Department of Molecular Physiology & Biophysics and Diabetes Research and Training Center, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
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ABSTRACT |
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Arteriovenous difference and tracer
([3-3H]glucose) techniques were used in 42-h-fasted
conscious dogs to identify any insulin-like effects of intraportally
administered glucagon-like peptide 1-(7-36)amide (GLP-1). Each study consisted of an equilibration, a basal, and three
90-min test periods (P1, P2, and P3) during which somatostatin, intraportal insulin (3-fold basal) and glucagon (basal), and
peripheral glucose were infused. Saline was infused intraportally
in P1. During P2 and P3, GLP-1 was infused intraportally at 0.9 and 5.1 pmol · kg1 · min
1
in eight dogs, at 10 and 20 pmol · kg
1 · min
1
in seven dogs, and at 0 pmol · kg
1 · min
1
in eight dogs (control group). Net hepatic glucose uptake was significantly enhanced during GLP-1 infusion at 20 pmol · kg
1 · min
1
[21.8 vs. 13.4 µmol · kg
1 · min
1
(control), P < 0.05]. Glucose utilization was
significantly increased during infusion at 10 and 20 pmol · kg
1 · min
1
[87.3 ± 8.3 and 105.3 ± 12.8, respectively, vs. 62.2 ± 5.3 and 74.7 ± 7.4 µmol · kg
1 · min
1
(control), P < 0.05]. The glucose infusion rate
required to maintain hyperglycemia was increased (P < 0.05) during infusion of GLP-1 at 5.1, 10, and 20 pmol · kg
1 · min
1
(22, 36, and 32%, respectively, greater than control). Nonhepatic glucose uptake increased significantly during delivery of GLP-1 at 5.1 and 10 pmol · kg
1 · min
1
(25 and 46% greater than control) and tended (P = 0.1)
to increase during GLP-1 infusion at 20 pmol · kg
1 · min
1
(24% greater than control). Intraportal infusion of GLP-1 at high
physiological and pharmacological rates increased glucose disposal
primarily in nonhepatic tissues.
incretin; net hepatic glucose uptake; muscle glucose uptake; blood glucose
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INTRODUCTION |
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TYPE 2 DIABETES (T2DM) is characterized by abnormal insulin secretion
from pancreatic -cells and insulin resistance (13). Sulfonylureas, commonly used in treatment of T2DM, have a strong insulinotropic action, but they are prone to cause fasting hypoglycemia because their action is not glucose dependent. Therefore, a drug that
effectively stimulates insulin secretion only under hyperglycemic conditions is desirable. Recently, glucagon-like peptide
1-(7-36)amide (GLP-1) has been shown to have a
glucose-dependent insulinotropic effect (2).
GLP-1 is derived from proglucagon in the L cells in the distal ileum
and colon and is rapidly released into the portal vein after meal
ingestion (2). It lowers postprandial glucose levels in
both healthy and T2DM subjects (24, 50). Its actions
include stimulation of insulin secretion via a specific receptor on
pancreatic -cells (46), inhibition of glucagon
secretion from
-cells, and delay of gastric emptying
(2).
Whether GLP-1 has any effects on glucose disposal, outside of its actions on pancreatic hormone secretion and gastric emptying, remains unclear. Although some in vitro data indicate that GLP-1 can stimulate glucose uptake by adipocytes (18, 33) and skeletal muscle (51), other investigations have found no GLP-1 stimulation of glucose transport or glycogen synthesis in isolated muscle (22). Similarly, some in vivo investigations have suggested that GLP-1 enhances either insulin-dependent (43) or -independent (8) glucose uptake, whereas others have failed to find such effects (20, 21, 40, 42). In these previous in vivo studies, GLP-1 was delivered via a peripheral or central vein, in contrast to its normal route of secretion into the hepatic portal vein. Recent evidence suggests that the peptide may act within the hepatoportal region (5, 36, 38, 39). Because GLP-1 is degraded very rapidly by dipeptidyl-peptidase IV (DPP-IV) in plasma (9, 10), it would be optimal to deliver the peptide into the portal vein to evaluate its physiological effects fully. Some of the divergent results obtained in earlier in vivo studies may have arisen because of the difficulty in delivering an effective dose of GLP-1 to the liver, especially in human subjects, where the portal vein is inaccessible as an infusion route. In the present study, therefore, we examined the physiological and pharmacological effects of GLP-1 on glucose metabolism by infusing it into the portal vein of conscious dogs. We hypothesized that intraportal delivery would stimulate hepatic and nonhepatic glucose uptake independently of changes in pancreatic hormone secretion. We performed the studies under a somatostatin-controlled pancreatic clamp because of GLP-1's effect on pancreatic hormone secretion, and we employed hyperglycemic and hyperinsulinemic conditions to mimic the postprandial state, when most GLP-1 release occurs.
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RESEARCH DESIGN AND METHODS |
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Animals and surgical procedures. Experiments were performed on twenty-three 42-h-fasted conscious mongrel dogs (23.5 ± 0.6 kg) of either sex that had been fed once daily a standard meat and chow diet (31% protein, 52% carbohydrate, 11% fat, and 6% fiber based on dry weight; Kal Kan, Vernon, CA and Purina Lab Canine Diet no. 5006, Purina Mills, St. Louis, MO). The dogs were housed in a facility that met American Association for the Accreditation of Laboratory Animal Care guidelines, and the protocols were approved by the Vanderbilt University Medical Center Animal Care Committee. At least 16 days before experimentation, a laparotomy was performed with animals under general anesthesia. Silastic catheters (Dow Corning, Midland, MI) for blood sampling were placed into the portal vein, a hepatic vein, and a femoral artery, and infusion catheters were inserted into a jejunal vein and a splenic vein, as previously described (35). Ultrasonic flow probes (Transonic Systems, Ithaca, NY) were placed around the portal vein and the hepatic artery. On the day of the experiment, the catheters were exteriorized under local anesthesia, and intravenous access was established in three peripheral veins (35). Dogs were used for an experiment only if they met established criteria (35).
Experimental design.
Each experiment in the three protocols consisted of a tracer
equilibration period (120 to
20 min), a basal period (
20 to 0 min), and three test periods during which hyperglycemia and hyperinsulinemia existed (0-120 min, 120-210 min, and
210-300 min). At
120 min, a primed (1.2 µCi/kg), continuous
(0.17 µCi/min) infusion of [3-3H]glucose (New England
Nuclear, Boston, MA) and a continuous infusion of indocyanine green dye
(0.08 mg/min; Sigma Chemical, St. Louis, MO) were started. At
time 0, a peripheral infusion of somatostatin (0.8 µg · kg
1 · min
1;
Bachem, Torrance, CA) was begun to inhibit endogenous pancreatic insulin and glucagon secretion. Intraportal infusions of insulin (1.2 mU · kg
1 · min
1;
Eli Lilly, Indianapolis, IN), to achieve hyperinsulinemia, and glucagon
(0.5 ng · kg
1 · min
1;
Bedford Laboratory, Bedford, OH), to maintain basal levels, were also
started. A 50% dextrose solution was infused peripherally at variable
rates starting at time 0 to clamp the arterial plasma glucose level at 220 mg/dl. The infusion rate of glucose was adjusted in response to the plasma glucose concentration, which was measured every 5 min. In the second and third test periods in one protocol, GLP-1-(7-36)amide (Sigma) was infused into the portal
vein at 0.9 and 5.1 pmol · kg
1 · min
1,
respectively [low-dose GLP-1 (LGLP), n = 8]. In the
second and third test periods of another protocol, GLP-1 was infused
intraportally at 10 and 20 pmol · kg
1 · min
1,
respectively [high-dose GLP-1 (HGLP), n = 7]. These
GLP-1 infusion rates were chosen to create a wide range of
physiological and pharmacological plasma GLP-1 concentrations. In the
third protocol, saline was infused intraportally during the second and
third test periods [control group (CONT), n = 8].
Blood samples were taken and processed as previously described
(35). Diprotin A (50 nmol/ml; Sigma) was added to the
blood as soon as it was obtained to inhibit DPP-IV activity.
Analytical procedures. Plasma glucose and glucose radioactivity (3H), insulin, glucagon, cortisol, nonesterifed fatty acids (NEFA), and blood lactate, glycerol, and alanine were measured as previously described (6, 35). Plasma GLP-1 levels were determined by an RIA method that specifically determines the biologically active form of GLP-1 [i.e., GLP-1-(7-36)amide or GLP-1-(7-37)] and binds to the NH2-terminal region (Linco Research, St. Charles, MO) (52).
Calculations.
Net hepatic substrate balance (NHB) was calculated using the formula
[H · Ft (A · Fa + P · Fp)], where A, P, and H are the arterial, portal vein, and hepatic vein substrate concentrations, and
Fa, Fp, and Ft are hepatic
arterial, portal vein, and total hepatic blood or plasma flows (as
appropriate), respectively. Hepatic substrate load was calculated as
A · Fa + P · Fp. Net hepatic fractional
extraction (FE) was calculated as NHB
hepatic load. For all
calculated data, plasma glucose concentrations were converted to blood
concentrations with correction factors compiled from extensive data
from our laboratory (25). During the basal period,
arterial, portal vein, and hepatic vein plasma glucose concentrations
were multiplied by 0.74, 0.74, and 0.73 to convert them to blood
glucose concentrations, and during the experimental period all plasma
concentrations were multiplied by 0.73. Use of blood, rather than
plasma, glucose concentrations ensures accurate NHB calculations
regardless of the characteristics of glucose entry into the erythrocyte.
Statistical analysis. Data are expressed as means ± SE and analyzed by SigmaStat (SPSS, Chicago, IL). Two-way repeated-measures analysis of variance was used to compare the time-course data of groups. Classification factors were treatment group and time period, as well as their interaction. "Dog within group" was used for an error term. For significant F values, the Student-Newman-Keuls multirange test was employed as a post hoc analysis. Differences were considered significant when P < 0.05.
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RESULTS |
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Hormone concentrations.
The mean arterial insulin concentrations increased to approximately
threefold basal and were not different among the groups (Table
1). The plasma glucagon concentrations
remained basal and similar in all of the groups. Likewise, the plasma
cortisol concentrations remained basal in all of the groups.
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Hepatic blood flow and cardiovascular parameters.
In response to somatostatin, portal and total hepatic blood flows
decreased in each group (Table 1). Arterial blood flow was similar in
LGLP and CONT during the test periods. High-dose GLP-1 infusion
increased hepatic artery flow significantly, with the mean for the
third period being 46% greater than that in CONT (P < 0.05). The increase in hepatic artery flow brought about an increase
(P < 0.05) in total hepatic blood flow during
periods 2 and 3 in HGLP (29 ± 1 ml · kg1 · min
1
in both periods vs. 26 ± 1 ml · kg
1 · min
1
during period 1). Neither the CONT nor the LGLP group
exhibited an increase in total hepatic blood flow during periods
2 and 3. GLP-1 did not change heart rate or blood
pressure at any infusion rate (data not shown).
Blood glucose levels and hepatic glucose balance.
In response to the peripheral glucose infusion, arterial blood glucose
concentrations increased to ~8.7 mmol/l, where they were maintained
(Fig. 2). The hepatic glucose loads (HGL)
during period 1 averaged 252 ± 17, 207 ± 13, and
221 ± 11 µmol · kg1 · min
1
in CONT, LGLP, and HGLP, respectively. The HGL remained relatively stable in CONT (
12 ± 6 and
1 ± 10 µmol · kg
1 · min
1
for periods 2 and 3, respectively, vs.
period 1), and it did not change significantly during
low-dose GLP-1 infusion (
15 ± 14 and 19 ± 19 µmol · kg
1 · min
1
vs. period 1). However, it increased significantly with the
10 and 20 pmol · kg
1 · min
1
infusion rates (
28 ± 10 and 31 ± 12 µmol · kg
1 · min
1
for periods 2 and 3, respectively, vs.
period 1, P < 0.05), primarily as a result
of the rise in hepatic artery blood flow.
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NEFA and glycerol concentrations and net hepatic uptake.
Concomitant hyperinsulinemia and hyperglycemia reduced the arterial
NEFA levels substantially from 811 ± 161, 761 ± 70, and 724 ± 67 µmol/l at 0 min to 96 ± 19, 102 ± 15, and
52 ± 11 µmol/l at 120 min in CONT, LGLP, and HGLP, respectively
(Table 2). NEFA concentrations then
remained low throughout each protocol. Net hepatic NEFA uptake fell to
near zero in response to hyperglycemia and hyperinsulinemia and did not
change significantly thereafter in any protocol.
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Lactate and alanine concentrations and balance data and net hepatic
glycogen synthesis.
After initiation of the hyperglycemic hyperinsulinemic clamp, arterial
blood lactate levels increased (Fig. 5),
and net hepatic lactate balance (NHLB) changed from uptake to output in
all groups. NHLB declined to near 0 µmol · kg1 · min
1
in all groups by 150 min, and the CONT group then returned to a low
rate of net hepatic lactate uptake. In the LGLP and HGLP groups, a low
rate of net hepatic lactate output was evident during period
3, consistent with a stimulation of glycolysis by GLP-1. During
infusion of GLP-1 at 20 pmol · kg
1 · min
1,
net hepatic glycogen storage was significantly enhanced, as indicated
by comparison of net hepatic carbon retention with CONT during test
period 3 (20.7 ± 3.2 vs. 12.9 ± 1.6 µmol · kg
1 · min
1,
P < 0.05). No significant changes were observed in the
arterial blood alanine levels, net hepatic alanine uptakes, or net
hepatic FE throughout the study (data not shown).
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DISCUSSION |
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GLP-1 was infused into the portal vein at four different rates.
GLP-1 infusion at 0.9 pmol · kg1 · min
1
increased the arterial plasma GLP-1 from a basal value of 12 ± 5 to 17 ± 6 pmol/l. The levels achieved during the lowest GLP-1 infusion rate are consistent with the concentrations of intact GLP-1
reported for conscious dogs after an oral glucose load (2 g/kg)
(32) or intragastric delivery of a mixed meal
(12) and for healthy humans after mixed meal ingestion
(52). These concentrations are also similar to the
arterialized venous concentrations obtained in individuals with type 1 diabetes (T1DM) and T2DM during peripheral GLP-1 infusion at 1.2 pmol · kg
1 · min
1
(49, 50). The infusion of GLP-1 at 5.1 pmol · kg
1 · min
1
resulted in arterial GLP-1 plasma levels of 51 ± 16 pmol/l,
approximately twofold higher than those normally reported after a mixed
meal in young adults (52) but similar to those attained in
elderly subjects with T2DM ingesting a mixed meal (14).
The portal GLP-1 concentrations at the 0.9 and 5.1 pmol · kg
1 · min
1
infusion rates were 25 ± 8 and 102 ± 31 pmol/l,
respectively. Because there are few reports in the literature that
include portal vein GLP-1 concentrations, we have measured intact
arterial and portal vein GLP-1 in three overnight-fasted dogs under
basal conditions and for 3 h after an intragastric glucose bolus
of 1.5 g/kg (Dardevet D, Moore MC, and Cherrington AD, unpublished
data). The basal GLP-1 concentrations in those animals were 17 ± 3 pmol/l in both the artery and portal vein. Peak values (20 min after
glucose administration) were 39 ± 12 (artery) and 65 ± 24 (portal vein) pmol/l, with a gradual decline thereafter. Two hours
after glucose delivery, the levels had reached 21 ± 5 and 25 ± 10 pmol/l in the artery and portal vein, respectively. Thus the two
lowest infusion rates in the current investigation resulted in
physiological or near-physiological concentrations of GLP-1. On the
other hand, intraportal infusion of GLP-1 at 10 and 20 pmol · kg
1 · min
1
resulted in arterial GLP-1 levels that were clearly pharmacological (383 ± 82 and 480 ± 70 pmol/l, respectively). The
portal concentrations achieved in our study (588 ± 90 and
914 ± 158 pmol/l with GLP-1 at 10 and 20 pmol · kg
1 · min
1,
respectively) were somewhat lower than would be predicted on the basis
of our infusion rates and portal flow (~800 and 1,600 pmol/l,
respectively). This probably resulted from the rapid degradation of
GLP-1 in the circulation (29). Also, GLP-1 was prepared in saline that contained 3% (vol/vol) of the dog's own plasma, as were
all of the other hormones, to reduce binding of the hormones to the
containers used for preparing them and to the infusion lines. No DPP-IV
inhibitor was added to the GLP-1 solution, and thus it is possible that
there was some degradation of GLP-1 during the infusion period.
The dogs studied tolerated all GLP-1 infusion rates without apparent
discomfort or other side effects. Gastrointestinal side effects, such
as nausea, have been reported in human subjects receiving GLP-1,
especially when the peptide was delivered intravenously at rates as
high as 2.7 pmol · kg1 · min
1
(41). GLP-1 receptor agonists have also caused nausea in
some investigations (1, 19) but not in others (17,
37), and calves have tolerated short-term GLP-1 infusion rates
as high as 35 pmol · kg
1 · min
1
(16). It is likely that the gastrointestinal side effects
are related to the well-known ability of GLP-1 to delay gastric
emptying (17), and thus the fact that the current studies
were conducted in fasted dogs may have helped to reduce any discomfort
that might otherwise have accompanied GLP-1 treatment.
In vitro data suggest that GLP-1 can directly stimulate glucose uptake
by nonhepatic tissues (18, 33, 51), although not all
investigations have confirmed this effect (22). Whether GLP-1 has an independent effect on tissue glucose uptake in vivo has
also been difficult to determine, largely because physiological doses
of GLP-1 stimulate insulin secretion (26, 47). Using the
minimal-model technique to evaluate the response of normal subjects to
a frequently sampled intravenous glucose tolerance test, D'Alessio et
al. (7) concluded that GLP-1 infusion increased glucose
effectiveness and insulin-independent glucose disposal. In a study
using the same technique in mice (3), GLP-1 was found to
augment insulin secretion but not insulin sensitivity. In agreement
with this, Freyse et al. (21) observed that intravenous infusion of GLP-1 at 10 pmol · kg1 · min
1
did not change insulin sensitivity in C-peptide-negative 90% pancreatectomized dogs. Similarly, Ryan et al. (42) found
no evidence of enhancement of insulin sensitivity by GLP-1 in normal volunteers during a euglycemic clamp. Somatostatin was not used, but
each subject underwent a control experiment during which insulin was
infused at a rate to mimic the levels observed during the infusion of
GLP-1. Even in studies utilizing the pancreatic clamp technique to
eliminate changes in insulin and glucagon secretion, differing
conclusions have been reached regarding the insulin-independent effects
of GLP-1. Healthy volunteers exhibited no augmentation of glucose
disposal associated with GLP-1 infusion during a hyperinsulinemic euglycemic clamp in which somatostatin was used to suppress pancreatic hormone secretion and glucagon and growth hormone were replaced at
basal levels (40). Similar conclusions were obtained with euinsulinemic and hyperinsulinemic clamp studies in hyperglycemic subjects with T2DM (50). On the other hand, a delayed
enhancement (during the 4th h of GLP-1 infusion) of whole body but not
splanchnic glucose disposal was observed during intraduodenal glucose
infusion under pancreatic clamp conditions in individuals with T1DM
(49). Sandhu et al. (43) reported that GLP-1
potentiated insulin-stimulated glucose utilization during a
hyperinsulinemic hyperglycemic clamp in depancreatized dogs but had no
effect in the presence of a euinsulinemic hyperglycemic clamp. GLP-1
appeared to augment the suppression of NEFA levels during the
hyperinsulinemic but not the euinsulinemic clamp, and this could at
least partly account for the stimulation of glucose utilization by
GLP-1 (43). In contrast to the normal route of GLP-1
secretion, i.e., the hepatic portal vein, a common feature of all of
the previous in vivo studies cited is that GLP-1 was administered via a
peripheral or central vein. The degradation of GLP-1 in vivo is very
rapid (plasma half-life 2-4 min) (11, 28).
Consequently, portal vein GLP-1 concentrations during peripheral GLP-1
administration were undoubtedly lower than the peripheral
concentrations. Thus these previous studies beg the question of what
effect GLP-1 might have on glucose metabolism when delivered via its
endogenous secretion route.
The current data are unique in that they were obtained in conscious
animals during the intraportal infusion of GLP-1. Recent reports
highlight the importance of the hepatoportal area in the physiological
activity of GLP-1. Nakabayashi et al. (36) reported that
intraportal GLP-1 infusion at a physiological dose stimulated afferent
vagal nerve activity in rats. This activation, in turn, stimulated
efferent signaling in the pancreatic branch of the vagus nerve,
suggesting a neural component of GLP-1's stimulation of insulin
secretion (36). This hypothesis was verified by a recent
investigation (4). In addition, the afferent limb of the
inhibitory effect of GLP-1 on gastric emptying is mediated by the vagus
nerve (27). Moreover, portal but not peripheral infusion
of the GLP-1 antagonist exendin-(9-39) inhibited the increase in glucose clearance observed during portal glucose infusion in normal mice (5). Also, glucose clearance did not
increase during portal glucose infusion in GLP-1 receptor knockout
mice, although it did so in wild-type mice. Despite the strong evidence in the literature linking the hepatoportal region with GLP-1 action, the current findings clearly show that the 0.9 pmol · kg1 · min
1
infusion rate, which resulted in physiological circulating GLP-1 levels, did not bring about an enhancement of glucose uptake by either
the liver or the nonhepatic tissues. During infusion of GLP-1 at 5.1 pmol · kg
1 · min
1,
there were no discernible effects on the liver, but the GIRs required
to maintain the hyperglycemic clamp and the non-HGU were elevated over
the rate evident in CONT. On the other hand, during delivery of
pharmacological doses of GLP-1, NHGU, Rd, GIR, and non-HGU
were increased. Because all of the GLP-1 infusion rates associated with
enhancement of glucose uptake by the peripheral tissues resulted in
high physiological or frankly pharmacological circulating GLP-1 levels,
it is impossible to distinguish between two possible interpretations of
the data: 1) GLP-1 does not have a direct effect on
peripheral tissues but instead acts within the portal vein to stimulate
non-HGU indirectly, or 2) GLP-1 can act via a hepatoportal
receptor but also may have direct peripheral effects at higher
concentrations. In regard to the first possibility, release of a
humoral factor from the liver has been hypothesized to enhance insulin
sensitivity in skeletal muscle (30), but it remains
unclear whether our current observation relates to that factor.
A second unique feature of the current results is that they clearly
indicate, by two independent measures (GIR and glucose Rd),
that GLP-1 stimulated whole body glucose disposal in normal conscious
dogs in which pancreatic hormone concentrations were fixed and
hyperglycemia was present. The plasma NEFA levels were low, and there
were no significant differences among the groups. This suggests that
the combination of hyperglycemia and hyperinsulinemia brought about a
maximum antilipolytic effect, so that we were able to examine the
effect of GLP-1 on glucose disposal independent of an indirect effect
resulting from suppression of lipolysis. During infusion of GLP-1 at
5.1 and 10 pmol · kg1 · min
1,
non-HGU was significantly increased, and non-HGU tended to be increased
during delivery of GLP-1 at 20 pmol · kg
1 · min
1.
In fact, there were no differences among the rates of non-HGU during
GLP-1 infusion at 5.1, 10, and 20 pmol · kg
1 · min
1,
but the increased variance evident during the highest infusion rate
precluded statistical significance. The time course of GLP-1's action
is unclear. If infused over a longer period, it is possible that it
would have had larger effects at lower dosages (49). This
is consistent with the enhancement of GIR and non-HGU by GLP-1 at 5.1 pmol · kg
1 · min
1,
because that infusion rate occurred during the third test period. This
raises the question of whether the results during test
period 3 were confounded by pretreatment with GLP-1 during
period 2. Although it is possible that prolonged
administration of GLP-1 affected the results during period
3, this in no way negates the primary conclusion of the current
investigation. It is clear that infusion of GLP-1 at 10 pmol · kg
1 · min
1,
which occurred during period 2 and thus without GLP-1
pretreatment, significantly stimulated non-HGU, showing the GLP-1 is
capable of acute actions. Non-HGU also increased significantly during GLP-1 infusion at 5.1 pmol · kg
1 · min
1.
Although this may have been a result of prolonged GLP-1 infusion, since
it occurred during period 3, it nevertheless demonstrates that physiological or near-physiological GLP-1 levels can stimulate non-HGU. Therefore, we can conclude that GLP-1 is capable of enhancing glucose uptake in nonhepatic tissues, independent of any effect it has
on pancreatic hormone secretion. NHGU was significantly enhanced only
during GLP-1 infusion at 20 pmol · kg
1 · min
1
and in the presence of an increase in hepatic artery flow (see below).
Thus GLP-1's stimulation of glucose uptake under these conditions was
exerted primarily on nonhepatic tissues. The enhancement of NHGU at the
highest infusion rate may help to explain why non-HGU did not increase
more during the 20 pmol · kg
1 · min
1
infusion than during the 10 pmol · kg
1 · min
1
rate. We have previously emphasized the reciprocal nature of glucose
uptake into the liver and the nonhepatic tissues (primarily skeletal
muscle) (23, 25, 34).
The increase in hepatic artery blood flow at a GLP-1 infusion rate of
20 pmol · kg1 · min
1
led to an increment in HGL, which could have contributed to the increase in NHGU that we observed. On the other hand, FE was also significantly increased by GLP-1 at 20 pmol · kg
1 · min
1,
suggesting that GLP-1 had an effect independent of a change in HGL.
Nevertheless, FE could have changed as a result of whatever factor
caused the increase in blood flow. In a previous study, we found that
intraportal acetylcholine (ACh) stimulated NHGU in dogs during
a hyperglycemic hyperinsulinemic pancreatic clamp (45).
However, ACh administration was associated with an increase in hepatic
arterial flow. Subsequently, we administered ACh intraportally to dogs
with hepatic artery ligation and determined that, in the absence of an
increase in hepatic artery flow, ACh did not enhance NHGU during a
hyperglycemic, hyperinsulinemic clamp (Moore MC and Cherrington AD,
unpublished results). The increase in NHGU during the highest GLP-1
infusion rate was accompanied by a tendency toward enhancement of net
hepatic carbon retention, reflecting an augmentation of hepatic
glycogen synthesis. This is in agreement with results of in vitro
studies in which GLP-1 was determined to stimulate glycogen synthase
activity (31) and glycogen synthesis (48) in
hepatocytes from both normal and diabetic rats.
In conclusion, under conditions in which neither insulin nor glucagon secretion, gastric emptying, nor lipolysis was altered by GLP-1, the peptide stimulated glucose Rd and increased the GIR required to maintain the hyperglycemic clamp. This was primarily a consequence of an increase in glucose uptake by the nonhepatic tissues, because significant stimulation of NHGU was observed only during the highest dosage of GLP-1 and in a setting in which hepatic artery flow changed, consistent with stimulation of NHGU by either the change in blood flow or a factor that contributed to the change in flow. These data demonstrate that there is a potential for GLP-1 to affect glucose uptake in both the liver and the nonhepatic tissues (presumably primarily skeletal muscle), but they leave unresolved the issue of dose dependence and time course of action. Nevertheless, our findings support a role for GLP-1 as a tool for the reduction of postprandial hyperglycemia in individuals with diabetes.
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ACKNOWLEDGEMENTS |
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These studies were supported by National Institutes of Health Grant R01-DK-43706 and the Diabetes Research and Training Center Grant SP-60-AM-20593.
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FOOTNOTES |
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Address for reprint requests and other correspondence: M. C. Moore, 702 Light Hall, Vanderbilt Univ. School of Medicine, Nashville, TN 37232-0615 (E-mail: genie.moore{at}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.
First published February 4, 2003;10.1152/ajpendo.00503.2002
Received 14 November 2002; accepted in final form 28 January 2003.
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