Portal Glucagon Research Articles

Glucagon's effect on liver protein metabolism in vivo.

The postprandial state is characterized by a storage of nutrients in the liver, muscle, and adipose tissue for later utilization. In the case of a protein-rich meal, amino acids (AA) stimulate glucagon secretion by the α-cell. The aim of the present study was to determine the impact of the rise in glucagon on AA metabolism, particularly in the liver. We used a conscious catheterized dog model to recreate a postprandial condition using a pancreatic clamp. Portal infusions of glucose, AA, and insulin were used to achieve postprandial levels, while portal glucagon infusion was either maintained at the basal level or increased by three-fold. The high glucagon infusion reduced the increase in arterial AA concentrations compared with the basal glucagon level (-23%, P < 0.05). In the presence of high glucagon, liver AA metabolism shifted toward a more catabolic state with less protein synthesis (-36%) and increased urea production (+52%). Net hepatic glucose uptake was reduced modestly (-35%), and AA were preferentially used in gluconeogenesis, leading to lower glycogen synthesis (-54%). The phosphorylation of AMPK was increased by the high glucagon infusion (+40%), and this could be responsible for increasing the expression of genes related to pathways producing energy and lowering those involved in energy consumption. In conclusion, the rise in glucagon associated with a protein-rich meal promotes a catabolic utilization of AA in the liver, thereby, opposing the storage of AA in proteins.

Higher Endogenous Glucose Production During OGTT vs Isoglycemic Intravenous Glucose Infusion

Oral glucose ingestion elicits a larger insulin response and delayed suppression of glucagon compared to isoglycemic IV glucose infusion (IIGI). We studied whether these differences translate into effects on endogenous glucose production (EGP) and glucose disposal in patients with type 2 diabetes and nondiabetic control subjects. This was a single-blinded, randomized, crossover study. The study was conducted at a specialized research unit. Ten patients with type 2 diabetes (age, [mean ± SD] 57.1 ± 6.7 years; body mass index, 29.0 ± 4.3 kg/m2; hemoglobin A1c, 53.8 ± 11.0 mmol/mol; duration of diabetes, 9.2 ± 5.0 years) and 10 matched nondiabetic control subjects (age, 56.0±10.7 years; body mass index, 29.8 ± 2.9 kg/m2; hemoglobin A1c, 33.8 ± 5.5 mmol/mol) participated. Three experimental days: 75 g-oral glucose tolerance test (OGTT), IIGI, and IIGI+glucagon (IIGI with a concomitant IV glucagon infusion [0.8 ng/kg/min from 0 to 25 minutes] designed to mimic portal glucagon concentrations during OGTT in the type 2 diabetic group) were undertaken. Glucose kinetics were assessed by tracer methodology. Glucose rate of disappearance was higher during the OGTT vs IIGI in the control group, but similar on all days in the diabetic group. Surprisingly, in both groups, EGP was more suppressed during IIGI than during OGTT, and exogenous glucagon infusion during IIGI did not restore EGP to the levels observed during OGTT. EGP was less suppressed during OGTT than during IIGI in both patients with type 2 diabetes and in nondiabetic control subjects. Based on the present experimental design, it was not possible to attribute this difference to the delayed glucagon suppression observed in the initial phase of the OGTT.

Hypothalamic Brain‐Derived Neurotrophic Factor Regulates Glucagon Secretion Mediated by Pancreatic Efferent Nerves

Understanding the molecular mechanism of the regulation of glucagon secretion is critical for treating the dysfunction of α cells observed in diabetes. Glucagon-like peptide (GLP)-1 analogues reduce plasma glucagon and are assumed to contribute to their action to lower blood glucose. It has previously been demonstrated that the central administration of brain-derived neurotrophic factor (BDNF) improves glucose metabolism by a mechanism independent of feeding behaviour in obese subjects. Using male rats, we examined whether BDNF influences glucagon secretion from α cells via the the central nervous system. We investigate whether: (i) the central infusion of BDNF stimulates glucagon and/or insulin secretion via the pancreatic efferent nerve from the hypothalamus; (ii) the intraportal infusion of GLP-1 regulates glucose metabolism via the central and peripheral nervous system; and (iii) BDNF receptor and/or BDNF-positive fibres are localised near α cells of islets. The portal glucagon level decreased with the central administration of BDNF (n=6, in each; P<0.05); in contrast, there was no significant change in portal insulin, peripheral glucagon and insulin levels with the same treatment. This reduction of glucagon secretion was abolished by pancreatic efferent denervation (n=6, in each; P<0.05). In an immunohistochemical study, pancreatic α cells were stained specifically with BDNF and tyrosine-related kinase B, a specific receptor for BDNF, and α cells were also co-localised with BDNF. Moreover, intraportal administration of GLP-1 decreased glucagon secretion, as well as blood glucose, whereas it increased the BDNF content in the pancreas; these effects were inhibited with the central infusion of BDNF antibody (n=6, in each; P<0.05). BDNF and GLP-1 affect glucose metabolism and modulate glucagon secretion from pancreatic α cells via the central and peripheral nervous systems.

Mechanism of Action of DPP-4 Inhibitors—New Insights

Mechanism of Action of DPP-4 Inhibitors—New Insights

Physiological and Pharmacological Mechanisms through which the DPP-4 Inhibitor Sitagliptin Regulates Glycemia in Mice

Inhibition of dipeptidyl peptidase-4 (DPP-4) activity improves glucose homeostasis through a mode of action related to the stabilization of the active forms of DPP-4-sensitive hormones such as the incretins that enhance glucose-induced insulin secretion. However, the DPP-4 enzyme is highly expressed on the surface of intestinal epithelial cells; hence, the role of intestinal vs. systemic DPP-4 remains unclear. To analyze mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice, we administered low oral doses of the DPP-4 inhibitor sitagliptin that selectively reduced DPP-4 activity in the intestine. Glp1r(-/-) and Gipr(-/-) mice were studied and glucagon-like peptide (GLP)-1 receptor (GLP-1R) signaling was blocked by an i.v. infusion of the corresponding receptor antagonist exendin (9-39). The role of the dipeptides His-Ala and Tyr-Ala as DPP-4-generated GLP-1 and glucose-dependent insulinotropic peptide (GIP) degradation products was studied in vivo and in vitro on isolated islets. We demonstrate that very low doses of oral sitagliptin improve glucose tolerance and plasma insulin levels with selective reduction of intestinal but not systemic DPP-4 activity. The glucoregulatory action of sitagliptin was associated with increased vagus nerve activity and was diminished in wild-type mice treated with the GLP-1R antagonist exendin (9-39) and in Glp1r(-/-) and Gipr(-/-) mice. Furthermore, the dipeptides liberated from GLP-1 (His-Ala) and GIP (Tyr-Ala) deteriorated glucose tolerance, reduced insulin, and increased portal glucagon levels. The predominant mechanism through which DPP-4 inhibitors regulate glycemia involves local inhibition of intestinal DPP-4 activity, activation of incretin receptors, reduced liberation of bioactive dipeptides, and activation of the gut-to-pancreas neural axis.

The Glucagonostatic and Insulinotropic Effects of Glucagon-Like Peptide 1 Contribute Equally to Its Glucose-Lowering Action

OBJECTIVEGlucagon-like peptide 1 (GLP-1) exerts beneficial antidiabetic actions via effects on pancreatic β- and α-cells. Previous studies have focused on the improvements in β-cell function, while the inhibition of α-cell secretion has received less attention. The aim of this research was to quantify the glucagonostatic contribution to the glucose-lowering effect of GLP-1 infusions in patients with type 2 diabetes.RESEARCH DESIGN AND METHODSTen male patients with well-regulated type 2 diabetes (A1C 6.9 ± 0.8%, age 56 ± 10 years, BMI 31 ± 3 kg/m2 [means ± SD]) were subjected to five 120-min glucose clamps at fasting plasma glucose (FPG) levels. On day 1, GLP-1 was infused to stimulate endogenous insulin release and suppress endogenous glucagon. On days 2–5, pancreatic endocrine clamps were performed using somatostatin infusions of somatostatin and/or selective replacement of insulin and glucagon; day 2, GLP-1 plus basal insulin and glucagon (no glucagon suppression or insulin stimulation); day 3, basal insulin only (glucagon deficiency); day 4, basal glucagon and stimulated insulin; and day 5, stimulated insulin. The basal plasma glucagon levels were chosen to simulate portal glucagon levels.RESULTSPeptide infusions produced the desired hormone levels. The amount of glucose required to clamp FPG was 24.5 ± 4.1 (day 1), 0.3 ± 0.2 (day 2), 10.6 ± 1.1 (day 3), 11.5 ± 2.7 (day 4), and 24.5 ± 2.6 g (day 5) (day 2 was lower than days 3 and 4, which were both similar and lower than days 1 and 5).CONCLUSIONSWe concluded that insulin stimulation (day 4) and glucagon inhibition (day 3) contribute equally to the effect of GLP-1 on glucose turnover in patients with type 2 diabetes, and these changes explain the glucose-lowering effect of GLP-1 (day 5 vs. day 1).

The Pancreatohepatorenal cAMP-Adenosine Mechanism

Stimulation of adenylyl cyclase causes cellular efflux of cAMP, and cAMP (unlike adenosine) is stable in blood. Therefore, it is conceivable that cAMP could function as a circulating adenosine prohormone by local target-organ conversion of distally released cAMP to adenosine via the sequential actions of ectophosphodiesterase and ecto-5'-nucleotidase (cAMP==> AMP==> adenosine; called the cAMP-adenosine pathway). A possible specific representation of this general concept is the pancreatohepatorenal cAMP-adenosine mechanism. The pancreas secretes glucagon into the portal circulation, and glucagon is a stimulant of hepatic adenylyl cyclase. Therefore, we hypothesize that the pancreas, via glucagon, stimulates hepatic cAMP production, which provides circulating cAMP for conversion to adenosine in the kidney via the cAMP-adenosine pathway. In normal rats, intravenous cAMP increased urinary and renal interstitial (assessed by renal microdialysis) cAMP and adenosine. Intraportal infusions of glucagon increased plasma cAMP 10-fold, it did not affect plasma adenosine, and it increased urinary and renal interstitial cAMP and adenosine. Local renal interstitial blockade (by adding inhibitors directly to the microdialysis perfusate) of ectophosphodiesterase (using 3-isobutyl-1-methylxanthine or 1,3-dipropyl-8-p-sulfophenylxanthine) or ecto-5'-nucleotidase (using alpha,beta-methyleneadenosine-5'-diphosphate) prevented the cAMP-induced and glucagon-induced increases in renal interstitial adenosine, but not cAMP. In ZSF1 rats with the metabolic syndrome, an oral glucose load increased plasma glucagon and urinary cAMP and adenosine excretion. We conclude that circulating cAMP is a substrate for local conversion to adenosine via the cAMP-adenosine pathway. A specific manifestation of this is the pancreatohepatorenal cAMP-adenosine mechanism (pancreas==> portal glucagon==> liver==> circulating cAMP==> kidney==> local cAMP-adenosine pathway).

Interaction of insulin and prior exercise in control of hepatic metabolism of a glucose load.

To determine if prior exercise enhances insulin-stimulated extraction of glucose by the liver, chronically catheterized dogs were submitted to 150 min of treadmill exercise or rest. After exercise or rest, dogs received portal glucose (18 micro mol x kg(-1) x min(-1)), peripheral somatostatin, and basal portal glucagon infusions from t = 0 to 150 min. A peripheral glucose infusion was used to clamp arterial blood glucose at 8.3 mmol/l. Insulin was infused into the portal vein to create either basal levels or mild hyperinsulinemia. Prior exercise did not increase whole-body glucose disposal in the presence of basal insulin (25.5 +/- 1.5 vs. 20.3 +/- 1.7 micro mol x kg(-1) x min(-1)), but resulted in a marked enhancement in the presence of elevated insulin (97.2 +/- 15.1 vs. 64.4 +/- 7.4 micro mol x kg(-1) x min(-1)). Prior exercise also increased net hepatic glucose uptake in the presence of both basal insulin (7.5 +/- 1.2 vs. 2.9 +/- 2.4 micro mol x kg(-1) x min(-1)) and elevated insulin (22.0 +/- 3.5 vs. 11.5 +/- 1.8 micro mol x kg(-1) x min(-1)). Likewise, net hepatic glucose fractional extraction was increased by prior exercise with both basal insulin (0.04 +/- 0.01 vs. 0.01 +/- 0.01 micro mol x kg(-1) x min(-1)) and elevated insulin (0.10 +/- 0.01 vs. 0.05 +/- 0.01). Hepatic glycogen synthesis was increased by elevated insulin, but was not enhanced by prior exercise. Although the increase in glucose extraction after exercise could be ascribed to increased insulin action, the increase in hepatic glycogen synthesis was independent of it.

Lack of suppression of glucagon contributes to postprandial hyperglycemia in subjects with type 2 diabetes mellitus.

We tested the hypothesis that a lack of suppression of glucagon causes postprandial hyperglycemia in subjects with type 2 diabetes. Nine diabetic subjects ingested 50 g glucose on two occasions. On both occasions, somatostatin was infused at a rate of 4.3 nmol/kg x min, and insulin was infused in a diabetic insulin profile. On one occasion, glucagon was also infused at a rate of 1.25 ng/kg x min to maintain portal glucagon concentrations constant (nonsuppressed study day). On the other occasion, glucagon infusion was delayed by 2 h to create a transient decrease in glucagon (suppressed study day). Glucagon concentrations on the suppressed study day fell to about 70 ng/L during the first 2 h, rising thereafter to approximately 120 ng/L. In contrast, glucagon concentrations on the nonsuppressed study day remained constant at about 120 ng/L throughout. The decrease in glucagon resulted in substantially lower (P < 0.001) glucose concentrations on the suppressed compared with the nonsuppressed study days (9.2+/-0.7 vs. 10.9+/-0.8 mmol/L) and a lower (P < 0.001) rate of release of [14C]glucose from glycogen (labeled by infusing [1-14C]galactose). On the other hand, flux through the hepatic UDP-glucose pool (and, by implication, glycogen synthesis), measured using the acetaminophen glucuronide method, did not differ on the two occasions. We conclude that lack of suppression of glucagon contributes to postprandial hyperglycemia in subjects with type 2 diabetes at least in part by accelerating glycogenolysis. These data suggest that agents that antagonize glucagon action or secretion are likely to be of value in the treatment of patients with type 2 diabetes.

Effects of Physical Exercise on Liver ATP Levels in Fasted and Phosphate-Injected Rats

The purpose of the present study was to investigate the effects of exercise (30 min, 23 m/min, 0% grade) on the hepatic levels of ATP in fasted adrenodemedullated rats, with an intraperitoneal injection of sodium phosphate (Na 2 PO 4, 0.91 mM) or saline (NaCl). Sodium phosphate was injected to determine if the postulated decrease in liver ATP during exercise may be changed by providing an excess of phosphate. At the end of exercise, a piece of liver was rapidly freeze clamped and used for the enzymatic determination of ATP levels. Liver ATP, in saline-injected rats, was significantly (P < 0.05) decreased by fasting, compared to fed rats (X ± SE: 3.21 ± 0.2 vs 2.86 ± 0.2 µmol/g). Exercise in fasted rats decreased even more the ATP response in liver (2.58 ± 0.14 µmol/g). Injection of Na 2 PO 4 did not significantly (P < 0.05) alter the pattern of ATP response following these 3 conditions (3.35 ± 0.14 vs 3.0 ± 0.12 vs 2.57 ± 0.1 µmol/g), ATP levels being significantly (P < 0.05) decreased by the fast and the exercise in the fasted state. Fasting and exercise resulted in a significant (P < 0.05) decrease in liver glycogen and plasma glucose concentrations and an increase in free fatty acid levels in both NaCl- and Na 2 PO 4 -injected groups. In both injection conditions, ß-hydroxybutyrate and peripheral insulin concentrations were respectively, increased and decreased (P < 0.05) by fasting, while norepinephrine and portal glucagon were decreased (P < 0.05) following exercise. The main effect of the injection of Na 2 PO 4 was a stimulation (P < 0.05) of peripheral glucagon response following exercise. It is concluded that exercise results in a decrease in liver ATP levels even in fasted rats and that this decrease is not corrected by Na 2 PO 4 administration. The decreased liver ATP levels might be involved in the metabolic adaptations to exercise.

Effects of hepatic portal infusion of deionized water on metabolic and hormonal responses to exercise in rats.

The present study was conducted to investigate the in vivo effects of an intrahepatic infusion of deionized water during exercise in rats. Adrenodemedullated male Sprague-Dawley rats were continuously infused for 30 min either at rest or during treadmill exercise (26 m/min, 0% grade). Rats were randomly assigned to one of three infusion conditions (52 micro ul/min) with either deionized water (PW) or saline (PS; NaCl; 0.9%) via the hepatic portal vein or deionized water through the jugular vein (JW). The exercise period caused a significant (P < 0.05) decrease in liver glycogen and relative liver water content and peripheral and portal blood glucose and insulin while increasing peripheral and portal glucagon and K+ plasma concentrations. These responses, with the exception of K+, were not influenced by the different types of infusions. The increase in K+ during exercise was significantly (P < 0.05) higher in JW rats than in the PW and PS groups. Both the infusion and exercise protocols did not significantly alter the liver weight-to-body weight ratio, plasma osmolality, free fatty acids, beta-hydroxybutyrate, Na+, Cl-, vasopressin, and catecholamine concentrations. It is concluded that an hepatic portal infusion of deionized water does not specifically alter the metabolic and hormonal responses to exercise in rats.

Indirect effect of insulin to suppress endogenous glucose production is dominant, even with hyperglucagonemia.

Suppression of endogenous glucose production (EGP) is one of insulin's primary metabolic effects and failure of this action is a major contributor to fasting hyperglycemia of type 2 diabetes mellitus. Classically, insulin was thought to suppress the liver directly, via hyperinsulinemia in the portal vein. Recently, however, we and others have demonstrated that at least part, and possibly most of insulin's action to suppress EGP is normally mediated via an extrahepatic (i.e., indirect) mechanism. We have suggested that this mechanism involves insulin suppression of adipocyte lipolysis, leading to lowered FFA and reduced EGP ("Single Gateway Hypothesis"). Previous studies of the indirect insulin effect from this laboratory were done under conditions of lowered portal glucagon. Because of the possibility that the direct (i.e., portal) effect of insulin may have been underestimated with hypoglucagonemia, these studies examined the relative importance of portal insulin, versus peripheral insulin (administered at one-half the dose to equalize peripheral insulin levels) at four rates of portal glucagon infusion: 0, 0.65 (under-), 1.5 (basal-), and 3.0 ng/kg per min (over-replacement). Portal versus peripheral insulin suppressed steady-state EGP to the same extent (52%), confirming that the primary effect of insulin to suppress EGP is via the peripheral mechanism. This conclusion was maintained regardless of portal glucagonemia, although there was some evidence for an increase in the direct insulin effect at hyperglucagonemia. The indirect effect of insulin is the primary mechanism of steady-state EGP suppression under normal conditions. The direct effect increases with hyperglucagonemia; however, the indirect effect remains predominant even under those conditions.

Direct and indirect effects of insulin in suppressing glucose production in depancreatized dogs: role of glucagon.

We have previously shown that during glucose clamps in moderately hyperglycemic depancreatized dogs: 1) peripheral insulin infusion, resulting in greater systemic insulinemia and greater suppression of glucagon than equidose portal infusion, inhibited glucose production (GP) to a greater extent; and 2) portal and half-dose peripheral infusions, resulting in matched peripheral insulinemia and similar suppression of glucagon, inhibited GP equally. These findings are consistent with an indirect effect of insulin in suppressing GP in diabetic dogs, which might be partly mediated by the differential suppression of glucagon. To address this question, we performed the experimental protocols of the previous study under conditions of constant glucagon levels (approximately 550 ng/liter), achieved by a high rate portal glucagon infusion (5 ng/kg.min). As in the previous study (basal glucagon levels, approximately 170 ng/liter), we used depancreatized dogs and assessed GP with HPLC-purified [6(-3)H]glucose. After obtaining constant basal hyperglycemia (approximately 10 mM) with portal infusions of insulin (4.8 +/- 0.5 pmol/kg.min) and glucagon, an additional infusion of insulin was administered for 180 min, either portally (portal; n = 7) or peripherally (peripheral; n = 8) at the same rate (5.4 pmol/kg.min) or at half that rate peripherally (1/2 periph; n = 5). Plasma glucose and glucose specific activities were clamped at basal levels. Systemic insulin levels increased by 215 +/- 16,310 +/- 26, and 184 +/- 15 pM, and estimated hepatic insulin levels increased by 398 +/- 20, 310 +/- 26, and 184 +/- 15 pM with portal, peripheral, and 1/2 periph, respectively. GP was suppressed to the same extent with portal and peripheral (53 +/- 6% and 50 +/- 6%), but less with 1/2 periph (35 +/- 5%). FFA levels were suppressed to a greater extent with peripheral than portal or 1/2 periph, whereas the responses of lactate alanine and glycerol to insulin infusion were similar in the three groups. Thus, in the present report, unlike in our previous study, 1) suppression of GP was proportional to the hepatic insulin levels; and 2) systemic insulin levels did not dominate suppression of GP. We, therefore, conclude that in hyperglycemic depancreatized dogs 1) glucagon, at concentrations seen in poorly controlled diabetes, can unmask a direct effect of hepatic insulin levels on GP; and 2) the suppression of glucagon may play a role in the peripheral effect of exogenously delivered insulin on GP. This is the first in vivo study to show that the main direct effect of insulin on the liver is to counteract the effect of glucagon.

The Satiating Potency of Hepatic Portal Glucagon in Rats and Insulin or Insulin Antibodies

Geary, N., L. Asarian and W. Langhans. The satiating potency of hepatic portal glucagon in rats and insulin or insulin antibodies. Physiol Behav 61(2) 199–208, 1997.—To characterize the interactive effects of acute prandial manipulations of insulin and glucagon on spontaneous feeding in adult male rats fed ad lib, glucagon (G) and insulin (I) or insulin antibodies (IAb) were coinfused into the hepatic portal vein during the first meal of the dark phase. Infusions (3–6 min, 33 μl/min) were remotely controlled, and a computerized system recorded meal patterns. In Experiment 1, five separate factorial designs were used to test the effects of G (1.3 or 13 μg/meal) alone, I (1.3 or 2.7 mU/meal) alone, or both G + I. The peptides were infused either simultaneously or sequentially (G before I). The larger dose of G alone reduced meal size. I neither inhibited feeding nor increased the effects of either G dose. In one test, 13 μg/meal G did not block meal size when followed by 2.7 mU I, but this antagonism did not occur in a replication. In several tests, there was a trend for I to decrease the size of the spontaneous meal that followed the meal during which I was infused, but this was statistically significant only once. Intermeal intervals were not affected in any test. Experiment 2 tested coinfusions of 20 μg G and polyclonal IAb with an in vitro binding capacity of 40 mU rat insulin. G alone reduced meal size, IAb alone increased meal size, and G + IAb produced an additive effect. These data extend previous investigations of the satiating action of G and I in the rat and indicate 1. that exogenous I does not affect the satiating potency of G; 2. that exogenous G and endogenous I elicit an additive synergistic inhibition of spontaneous meal size; and 3. that G-induced I secretion does not mediate the satiating effect of G.

Chronic Administration of Octreotide Increases Vascular Responsiveness in Rats with Portal Hypertension

1. It has been reported that ortreotide partially corrects the hyperdynamic state in patients and animals with portal hypertension. The aim of the present study was to investigate whether chronic administration of octreotide can increase vascular responsiveness in rats with portal hypertension. 2. Portal hypertension was induced by partial portal vein ligation. Octreotide was given for 9 days subcutaneously (100 micrograms/kg every 12 h) starting 1 day before ligation. The aorta and mesenteric artery were then removed to study contraction after pressure recording. 3. Octreotide treatment significantly reduced portal pressure and plasma glucagon concentrations compared with the vehicle-treated group. Both phenylephrine and vasopressin induced concentration-dependent contractile responses in the aorta and mesenteric artery from both groups. The maximum contractile responses to phenylephrine and vasopressin in aorta and mesenteric artery were significantly greater in the octreotide-treated group than in the vehicle-treated group. The EC50 values for phenylephrine and vasopressin were significantly different in the aorta, but not in the mesenteric artery, but not in the mesenteric artery, between the two groups. In contrast, octreotide treatment did not alter the contractile responsiveness of arteries rom sham-operated rats. 4. These results show that, in rats with portal vein stenosis, octreotide increases arterial contractile responsiveness and reduces portal pressure.

Direct effects of catecholamines on hepatic glucose production in conscious dog are due to glycogenolysis.

The effects of catecholamines (CATS) infused into the hepatic portal vein were studied in ten 18-h-fasted conscious dogs. Glucose production (GP) and gluconeogenesis (GNG) were assessed using tracer ([3H]glucose, [14C]alanine) and arteriovenous difference techniques. Each experiment consisted of a 100-min equilibration, a 40-min basal, and two 90-min test periods. A pancreatic clamp (somatostatin + basal portal insulin and glucagon) was used to fix insulin and glucagon at basal levels. Propranolol (1 microgram.kg-1.min-1) and phentolamine (2 micrograms.kg-1.min-1) were infused intraportally during both test periods of the blockade group while a carrier solution was infused in the control group. Norepinephrine (NE; 100 ng.kg-1.min-1) and epinephrine (Epi; 40 ng.kg-1.min-1) were infused intraportally during the second test period of both protocols. Portal NE (70 +/- 46 to 8,404 +/- 674 and 162 +/- 57 to 6,530 +/- 624 pg/ml, respectively) and portal Epi (21 +/- 11 to 3,587 +/- 309 and 29 +/- 6 to 2,989 +/- 406 pg/ml, respectively) rose in the control and adrenergic blockade groups, respectively. The increases in arterial NE and Epi were modest in both groups. Intraportal infusion of CATS increased GP from 2.1 +/- 0.2 to 6.2 +/- 1.0 mg.kg-1.min-1 in the control group but did not change it (2.7 +/- 0.4 to 2.7 +/- 0.3 mg.kg-1.min-1) in the blockade group. Portal CATS had no effect on GNG in the presence or absence of adrenergic blockade (GNG rose from 0.7 +/- 0.2 to 0.9 +/- 0.2 and 0.8 +/- 0.2 to 1.0 +/- 0.2 mg.kg-1.min-1 in the control and blockade groups, respectively). In conclusion, portal infusion of catecholamines significantly augmented GP by selectively stimulating glycogenolysis. The increase in hepatic GP could be completely inhibited by intraportal adrenergic blockade.

Effect of hepatic vagotomy on plasma catecholamines during exercise-induced hypoglycemia.

The existence of a hepatosympathetic reflex active during insulin-induced hypoglycemia has recently been reported. The purpose of the present investigation was to test the hypothesis that the liver, through the afferent innervation of the vagus nerve, contributes to plasma epinephrine and norepinephrine responses during exercise-induced hypoglycemia. Hepatic vagotomized and sham-operated rats were killed at rest or after 30, 60, and 120 min of running exercise (26 m/min, 0% grade). At the end of the 120-min exercise period, liver glycogen, glucose, and insulin levels measured in the portal and peripheral plasma were all significantly reduced (P < 0.05) while epinephrine and norepinephrine concentrations, beta-hydroxybutyrate, lactate, and portal and peripheral glucagon plasma levels were all significantly increased (P < 0.05). However, no significant differences were observed between hepatic vagotomized and sham-operated rats at rest and after exercise for the metabolic and hormonal responses. These results suggest that if a hepatosympathetic reflex is active during an exercise-induced hypoglycemia situation, then this contribution is probably hidden by more important regulatory mechanisms.

Regulation of urea synthesis by glucose and glucagon in normal man

The separate effects of glucose and glucagon on alanine stimulated hepatic amino-N to urea-N conversion, quantified by the Functional Hepatic Nitrogen Clearance (FHNC) (i.e. the linear slope of the relation between urea synthesis rate and blood α-amino-N concentration), were studied in 7 healthy subjects. FHNC was measured four times in each: during constant infusion of alanine alone; alanine superimposed on constant glucose infusion; alanine superimposed on glucose and low stepwise glucagon infusions; and alanine super-imposed on glucose and high constant glucagon infusions. Glucose halved the glucagon response to alanine. This reduction was abolished by the low stepwise glucagon infusion, aimed at re-establishing portal glucagon levels. The high glucagon infusion resulted in 3-fold elevated glucagon levels. During alanine infusion alone FHNC was (mean ± SEM) 32.5 ± 1.9 l/h. Glucose reduced FHNC by 43% to 18.4 ± 0.9 l/h (p < 0.01). The low stepwise glucagon infusion only partially normalized FHNC as reduced by glucose (to 24.6 ± 1.5 l/h, (p < 0.01 vs alanine alone)). The high glucagon infusion increased FHNC by 35% despite hyperglycaemia (to 44.1 ± 1.5 l/h, (p < 0.01 vs alanine alone)). The results show that both glucose and glucagon are independent but opposite regulators of hepatic amino-N conversion. The physiological glucose effect is accomplished by a combination of both the effect of glucose itself and the inhibition by glucose of the glucagon response to alanine. Hyperglucagonaemia increases FHNC and overrules the inhibition by glucose. This may explain the defect nitrogen sparing by glucose and to some extent the catabolism in hyperglucagonaemic stress conditions, despite prevailing hyperglycaemia.

Presinusoidal and proximal intrasinusoidal confluence of hepatic artery and portal vein in rat liver: Functional evidence by orthograde and retrograde bivascular perfusion

The site of confluence of the artery and the portal vein in the liver still appears to be controversial. Anatomical studies suggested a presinusoidal or an intrasinusoidal confluence in the first, second or even final third of the sinusoids. The objective of this investigation was to study the problem with functional biochemical techniques. Rat livers were perfused through the hepatic artery and simultaneously either in the orthograde direction from the portal vein to the hepatic vein or in the retrograde direction from the hepatic vein to the portal vein. Arterial flow was linearly dependent on arterial pressure between 70 cm H2O and 120 cm H2O at a constant portal or hepatovenous pressure of 18 cm H2O. An arterial pressure of 100 cm H2O was required for the maintenance of a homogeneous orthograde perfusion of the whole parenchyma and of a physiologic ratio of arterial to portal flow of about 1:3. Glucagon was infused either through the artery or the portal vein and hepatic vein, respectively, to a submaximally effective "calculated" sinusoidal concentration after mixing of 0.1 nmol/L. During orthograde perfusions, arterial and portal glucagon caused the same increases in glucose output. Yet during retrograde perfusions, hepatovenous glucagon elicited metabolic alterations equal to those in orthograde perfusions, whereas arterial glucagon effected changes strongly reduced to between 10% and 50%. Arterially infused trypan blue was distributed homogeneously in the parenchyma during orthograde perfusions, whereas it reached clearly smaller areas of parenchyma during retrograde perfusions. Finally, arterially applied acridine orange was taken up by all periportal hepatocytes in the proximal half of the acinus during orthograde perfusions but only by a much smaller portion of periportal cells in the proximal third of the acinus during retrograde perfusions. These findings suggest that in rat liver, the hepatic artery and the portal vein mix before and within the first third of the sinusoids, rather than in the middle or even last third.

Splanchnic haemodynamics and vasoactive agents in experimental cirrhosis.

It is well known that portal hypertension is associated with a hyperdynamic systemic circulatory state. This study measures systemic and splanchnic haemodynamics in an experimental rat model of hepatic cirrhosis. It also investigates the association between haemodynamic changes in cirrhotic animals and circulating levels of the vasoactive hormones glucagon and vasoactive intestinal polypeptide (VIP). Splanchnic blood flow was significantly increased in the cirrhotic group (13.2 ± 1.3 vs. 9.2 ± 1.6 ml/min, P < 0.05). Circulating levels of glucagon and VIP were two and five fold increased respectively in cirrhotic animals compared to controls. There was a strong correlation between portal pressure and glucagon levels in the cirrhotic group (r = 0.85). Raised splanchnic blood flow is partly responsible for elevated portal pressure in this model and this rise may be humorally mediated.