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*
U.S. Department of Agriculture/ARS Childrens Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030 and
Department of Medical Physiology, The Panum Institute, University of Copenhagen, DK-2200, Copenhagen, Denmark
3To whom correspondence should be addressed at 1100 Bates Street. E-mail: dburrin{at}bcm.tmc.edu
| ABSTRACT |
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KEY WORDS: intestinal growth bolus feeding continuous feeding glucagon-like peptide-2 peptide YY gastric inhibitory polypeptide
| INTRODUCTION |
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Glucagon-like peptide-2
(GLP-2)4
and peptide YY (PYY) have been implicated as humoral signals that
mediate the intestinal trophic effects of enteral nutrition. PYY and,
to a lesser extent, GLP-2 have also been reported to suppress gastric
motility, gastric and pancreatic secretion and thus are implicated as
putative endocrine signals in the ileal brake phenomenon (Holst 1997
, Pappas et al. 1986
, Wojdemann et al. 1999
). Both PYY and GLP-2 are produced locally in the
endocrine L cells distributed along the distal ileum and colon
(Adrian et al. 1985
, Polak et al. 1971
).
Feeding is a potent stimulus for the secretion of both GLP-2 and PYY
into the circulation (Xiao et al. 1999
). We recently
showed in neonatal pigs that circulating concentrations of GLP-2 and
PYY are directly correlated with the levels of enteral nutrient intake
as well as with gut growth (Burrin et al. 2000a
).
Moreover, there is evidence that GLP-2 and, to a lesser extent PYY have
a trophic effect on the gut, including in neonates (Burrin et al. 2000b
, Gomez et al. 1995
, Lovshin and Drucker, 2000
). Another gut peptide secreted in response to
feeding, gastric inhibitory polypeptide (GIP), is produced in the K
cells, which are located mainly in the duodenum (Tseng et al. 1993
). GIP not only stimulates the secretion of intestinal
glucagon-like peptides (Roberge and Brubaker 1993
)
but also may have an indirect growth-promoting effect via
stimulation of insulin secretion (Fehmann et al. 1995
).
We hypothesized that bolus feeding compared with continuous feeding is
associated with a higher secretion of gut peptides, especially GLP-2
and PYY. Furthermore, we hypothesized that the pattern of secretion of
GLP-2 and PYY would parallel that of GIP.
| METHODS |
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The protocol was approved by the Animal Care and Use Committee of Baylor College of Medicine and was conducted in accordance with the National Research Councils Guide for the Care and Use of Laboratory Animals. The study involved five 30-d-old female crossbred piglets (Large White x Hampshire x Duroc) purchased from the Texas Department of Criminal Justice, Huntsville, TX. The pigs were received at 2 wk of age and fed a cows milk replacer formula (Litterlife; Merrick, Union, WI), at a rate of 50 g · kg body-1 · d-1. The composition (per kg dry matter) of Litterlife is 500 g lactose, 100 g fat and 250 g protein. The calculated energy density is 18 MJ gross energy/kg dry matter. The formula powder was thoroughly mixed with water before feeding to achieve 215 g powder/L.
Study design.
The surgical procedures and postoperative care have been described
previously (Stoll et al. 1999
). In brief, at a postnatal
age of 3 wk, piglets underwent surgery after being deprived of food
overnight. Catheters were implanted into the stomach, the portal and
jugular veins and the common carotid artery. An ultrasonic blood flow
probe (Transonic Systems, Ithaca, NY) was placed around the portal
vein. After surgery, pigs received intravenous nutrition for 2 d.
Weight gain was restored to presurgical rates within 4 d after
surgery.
At a postnatal age of 30 d, the piglets were deprived of food from
1800 to 700 h. Baseline (time = 0) arterial and portal blood
samples were taken, and the portal blood flow (PBF) was measured for 30
min. In a randomized, crossover design, the piglets received either a
bolus feed containing one third of their daily intake (
80 mL/kg) in
their feeder on study day 1 or a priming feed of one twelfth (
20
mL/kg) of their daily intake, directly followed by a continuous
intragastric infusion at a rate of one twenty-fourth of their daily
intake per hour (
10 mL/kg) on study d 2. Importantly, the total
formula intake during the 6-h period was the same (80 mL/kg) in both
treatment groups. Arterial and portal blood samples were drawn at
hourly intervals until 6 h from the start of the feeding. PBF was
measured continuously.
Sample preparation.
Blood samples were drawn into EDTA (4.5 mg) tubes, mixed gently and
immediately centrifuged at 2000 x g at 4°C for
10 min to obtain plasma. The chilled plasma samples were quickly frozen
in liquid nitrogen and stored at -70°C until analysis. All samples
assayed for a given hormone were run in one assay. Plasma GLP-2
concentrations were quantified using a specific N-terminal
radioimmunoassay as described previously (Hartmann et al. 2000
). In short, plasma samples were extracted with 75%
ethanol (final concentration) and centrifuged at 3000 x and 4°C
for 30 min. The supernatant was decanted, lyophilized, and
reconstituted to the original plasma volume in assay buffer of 80 mmol
sodium phosphate/L buffer, pH 7.5, containing 1 g
valine-pyrrolidide/L (courtesy of Novo Nordisk, Bagsvaerd, Denmark),
0.1% wt/vol human serum albumin (ORHA; 20/21, Behring, Marburg,
Germany), 10 mmol EDTA/L and 0.6 mmol thimerosal/L (Sigma-Aldrich
Chemical, St. Louis, MO). For standards, we used human GLP-2, and the
tracer was bovine GLP-2 with a Thr12
Tyr12 substitution,
125I labeled according to the chloramine-T method.
Approximately 300-µL extracted samples and human GLP-2 standards were
incubated with 100 µL rabbit GLP-2 antiserum, code no. 92160 (final
dilution 1:25,000), raised against an N-terminal fragment of human
GLP-2; this antiserum specifically recognizes the N-terminal region
of both the human and porcine GLP-2. Free and bound moieties were
separated with plasma-coated charcoal (E. Merck, Darmstadt,
Germany). The experimental detection limit of this assay is <5 pmol/L,
and the intra-assay coefficient of variation is 5% at a
concentration of 40 pmol/L. PYY and GIP were measured using a
commercially available radioimmunoassay kit specific for the porcine
peptides (Peninsula Laboratories, San Carlos, CA). Plasma samples (100
µL) and porcine PYY standards were incubated with rabbit PYY
antiserum (final dilution, 1:10,000) raised against the full-length
porcine PYY (136) peptide. Porcine 125I-PYY was used as a
tracer. The sensitivity of this assay is
5 pg/tube, and the
intra-assay coefficient of variation was 15%. GIP was measured in
plasma samples (100 µL) incubated with rabbit antiserum raised
against porcine GIP and compared with porcine GIP standards. Porcine
125I-GIP was used as a tracer. The sensitivity of the assay
was 25 pg/tube, and the intra-assay coefficient of variation was
7%.
Statistical analysis.
All values are shown as the means ± SE. Differences in peptide production rates between the feeding modalities were assessed by one-way ANOVA. Differences in PBF and arterial peptide concentrations during the 6-h sampling period were analyzed by repeated measures ANOVA. A difference of P < 0.05 was considered statistically significant.
| RESULTS |
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There was a significant increase in PBF in response to feeding
(Fig. 1
) that occurred 2 h after the start of the feeding. However, the
increase was not related to the modality of feeding. On average, PBF
was elevated to
130150% of baseline, reaching a maximum at 3 h in the bolus-fed group and at 6 h in the continuously fed
piglets. PBF remained above baseline flow rates from 2 h
throughout the entire study period, with no significant differences
between the two feeding modalities at any time.
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The baseline arterial and portal concentrations of GLP-2 were 18
± 8 pmol/L (bolus-fed pigs) and 26 ± 10 pmol/L
(continuously fed pigs). The largest increase in concentration occurred
within 1 h after the start of the feeding. The concentration at
1 h was significantly different from the baseline concentration in
both groups (P < 0.001, Fig. 2
). No differences were found in GLP-2 concentrations between the two
feeding modalities during the 6-h experiment. Hourly production of
GLP-2 by the portal drained viscera was calculated on the basis of the
difference in arterial and portal concentration multiplied by the PBF.
Almost identical production rates for the two groups were obtained for
GLP-2 measured over 6 h (Table 1
).
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The pattern of PYY secretion was similar to that of GLP-2 (Fig. 2)
.
However, the magnitude of change in arterial concentrations after the
start of feeding was much smaller. Although GLP-2 concentrations
increased fourfold to eightfold, PYY concentrations increased by only
50%. Again, we found a significant increase in concentration within
1 h, independent of feeding modality (P < 0.01).
Neither arterial nor portal concentrations were different in response
to either bolus or continuous feeding. The total PYY production rate by
the PDV was not significantly different between feeding modalities
(Table 1)
.
GIP.
Arterial GIP levels increased significantly within 1 h of feeding
and remained elevated throughout the experiment in the continuously fed
and bolus-fed piglets (P < 0.01, Fig. 2
). The
magnitude of response to feeding resembled the response of GLP-2, with
a fivefold to ninefold increase above baseline concentrations. The
total GIP production rate by the PDV was not significantly different
between groups (Table 1)
.
| DISCUSSION |
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Although the K and L cells producing either GIP or GLP-2 and PYY,
respectively, are found at different sites along the gastrointestinal
tract (Adrian et al. 1985
, Polak et al. 1971
, Tseng et al. 1993
), all peptide
concentrations increased significantly within the 1st h. That GIP
concentrations would rise within 1 h was predictable, because K
cells are located in the proximal small intestine (Knapper et al. 1995
). However, it appears unlikely that within 1 h
enteral nutrients would come in direct contact with the L cells located
in the distal bowel, which implies that other mechanisms are probably
involved in the secretion of GLP-2 and PYY. Indeed, GIP stimulates
GLP-1 secretion from the L cell, and thus probably GLP-2 as well
(Roberge and Brubaker 1993
). If so, it seems that GIP
has a differential effect on L-cell hormone secretion, because the
pattern of secretion of PYY was clearly different from that of GLP-2.
Besides the endocrine system, the parasympathetic nerves and the
adrenergic system may play a role in the secretion of these gut
peptides (Rocca and Brubaker 1999
, Sheikh et al. 1989
). Thus, it is evident from the present study that direct
interaction of nutrients with the L cells per se is not required to
obtain a surge in GLP-2 or PYY secretion. Furthermore, based on the
differences in the rise in arterial concentrations of PYY and GLP-2, it
is likely that PYY secretion, although produced in the same cell as
GLP-2, is regulated quite differently than GLP-2 secretion.
Another striking difference between the secretion pattern of GLP-2 and
GIP with that of PYY was the magnitude of the increase after feeding.
Although maximal PYY concentrations showed a 50% increase, GLP-2 and
GIP increased fourfold to eightfold. The magnitude of increase in GLP-2
concentrations after a meal in our study was higher than that found in
adult humans (Hartmann et al. 2000
, Orskov and Holst 1987
, Xiao et al. 1999
), whose GLP-2
levels increased 1.3- to 4-fold. This might be a consequence of the
stage of development, because circulating concentrations of PYY
decrease with age (Adrian et al. 1986
). However, the
increase in GLP-2 levels in bolus-fed pigs was even greater than
the difference that we found previously between neonatal pigs fed
either enterally or parenterally (
2-fold) (Burrin et al. 2000a
). GIP secretion increases threefold in response to
feeding in
8-wk-old pigs to a maximum of 400 pmol/L, yet the
relative increase in the postfeeding plasma concentrations of our
younger piglets was much higher.
The circulating concentration of these gut peptides is determined not
only by the production rate but also by the clearance rate. Recent
evidence shows that the kidneys are the major site of GLP-2 clearance
(Tavares et al. 2000
). That neonates have lower renal
clearance rates is well known and might explain the higher levels we
found in our neonatal piglets compared with those of older mammals. We
are not aware of any studies that measured the production of gut
hormones by the PDV, so we cannot determine whether the relatively high
concentrations of hormones we found were due to a lower clearance rate
or a higher production rate in the neonatal pig.
In conclusion, we did not find a significant difference in the concentrations of GLP-2, PYY or GIP in response to a primed continuous versus bolus feeding. Moreover, the overall production rate of these trophic peptides by the PDV was not significantly different. Thus, if indeed bolus feeding is more trophic to the gut mucosa than continuous feeding, the response does not appear to be mediated via acute differences in GLP-2, PYY or GIP secretion.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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2 J.B. van G. was supported by the Sophia Foundation for Scientific Research, the Nutricia Research Foundation
and the Royal Netherlands Academy of Science and Arts (Ter Meulen Fund). ![]()
4 Abbreviations used: GIP, gastric inhibitory peptide; GLP-2, glucagon-like peptide-2; PBF, portal blood flow, PDV, portal-drained viscera; PYY, peptide YY. ![]()
Manuscript received September 11, 2000. Revision accepted November 30, 2000.
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