Nonhepatic response to portal glucose delivery in conscious dogs

Mary Courtney Moore1, Po-Shiuan Hsieh1, Doss W. Neal2, and Alan D. Cherrington1,2

1 Department of Molecular Physiology and Biophysics and 2 Diabetes Research and Training Center, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The glycemic and hormonal responses and net hepatic and nonhepatic glucose uptakes were quantified in conscious 42-h-fasted dogs during a 180-min infusion of glucose at 10 mg · kg-1 · min-1 via a peripheral (Pe10, n = 5) or the portal (Po10, n = 6) vein. Arterial plasma insulin concentrations were not different during the glucose infusion in Pe10 and Po10 (37 ± 6 and 43 ± 12 µU/ml, respectively), and glucagon concentrations declined similarly throughout the two studies. Arterial blood glucose concentrations during glucose infusion were not different between groups (125 ± 13 and 120 ± 6 mg/dl in Pe10 and Po10, respectively). Portal glucose delivery made the hepatic glucose load significantly greater (36 ± 3 vs. 46 ± 5 mg · kg-1 · min-1 in Pe10 vs. Po10, respectively, P < 0.05). Net hepatic glucose uptake (NHGU; 1.1 ± 0.4 vs. 3.1 ± 0.4 mg · kg-1 · min-1) and fractional extraction (0.03 ± 0.01 vs. 0.07 ± 0.01) were smaller (P < 0.05) in Pe10 than in Po10. Nonhepatic (primarily muscle) glucose uptake was correspondingly increased in Pe10 compared with Po10 (8.9 ± 0.4 vs. 6.9 ± 0.4 mg · kg-1 · min-1, P < 0.05). Approximately one-half of the difference in NHGU between groups could be accounted for by the difference in hepatic glucose load, with the remainder attributable to the effect of the portal signal itself. Even in the absence of somatostatin and fixed hormone concentrations, the portal signal acts to alter partitioning of a glucose load among the tissues, stimulating NHGU and reducing peripheral glucose uptake.

portal vein; liver; insulin sensitivity; glucose load


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

THE RATE OF NET HEPATIC GLUCOSE UPTAKE (NHGU) is two to three times greater when glucose is infused via the portal vein, compared with a peripheral vein, under conditions in which insulin and glucagon concentrations and the hepatic glucose load are fixed at similar values with the two routes of delivery (1, 15, 16, 20). Evidence suggests that the enhancement of NHGU during portal glucose delivery results from a neurally mediated signal (the "portal signal") induced by the presence of a negative arterial-portal gradient, i.e., the portal vein glucose concentration is greater than the arterial concentration (1, 2, 21). The effect of this portal signal is not restricted to the liver. Nonhepatic tissues, particularly skeletal muscle, reduce their net glucose uptake in response to the portal signal (5). Thus it appears that the portal signal provides a mechanism for coordinating the disposition of glucose entering the body via the portal vein.

We have never quantified the effect of the portal signal when insulin and glucagon were free to change. Ishida et al. (6) infused glucose into the jugular or portal veins of dogs for 120 min at rates varying from 3 to 13 mg · kg-1 · min-1 (identical for the two routes of glucose infusion) and found that the arterial, portal, and hepatic vein insulin responses did not differ significantly with the two glucose infusion routes. Moreover, the arterial glucose concentrations were the same with the two routes of delivery. Nevertheless, NHGU was nearly threefold greater during portal vs. peripheral glucose infusion. Ishida et al., however, were interested primarily in the insulin response to glucose infusion and therefore did not attempt to confirm that mixing of the glucose infusate with the blood in the portal vein had occurred. Thus it was not clear that their hepatic balance data were completely reliable. Moreover, they did not report the rate of nonhepatic glucose uptake. Therefore the first purpose of the current studies was to assess the glycemic and hormonal responses to glucose delivered at the same rate via the peripheral and portal routes. We designed the study to use data only from dogs in which we could confirm that mixing of the portal infusate with the portal vein blood had occurred. Our second goal was to quantify the hepatic and the nonhepatic contributions to glucose disposal. These studies provide a quantitative assessment of any glycemic advantage inherent in the presence of the portal signaling mechanism.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Animals, diets, and experimental preparation. Studies were carried out in conscious 42-h-fasted adult dogs of either sex with a mean weight of 25 ± 1 kg. Housing and diet have been described previously (12). The protocol was approved by the Vanderbilt University Medical Center Animal Care Subcommittee, and animals were housed according to American Association for the Accreditation of Laboratory Animal Care International guidelines. All dogs underwent a laparotomy under general anesthesia ~16 days before the initial study, and silicone rubber catheters (Dow Corning, Midland, MI) were inserted in the portal and left common hepatic veins, a splenic and a jejunal vein, the inferior cava, and the femoral artery, as previously described (12, 13). Ultrasonic flow probes (Transonic Systems, Ithaca, NY) were positioned around the portal vein and hepatic artery, and their proximal ends were placed in subcutaneous pockets. Criteria for use of an animal in an experiment were as previously published (13).

Experimental design. At -90 min, a continuous peripheral infusion of indocyanine green dye (ICG; Sigma, St. Louis, MO; 4 µg · kg-1 · min-1) was begun. The latter provided confirmation of hepatic vein catheter placement and a second measurement of hepatic blood flow. After 70 min (-100 to -30) of dye equilibration, there was a 30-min (-30 to 0) control or basal period, followed by a 180-min (0-180 min) period of continuous glucose infusion (Dextrose 20%, Baxter Healthcare, Deerfield, IL). There were two different protocols, based on the route of glucose infusion: glucose via peripheral vein (inferior vena cava) at 10 mg · kg-1 · min-1 (Pe10, n = 5) or glucose via portal vein (i.e., jejunal and splenic infusion catheters) at 10 mg · kg-1 · min-1 (Po10, n = 6). The dogs received glucose at 3.3 mg · kg-1 · min-1 for the first 15 min of the glucose infusion period, 6.7 mg · kg-1 · min-1 for the next 15 min, and 10 mg · kg-1 · min-1 thereafter. For the Po10 protocol, p-aminohippuric acid (PAH; Sigma) was mixed with the glucose so that it was delivered at 0.4 mg · kg-1 · min-1. PAH was used to assess mixing of the infused glucose with blood in the portal and hepatic veins, as described previously (13). In both protocols, blood samples (3-9 ml each) were obtained from the artery every 15-30 min and from the portal and hepatic veins (to allow calculation of hepatic balance via the arteriovenous difference technique) every 30-60 min throughout the basal and glucose infusion periods. The collection, processing, and analysis of blood samples have been described in detail elsewhere (14, 20).

Processing and analysis of samples. Hematocrit; blood glucose, lactate, alanine, and glycerol; and plasma glucose, insulin, and glucagon concentrations were determined as described previously (12, 13). Plasma nonesterified fatty acid (NEFA) concentrations were determined enzymatically (NEFA C kit, Wako Chemicals, Richmond, VA).

Calculations. Hepatic blood flow (HBF) was calculated by two methods, ultrasonic flow probes and ICG dye extraction (12, 13). The results obtained with the two methods were not significantly different. Because the flow probes make it possible to determine the relative proportions of the HBF provided by the hepatic artery and the portal vein, the results reported in this paper utilize HBF obtained from the flow probes.

In the Po10 group, the recovery of PAH in the portal and hepatic veins was compared with the PAH infusion rate as previously described to determine the adequacy of mixing of the glucose infusate with the blood in the portal vein (13). Because the portal glucose infusion rate in these studies was so high, making up nearly one-half of the increase in the hepatic glucose load during the glucose infusion period, adequate mixing was both more difficult to achieve than in studies with lower infusion rates and also more crucial to obtaining reliable data. Ten Po10 studies were performed; four were excluded because of poor mixing. PAH recovery was calculated with both Transonic and dye-derived hepatic blood flow measurements, and all dogs retained in the database exhibited acceptable mixing of the infusate with both blood flow measurements. In the six dogs included in this report, the recovery ratios for PAH in both the portal and hepatic veins were 0.8 ± 0.1 with Transonic flows. When ICG flows were used, the recovery ratios were 0.9 ± 0.1 in both veins. A ratio of 1.0 would represent ideal mixing. Because mixing errors, when they occur, are random, individual data points were not excluded if the experiment as a whole was included.

The rate of substrate delivery to the liver, or hepatic substrate load, was calculated by a direct (D) method as
load<SUB>in(D)</SUB><IT>=</IT>([S]<SUB>A</SUB><IT>×</IT>ABF)<IT>+</IT>([S]<SUB>P</SUB><IT>×</IT>PBF)
where [S] is the substrate concentration, A and P refer to artery and portal vein, respectively, and ABF and PBF refer to blood or plasma flow (as appropriate) through the hepatic artery and portal vein, respectively. Hepatic sinusoidal insulin concentrations were calculated in a manner similar to loadin, with plasma flows, and the results were divided by the total hepatic plasma flow.

To avoid any potential errors arising from either incomplete mixing of glucose during intraportal glucose infusion or lack of precise measurements of the distribution of HBF, hepatic glucose load was also calculated by an indirect (I) method
load<SUB>in(I)</SUB><IT>=</IT>(G<SUB>A</SUB><IT>×</IT>HBF)<IT>+</IT>GIR<SUB>Po</SUB><IT>−</IT>GUG
where G is the blood glucose concentration, GIRPo is the intraportal glucose infusion rate (corrected for any incomplete recovery of the PAH in the portal vein), and GUG is the uptake of glucose by the gastrointestinal tract, calculated on the basis of the previously described relationship between the arterial blood glucose concentration and GUG (15, 16).

The load of a substrate exiting the liver was calculated as
load<SUB>out</SUB><IT>=</IT>[S]<SUB>H</SUB><IT>×</IT>HBF
where H represents the hepatic vein.

Direct and indirect methods were used in calculation of net hepatic balance (NHB). The direct calculation (used for substrates other than glucose) was NHBD = loadout - loadin(D). The indirect calculation (used for glucose, because this method minimizes any errors introduced by inadequacies of mixing) was NHBI = loadout - loadin(I). A negative value indicates net uptake. Net fractional glucose extraction by the liver was calculated as the ratio of NHB to loadin. Net nonhepatic glucose uptake was calculated as glucose infused minus NHGU.

The trapezoidal rule was used to determine the area under the curve. Both positive and negative excursions (i.e., net balance) were included in the calculations for net hepatic disposition of substrates.

Data are presented as means ± SE. SYSTAT (Evanston, IL) was used for statistical analysis. Time-course data were analyzed with repeated-measures ANOVA, and individual pieces of data (e.g., area under the curve) were analyzed with independent t-tests. Results were considered statistically significant at P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Insulin and glucagon concentrations. The mean basal arterial plasma insulin concentrations in Pe10 and Po10 were 8 ± 2 and 11 ± 3 µU/ml, respectively (P = 0.5; Fig. 1). The arterial insulin concentrations during the glucose infusion period (0-180 min) did not differ significantly between Pe10 and Po10 (mean values 37 ± 6 and 43 ± 12 µU/ml, respectively; P = 0.4). On the other hand, the portal vein insulin concentrations (data not shown; 72 ± 13 and 112 ± 23 µU/ml during the last 2 h of the studies in Pe10 and Po10, respectively) and the estimated hepatic sinusoidal insulin concentrations (Table 1) were only about two-thirds as great in Pe10 as the corresponding concentrations in Po10, although these findings did not reach significance (P = 0.3).


View larger version (17K):
[in this window]
[in a new window]
 
Fig. 1.   Arterial plasma insulin (A) and glucagon (B) concentrations in 42-h-fasted conscious dogs in the basal condition and during the peripheral iv (Pe10, n = 5) or portal (Po10, n = 6) infusion of glucose at 10 mg · kg-1 · min-1. Glucose was infused at 3.3 mg · kg-1 · min-1 from 0 to 15 min, 6.7 mg · kg-1 · min-1 from 15 to 30 min, and 10 mg · kg-1 · min-1 for the remainder of the infusion period. There are no significant differences between groups.


                              
View this table:
[in this window]
[in a new window]
 
Table 1.   Estimated sinusoidal insulin concentrations in dogs receiving glucose at 10 mg · kg-1 · min-1 via a peripheral or the portal vein

The mean basal arterial plasma glucagon concentrations in Pe10 and Po10 were 40 ± 7 and 44 ± 7 pg/ml, respectively (P = 0.6). The glucagon concentrations showed a progressive and significant decline during glucose infusion but were not different (P = 0.8) between the groups at any time (final concentrations 31 ± 5 and 31 ± 6, respectively, P < 0.05 vs. basal concentrations).

HBF. HBF did not change between the basal and the infusion periods, and it was not different between the treatments (data not shown). The mean HBFs during the last 2 h were 30 ± 1 and 32 ± 3 ml · kg-1 · min-1 in Pe10 and Po10, respectively.

Glucose concentrations and balance data. The arterial blood glucose concentrations in the basal period were not different in the two groups [80 ± 2 (Pe10) and 78 ± 2 (Po10) mg/dl; Fig. 2]. During the glucose infusion period, the arterial blood glucose concentrations were indistinguishable in Pe10 and Po10 (mean during the last 2 h, when the glucose infusion rate was 10 mg · kg-1 · min-1: 125 ± 13 and 120 ± 6 mg/dl, respectively). The portal blood glucose concentration during the infusion period was lower in Pe10 than in Po10 (121 ± 13 and 146 ± 4 mg/dl during the last 2 h; P < 0.05). The hepatic glucose loads did not differ between the groups during the basal period, but during glucose infusion, the hepatic glucose load was lower in Pe10 than in Po10 (36 ± 3 vs. 45 ± 5 mg · kg-1 · min-1, respectively, P < 0.05).


View larger version (15K):
[in this window]
[in a new window]
 
Fig. 2.   Arterial and portal blood glucose concentrations and hepatic glucose loads in 42-h-fasted conscious dogs in the basal condition and during the peripheral iv (Pe10, n = 5) or portal (Po10, n = 6) infusion of glucose at 10 mg · kg-1 · min-1. Glucose was infused at 3.3 mg · kg-1 · min-1 from 0 to 15 min, 6.7 mg · kg-1 · min-1 from 15 to 30 min, and 10 mg · kg-1 · min-1 for the remainder of the infusion period. *P < 0.05 vs. Pe10.

During the basal period, the livers exhibited net release of glucose at a similar rate in both groups (1.6 ± 0.2 and 1.8 ± 0.2 mg · kg-1 · min-1 in Pe10 and Po10, respectively; Fig. 3). The rate of NHGU during the glucose infusion period was almost threefold larger in Po10 than in Pe10, when the indirect calculation (3.1 ± 0.4 vs. 1.1 ± 0.4 mg · kg-1 · min-1 during the last 2 h; P < 0.05) was used. NHGU in Po10 was slightly less (2.5 ± 0.2 mg · kg-1 · min-1) when the direct calculation was used. Net hepatic fractional extraction of glucose (NHFEG) was also over twofold higher in Po10 than in Pe10 (0.07 ± 0.01 vs. 0.03 ± 0.01; P < 0.05). The rate of nonhepatic glucose uptake (indirect calculation) was 6.9 ± 0.4 vs. 8.9 ± 0.4 mg · kg-1 · min-1 in Po10 and Pe10 respectively, P < 0.05. 


View larger version (17K):
[in this window]
[in a new window]
 
Fig. 3.   Net hepatic glucose balance, net hepatic fractional extraction of glucose, and nonhepatic glucose uptake in 42-h-fasted conscious dogs in the basal condition and during the peripheral iv (Pe10, n = 5) or portal (Po10, n = 6) infusion of glucose at 10 mg · kg-1 · min-1. Glucose was infused at 3.3 mg · kg-1 · min-1 from 0 to 15 min, 6.7 mg · kg-1 · min-1 from 15 to 30 min, and 10 mg · kg-1 · min-1 for the remainder of the infusion period. *P < 0.05 between groups.

Lactate concentrations and balance data. The arterial blood lactate concentrations and net hepatic lactate balances were very similar in the two groups throughout the basal and glucose infusion periods (Fig. 4). During the basal period, the dogs exhibited net hepatic lactate uptake. After the beginning of the glucose infusion, the livers shifted to net hepatic lactate output, with a peak rate of ~13 µmol · kg-1 · min-1 in both groups.


View larger version (18K):
[in this window]
[in a new window]
 
Fig. 4.   Arterial blood lactate concentrations and net hepatic lactate balance in 42-h-fasted conscious dogs in the basal condition and during the peripheral iv (Pe10, n = 5) or portal (Po10, n = 6) infusion of glucose at 10 mg · kg-1 · min-1. Glucose was infused at 3.3 mg · kg-1 · min-1 from 0 to 15 min, 6.7 mg · kg-1 · min-1 from 15 to 30 min, and 10 mg · kg-1 · min-1 for the remainder of the infusion period. There are no differences between groups.

Subtraction of the cumulative net hepatic lactate output from the cumulative NHGU yields an estimate of the carbon available for glycogen synthesis. In Pe10, the net hepatic uptake of carbon was only ~195 ± 58 mg glucose equivalents/kg, whereas in Po10, it was ~395 ± 86 mg glucose equivalents/kg (P < 0.05). Glucose oxidation rates during hyperinsulinemic, hyperglycemic clamps (with similar insulinemia and glycemia as in the current studies) are only ~0.07 mg · kg-1 · min-1 (S. Satake, M. C. Moore, and A. D. Cherrington, unpublished observations). At that rate, glucose oxidation over the course of these studies would total <13 mg/kg.

Glycerol and NEFA data. Arterial blood glycerol concentrations tended to be higher in Po10 during the basal period (86 ± 5 and 107 ± 23 µ mol/l in Pe10 and Po10, respectively; P = 0.2), but they decreased similarly with both routes of glucose infusion (Delta  from basal to lowest value: -56 ± 5 and -56 ± 15 µ mol/l in Pe10 and Po10, respectively, P = 0.7; Table 2). The nadir of the glycerol concentrations tended to be lower in Pe10 than in Po10 (30 ± 8 vs. 51 ± 18 µmol/l, P = 0.3). The basal and nadir rates of net hepatic glycerol uptake were 1.6 ± 0.1 and 0.4 ± 0.1 µmol · kg-1 · min-1 in Pe10 and 1.7 ± 0.2 and 0.1 ± 0.3 µmol · kg-1 · min-1 in Po10 (P = 1.0 between groups). Net hepatic fractional extraction of glycerol did not differ among groups [0.6 ± 0.1 (basal) to 0.4 ± 0.1 (nadir) in Pe10 and 0.5 ± 0.1 to 0.3 ± 0.1 in Po10; data not shown].

                              
View this table:
[in this window]
[in a new window]
 
Table 2.   Blood glycerol and plasma NEFA concentrations and hepatic balance data in dogs receiving glucose at 10 mg · kg-1 · min-1 via a peripheral or the portal vein

Plasma NEFA concentrations and net hepatic uptakes also declined in both groups. Both the basal values [635 ± 30 (Pe10) vs. 884 ± 112 (Po10) µmol/l, P = 0.06] and the change from basal concentrations during glucose infusion [-538 ± 26 (Pe10) and -797 ± 113 (Po10) µmol/l; P = 0.05] tended to be greater in Po10. The net hepatic NEFA uptake rates fell from 2.5 ± 0.3 to 0.0 ± 0.0 µmol · kg-1 · min-1 in Pe10 and from 2.9 ± 0.2 to 0.0 ± 0.1 µmol · kg-1 · min-1 in Po10 (P = 0.8 between groups). NHFEG fell from 0.2 ± 0.0 to 0.0 ± 0.0 in both groups (data not shown).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

We have previously demonstrated that NHGU is about two- to threefold greater with portal vs. peripheral glucose infusion, when somatostatin is infused and the insulin and glucagon concentrations and the hepatic glucose load are kept the same between the two infusion routes (1, 15, 16). The present experiments were the first in which we assessed the impact of the portal signal in a model in which we did not infuse somatostatin or fix the insulin and glucagon levels and the hepatic glucose load. Nevertheless, NHGU was ~2.5-fold (~2 mg · kg-1 · min-1) greater during portal infusion of glucose at 10 mg · kg-1 · min-1 than during peripheral infusion of glucose at the same rate. Approximately one-half of the difference in NHGU between the groups can be accounted for by the larger hepatic glucose load in Po10 than in Pe10 (15), and the remainder represents a portal signal effect. This conclusion is reinforced by the NHFEGs, which take into account differences in hepatic glucose load. The NHFEG in Po10 was approximately twofold greater than that evident in Pe10. Because more glucose entered the general circulation in Pe10 than in Po10, it might be expected that the arterial blood glucose concentrations would be higher during peripheral glucose delivery. Glucose uptake by the nonhepatic tissues was ~29% greater in Pe10 than in Po10, however, resulting in a virtually identical arterial blood glucose response in the two protocols.

The difference in nonhepatic glucose uptake was remarkable in that the circulating insulin concentrations were very similar between groups, and in fact tended to be higher in Po10, particularly at 60 min (Fig. 1). The portal and hepatic sinusoidal insulin concentrations were ~50% greater in Po10 than in Pe10, although the responses in both groups were highly variable and were not significantly different (Table 1). There are two potential explanations for the insulin data in these studies. First, it is possible that the portal delivery of glucose enhanced insulin secretion. It is well established that hyperglycemia resulting from oral glucose consumption stimulates insulin secretion more than equivalent hyperglycemia resulting from peripheral glucose infusion (19). This "incretin effect" has been largely attributed to the actions of gut hormones, such as gastric inhibitory peptide and glucagon-like peptide 1 (17, 18). However, Dunning et al. (4) in our laboratory demonstrated that portal glucose delivery also enhances insulin secretion over that observed with peripheral glucose infusion. These investigators achieved identical hyperglycemia (~150 mg/dl) in two groups of dogs by infusing glucose either peripherally or portally. The arterial insulin concentrations and pancreatic output of insulin increased ~75% more with portal vs. peripheral glucose infusion, and these effects were determined to be neurally mediated (4). The arterial insulin concentrations were only 15% greater (~6 µU/ml; nonsignificant) in Po10 than in Pe10, compared with the 75% difference noted in our previous study. Because portal insulin concentrations were ~50% greater (~30 µU/ml; nonsignificant) in Po10 than in Pe10, clearance of insulin may have been enhanced by portal glucose delivery, thus causing arterial insulin to reflect insulin secretion less precisely. However, Ishida et al. (6) found no difference in fractional hepatic extraction of insulin after portal vs. peripheral glucose infusion. An alternative explanation for the discrepancy in the percent increase in the arterial and portal vein insulin in the current studies is that the tendency for the portal and sinusoidal concentrations to be higher in Po10 was a random event resulting from incomplete mixing of the pancreatic hormones with the portal vein blood. This would mean that the dogs experienced only a small increase in insulin secretion with portal vs. peripheral glucose delivery. As mentioned above, this explanation is supported by the findings of Ishida et al., who also infused glucose at identical rates via the portal and peripheral routes and found no difference in arterial, portal, or hepatic vein insulin concentrations with the two routes of delivery.

Clearly the portal signal by itself enhances NHGU in the presence of fixed hormone concentrations (1, 13, 15, 20). When hormones are free to change, however, the stimulation of insulin secretion may be an important component of the action of the portal signal. Even if an increase in insulin secretion was responsible for a portion of the enhancement of NHGU during portal glucose delivery in these studies, the insulin response cannot explain the impact of portal glucose delivery on peripheral glucose uptake. Despite the tendency for arterial insulin concentrations to be higher in Po10 than in Pe10, there was significantly less glucose uptake by the nonhepatic tissues in Po10, implying that some factor in addition to insulin was involved in regulation of peripheral glucose uptake. The similarity in the circulating insulin concentrations confirms the findings of Ishida et al. (6), who also infused identical amounts of glucose via the peripheral and portal routes in conscious dogs. The reduction in glucose uptake by the nonhepatic tissues (primarily skeletal muscle) during portal glucose delivery in the current studies was consistent with previous observations from our laboratory (1, 20). Recently we have been able to demonstrate, by direct measurement of glucose uptake across the canine hindlimb, that the suppression of nonhepatic glucose uptake in response to portal glucose delivery is due primarily to a decrease in skeletal muscle glucose uptake (5). It is not known how this suppression of skeletal muscle glucose uptake is mediated, but either neural (8, 10, 11, 24) or humoral (26, 27) mechanisms, or both, might serve to allow communication between the hepatoportal region and the muscle.

The basal arterial concentrations of glycerol and NEFA tended to be higher in Po10 than in Pe10. This was a chance finding, because all of the dogs were managed and fasted in the same manner before study and were randomly assigned to their treatment groups. Despite this, the net hepatic uptakes of glycerol and NEFA were no different basally, indicating that there was no systematic difference in hepatic substrate uptake between groups. When NEFA and glycerol concentrations are maintained at basal concentrations, rather than being allowed to fall in the presence of hyperglycemia and hyperinsulinemia, hepatic glucose output is not suppressed as it normally would be by insulin (7, 9, 22, 23, 25). High levels of NEFA are associated with decreases in glucose disappearance, glucose oxidation, muscle glycogen synthase activity, and glycogen synthesis (3). Thus differences in NEFA during glucose administration in the current studies would have been expected to minimize differences in NHGU and accentuate the differences in nonhepatic uptake. However, during the administration of glucose, the blood glycerol and plasma NEFA concentrations decreased by similar percentages in the two groups (50-60% decrease in glycerol, 84-87% decrease in NEFA), making the availability of these substrates to the livers very similar (Table 2). In absolute terms, NEFA concentrations were decreased more in Po10 than in Pe10, suggesting that lipolysis may have been reduced more by portal than by peripheral glucose delivery. If this were the case, then our findings would be consistent with a simultaneous increase in insulin responsiveness in adipose tissue and decrease in insulin responsiveness in skeletal muscle during portal glucose delivery. This suggests a very high degree of tissue specificity in the response to the portal signal.

Net hepatic uptake of carbon (net glucose uptake - net lactate output) was about twofold greater in Po10 than in Pe10. The likely major fate of this carbon was glycogen synthesis. We have previously shown that the portal signal, in the absence of a rise in insulin concentration, enhances glycogen synthase activity and net hepatic glycogen synthesis (20), but whether the enhancement of glycogen synthase is the primary effect of the portal signal on the liver is unknown. Glycogen synthase is allosterically regulated by glucose 6-phosphate, and it may be that the portal signal stimulates glycogen synthase secondarily by enhancing the hepatic uptake or phosphorylation of glucose. Glycogen synthase activity would also be stimulated if there were enhancement of insulin secretion in Po10.

In conclusion, the current data and other recent data from our laboratory indicate that the portal signal impacts upon the liver but also upon nonhepatic tissues, particularly skeletal muscle (5). Portal glucose delivery in the current study enhanced NHGU nearly threefold. This could be attributed, in approximately equal proportions, to the difference in the hepatic glucose loads during peripheral and portal glucose infusion and to portal signal effects, possibly including a stimulation of insulin secretion. The portal signal also suppressed glucose uptake in nonhepatic tissues. This is evident because the arterial blood glucose concentrations plateaued at a similar level with the two infusion routes, whereas circulating insulin levels were virtually identical in the two groups during the final 2 h of study. Because the liver's role in glucose removal was enhanced in Po10 compared with Pe10, the circulating glucose concentrations would have been expected to be lower in Po10 if the nonhepatic tissues had not displayed a reduction in glucose uptake. Thus, even in the absence of somatostatin and fixed insulin and glucagon concentrations, the portal signal impacted upon the distribution of a glucose load between the hepatic and nonhepatic tissues and stimulated the hepatic storage of carbon.


    ACKNOWLEDGEMENTS

The authors appreciate the technical assistance of Wanda Snead and Angelina Penaloza in the Hormone Core Laboratory of the Vanderbilt Diabetes Research and Training Center and of Margaret Converse.


    FOOTNOTES

These studies were supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant R-01-DK-43706 and Diabetes Research and Training Center Grant SP-60-AM-20593.

Current address for P.-S. Hsieh: Dept. of Physiology and Biophysics, National Defense Medical Center, Taipei, Taiwan.

Address for reprint requests and other correspondence: M. C. Moore, 702 Light Hall, Dept. of Molecular Physiology & Biophysics, Vanderbilt Univ. School of Medicine, Nashville, TN 37232-0615 (E-mail: genie.moore{at}mcmail.vanderbilt.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 14 February 2000; accepted in final form 12 July 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Adkins, BA, Myers SR, Hendrick GK, Stevenson RW, Williams PE, and Cherrington AD. Importance of the route of intravenous glucose delivery to hepatic glucose balance in the conscious dog. J Clin Invest 79: 557-565, 1987[ISI][Medline].

2.   Adkins-Marshall, B, Pagliassotti MJ, Asher JR, Connolly CC, Neal DW, Williams PE, Myers SR, Hendrick GK, Adkins RB, Jr, and Cherrington AD. Role of hepatic nerves in response of liver to intraportal glucose delivery in dogs. Am J Physiol Endocrinol Metab 262: E679-E686, 1992[Abstract/Free Full Text].

3.   Boden, G, Chen X, Ruiz J, White JV, and Rossetti L. Mechanisms of fatty acid-induced inhibition of glucose uptake. J Clin Invest 93: 2438-2446, 1994[ISI][Medline].

4.   Dunning, B, Neal D, Scott M, and Cherrington AD. Neurally-mediated augmentation of the pancreatic response to glucose with portal glucose delivery in conscious dogs (Abstract). Diabetes 44: 191A, 1995.

5.   Galassetti, P, Shiota M, Zinker BA, Wasserman DH, and Cherrington AD. A negative arterial-portal venous glucose gradient decreases skeletal muscle glucose uptake. Am J Physiol Endocrinol Metab 275: E101-E111, 1998[Abstract/Free Full Text].

6.   Ishida, T, Chap Z, Chou J, Lewis R, Hartley C, Entman M, and Field JB. Differential effects of oral, peripheral intravenous, and intraportal glucose on hepatic glucose uptake and insulin and glucagon extraction in conscious dogs. J Clin Invest 72: 590-601, 1983[ISI][Medline].

7.   Kruszynska, YT, Mulford MI, Yu JG, Armstrong DA, and Olefsky JM. Effects of nonesterified fatty acids on glucose metabolism after glucose ingestion. Diabetes 46: 1586-1593, 1997[Abstract].

8.   Le Marchand, Y, Freychet P, and Jeanrenaud B. Longitudinal study on the establishment of insulin resistance in hypothalamic obese mice. Endocrinology 102: 74-85, 1978[Abstract].

9.   Lewis, GF, Zinman B, Groenewoud Y, Vranic M, and Giacca A. Hepatic glucose production is regulated both by direct hepatic and extrahepatic effects of insulin in humans. Diabetes 45: 454-462, 1996[Abstract].

10.   Miles, PDG, Yamatani K, Brown MR, Lickley LA, and Vranic M. Intracerebroventricular administration of somatostatin octapeptide counteracts the hormonal and metabolic responses to stress in normal and diabetic dogs. Metabolism 43: 1134-1143, 1994[ISI][Medline].

11.   Minokoshi, Y, Okano Y, and Shimazu T. Regulatory mechanism of the ventromedial hypothalamus in enhancing glucose uptake in skeletal muscles. Brain Res 649: 343-347, 1994[ISI][Medline].

12.   Moore, MC, Cherrington AD, Cline G, Pagliassotti MJ, Jones EM, Neal DW, Badet C, and Shulman GI. Sources of carbon for hepatic glycogen synthesis in the conscious dog. J Clin Invest 88: 578-587, 1991[ISI][Medline].

13.   Moore, MC, Flakoll PJ, Hsieh P-S, Pagliassotti MJ, Neal DW, Monohan MT, Venable C, and Cherrington AD. Hepatic glucose disposition during concomitant portal glucose and amino acid infusions in the dog. Am J Physiol Endocrinol Metab 274: E893-E902, 1998[Abstract/Free Full Text].

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

15.   Myers, SR, Biggers DW, Neal DW, and Cherrington AD. Intraportal glucose delivery enhances the effects of hepatic glucose load on net hepatic glucose uptake in vivo. J Clin Invest 88: 158-167, 1991[ISI][Medline].

16.   Myers, SR, McGuinness OP, Neal DW, and Cherrington AD. Intraportal glucose delivery alters the relationship between net hepatic glucose uptake and the insulin concentration. J Clin Invest 87: 930-939, 1991[ISI][Medline].

17.   Nauck, MA, Bartels E, Orskov C, Ebert R, and Creutzfeldt W. Additive insulinotropic effects of exogenous synthetic human gastric inhibitory polypeptide and glucagon-like peptide-1-(7-36) amide infused at near-physiological insulinotropic hormone and glucose concentrations. J Clin Endocrinol Metab 76: 912-917, 1993[Abstract].

18.   Nauck, MA, Holst JJ, Willms B, and Schmiegel W. Glucagon-like peptide 1 (GLP-1) as a new therapeutic approach for type 2-diabetes. Exp Clin Endocrinol Diabetes 105: 187-195, 1997[ISI][Medline].

19.   Nauck, MA, Homberger E, Siegel EG, Allen RC, Eaton RP, Ebert R, and Creutzfeldt W. Incretin effects of increasing glucose loads in man calculated from venous insulin and C-peptide responses. J Clin Endocrinol Metab 63: 492-498, 1986[Abstract].

20.   Pagliassotti, MJ, Holste LC, Moore MC, Neal DW, and Cherrington AD. Comparison of the time courses of insulin and the portal signal on hepatic glucose and glycogen metabolism in the dog. J Clin Invest 97: 81-91, 1996[Abstract/Free Full Text].

21.   Pagliassotti, MJ, Myers SR, Moore MC, Neal DW, and Cherrington AD. Magnitude of negative arterial-portal glucose gradient alters net hepatic glucose balance in conscious dogs. Diabetes 40: 1659-1668, 1991[Abstract].

22.   Prager, R, Wallace P, and Olefsky JM. Direct and indirect effects of insulin to inhibit hepatic glucose output in obese subjects. Diabetes 36: 607-611, 1987[Abstract].

23.   Rebrin, K, Steil GM, Mittelman SD, and Bergman RN. Causal linkage between insulin suppression of lipolysis and suppression of liver glucose output in dogs. J Clin Invest 98: 741-749, 1996[Abstract/Free Full Text].

24.   Shimazu, T, Sudo M, Minokoshi Y, and Takahashi A. Role of the hypothalamus in insulin-independent glucose uptake in peripheral tissues. Brain Res Bull 27: 501-504, 1991[ISI][Medline].

25.   Sindelar, DK, Chu CA, Rohlie M, Neal DW, Swift LL, and Cherrington AD. The role of fatty acids in mediating the effects of peripheral insulin on hepatic glucose production in the conscious dog. Diabetes 46: 187-196, 1997[Abstract].

26.   Xie, H, and Lautt WW. Insulin resistance caused by hepatic cholinergic interruption and reversed by acetylcholine administration. Am J Physiol Endocrinol Metab 271: E587-E592, 1996[Abstract/Free Full Text].

27.   Xie, H, and Lautt WW. Insulin resistance of skeletal muscle produced by hepatic parasympathetic interruption. Am J Physiol Endocrinol Metab 270: E858-E863, 1996[Abstract/Free Full Text].


Am J Physiol Endocrinol Metab 279(6):E1271-E1277
0193-1849/00 $5.00 Copyright © 2000 the American Physiological Society