes in the presence of a peripheral isoglycemic clamp. Net gut glucose output (NGGO) was 5.2 +/- 0.6 and 4.6 +/- 0.8 mg.kg(-1).min(-1) in INS and SAL, respectively, from t = 20 to 60 minutes. Transporter-mediated absorption was 87% +/- 2% of NGGO in both INS and SAL. Passive gut glucose absorption was 13% +/- 2% of NGGO in both INS and SAL. Phloridzin-induced inhibition of transporter-mediated absorption completely abolished passive absorption of L-GLC in both groups. This study shows that under physiological conditions, a portal vein insulin infusion that results in circulating hyperinsulinemia does not increase either transporter-mediated or passive absorption of an intraduodenal glucose load.

Pregnancy impairs the counterregulatory response to insulin-induced hypoglycemia in the dog.

The impact of pregnancy on the counterregulatory response to insulin-induced hypoglycemia was examined in six nonpregnant (NP) and six pregnant (P; 3rd trimester) conscious dogs by tracer and arteriovenous difference techniques. After basal sampling, insulin was infused intraportally at 30 pmol.kg(-1).min(-1) for 180 min. Insulin rose from 70 +/- 15 to 1,586 +/- 221 pmol/l and 27 +/- 4 to 1,247 +/- 61 pmol/l in the 3rd h in NP and P, respectively. Arterial glucose fell from 5.9 +/- 0.2 to 2.3 +/- 0.2 mmol/l in P. Glucose was infused in NP to equate the rate of fall of glucose and the steady-state concentrations in the groups (5.9 +/- 0.2 to 2.3 +/- 0.1 mmol/l in NP). Glucagon was 32 +/- 6, 69 +/- 11, and 48 +/- 10 ng/l (basal and 1st and 3rd h) in NP, but the response was attenuated in P (34 +/- 5, 46 +/- 6, 41 +/- 9 ng/l). Cortisol and epinephrine rose similarly in both groups, but norepinephrine rose more in NP (Delta3.01 +/- 0.46 and Delta1.31 +/- 0.13 nmol/l, P < 0.05). Net hepatic glucose output (NHGO; micromol.kg(-1).min(-1)) increased from 10.6 +/- 1.8 to 21.2 +/- 3.3 in NP (3rd h) but did not increase in P (15.1 +/- 1.5 to 15.3 +/- 2.8 micromol.kg(-1).min(-1), P < 0.05 between groups). The glycogenolytic contribution to NHGO in NP increased from 5.8 +/- 0.7 to 10.4 +/- 2.5 micromol.kg(-1).min(-1) by 90 min but steadily declined in P. The increase in glycerol levels and the gluconeogenic contribution to NHGO were 50% less in P than in NP, but ketogenesis did not differ. The glucagon and norepinephrine responses to insulin-induced hypoglycemia are blunted in late pregnancy in the dog, impacting on the magnitude of the metabolic responses to the fall in glucose.

Hepatic glucose autoregulation: responses to small, non-insulin-induced changes in arterial glucose.

The purpose of this study was to determine whether the sedentary dog is able to autoregulate glucose production (R(a)) in response to non-insulin-induced changes (<20 mg/dl) in arterial glucose. Dogs had catheters implanted >16 days before study. Protocols consisted of basal (-30 to 0 min) and bilateral renal arterial phloridzin infusion (0-180 min) periods. Somatostatin was infused, and glucagon and insulin were replaced to basal levels. In one protocol (Phl +/- Glc), glucose was allowed to fall from t = 0-90 min. This was followed by a period when glucose was infused to restore euglycemia (90-150 min) and a period when glucose was allowed to fall again (150-180 min). In a second protocol (EC), glucose was infused to compensate for the renal glucose loss due to phloridzin and maintain euglycemia from t = 0-180 min. Arterial insulin, glucagon, cortisol, and catecholamines remained at basal in both protocols. In Phl +/- Glc, glucose fell by approximately 20 mg/dl by t = 90 min with phloridzin infusion. R(a) did not change from basal in Phl +/- Glc despite the fall in glucose for the first 90 min. R(a) was significantly suppressed with restoration of euglycemia from t = 90-150 min (P < 0.05) and returned to basal when glucose was allowed to fall from t = 150-180 min. R(a) did not change from basal in EC. In conclusion, the liver autoregulates R(a) in response to small changes in glucose independently of changes in pancreatic hormones at rest. However, the liver of the resting dog is more sensitive to a small increment, rather than decrement, in arterial glucose.

Insulin action during late pregnancy in the conscious dog.

Our aim was to assess the magnitude of peripheral insulin resistance and whether changes in hepatic insulin action were evident in a canine model of late (3rd trimester) pregnancy. A 3-h hyperinsulinemic (5 mU.kg(-1).min(-1)) euglycemic clamp was conducted using conscious, 18-h-fasted pregnant (P; n = 6) and nonpregnant (NP; n = 6) female dogs in which catheters for intraportal insulin infusion and assessment of hepatic substrate balances were implanted approximately 17 days before experimentation. Arterial plasma insulin rose from 11 +/- 2 to 192 +/- 24 and 4 +/- 2 to 178 +/- 5 microU/ml in the 3rd h in NP and P, respectively. Glucagon fell equivalently in both groups. Basal net hepatic glucose output was lower in NP (1.9 +/- 0.1 vs. 2.4 +/- 0.2 mg.kg(-1).min(-1), P < 0.05). Hyperinsulinemia completely suppressed hepatic glucose release in both groups (-0.4 +/- 0.2 and -0.1 +/- 0.2 mg.kg(-1).min(-1) in NP and P, respectively). More exogenous glucose was required to maintain euglycemia in NP (15.2 +/- 1.3 vs. 11.5 +/- 1.1 mg.kg(-1).min(-1), P < 0.05). Nonesterified fatty acids fell similarly in both groups. Net hepatic gluconeogenic amino acid uptake with high insulin did not differ in NP and P. Peripheral insulin action is markedly impaired in this canine model of pregnancy, whereas hepatic glucose production is completely suppressed by high circulating insulin levels.

Suppression of endogenous glucose production by mild hyperinsulinemia during exercise is determined predominantly by portal venous insulin.

Hyperinsulinemia during exercise in people with diabetes requiring exogenous insulin is a major clinical problem. The aim of this study was to assess the significance of portal vein versus arterial insulin to hepatic effects of hyperinsulinemia during exercise. Dogs had sampling (artery, portal vein, and hepatic vein) and infusion (vena cava and portal vein) catheters and flow probes (hepatic artery and portal vein) implanted >16 days before a study. Protocols consisted of equilibration (-130 to -30 min), basal (-30 to 0 min), and treadmill exercise (0-150 min) periods. Somatostatin was infused and glucagon and insulin were replaced in the portal vein to achieve basal arterial and portal vein levels at rest and simulated levels during the first 60 min of exercise. From 60 to 150 min of exercise, the simulated insulin infusion was sustained (C; n = 7), modified to selectively create a physiologic increment in arterial insulin (Pe; n = 7), or altered to increase arterial insulin as in Pe but with a concomitant increase in portal insulin (PePo; n = 7). Euglycemic clamps were performed in all studies. Portal and arterial insulin were 15 +/- 2 and 4 +/- 1 micro U/ml (mean +/- SE of all groups), respectively, at t = 60 min in all groups. Insulin levels were unchanged for the remainder of the exercise period in C. Arterial insulin was increased from 3 +/- 1 to 14 +/- 2 micro U/ml, whereas portal insulin did not change in Pe after t = 60 min. Arterial insulin was increased from 3 +/- 1 to 15 +/- 2 micro U/ml, and portal insulin was increased from 16 +/- 3 to 33 +/- 3 micro U/ml in PePo after t = 60 min. Endogenous glucose production (R(a)) rose similarly from basal during the first 60 min of exercise in all groups (mean +/- SE of all groups was from 2.2 +/- 0.1 to 6.8 +/- 0.5 mg. kg(-1). min(-1)). The increase in R(a) was sustained for the remainder of the exercise period in C. R(a) was suppressed by approximately 40%, but only after 60 min of hyperinsulinemia, and by approximately 20% after 90 min of hyperinsulinemia in Pe. In contrast, the addition of portal venous hyperinsulinemia caused approximately 90% suppression of R(a) within 20 min and for the remainder of the experiment in PePo. Measurements of net hepatic glucose output were similar to R(a) responses in all groups. Arterial free fatty acids (FFAs), a stimulus of R(a), were increased to 1,255 +/- 258 micro mol/l in C but were only 459 +/- 67 and 312 +/- 42 micro mol/l in Pe and PePo, respectively, by 150 min of exercise. Thus, during exercise, the exquisite sensitivity of R(a) to hyperinsulinemia is due entirely to portal venous hyperinsulinemia during the first 60 min, after which peripheral hyperinsulinemia may control approximately 20-40%, possibly as a result of inhibition of the exercise-induced increase in FFA.

Impact of infection on glucose-dependent liver glucose uptake during TPN: interaction with insulin.

Chronic total parenteral nutrition (TPN) markedly augments net hepatic glucose uptake (NHGU). This adaptive increase is impaired by an infection despite accompanying hyperinsulinemia. In the nonadapted state, NHGU is dependent on the prevailing glucose levels. Our aims were to determine whether the adaptation to TPN alters the glucose dependence of NHGU, whether infection impairs this dependence, and whether insulin modulates the glucose dependence of NHGU during infection. Chronically catheterized dogs received TPN for 5 days. On day 3 of TPN, dogs received either a bacterial fibrin clot to induce a nonlethal infection (INF, n = 9) or a sterile fibrin clot (Sham, n = 6). Forty-two hours after clot implantation, somatostatin was infused. In Sham, insulin and glucagon were infused to match the level seen in Sham (9 +/- 1 microU/ml and 23 +/- 4 pg/ml, respectively). In infected animals, either insulin and glucagon were infused to match the levels seen in infection (25 +/- 2 microU/ml and 101 +/- 15 pg/ml; INF-HI; n = 5) or insulin was replaced to match the lower levels seen in Sham (13 +/- 2 microU/ml), whereas glucagon was kept elevated (97 +/- 9 pg/ml; INF-LO; n = 4). Then a four-step (90 min each) hyperglycemic (120, 150, 200, or 250 mg/dl) clamp was performed. NHGU increased at each glucose step in Sham (from 3.6 +/- 0.6 to 5.4 +/- 0.7 to 8.9 +/- 0.9 to 12.1 +/- 1.1 mg.kg(-1).min(-1)); the slope of the relationship between glucose levels and NHGU (i.e., glucose dependence) was higher than that seen in nonadapted animals. Infection impaired glucose-dependent NHGU in both INF-HI (1.3 +/- 0.4 to 2.9 +/- 0.5 to 5.5 +/- 1.0 to 7.7 +/- 1.6 mg.kg(-1).min(-1)) and INF-LO (0.5 +/- 0.7 to 2.2 +/- 0.6 to 4.2 +/- 1.0 to 5.8 +/- 0.8 mg.kg(-1).min(-1)). In summary, TPN augments glucose-dependent NHGU, the presence of infection decreases glucose-dependent NHGU, and the accompanying hyperinsulinemia associated with infection does not sustain the glucose dependence of NHGU.