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Department of Molecular Physiology and Biophysics, and Diabetes Research and Training Center, Vanderbilt University School of Medicine, Nashville, Tennessee 37232
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ABSTRACT |
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 Top
 Abstract
 Introduction
METHODS
RESULTS
DISCUSSION
References
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The effects of prior
fast duration (18 h, n = 8;
42 h, n = 8) on the glycemic and
tissue-specific responses to an intraduodenal glucose load were studied
in chronically catheterized conscious dogs.
[3-3H]glucose was
infused throughout the study. After basal measurements, glucose spiked
with [U-14C]glucose
was infused for 150 min intraduodenally. Arterial insulin and glucagon
were similar in the two groups. Arterial glucose (mg/dl) rose ~70%
more during glucose infusion after 42 h than after an 18-h fast. The
net hepatic glucose balance
(mg · kg
1 · min1)
was similar in the two groups (basal: 1.8 ± 0.2 and 2.0 ± 0.3; glucose infusion: 
2.2 ± 0.5 and 2.2 ± 0.7). The
intrahepatic fate of glucose was 79% glycogen, 13% oxidized, and 8%
lactate release after a 42-h fast; it was 23% glycogen, 21% oxidized, and 56% lactate release after an 18-h fast. Net hindlimb glucose uptake was similar between groups. The appearance of intraduodenal glucose during glucose infusion (mg/kg) was 900 ± 50 and 1,120 ± 40 after 18- and 42-h fasts (P < 0.01).
Conclusion: glucose administration after prolonged fasting induces
higher circulating glucose than a shorter fast (increased appearance of
intraduodenal glucose); liver and hindlimb glucose uptakes and the
hormonal response, however, are unchanged; finally, an intrahepatic
redistribution of carbons favors glycogen deposition.
fasting; glycogen; glucose uptake; gut; liver; skeletal muscle
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INTRODUCTION |
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Top
Abstract
Introduction
METHODS
RESULTS
DISCUSSION
References
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PRIOR EXERCISE that diminishes tissue glycogen stores increases the ability of skeletal muscle to store glucose as glycogen (20, 27). This is due in part to an increase in muscle insulin sensitivity, but also in part to extramuscular adaptations (14, 20). Intestinal glucose absorption (13), the net rate of splanchnic glucose release (14, 20), and arterial glucose are higher (14, 18, 20) in response to a glucose load immediately after exercise than after rest. The signals that cause these adaptations in the postexercise state are still unknown. It is possible that the effects of prior exercise merely reflect the accelerated transition to a more fasted state. Prolonged fasting can markedly affect the absorption dynamics and fate of an oral carbohydrate load (22). One of the main determinants of the metabolic response to food deprivation is the depletion of glycogen stores. Hepatic glycogen is stored more rapidly in human subjects administered a glucose load after a prolonged fast as opposed to a shorter-duration fast (9, 10). Moreover, a strong positive correlation exists between the degree of hepatic glycogen depletion and the rate of hepatic glycogen turnover (1, 6, 21).
Despite the importance of the nutritional state in determining the metabolic response to feeding, very little is known about how splanchnic tissues adapt to substrate depletion induced by a prolonged fast. It was the aim of this study to determine whether, like prior exercise, a prolonged fast causes adaptations in the gut and liver that facilitate the disposition of an intraduodenal glucose load. To assess this, hepatic glycogen was depleted by prolonged fasting (42 h) to a degree similar to that observed after prolonged moderate exercise. Glucose fluxes and metabolism were quantified in 18- and 42-h-fasted conscious dogs by use of a dual isotopic method and arteriovenous difference techniques.
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METHODS |
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Top
Abstract
Introduction
METHODS
RESULTS
DISCUSSION
References
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Animals and surgical procedures. Sixteen mongrel dogs of either gender (mean weight 23 ± 2 kg) were studied. Animals were housed in a facility that met American Association for the Accreditation of Laboratory Animals guidelines and were fed a standard diet of meat and chow (34% protein, 14.5% fat, 46% carbohydrate, and 5.5% fiber based on dry weight). Experimental protocols were approved by the Vanderbilt University School of Medicine Animal Care Subcommittee. At least 16 days before each experiment, a laparotomy was performed under general anesthesia. Silastic catheters (0.03 ID) were inserted in the vena cava for tracer and indocyanine green (ICG) infusion. A Silastic catheter (0.08 ID) was inserted through the duodenal wall 3-4 cm below the pylorus for infusion of cold glucose and [U-14C]glucose. Silastic catheters (0.04 ID) were inserted in the portal vein and left common hepatic vein for blood sampling. Incisions were also made in the neck region and the inguinal region for the insertion of Silastic sampling catheters (0.04 ID) in the carotid artery (advanced so that its tip rested at the aortic arch) and in a lateral circumflex vein (advanced so that its tip reached the common iliac vein distal to the anastomosis with the vena cava). After insertion, the catheters were filled with saline containing heparin, and their free ends were knotted.
Doppler flow probes (Transonic Systems, Ithaca, NY) were used to measure portal vein, hepatic artery, and external iliac artery blood flows. A small section of the portal vein upstream from its junction with the gastroduodenal vein was cleared of tissue, and a 6.0-mm-ID flow cuff was secured around the vessel. The gastroduodenal vein was isolated and ligated proximal to its coalescence with the portal vein. A section of the main hepatic artery proximal to the portal vein was isolated, and a 3.0-mm-ID flow cuff was placed around the vessel and secured. The external iliac artery was accessed from the abdominal incision, dissected free of surrounding tissue, and fitted with a 4.0-mm-ID flow probe cuff that was then secured around the vessel. The Doppler probe leads and the knotted free catheter ends, with the exception of the carotid artery and the common iliac vein catheters, were stored in subcutaneous pockets in the abdominal region so that complete closure of the skin incision was possible. The carotid artery and common iliac vein catheters were stored in subcutaneous pockets in the neck and inguinal regions, respectively. Only animals that had 1) a leukocyte count <18,000/mm3, 2) a hematocrit >36%, 3) normal stools, and 4) a good appetite (consuming all of the daily ration) were used for experiments. Studies were conducted on conscious dogs after either an 18-h or a 42-h fast. In the dog, an 18-h fast allows complete gut absorption of a standard meal but induces only a partial depletion of muscle and liver glycogen stores (7). A 42-h fast, on the other hand, results in the reduction of muscle and liver glycogen to a stable minimum, which is <50% of the tissue glycogen content measured after 18 h of fasting. On the day of the experiment, the subcutaneous ends of the catheters were freed through small skin incisions made under local anesthesia (2% lidocaine; Astra Pharmaceutical, Worcester, MA) in the abdominal, inguinal, and neck regions. The contents of each catheter were aspirated and were flushed with saline. Silastic tubing was connected to the exposed catheters and brought to the back of the dog, where they were secured with quick-drying glue. Saline was infused through the arterial catheters throughout the experiment (0.1 ml/min). Data from several of the dogs in the 18-h-fasted group were included in the database of a previously published study (14).Experimental procedures.
The study was divided into an equilibration period
(t = 180 to 30 min), a
baseline period (t = 20 to 0 min), and a glucose infusion period (t = 0 to 150 min). At t = 110
min, primers of [3-3H]glucose (30 µCi) and sodium
[14C]bicarbonate (0.64 µCi/min) were given, followed by constant-rate venous infusions of
[3-3H]glucose (0.3 µCi/min) and ICG (0.1 mg/min), which were continued for the duration
of the study. From t = 0 to 150 min,
glucose mixed with
[U-14C]glucose to give
a specific activity (SA) of ~8,700 dpm/mg was given as a primed
infusion (150 mg/kg; 8 mg · kg1 · min1)
into the duodenum. Isotopes were obtained from New England Nuclear (Boston, MA), and ICG was purchased from Hynson, Westcott & Dunning (Baltimore, MD). Arterial samples were drawn at 5-min intervals from
t = 20 to 0 min and
at 15-min intervals from t = 0 to 150 min. Portal, hepatic, and common iliac vein samples were drawn at
t = 20, 10, 0, 15, 30, 60, 90, 120, and 150 min. Portal vein, hepatic artery, and external
iliac artery blood flows were recorded continuously from the frequency
shifts of the pulse sound signal emitted from the Doppler flow probes
(15). At the cessation of the experiment, dogs were euthanized with an
overdose of pentobarbital sodium, an abdominal midline incision was
made, and ~2-g biopsies were taken from the liver. An incision was
then made on the medial aspect of the left limb, and ~2-g biopsies
were taken from three hindlimb skeletal muscles (gastrocnemius,
gracilis, and flexor halucis longus). Upon excision, all tissue samples
were immediately frozen with clamps cooled in liquid nitrogen. The time
interval between the pentobarbital injection and the last biopsy was
<5 min.
Processing of blood and tissue samples.
Plasma glucose levels were determined during the experiments by the
glucose oxidase method with a glucose analyzer (Beckman Instruments,
Fullerton, CA). After the completion of the experiment, plasma and
deproteinized blood samples were stored at 70°C until later
analysis. For the determination of plasma glucose radioactivity, 3H and
14C samples were deproteinized
with barium hydroxide and zinc sulfate and placed over Dowex 50W-X8
(Bio-Rad Laboratories, Richmond, CA) and Amberlite (Rohm and Haas,
Philadelphia, PA) resins. Samples were centrifuged, and the supernatant
was evaporated and reconstituted in 1 ml of water and 10 ml of
Ecolite+ (ICN Biomedicals, Irvine,
CA). Radioactivity was then determined by liquid scintillation counting
with a Beckman LS 5000TD. Whole blood [samples deproteinized by
1:3 dilution in 4% perchloric acid (PCA)] lactate, glycerol,
alanine, glucose, and plasma free fatty acids (FFA) were measured by
enzymatic methods (19) on a Technicon Autoanalyzer (Tarrytown, NY) or
on a Monarch 2000 centrifugal analyzer (Instrumentation Laboratories,
Lexington, MA). Whole blood lactate and glucose radioactivities were
determined on a separate set of deproteinized samples (diluted 1:1 in
8% PCA) according to Okajima et al. (26). The content of
14CO2
in whole blood was determined as previously described (11). Liver and
muscle glycogen mass and radioactivity were measured by a previously
described method (3).
Calculations. The tracer-determined total rate of glucose appearance (Ra) was determined by steady-state equations for isotope [3-3H]glucose dilution with a pool fraction of 0.65 (5, 8). The systemic Ra of the intraduodenal glucose load was determined by dividing the hepatic [14C]glucose production by the glucose SA of the intraduodenal infusate. The Ra of [14C]glucose was determined using non-steady-state equations for isotope [3-3H]glucose dilution with a pool fraction of 0.65 (5, 8) by using as SA the ratio of [3H]glucose to [14C]glucose. The systemic Ra of intraduodenal glucose calculated in this manner will be overestimated by an amount that is dependent on the rate that [14C]glucose is recycled. Endogenous glucose Ra was calculated by subtracting the systemic Ra of the intraduodenal glucose from the total glucose Ra.
Net hepatic balances of lactate, glucose, 14CO2, alanine, FFA, and glycerol were determined by the formula HAF × ([H] [A]) + PVF × ([H] [P]), where [A],
[P], and [H] are the arterial, portal vein, and
hepatic vein substrate concentrations, and HAF and PVF are the hepatic
artery and portal vein blood flows (with the exception of FFA, for
which plasma concentrations and flows were used). With this formula,
net substrate output appears as a positive number and net uptake as a
negative number, unless indicated differently. The load of a substrate
reaching the liver was calculated as [P] × PVF + [A] × HAF. Net hepatic fractional substrate
extractions were calculated as the ratio between net hepatic balance
and hepatic load. Net gut balances were calculated as PVF × ([P] [A]), and net splanchnic balances
were calculated as (HAF + PVF) × ([H] [A]).
Net limb balances were calculated as LF × ([A] [I]). LF is limb blood flow through the external
iliac artery, and [I] is the substrate level in the common
iliac vein. Net limb fractional substrate extraction was calculated as
the net limb substrate uptake divided by the limb substrate load, which
was equal to LF × [A]. The mean ratio of blood to
plasma glucose was calculated for the basal period and the glucose
infusion period for each of the four sampling sites. Plasma glucose
values were then multiplied by their corresponding ratio (i.e., blood
glucose/plasma glucose) to convert them to blood glucose
concentrations. The advantage of using plasma glucose measurements is
that a large number of replicates can be obtained, reducing the
measurement CV. The conversion to blood values alleviates the need for
assumptions regarding the equilibration of substrates between red cell
and plasma water.
Hepatic conversion of glucose to
CO2 was calculated as the net
hepatic
14CO2
production rate divided by the hepatic
[14C]glucose precursor
SA. The precursor SA was considered to be the
[14C]glucose SA in the
inflowing blood to the liver and was calculated as (portal vein
[14C]glucose SA × PVF + artery
[14C]glucose SA × HAF)/(HAF + PVF). Because during a
[14C]glucose infusion
[14C]lactate
accumulates, it is necessary to consider lactate SA when net liver
[14C]lactate uptake is
present. In these experiments, lactate was consistently produced by the
liver and therefore was not included in the calculation of hepatic
glucose metabolism. Assumptions involved in these calculations have
been described in detail previously (28, 30).
Net deposition of glycogen deriving from circulating glucose was
calculated in liver and muscle during the intraduodenal glucose infusion as the fraction of glycogen derived from circulating glucose
([14C]glycogen SA/mean
[14C]glucose precursor
SA during intraduodenal glucose infusion) multiplied by the tissue
glycogen content. It should be noted that, in the liver, this
calculation does not include net deposition of glycogen derived from
the indirect pathway. Cold and radioactive liver
glycogen concentrations were the means of biopsy measurements from the
left lateral and left central lobes. Cold and radioactive muscle
glycogen concentrations were the means of measurements from the
gastrocnemius, gracilis, and flexor halucis longus biopsies.
Statistics were performed using SuperAnova (Abacus Concepts, Berkeley,
CA) on a Macintosh PowerPC. Statistical comparison between groups and
over time were made using ANOVA designed to account for repeated
measures. Specific time points were examined for significance by using
contrasts solved by univariate repeated measures. Statistics are
reported in the corresponding table or figure legend for each variable.
Differences were considered significant when
P < 0.05. Data are expressed as
means ± SE.
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RESULTS |
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Top
Abstract
Introduction
METHODS
RESULTS
DISCUSSION
References
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Pancreatic hormone levels.
Arterial plasma insulin levels were similar in the two groups during
the baseline period and rose similarly during the intraduodenal glucose
infusion (Fig.
1A).
Arterial plasma glucagon levels were similar between groups at baseline
and were not significantly affected by glucose infusion in either group
(Fig. 1B).
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Blood flows.
Portal vein, hepatic artery, and external iliac artery blood flows
(Table 1) were similar in 42- and
18-h-fasted dogs throughout the study.
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Plasma levels, rates of appearance, and net gut output of glucose.
Basal blood glucose concentrations (Fig. 2)
were similar between groups. During intraduodenal glucose infusion,
glucose concentrations were consistently higher in all vessels in the
42- compared with 18-h-fasted animals
(P < 0.05-0.001 throughout the
glucose infusion period). The arterial-portal vein glucose gradient
(Table 2), positive in both groups in the
basal period, was markedly negative during intraduodenal glucose
infusion. Gradient values were slightly more negative in the
42-h-fasted animals, although a significant difference between groups
was detected only at t = 150 min
(P < 0.05).
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Hepatic glucose metabolism.
Basal net hepatic glucose output was ~1.8
mg · kg1 · min1
in both groups. During the intraduodenal glucose infusion, both groups shifted to net hepatic glucose uptake (NHGU), with values progressively increasing for the first 60 min of glucose infusion, achieving a
plateau during the final 90 min. There was no difference between 18- and 42-h-fasted animals (Fig.
4C). Net
hepatic glucose fractional extraction was also similar between the two
groups throughout the glucose infusion period (Fig.
4B). The hepatic glucose load was
similar between groups in the basal period but was significantly higher
in the 42-h-fasted animals during the intraduodenal glucose infusion
(P < 0.05-0.001 at
t = 30, 60, 90, and 150 min). Despite similar rates of net hepatic uptake, the intrahepatic fate of the
glucose taken up by the liver was different in the two groups of dogs.
The rate of glucose oxidation was ~35% lower in the 42-h-fasted than
in the 18-h-fasted dogs (Fig. 5).
Conversely, the rate of net liver glycogen synthesis was significantly
higher in the 42-h-fasted than in the 18-h-fasted animals (Fig. 5).
Despite the higher glycogen synthetic rate, the liver glycogen content
was still lower at the end of the intraduodenal glucose infusion in the
42-h-fasted compared with the 18-h-fasted dogs [27.2 ± 5.8 vs.
38.1 ± 6.4 mg/g liver tissue; not significant (NS)] because of
the lower initial glycogen concentration in the 42-h-fasted dogs.
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Hindlimb glucose uptake and fractional extraction, muscle glycogen
content, and net muscle glycogen synthetic rate.
Basal net hindlimb glucose uptake was similar between groups (Table
4). During intraduodenal glucose infusion,
net hindlimb glucose uptake increased similarly in the two groups. Net
hindlimb glucose fractional extraction (Table 4) was also similar in
the two groups throughout the study. The mean glycogen content in skeletal muscle at the end of the experiment was 6.2 ± 0.8 mg/g muscle tissue in the 18-h-fasted dogs and 5.2 ± 0.5 mg/g in
42-h-fasted animals (NS). The net muscle glycogen synthetic rate was
3.8 ± 0.9 mg · g1 · min1
in the 18-h-fasted group and 4.5 ± 1.2 mg · g1 · min1
in the 42-h-fasted group (NS).
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Blood levels and hepatic and hindlimb balances of lactate.
Basal lactate concentrations (Fig. 6) were
significantly lower in the 42-h-fasted dogs compared with 18-h-fasted
dogs in arterial, portal vein, hepatic vein, and common iliac vein
blood. Although both groups displayed a similar pattern of change in
circulating lactate levels during intraduodenal glucose, lactate
concentrations remained significantly lower in the 42-h-fasted animals
throughout the study.
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1 · min1),
whereas 42-h-fasted animals displayed a net uptake of ~8
µmol · kg1 · min1
(Fig.
7A).
During intraduodenal glucose, 18-h-fasted dogs increased their net
hepatic lactate output to ~15
µmol · kg1 · min1.
Although 42-h-fasted animals also became net producers of lactate during intraduodenal glucose, the rates were much lower than in 18-h-fasted dogs (~3
µmol · kg1 · min1).
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1 · min1,
which increased to >30
µmol · kg1 · min1
during intraduodenal glucose (Fig.
7B). Conversely, 42-h-fasted animals
had a basal net hindlimb lactate output of ~20
µmol · kg1 · min1,
which was reduced by intraduodenal glucose but never shifted to net uptake.
Arterial levels, net hepatic uptake and fractional extraction, and
net hindlimb output of alanine.
Basal arterial alanine (Table 5) was lower
in 42- than in 18-h-fasted dogs. The level increased in 42-h-fasted
dogs but was unchanged in 18-h-fasted dogs in response to the glucose
load. Net hepatic alanine uptake (Table 5), although virtually the same
in the two groups of dogs, increased significantly in the 42-h-fasted
animals as a result of the increase in alanine concentration.
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Arterial levels and net hepatic uptakes and fractional extractions
of glycerol and FFA.
Arterial glycerol (Table 6) was
consistently higher in 42- than in 18-h-fasted dogs. Net hepatic
glycerol uptake was moderately elevated in the 42-h-fasted dogs
compared with 18-h-fasted dogs (P < 0.05 at t = 60 and 120 min) despite a
reduced net hepatic fractional glycerol extraction
(P < 0.05 at
t = 15, 30, 90, and 150 min). Arterial
FFA concentrations were similar in 42- and 18-h-fasted dogs except at
t = 15 min, when it was reduced in 42-h-fasted dogs (Table 6). Net hepatic FFA uptake and fractional extraction, lower in the 42-h-fasted dogs at baseline, were similar between the two groups during intraduodenal glucose.
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DISCUSSION |
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Top
Abstract
Introduction
METHODS
RESULTS
DISCUSSION
References
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The results of the present study show how a prolonged fast (42 h) affects the metabolic response to an intraduodenal glucose load. There was a 60% greater increase in the blood glucose response to intraduodenal glucose in 42-h-fasted dogs than in the animals fasted for 18 h. Although the greatest difference in blood glucose concentrations was measured in the portal vein, blood glucose was also consistently higher in the artery and hepatic vein of the animals that underwent a longer fast. The onset of the glycemic response was very rapid. In both 18- and 42-h-fasted dogs, ~75% of the increase in circulating glucose occurred in the first 15 min after the start of the intraduodenal glucose infusion; the remaining 25% occurred in the ensuing 15 min. Subsequent changes in glucose were similar in 18- and 42-h-fasted dogs, but at this time the difference in circulating levels between the two groups had already been established.
The cause of the greater blood glucose response observed in the dogs that underwent a longer fast was not a difference in net glucose uptake by the liver or by the skeletal muscle, as similar rates of these variables were measured in both groups of animals. However, because insulin levels were similar and glucose levels were higher after a more prolonged fast, it is possible that the longer fast duration induced some degree of resistance to insulin or hyperglycemia. This resistance may therefore have been one of the determinants of the greater blood glucose response after prolonged fasting. Part of the greater blood glucose response after the prolonged fast can be accounted for by greater intestinal absorption. This is suggested by the fact that the systemic appearance of intraduodenal glucose, as determined by tracing the rate of appearance of [14C]glucose mixed into the intraduodenal glucose infusate (area under time-course curve, P < 0.01), was higher in 42- than in 18-h-fasted dogs. Net gut glucose output, as determined by direct measurement of arteriovenous differences, also tended to be higher in these animals (area under time-course curve, P = 0.065). Shortly after the start of the glucose infusion, when the greater blood glucose response was established, the absolute values of systemic appearance of intraduodenal glucose and net gut glucose output were similar within each group. Toward the end of the study, on the other hand, systemic appearance of intraduodenal glucose was greater than net gut glucose output. This discrepancy can probably be explained by the recycling of labeled [14C]glucose causing a gradual increase in the systemic rate of appearance of intraduodenal glucose (0-120 min). Because the gradual increase was similar in the 18- and 42-h-fasted groups, the absolute rates rose equally regardless of the fast duration. Net gut glucose output, on the other hand, remained constant in both groups after the initial increase after the onset of the glucose infusion. The kinetic behavior of this variable, therefore, paralleled the time course of the circulating glucose levels, although the difference in net gut glucose output was of a smaller magnitude (~10%). Because, as described below, uptake of glucose occurs in the gut during intraduodenal glucose infusion, net gut glucose output underestimates intestinal glucose absorption.
The rate of net gut glucose output measured in the 42-h-fasted dogs,
when we consider that net gut glucose uptake is ~0.8 mg · kg1 · min1
under hyperglycemic conditions (22), shows that ~65% of the infused
sugar was absorbed as glucose. This is similar to the finding of Moore
el al. (24). These investigators showed that, of the total mass of
glucose infused over 4 h, ~27% was not absorbed as glucose. Of this
fraction, a small amount (<5%) could be accounted for by the net gut
output of lactate, alanine, and glycerol. The authors speculated that
the rest could be accounted for by gastrointestinal glucose oxidation
or by glucose that remained in the gut at the end of the study.
The potential mechanisms that might modulate intestinal glucose absorption after a fast, such as exercise or other physiological stimuli, remain to be determined. Prior exercise is unable to induce a significant increase in the absorption of water and solutes unless carbohydrates are part of the ingested solution (13). It is therefore unlikely that a nonspecific increase in gut permeability might explain the postexercise increase in carbohydrate absorption. The presence of glucose in the gut stimulates its own transport into the enterocytes. This involves Na-dependent glucose transporters on the brush-border membrane (sodium-dependent D-glucose transporter 1 and 2) and the Na-independent transporter GLUT-5 (29). The density and activity of these transporters may be up- or downregulated by transcriptional/translational processes, dietary manipulation (particularly absence or excess of dietary lipids), and pathological states such as diabetes and inflammatory bowel disease (28). Although the regulatory signals responsible for transport induction across the enterocytic membrane are not clearly understood, several gut regulatory peptides, such as epidermal growth factor and peptide YY, have been proposed to have important roles in the control of this process (2).
Among the known determinants of NHGU, the arterial and portal vein
insulin and glucagon levels, as well as the arterial-portal vein
glucose gradient, were similar between the two groups. Only the hepatic
glucose load was higher (~15%) in 42-h-fasted dogs. This did not,
however, provide a sufficient stimulus to enhance NHGU detectably in
42-h-fasted dogs beyond rates in the 18-h-fasted dogs. Virtually all
the glucose taken up by the liver in these studies could be accounted
for by oxidation, incorporation into liver glycogen, or release as
lactate. The total amount of glucose that was disposed of in the liver
by the summation of the above processes was the same in 42- and
18-h-fasted dogs (2.3 mg · kg1 · min1
in either group). When each pathway is analyzed separately, however, marked differences between groups can be seen. The mean rate of net
glycogen synthesis was over threefold greater in 42- than in
18-h-fasted dogs (1.8 vs. 0.5 mg · kg1 · min1).
Conversely, 18-h-fasted dogs displayed a sevenfold greater net lactate
output (1.3 vs. 0.2 mg · kg1 · min1)
and a 60% greater rate of liver glucose oxidation (0.5 vs. 0.3 mg · kg1 · min1)
than 42-h-fasted animals. These data show how prior fast duration can
markedly alter the intrahepatic flow of carbons. Also, our data suggest
that the suppression of net hepatic lactate production is a mechanism
that determines the blunting of hyperlactatemia in response to glucose
administration after prolonged fasting (10). Changes in the
intracellular levels of some key intermediates in the glycolytic
pathway, such as glucose-6-phosphate, may be the basis for the
differences in the intrahepatic fate of glucose that occur with fasting.
The basal liver glycogen content of our experimental animals can be
estimated by subtracting the net hepatic glycogen synthesis, measured
during glucose infusion, from the final glycogen content. In
18-h-fasted dogs, the mean rate of net liver glycogen synthesis was
~0.5
mg · kg1 · min1,
which corresponds to ~2.5 mg of net glycogen storage per gram of
liver tissue over the 150 min of glucose infusion. Subtraction of this
amount from the final glycogen content of 38 ± 5 mg/g yields an
estimated initial glycogen content of ~35.5 mg/g. In 42-h-fasted
dogs, the mean rate of net liver glycogen synthesis was ~1.8
mg · kg1 · min1,
which corresponds to ~9 mg of net glycogen storage per gram of liver
tissue over the 150 min of glucose infusion. Subtraction of this amount
from the final glycogen content of 27 ± 6 mg/g yields an estimated
initial glycogen content of ~18 mg/g liver, or one-half the baseline
glycogen estimated to be present in the livers of 18-h-fasted animals.
These calculated values of basal liver glycogen content are similar to
those reported by other investigators for similar fast durations in the
dog (16, 24). It should be noted that the above calculation considers
only glycogen deposition via the direct pathway and therefore
overestimates the initial glycogen content by the amount of the
incorporation of amino acids and glycerol into glycogen. Although the
net hepatic uptakes of alanine and glycerol were measured, it is
impossible to estimate the percentage that was converted to glucose.
The difference in net uptake of either metabolite, however, was
quantitatively small, and greater uptake, if present, was always
measured in the 42-h-fasted group. This indicates that, although the
absolute value of initial glycogen content may have been overestimated, the estimate of the difference between the two groups was probably reasonable.
Fery et al. (10) previously investigated the response to a 75-g oral glucose load in 14- or 110-h-fasted humans. The arterial glucose level, whole body glucose oxidation, and storage responses to oral glucose in these studies are consistent with our results. These investigators, however, report a delayed and prolonged intestinal absorption of ingested glucose after the longer fast. This difference probably reflects the fast durations studied and the presence of delayed gastric emptying in the experimental model used by these authors (10). In the 110-h-fasted humans, marked changes in basal arterial glucagon (>100% increase), insulin (~60% decrease), and glucose (~45% decrease) occurred that are not present in the 42-h-fasted dogs of the current study. The results obtained in this study in the dog may reflect a response that is related to the depletion of glycogen per se, independent of changes in basal circulating glucose and pancreatic hormones.
Despite the fact that circulating glucose levels were consistently higher in 42- compared with 18-h-fasted animals, arterial insulin levels were similar between the two groups throughout the study. Findings similar to those of the 42-h-fasted dogs were reported by Hamilton et al. (14) in the postexercise state, and others have also shown that glucose-stimulated insulin secretion is attenuated by prior exercise in rats and humans (17, 23). The findings of the present study suggest that this blunting effect on insulin secretion may not be a specific postexercise effect but may result from the degree of glycogen depletion or some other adaptation that is common to fasting and exercise.
Arterial glycerol was higher in 42-h-fasted dogs compared with 18-h-fasted dogs, whereas FFA concentrations were similar between the two groups. This is consistent with observations for similar fast durations (12, 14) and may reflect a blunting of the antilipolytic effect of insulin associated with a higher rate of FFA reesterification after a longer fast (4). Consistent with the difference in arterial glycerol levels between 18- and 42-h-fasted dogs, a longer fast increased net hepatic glycerol uptake but did not affect the net hepatic balance of FFA of the circulating FFA levels.
In summary, prolongation of fast duration before an intraduodenal glucose load resulted in elevated circulating glucose levels and, to a smaller extent, increased glucose absorption from the gut. A prolonged fast did not change NHGU but was associated with increased hepatic storage of glucose as glycogen, as well as with reduced hepatic glucose oxidation and net lactate output. These effects occurred although circulating concentrations of pancreatic hormones were unaffected by the prior fast duration. The metabolic responses of gut and liver were similar to those observed after prior exercise. Net muscle glucose uptake and glycogen synthesis, on the other hand, were not altered by extending the fast duration from 18 to 42 h, as they are by prior exercise. Our data emphasize the role played by the interaction of splanchnic glucose metabolism and nutritional status in the determination of oral glucose tolerance.
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ACKNOWLEDGEMENTS |
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We thankfully acknowledge Wanda Snead, Pam Venson, and Brittina Murphy for excellent technical assistance.
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FOOTNOTES |
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This work was supported by the National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-50277. P. Galassetti was supported by the National Institutes of Health Training Grant DK-07061.
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. §1734 solely to indicate this fact.
Address for reprint requests: P. Galassetti, 702 Light Hall, Vanderbilt Univ. Medical Center, 22nd and Garland Sts., Nashville, TN 37232-0615.
Received 9 June 1998; accepted in final form 19 November 1998.
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REFERENCES |
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Top
Abstract
Introduction
METHODS
RESULTS
DISCUSSION
References
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