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Article

Net Hepatic Gluconeogenic Amino Acid Uptake in Response to Peripheral versus Portal Amino Acid Infusion in Conscious Dogs¹

Mary Courtney Moore^*2, Po-Shiuan Hsieh^*, Paul J. Flakoll^,**, Doss W. Neal and Alan D. Cherrington^*,

^* Department of Molecular Physiology and Biophysics, Diabetes Research and Training Center, and ^** Department of Surgery, Vanderbilt University School of Medicine, Nashville, TN 37232

²To whom correspondence should be addressed.

	ABSTRACT

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These studies were conducted to determine the effect of route of gluconeogenic amino acid delivery on the hepatic uptake of the amino acids. After a sampling period with no experimental intervention (basal period), conscious dogs deprived of food for 42 h received somatostatin, intraportal infusions of insulin (3-fold basal) and glucagon (basal), and a peripheral infusion of glucose to increase the hepatic glucose load 1.5-fold basal for 240 min. A mixture of alanine, glutamate, glutamine, glycine, serine and threonine was infused intraportally at 7.6 µmol · kg^-1 · min^-1 (PorAA group, n = 6) or peripherally at 8.1 µmol · kg^-1 · min^-1 (PerAA, n = 6), to match the hepatic load of gluconeogenic amino acids in PorAA. During the infusion period, there were no differences in PerAA and PorAA, respectively, with regard to arterial plasma insulin (144 ± 18 and 162 ± 18 pmol/L), glucagon (51 ± 8 and 47 ± 11 ng/L), hepatic glucose load (199.8 ± 22.2 and 210.9 ± 16.6 µmol · kg^-1 · min^-1), net hepatic glucose uptake (2.8 ± 2.2 and 2.2 ± 1.7 µmol · kg^-1 · min^-1), hepatic load of amino acids (68 ± 14 and 62 ± 7 µmol · kg^-1 · min^-1), or net hepatic glycogen synthesis (11.1 ± 2.2 and 8.9 ± 2.2 µmol · kg^-1 · min^-1). The net hepatic uptake of glutamine (2.1 ± 0.4 vs. 0.8 ± 0.3 µmol · kg^-1 · min^-1) and the net hepatic fractional extractions of glutamine (0.11 ± 0.02 vs. 0.05 ± 0.02) and serine (0.41 ± 0.03 vs. 0.34 ± 0.02) were greater in PorAA than in PerAA (P < 0.05). We speculate that one or more of the amino acids in the mixture causes enhancement of the net hepatic uptake and fractional extraction of glutamine, and perhaps other gluconeogenic amino acids, during intraportal amino acid delivery.

KEY WORDS: • amino acids • glutamine • liver nerves • dogs

	INTRODUCTION

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We recently reported the results of studies in conscious dogs receiving intraportal glucose infusion concomitant with intraportal or peripheral infusion of a mixture of gluconeogenic amino acids (alanine, glycine, glutamate, glutamine, serine and threonine) (Moore et al. 1999 ). Net hepatic glucose uptake (NHGU)³ was significantly greater with peripheral infusion of amino acids than with intraportal infusion. Moreover, the net hepatic uptakes (NHU) and/or fractional extractions (NHFE) of alanine, glutamate and glutamine were significantly greater during portal versus peripheral amino acid infusion. However, from those data we could not determine whether the NHU and/or NHFE of the amino acids were enhanced by portal amino acid administration per se or by an interaction between portal glucose and portal amino acid delivery.

Many amino acids are known to increase or decrease the afferent firing rate in the hepatic branch of the vagus nerve (Niijima and Meguid 1995 ). A decrease in the afferent firing rate was shown to be associated with changes in efferent vagal signaling (Niijima 1983 , Niijima 1989 ). We hypothesized that the gluconeogenic amino acids generate signals transmitted via the vagus that result in enhancement of their own net hepatic uptakes. As a first step in testing this hypothesis, we undertook the current studies to determine whether the NHU or NHFE of any of the gluconeogenic amino acids would be enhanced by their intraportal delivery in the absence of portal glucose infusion and thus in the absence of the portal glucose signal that results in enhanced NHGU.

	MATERIALS AND METHODS

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Animals, diets and experimental preparation.

Studies were carried out on conscious adult dogs of either sex with a mean weight of 21 ± 2 kg with breeds as previously described (Moore et al. 1993 ). Dogs were housed in a facility approved by the American Association for the Accreditation of Laboratory Animal Care International, and the protocols were approved by the Vanderbilt University Animal Care Committee. The dogs were fed once daily equal parts by weight of canned meat [Pedigree Meat Food (Kal Kan, Vernon, CA)] and nonpurified diet [Purina Lab Canine Diet no. 5006 (Purina, St. Louis, MO)] and provided, in g/100 g diet: protein, 17.5; fat, 7.9; carbohydrate, 23.5; crude fiber, 1.5; moisture, 44; and ash, 4.8. Micronutrient composition was as previously described (Moore et al. 1993 ). The animals were closely monitored, and the amount of food offered was adjusted to maintain a stable weight. However, these dogs were very homogeneous in terms of weight and activity level, and most received ~400 g each of meat and nonpurified diet daily.

All dogs underwent a laparotomy under general anesthesia ~16 d before study, for insertion of sampling catheters in the portal and left common hepatic veins and femoral artery and infusion catheters in a splenic and a jejunal vein to allow intraportal nutrient and hormone delivery (Pagliassotti et al. 1996 ). Ultrasonic flow probes (Transonic Systems, Ithaca, NY) were positioned around the portal vein and hepatic artery. Pre-study assessment and preparation were performed as previously described (Pagliassotti et al. 1996 ). Food was withheld for 42 h before each experiment, to allow glycogen to reach a stable minimum and all dogs to achieve a state of net hepatic lactate uptake.

Experimental design.

At -120 min, a primed (1.48 MBq), continuous (14.8 kBq/min) infusion of D-[3-³H] glucose and a continuous infusion of indocyanine green dye (Becton Dickinson, Cockeysville, MD; 4 µg · kg^-1 · min^-1) were begun via the left cephalic vein. After 80 min (-120 to -40) of tracer and dye equilibration, there was a 40 min (-40 to 0) control or basal period, followed by a 240 min (0 to 240) experimental period. At time 0, constant infusions of several solutions were begun, and they were continued throughout the experimental period. Somatostatin (0.8 µg · kg^-1 · min^-1 [Bachem, Torrance, CA]) was infused via a lateral saphenous vein to suppress endogenous insulin and glucagon secretion. Insulin (7.2 pmol · kg^-1 · min^-1; ~3-fold basal) and glucagon (basal; both hormones obtained from Eli Lilly & Co, Indianapolis, IN) were delivered intraportally. The dogs were divided into two groups (see below). Two dogs in each group received glucagon at 0.65 ng · kg^-1 · min^-1, but since this rate resulted in circulating glucagon levels slightly higher than basal, the infusion rate was lowered to 0.5 ng · kg^-1 · min^-1 in the remainder of the dogs. In one group of dogs (PorAA, n = 6), a mixture of gluconeogenic amino acids (L-isomers of glutamine, 126 mmol/L; glutamate, 69 mmol/L; threonine, 126 mmol/L; serine, 55 mmol/L; glycine, 111 mmol/L; and alanine, 131 mmol/L) was infused intraportally at 7.6 µmol · kg^-1 · min^-1. The amino acid mixture was prepared just before time 0 by dissolving the individual crystalline amino acids (Sigma Chemicals, St. Louis, MO) in deionized water. For the PorAA group, the amino acid mixture was mixed with p-aminohippuric acid (PAH; Sigma Chemicals; delivered at 0.4 mg · kg^-1 · min^-1). PAH was used to assess mixing of the infused amino acids with blood in the portal and hepatic veins as described previously for intraportally-infused glucose (Pagliassotti et al. 1996 ). The second group (PerAA, n = 6) received the amino acid mixture without PAH via a lateral saphenous vein at 8.1 µmol · kg^-1 · min^-1, a rate calculated to approximate the hepatic load of gluconeogenic amino acids in PorAA. This rate was estimated based on the proportion of hepatic flow provided by the hepatic artery and portal vein (~26% and 74%, respectively, during somatostatin infusion) and the expected fractional extraction of the amino acid mixture, derived from our previous studies in mixed-meal-fed dogs (Moore et al. 1994 ). We verified in the first two dogs that this infusion rate achieved a hepatic load that approximated the load in the PorAA group, and continued to evaluate the rate after each study. A primed, continuous infusion of 50% dextrose (Abbott Laboratories, North Chicago, IL) was begun in the right cephalic vein in both groups at time 0 so that the blood glucose could be quickly clamped at its desired value. Arterial plasma glucose concentrations were obtained every 5 min during the experimental period, and the glucose infusion rate was adjusted to maintain the hepatic glucose load at 150% of basal. The collection, processing and analysis of blood samples were described in detail elsewhere (Moore et al. 1998 ).

After completion of the experiment, each animal was killed with an overdose of sodium pentobarbital, the liver was rapidly removed and a tissue sample from each liver lobe was immediately freeze-clamped and stored at -70°C.

Processing and analysis of samples.

Blood glucose, glutamine, glutamate, lactate, alanine, serine, threonine, glycine, glycerol and hematocrit; plasma glucose, insulin and glucagon concentrations; and liver glycogen concentrations were determined as described previously (Moore et al. 1991 , Moore et al. 1998 ).

Calculations.

In the PorAA group, mixing of the infused amino acids in the portal vein was assessed by comparing recovery of PAH in the portal and hepatic veins with the PAH infusion rate as previously described (Moore et al. 1999, Pagliassotti et al. 1996 ). Mixing was good, as evidenced by the ratios of recovered to infused PAH in the portal and hepatic veins (0.9 ± 0.1 in both veins, with a ratio of 1.0 representing ideal mixing).

The ultrasonic flow probes and the dye extraction technique (Moore et al. 1991 ). yielded hepatic blood flow (HBF) rates that were not significantly different. Since the flow probes make it possible to determine the relative proportions of the HBF provided by the hepatic artery and the portal vein, calculations reported in this paper utilize HBF obtained from the flow probes when available. One or the other of the flow probes did not function in one dog in each group. In these animals indocyanine green-derived flows were used, and the portal vein was assumed to provide 80% of hepatic blood flow during the basal period and 74% during the experimental period (Moore et al. 1998 , Pagliassotti et al. 1996 ).

The rate of substrate delivery to the liver, or hepatic substrate load, was calculated by a direct (d) method as:

(1)

where [S] is the substrate concentration, A and P refer to artery and portal vein respectively, and ABF and PBF refer to blood flow through the hepatic artery and portal vein, respectively. To avoid any potential errors arising from either incomplete mixing of amino acids during intraportal infusion or lack of precise measurements of the distribution of hepatic blood flow, the hepatic amino acid load was also calculated by an indirect (i) method:

(2)

where G is the blood amino acid concentration, IR_Por is the intraportal amino acid infusion rate, and GU is the uptake of amino acids by the gastrointestinal tract during peripheral infusion.

The load of a substrate exiting the liver was calculated as:

(3)

where H represents the hepatic vein.

Direct and indirect methods were used in calculation of net hepatic balance (NHB). The direct calculation was: NHB_d = load_out - load_in(d). The indirect calculation was: NHB_i = load_out - load_in(i). A negative value indicates net hepatic uptake (NHU); where only NHU is evident, the negative sign is omitted and NHB is described as NHU. Both equations were used in calculation of net hepatic amino acid balance, but only the direct calculation was employed for other substrates. The results for net hepatic amino acid balance did not differ regardless of the method used in calculation. The results given in this report utilize the indirect calculation, because this method is less likely to be affected by any inadequate mixing in the portal vein. Net hepatic fractional extraction of substrates was calculated as the ratio of NHB to load_in.

Plasma glucose concentrations were converted to blood glucose as previously described (Pagliassotti et al. 1996 ). Use of blood concentrations ensures accurate measurements of net hepatic glucose and amino acid balance regardless of the characteristics of glucose or amino acid entry into the erythrocyte.

Net hepatic glycogen synthesis was calculated as the difference between post-study glycogen concentrations in the PerAA and PorAA dogs and basal concentrations in 11 dogs which were killed after 42 h of food deprivation (corresponding to time 0 in the experimental animals) (Moore et al. 1991 ). The glycogen concentrations for each dog represent the mean of the values for the seven liver lobes, weighted for the percentage of liver mass accounted for by each lobe (Moore et al. 1991 ).

Statistical methods.

Data are presented as mean ± SEM. SYSTAT 5.0 (SYSTAT, Inc, Evanston, IL) was used for statistical analysis. Time-course data were analyzed with repeated measures ANOVA with post-hoc analysis by univariate F tests. Independent-sample t-tests were used for analysis of glycogen data and area under the curve (AUC). Results were considered statistically significant at P < 0.05.

	RESULTS

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Insulin and glucagon.

The basal arterial plasma insulin concentrations in PorAA and PerAA did not differ (Table 1 ). They increased ~3-fold basal in both groups during the experimental period and the groups did not differ. The arterial plasma glucagon concentrations remained at basal level throughout the study in both groups.

The total hepatic blood flow did not differ between groups (30 ± 2 vs. 35 ± 4 mL · kg^-1 · min^-1 during the experimental period in PorAA and PerAA, respectively; P = 0.4). The arterial blood glucose concentration was clamped at a higher concentration in PorAA than in PerAA (6.9 ± 0.3 vs. 6.0 ± 0.2 mmol/L, P < 0.05; Fig. 1 ) in order to equalize the hepatic glucose loads in the two protocols. Indeed, the hepatic glucose loads increased to the same extent in both groups, from 155.4 ± 11.1 to 210.9 ± 16.6 µmol · kg^-1 · min^-1 in PorAA and from 144.3 ± 16.6 to 199.8 ± 22.2 µmol · kg^-1 · min^-1 in PerAA (P = 0.9 between groups).

View larger version (23K): [in this window] [in a new window]   Figure 1. Arterial blood glucose concentrations, hepatic glucose loads, net hepatic glucose balances and net hepatic fractional extractions of glucose in dogs in the basal state and during delivery of somatostatin, intraportal insulin (3-fold basal), intraportal glucagon (basal), and peripheral glucose infusion to raise the hepatic glucose load ~1.5-fold basal. Values are means ± SEM, n = 6 per group. Dogs received a gluconeogenic amino acid mixture intraportally (PorAA) or peripherally (PerAA). SRIF, somatostatin. Arterial blood concentrations were clamped at a higher level in PorAA to make the hepatic glucose load equivalent between groups.

Gluconeogenic amino acid levels and balance data.

The amino acid infusion rates brought about very similar hepatic loads for the sum of the gluconeogenic amino acids in the two groups (61.8 ± 7.1 and 67.7 ± 14.4 in PorAA and PerAA µmol · kg^-1 · min^-1, respectively, P = 0.8).

No significant differences between the two groups were present with respect to the arterial and portal vein concentrations or the hepatic loads of glutamine during the experimental period. However, the mean NHU (2.1 ± 0.4 vs. 0.8 ± 0.3 µmol · kg^-1 · min^-1, P < 0.05 for both repeated-measures ANOVA and t-test of AUC) and NHFE (0.11 ± 0.02 vs. 0.05 ± 0.02, P < 0.05 for both repeated-measures ANOVA and t-test of AUC) of glutamine were greater in PorAA than in PerAA (Fig. 2 ).

View larger version (26K): [in this window] [in a new window]   Figure 2. Arterial blood glutamine concentrations, hepatic glutamine load, net hepatic glutamine uptake and net hepatic fractional extraction of glutamine in dogs in the basal state and during delivery of somatostatin, intraportal insulin (3-fold basal), intraportal glucagon (basal), and peripheral glucose infusion to raise the hepatic glucose load ~1.5-fold basal. Values are means ± SEM, n = 6 per group. Dogs received a gluconeogenic amino acid mixture intraportally (PorAA) or peripherally (PerAA). SRIF, somatostatin.

There were no differences between the groups in concentration or net hepatic balance of lactate (data not shown). The arterial blood lactate concentrations rose progressively and similarly in both groups, from ~352 ± 50 µmol/L in the basal period to 865 ± 90 µmol/L at the last sampling time. Both groups shifted from net hepatic lactate uptake (9.2 ± 1.7 µmol · kg^-1 · min^-1) to net output within 30 min of beginning the hormone and substrate infusions (peak rate 3.6 ± 2.5 µmol · kg^-1 · min^-1). Both groups had returned to a low rate of net hepatic lactate uptake by 150 min, with the mean rate at the final sampling point being ~1.7 ± 1.3 µmol · kg^-1 · min^-1).

Glycogen data.

The net hepatic glycogen synthetic rates were 8.9 ± 2.2 and 11.1 ± 2.2 µmol · kg^-1 · min^-1 in PorAA and PerAA, respectively (P = 0.3). The net rates of glycogen synthesis via the direct pathway were not different in the two groups (5.5 ± 1.7 and 3.9 ± 1.7 µmol · kg^-1 · min^-1 in PorAA and PerAA, respectively).

	DISCUSSION

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We showed that intraportal glucose delivery enhances net hepatic glucose uptake and suggested that this occurs as a result of the generation of a signal by glucose sensors in the hepatoportal region (Adkins et al. 1987 ). Presumably this signal reaches the central nervous system via the hepatic branch of the vagus nerve (Adkins-Marshall et al. 1992 ). The afferent firing rate in the vagus nerve is inversely proportional to the portal vein glucose concentration (Niijima 1982 and Niijima 1996 ). However, the vagal firing rate is not modulated only by the portal glucose concentration. Niijima and Meguid (1995) found that intraportal injection or infusion of 15 common dietary amino acids also alters the afferent firing rate in the hepatic branch of the vagus nerve. Eight of the 15 amino acids studied by Niijima and Meguid (1995) , including alanine and serine, had an excitatory effect on the afferent firing rate in the hepatic branch of the vagus nerve in the rat. On the other hand, glycine, threonine and five other amino acids had an inhibitory effect (Niijima and Meguid 1995 ). Niijima and Meguid did not study glutamine and glutamate. We speculated that changes in afferent vagal firing might initiate a mechanism for directing selected substrates into the liver. Thus it might be possible for intraportal amino acids not only to modulate NHGU when administered concurrently with intraportal glucose (Moore et al. 1998 and Moore et al. 1999 ), but also to affect their own hepatic uptakes.

To examine the effect of intraportal amino acid infusion on NHU and NHFE of the amino acids themselves, it was necessary to conduct the current studies in the absence of a portal glucose infusion. In this way any potential interaction with the portal glucose signal could be avoided. The findings in our previous report in which glucose was infused portally (Moore et al. 1999 ), however, and those in the current study were similar in most respects. In the current study, portal delivery of amino acids was associated with ~2-fold greater NHU and NHFE of glutamine than peripheral delivery. Interestingly, this is the same magnitude of enhancement that portal glucose delivery has on glucose extraction by the liver (Myers et al. 1991a , Myers et al. 1991b , Pagliassotti et al. 1996 ). It is also virtually identical to the enhancement of glutamine NHU and NHFE that was observed with portal vs. peripheral amino acid delivery in the presence of portal glucose infusion (Moore et al. 1999 ).

The only statistically significant difference in the hepatic amino acid balance data between the investigation in which we infused glucose portally (Moore et al. 1999 ) and the current investigation in which we infused glucose peripherally occurred with glutamate. The NHU of glutamate was enhanced by portal amino acid delivery in the presence of portal glucose infusion but not in the presence of peripheral glucose infusion. It is possible that the portal glucose signal specifically enhances the uptake of glutamate. Since the portal glutamate concentration was higher during portal than peripheral amino acid infusion (Moore et al. 1999 ), it is possible that this was responsible for the enhancement of glutamate uptake. The NHU and NHFE of alanine were significantly increased with portal infusion in the presence of portal glucose delivery (Moore et al. 1999 ), while there was only a tendency (P = 0.08) for NHFE of alanine to be enhanced in the PorAA group in the current report. Similarly, the NHFE of serine tended to be greater during portal amino acid infusion in the presence of portal glucose delivery (Moore et al. 1999 ), but it was significantly enhanced during portal amino acid infusion in the absence of portal glucose infusion (current data).

There are at least two possible explanations for our findings. The first possibility is that the enhancement of hepatic amino acid uptake by portal amino acid delivery is actually a generalized phenomenon. In other words, the hepatic uptakes of all or most of the amino acids in our mixture may have been enhanced by portal delivery, but their hepatic loads and changes in net hepatic uptake were so small that only the findings for glutamine were consistently significant. The hepatic load of glutamine was ~2- to 10-fold greater than for the other five amino acids infused, and glutamine was unique among these amino acids in that the liver shifted from net output of the amino acid in the basal period to net uptake in the infusion period. Thus glutamine had a large "signal-to-noise" ratio in comparison to the other amino acids. The second possibility is that glutamine was really the only amino acid whose NHU and NHFE were significantly enhanced by portal delivery, and any statistically significant findings for other amino acids represent random events. In regard to this possibility, it is noteworthy that even when statistically significant enhancement of NHU and/or NHFE for alanine and serine during portal amino acid delivery was present, the enhancement was relatively modest (~20–30% greater than with peripheral amino acid infusion), in comparison to the enhancement of glutamine NHU and NHFE during portal amino acid delivery. Because it is impossible from our data to determine which of these explanations is correct, the remainder of this discussion will focus upon glutamine, for which there are clear cut differences in hepatic balance data between the portal and peripheral routes of amino acid delivery.

The question arises as to what metabolic event(s) would cause glutamine to be extracted by the liver more avidly during portal than peripheral amino acid infusion. The enhancement of glutamine uptake may result simply from the fact that the liver contains the enzymes for both synthesis and degradation of glutamine. The periportal region of the liver is high in glutaminase (Jungermann and Kietzmann 1996 ), and therefore the enhancement of hepatic glutamine uptake during portal amino acid delivery might have resulted from the ready availability of the enzyme for its degradation. However, glutamine apparently plays important roles in the regulation of key hepatic functions, including glycogen storage, protein synthesis, gluconeogenesis and urea cycle activity. Enhancement of glycogen synthase activity and glycogen synthesis by glutamine was documented in rat liver both in vitro (Lavoinne et al. 1987 ) and in vivo (Niewoehner and Nuttall 1996 ). In addition, glutamine is known to stimulate the transcription of certain genes, including ß-actin, phosphoenolpyruvate carboxykinase, and arginosuccinate synthetase, in hepatocytes (Husson et al. 1996 , Lavoinne et al. 1996 , Quillard et al. 1997 , Quillard et al. 1996 ). Glutamine’s enhanced uptake during portal amino acid delivery may have been related to one or more of these roles. Hepatic protein synthesis, in particular, was shown to be stimulated by portal, as opposed to peripheral vein, delivery of amino acids (Bennet and Haymond, 1991 , Bozzetti et al. 1993 ).

In the current studies, the total rate of net hepatic substrate uptake was very similar and quite low in both groups, and it closely approximated the rate of net hepatic glycogen synthesis. Thus it is not unexpected that glycogen storage rates should be similar in the two groups. We did not measure proteolysis, ureagenesis or protein synthetic rates, and therefore it is impossible to know what effect portal amino acid delivery had upon these processes. The amino acid mixture was imbalanced, and thus it was likely to promote proteolysis. Nevertheless, because both groups received the same amino acid mixture, our findings are unlikely to be explained simply by the composition of the infusate.

The negative arterial-portal (A-P) glucose gradient (i.e., concentration higher in the portal vein than in the artery) evident during portal glucose delivery is a crucial factor in the generation of the portal glucose signal (Adkins et al. 1987 , Gardemann et al. 1986 , Pagliassotti et al. 1991 , Stumpel and Jungermann 1997 ). It is possible that such a gradient between the portal vein and the arterial concentrations of amino acids could generate a portal amino acid signal. The situation in regard to amino acids is more complicated than in regard to glucose. Glutamine could generate its own signal, or another amino acid could generate a signal that would stimulate the hepatic uptake of glutamine. Of the six amino acids which were infused in the current report, only glutamine and serine displayed a positive A-P gradient in the basal state and during peripheral amino acid infusion but a negative gradient during at least a portion of the portal amino acid infusion (Fig. 2 and Table 2 ). It is also possible, however, that some other change, such as the widening of a negative A-P gradient during portal amino acid delivery (as in the case of alanine), could initiate a signal. Yet another possibility is that hepatic glutamine uptake was stimulated not by any signaling mechanism but simply by higher portal vein amino acid concentrations during intraportal amino acid infusion (Fig. 2 and Table 2 ). The data in our previous report (Moore et al. 1999 ), however, make this explanation unlikely. In those studies (Moore et al. 1999 ), the portal concentrations of all of the amino acids except glutamate were lower in the group receiving portal amino acids than in the group receiving peripheral amino acids. Thus glutamate would be the only candidate for stimulating the liver to increase its glutamine uptake via such a mechanism.

In conclusion, intraportal infusion of gluconeogenic amino acids during peripheral infusion of glucose, with the insulin and glucagon concentrations clamped at 3-fold basal and basal, respectively, enhanced the net hepatic uptake and fractional extraction of glutamine >2-fold (P < 0.05), compared with peripheral infusion of the same amino acid mixture. The net hepatic fractional extraction of serine was significantly increased and that of alanine tended to be increased with portal vs. peripheral amino acid delivery. However, the difference in the net hepatic fractional extraction of glutamine between portal and peripheral amino acid delivery was much greater than for serine and alanine. Amino acid sensors were identified in the hepatoportal region (Niijima and Meguid 1995 ; Tanaka et al. 1990 ), and they might be involved in the enhancement of the net hepatic glutamine uptake during portal amino acid infusion. The current data provide further evidence that the route of delivery alters the partitioning of some substrates between the liver and the nonhepatic tissues.

	FOOTNOTES

 
¹ Supported by the National Institute of Diabetes and Digestive and Kidney Diseases Grant R-01-DK-43706 and Diabetes Research and Training Center Grant SP-60-AM20593.

³ Abbreviations used: A, artery (femoral); ABF, arterial blood flow; A-P (arterial-portal); AUC, area under the curve; d, direct; gngAA, gluconeogenic amino acids; HBF, hepatic blood flow; i, indirect; NHB, net hepatic balance; NHFE, net hepatic fractional extraction; NHGU, net hepatic glucose uptake; NS, not statistically significant; PAH, p-aminohippuric acid; P, portal vein; PBF, portal blood flow; PerAA, group receiving amino acids via the peripheral route; PorAA, group receiving amino acids via the portal route; SRIF, somatostatin.

Manuscript received January 21, 1999. Initial review completed March 16, 1999. Revision accepted August 24, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

1. Adkins B. A., Myers S. R., Hendrick G. K., Stevenson R. W., Williams P. E., Cherrington A. D. Importance of the route of intravenous glucose delivery to hepatic glucose balance in the conscious dog. J. Clin. Invest. 1987;79:557-565

2. Adkins-Marshall B. A., Pagliassotti M. J., Asher J. R., Connolly C. C., Neal D. W., Williams P. E., Myers S. R., Hendrick G. K., Adkins R. B., , Jr & Cherrington A. D. Role of hepatic nerves in response of liver to intraportal glucose delivery in dogs. Am. J. Physiol. 1992;262:E679-E686[Abstract/Free Full Text]

3. Bennet W. M., Haymond M. H. Plasma pool source for fibrinogen synthesis in postabsorptive conscious dogs. Am. J. Physiol. 1991;260:E581-E587[Abstract/Free Full Text]

4. Bozzetti F., Regalia E., Barbieri A., Facchetti G., Bombardieri E., Montalto F., Cozzaglio L., Gennari L. Does portal nutrition benefit liver protein synthesis?. J. Parent. Ent. Nutr. 1993;17:20-24

5. Gardemann A., Strulik H., Jungermann K. A portal-arterial glucose concentration gradient as a signal for an insulin-dependent net glucose uptake in perfused rat liver. FEBS Lett 1986;202:255-259[Medline]

6. Husson A., Quillard M., Fairand A., Chedeville A., Lavoinne A. Hypoosmolarity and glutamine increased the beta-actin gene transcription in isolated rat hepatocytes. FEBS Lett 1996;394:353-355[Medline]

7. Jungermann K., Kietzmann T. Zonation of parenchymal and nonparenchymal metabolism in liver. Ann. Rev. Nutr. 1996;16:179-203[Medline]

8. Lavoinne A., Baquet A., Hue L. Stimulation of glycogen synthesis and lipogenesis by glutamine in isolated rat hepatocytes. Biochem. J. 1987;248:429-437[Medline]

9. Lavoinne A., Husson A., Quillard M., Chedeville A., Fairand A. Glutamine inhibits the lowering effect of glucose on the level of phosphoenolpyruvate carboxykinase mRNA in isolated hepatocytes. Eur. J. Biochem. 1996;242:537-543[Medline]

10. Moore M. C., Cherrington A. D., Cline G., Pagliassotti M. J., Jones E. M., Neal D. W., Badet C., Shulman G. I. Sources of carbon for hepatic glycogen synthesis in the conscious dog. J. Clin. Invest. 1991;88:578-587

11. Moore M. C., Flakoll P. J., Hsieh P. S., Pagliassotti M. J., Neal D. W., Monohan M. T., Venable C., Cherrington A. D. Hepatic glucose disposition during concomitant portal glucose and amino acid infusions in the dog. Am. J. Physiol. 1998;274:E893-E902[Abstract/Free Full Text]

12. Moore M. C., Hsieh P. S., Flakoll P. J., Neal D. W., Cherrington A. D. Differential effect of amino acid infusion route on net hepatic glucose uptake in the dog. Am. J. Physiol. 1999;276:E295-E302[Abstract/Free Full Text]

13. Moore M. C., Pagliassotti M. J., Swift L. L., Asher J., Murrell J., Neal D., Cherrington A. D. Disposition of a mixed meal by the conscious dog. Am. J. Physiol. 1994;266:E666-E675[Abstract/Free Full Text]

14. Moore M. C., Pagliassotti M. J., Wasserman D. H., Goldstein R., Asher J., Neal D. W., Cherrington A. D. Hepatic denervation alters the transition from the fed to the food-deprived state in conscious dogs. J. Nutr. 1993;123:1739-1746

15. Moore M. C., Rossetti L., Pagliassotti M. J., Monohan M., Venable C., Neal D., Cherrington A. D. Neural and pancreatic influences on net hepatic glucose uptake and glycogen synthesis. Am. J. Physiol. 1996;271:E215-E222[Abstract/Free Full Text]

16. Myers S. R., Biggers D. W., Neal D. W., Cherrington A. D. Intraportal glucose delivery enhances the effects of hepatic glucose load on net hepatic glucose uptake in vivo. J. Clin. Invest. 1991;88:158-167

17. Myers S. R., McGuinness O. P., Neal D. W., Cherrington A. D. Intraportal glucose delivery alters the relationship between net hepatic glucose uptake and the insulin concentration. J. Clin. Invest. 1991;87:930-939

18. Niewoehner C. B., Nuttall F. Q. Oral glutamine stimulates liver glycogen synthesis in fasted rats. Diabetes 1996;45:164A

19. Niijima A. Nervous regulation of metabolism. Prog. Neurobiol. 1989;33:135-147[Medline]

20. Niijima A. Glucose-sensitive afferent nerve fibres in the hepatic branch of the vagus nerve in the guinea-pig. J. Physiol. (London) 1982;332:315-323[Abstract/Free Full Text]

21. Niijima A. Glucose-sensitive afferent nerve fibers in the liver and their role in food intake and blood glucose regulation. J. Autonom. Nerv. Syst. 1983;9:207-220[Medline]

22. Niijima A. Afferent impulse discharges from glucoreceptors in the liver of the guinea pig. Nutrition 1996;12:390-393

23. Niijima A., Meguid M. M. An electrophysiological study on amino acid sensors in the hepato-portal system in the rat. Obesity Res 1995;3:741S-745S[Medline]

24. Pagliassotti M. J., Holste L. C., Moore M. C., Neal D. W., Cherrington A. D. Comparison of the time courses of insulin and the portal signal on hepatic glucose and glycogen metabolism in the dog. J. Clin. Invest. 1996;97:81-91[Medline]

25. Pagliassotti M. J., Myers S. R., Moore M. C., Neal D. W., Cherrington A. D. Magnitude of negative arterial-portal glucose gradient alters net hepatic glucose balance in conscious dogs. Diabetes 1991;40:1659-1668[Abstract]

26. Quillard M., Husson A., Lavoinne A. Glutamine increases argininosuccinate synthetase mRNA levels in rat hepatocytes: the involvement of cell swelling. Eur. J. Biochem. 1996;236:56-59[Medline]

27. Quillard M., Renouf S., Husson A., Meisse D., Lavoinne A. Glutamine and regulation of gene expression in mammalian cells: special reference to phosphoenolpyruvate carboxykinase (PEPCK). Biochimie 1997;79:125-128[Medline]

28. Stumpel F., Jungermann K. Sensing by intrahepatic muscarinic nerves of a portal-arterial glucose concentration gradient as a signal for insulin-dependent glucose uptake in the perfused rat liver. FEBS Lett 1997;406:119-122[Medline]

29. Tanaka K., Inoue S., Nagase H., Takamura Y., Niijima A. Amino acid sensors sensitive to alanine and leucine exist in the hepato-portal system in the rat. J. Autonom. Nerv. Syst. 1990;31:41-46[Medline]

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