Download partition and absorption of volatile fatty acids in the alimentary canal

PARTITION AND ABSORPTION OF VOLATILE
FATTY ACIDS IN THE ALIMENTARY CANAL OF
THE RAT
C. Remesy, C. Demigne
To cite this version:
C. Remesy, C. Demigne. PARTITION AND ABSORPTION OF VOLATILE FATTY ACIDS
IN THE ALIMENTARY CANAL OF THE RAT. Annales de Recherches Vétérinaires, INRA
Editions, 1976, 7 (1), pp.39-55. <hal-00900868>
HAL Id: hal-00900868
https://hal.archives-ouvertes.fr/hal-00900868
Submitted on 1 Jan 1976
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PARTITION AND ABSORPTION
OF VOLATILE FATTY ACIDS
IN THE ALIMENTARY CANAL OF THE RAT
C. REMESY
C. DEMIGNE
Station de Physiopathologie de la Nutrition,
Centre de Recherches de Clermont-Fevvand, I. N. R. A.,
Theix, Saint Genès Champanelle, 63110 Beaumont
RÉSUMÉ
RÉPARTITION
ET ABSORPTION DES ACIDES GRAS VOLATILS
DANS LE TUBE DIGESTIF DU RAT
La répartition des acides gras volatils (AGV) dans le tube digestif et leur absorption ont été
étudiés chez des rats recevant un régime comportant un mélange de céréales et de tourteaux
de soja.
Les concentrations en acide acétique et en acide D ou L lactique ont été comparées entre
les deux zones de l’estomac du Rat : zone sécrétoire et zone non sécrétoire (cul-de-sac). Les concentrations les plus élevées de ces deux acides ont été trouvées dans le cul-de-sac, où le pH est le
,o). Les concentrations en AGV dans les contenus de l’intestin grêle ainsi que leur
5
plus élevé (
absorption sont négligeables.
Des concentrations élevées en AGV ont été trouvées dans les contenus cacaux (environ
0 mM), alors que les concentrations en acide lactique sont très faibles. Le volume caecal et les
6
1
concentrations en AGV varient peu durant le nycthémère mais diminuent rapidement au cours
des premières z
4 h de jeûne Dans le côlon proximal, les concentrations en AGV sont inférieures
à celles du cxcum, en particulier pour l’acide butyrique.
La caecumectomie provoque une diminution considérable des concentrations en AGV dans
le côlon.
La mesure des différences artérioveineuses au niveau de la veine caecale et de la veine du
côlon a permis d’évaluer l’absorption des différents AGV ainsi que la cétogenèse. Il existe une
corrélation étroite entre les concentrations en AGV des contenus cxcaux et les différences artérioveineuses. Chez le Rat, la cétogenèse au niveau de la paroi caecale est faible (
12 p. 100 de l’acide
butyrique absorbé) et nulle au niveau du côlon. Les AGV sont entièrement absorbés sous forme
ionisée, lorsque le pH atteint !,5 chez les rats à jeun. Des mécanismes particuliers au caecum
doivent être envisagés, peut-être en corrélation avec l’importante absorption de sodium à ceniveau.
- ,,
,,,
,
_-,,
.
Volatile fatty acids (VI~A), produced mainly from fermentation of carbohydrates
in the ration, constitute an important source of energy for ruminants. However,
the role of the microflora in the digestion of carbohydrates and the production of
VFA have not been extensively studied in monogastrics animals.
The utilization of carbohydrates in the production of VFA by the microflora
of non-ruminants implies that they reach the caecum in measurable quantities. This
is not the case for simple sugars (fructose, glucose) which are completely absorbed
in the small intestine. However, even with very digestible diets, some food particles
have a very rapid transit and may reach the caecum without having been entirely
hydrolysed or absorbed, as result of gastric discharge and intestinal peristalsis.
This can only occur if the diet is not entirely soluble. In adults rats, certain disaccharides such as lactose are not hydrolysed completely in the small intestine for the
lactase activity falls abruptly in weanling rats HT
). In the absence
W
(I,
,
R
E
c 1973
of enzymatic digestion it is evident that celluloses and membrane sugars also will
reach the caecum.
Sugars of endogenous origin can also constitute substrates for bacterial metabolism. Indeed in germ free rats, the caecum contains considerable amounts of
T, G
D
STF
IND
STAFSON, ig&!).
U
carbohydrates INS
(L
T
D
TE
, I,
In addition to carbohydrates fraction, the microbial flora can utilize proteic
nitrogen for its own synthesis (after hydrolysis and deamination) or also as an energy
This bacterial metabolism results in the production of various compounds
VFA from various substrates.
ANOH and MICI!LS>;N
M
N
R
YANG, A
1970 demonstrated the production of
(
)
VFA in the rat caecum although this rodent is not a strict herbivore. In the rat
the development of the caecum, and consequently that of the flora, varies according
to the diet and is thus to a certain extent controlable. Since a measurable flora is
present in almost the whole length of the alimentary canal, studies were made
of the VFA production in the stomach, small intestine, caecum and colon. Absorption of VFA by the caecum wall in rat has been show by YANG, MnNOxAxArr and
OTTAZ and E
MICI!>;I,S!N
ORB (ig
W
1970 M
(
),
2). We determined the extent of this
7
absorption in each part of the alimentary canal by arteriovenous differences in VFA
concentration compared with the VFA levels in the digestive contents of each region
studied. Ketone body production was also studied by arteriovenous differences in
the caecum and the colon walls. This paper describes a part of a thesis work
SY 1973
E
M
(RE
,
) in which a great number of animals was used.
source.
such
as
EXPERIMENTAL
Animals and diets
All the experiments were carried out with Sherman rats fed to appetite, the dark period was
from 20
00 h to 8.
.
00 h. The diet was commercial pellets from Sanders (
iz6o Juvisy-sur-Orge,
9
France) comprising (parts of weight) : wheat 31
, maize 31
, soya-bean meal (
50 p. 100 protein)
8, molasses 5
2
, salts and vitamins 5
. The overall composition of this diet was : 20 p. 100 crude
protein, 2 p. 100 lipid, 4.! p. 100 crude fiber, 7 p. 100 minerals and vitamins and y p. too water.
Germ-free rats were from C.N.R.S. Laboratories, 9
o Gif-sur-Yvette (France) (Courtesy
m
of Dr Snc!uEr).
Physiological techniques
Cectomy.
n
l
aecu
The rats are anaesthetized with ether. After laparotomy on the white line the caecum was
exteriorized and the attachments of the appendix to the ileum were cut. A sterile cord was knotted
above the ileocaecal valve, the caecum was cut and the caecal walls remaining were carefully
cleaned with sterile saline solution. The ligature stops loss of blood and its impermeability made
a gathered-suture unnecessary. Conjunctive attachments then formed at the level of the cicatrice
without affecting the functionning of the ileocaecal valve. The operation lasted 3 to 4 minutes
and losses from a break in passage are very low (less than 10
p. 100
). Recuperation was very rapid.
The following symptoms resulted from caecumectomy : hypertrophy of the colon, light diarrhea
and slowed growth.
Blood
This
sampling.
carried out on animals anaesthetized with ether. After median laparotomy the ves6 mm). The portal blood was collected
punctured directly using a needle (
5 X 1
.
0
just above the branching of the portal vein, where it enters the liver. Blood from the stomach
was removed from a length of the gastric vein after separating the spleen from the stomach.
The blood draining the caecum was sampled as high as possible before the confluence with the
blood from the ileum and that draining the colon at the limit of the ascending colon. To sample
the blood irrigating the ileum or the colon, the caecal vein had first to be ligatured, in order
to eliminate mixture due to blood coming from the caecum by way of anastomoses.
sels
was
were
<Sampling of digestive contents.
Samples of approximately I g fresh weight from the various parts of
were placed in Eppendorf microtubes and immediately frozen at
C.
0
20
the
alimentary
canal
-
Biochemical
techniques
Plasma V FA.
VFA are extracted from plasma with ethanol in the presence of a known quantity of internal
standard (isobutyric acid). After addition of NaOH and evaporation of the ethanolic solution of
the sodium salts, the residue is dissolved in a dilute solution of orthophosphoric acid to permit
MIGNE, 1974
E
analyses by gas liquid chromatography (REMESY and D
).
Plasma ketone bodies.
-hydroxybutyric and acetoacetic acids were measured in plasma using an enzymatic
3
DLA_VBY and KREBS, 19
L
method (!’ILLIAMSOLB, ME
62).
VFA
the
digestive contents.
To test whether a single centrifugation was sufficient to extract the VFA from the precipitate, a comparison was made of the results after distillation and after centrifugation of a mixture
from
of caecal contents.
Steam distillation. As for measurement of plasma VFA, isobutyrate was chosen as internal
standard since there are only traces in the caecal content. Distillation was by the following
5 p. 100
5 ml 8
.
procedure : I g of fresh material was diluted in 20 ml of water, acidified with 0
(w/v) 4
P0 and I ml 40 mM/1 of Na isobutyrate was also added before distillation. One hunS
H
dred and fifty ml of distillate were collected in a beaker containing excess sodium hydroxide.
The distillate was evaporated and redissolved in 2 ml of water and o.I ml I
o p. 100 (w/v) H
4
o
p
3
before gas chromatography.
of the digestive contents were centrifuged in Eppendorf microtubes
Centrifugation.
after storage at
C to obtain the maximum volume of supernatant. To one volume of super0
20
1 volume of 10 p. 100 (w/v) H,P0
.
natant (
4
50 !,1), I volume of 40 mM internal standard and 0
were added. If the results obtained by distillation are expressed as a function of the caecal
water (!!-8o p. 100 of the fresh weight), similar concentrations and percentages are obtained
in both cases. The analyses of the digestive contents were therefore made using centrifugation.
Samples
-
Lactic acid
from
the
digestive
contents.
D and L lactic acid from the digestive contents were measured after centrifugation and
dilution of the supernatant (
40
4 M (
.
19 v). The enzymatic determination was made
I v) in -’C’0
with L-lactate dehydrogenase (EC 1
. 27
8) from
2
.
) or D-lactate dehydrogenase (EC 1
OHORST procedure 6
H
S
19 but with a diluted buffer (
(
)
5
25 M glycine,
.
0
Boehringer, according to ’
20 M hydrazine and 0.
.
0
03 M S
CO
2
K
’
)
RESULTS
Stomaclc and small intestine contents
Sto!nach.
samples were taken in the morning after separation of the complete gastric
parts : one from the translucent zone (non secretory region)
and the other from the opaque zone (secretory region).
The VFA in these two parts of the stomach were analysed (see table z). There
were significant differences (P <
o.ooi) between the two zones for acetic acid
6.o mM compared with 13
(translucent zone 5
9 mM in the opaque zone) and for pH
.
.o and 3
5
(
.6). Only traces of the other VFA (propionic and butyric acids) were present. The difference between the dry matter percentage measured in the two regions
4 p. 100 and opaque zone 3
.
i.o p. 100
(translucent zone 45
) was not sufficient to be
of any importance. The lactic acid in two parts of the gastric content was measured.
The concentration was 141
1 mM in non secretory region (6
.
5 p. 100 of I,-isomer)
and 4
mM
in
the
100
of
secretory region (55 p.
8.
3
I,-isomer). Thus the same gradient
The
contents into two
occured
as
for VFA but the concentration of lactic acid
was
greater.
Small intestine.
Only small quantities of acetic acid were found in the duodenum (
5 mM)
.
1
and only traces in the jejunum. Although concentrations of this acid were higher
in the ileum they remain relatively low (6.
1 mM) (table z).
Caecum contents
The VFA concentrations were practically uniform in the entire caecal contents
the values obtained in the appendix and near the ileo-caecal valve indicate.
The lactic acid concentrations were always very low ( < 5 mM) in contrast to those
in the gastric contents (table 2
).
as
Caecal
development.
The caecum of young rats developed very rapidly after weaning and its relative weight reached a maximum in I
oo g rats (
7 p. 100 of body weight against
.
2
. p. 100 for 100 g rats and i.
2
o g rats). The differences between 100 g
9 p. ioo for 33
rats and 330 g rats
were
not
significant.
Diurnal variations.
Diurnal variations in the relative weight of the caecum were negligible. For
in 1
0 g rats the variation was from 2
6
00 h to 2
.
. p. 100 at
4 p. 100 at 10
.
i8.oo h. Between the groups of rats used during the year VFA concentration remained
very stable, particularly in the morning, but a certain variation may be noted in
the evening when the animals began to go hungry (table 3
).
example,
Fasted animals.
The animals were deprived of food in the morning and killed after 24 or 4
8 hours.
A considerable fall in VFA concentrations resulted during the first 24 hours (more
than 6
0 p. ioo). Thereafter, as the starvation was prolonged, the VFA concentrations tended to diminish more slowly. In addition the percentage of butyric acid
diminished noticeably, from 15 to 10 p. 100
. The decrease of VFA concentration
1 after
.
1 and 7.6 ! 0
.
gives rise to an increased pH of the caecal contents : 7..! ! 0
24 or 4
8 hours (table q.).
Germ-free
In
aszimals.
rats we determined that there
traces of acetic acid (table 4
).
germ-free
ting only
Colon content
was no
VFA in the
caecum
excep-
and faeces
Animals fed and fasted.
The concentrations of VFA in the contents of the first part of the colon was
of the caecum in the same animals. During the passage of the
. 4 mM.
contents in the colon, the VFA level decreased from, for example, 1
4 to 112
7
8
This was particularly clear for butyric acid, the percentage of which decreases
from 14
2 p. 100
.
. This phenomenon was also found after fasting (total VFA :
1 to 10
.
caecum 4
o.imM for a 4
51
, and colon 3
8 hours fast) (table 5
). These differences cannot
be explained by variations in the levels of dry matter, which were very similar in
the caecum and the proximal colon.
compared with those
Caeciimectom
,
3
.
Samples were taken from the colon and the ileum (and also in the caecum of
controls) 21 days after the caecumectomy. The reduction of VFA levels in the colon
was considerable and the proportions of propionic and butyric acids were low and
unstable whereas in the ileum the VFA concentrations were always very low (table 6).
Faeces.
VFA concentrations, expressed per gram of dry matter, are about 75 p. 100
of those found in the colon content: the proportion of each acid were similar to those
of caecum and colon contents (table 2
).
A bso
j5tion
y
bil di!’erent Parts of the alimentary
canal
Stomach aaad small intestine.
The low concentrations of acetic acid (6.
2 mM) observed in the pyloric region
shows that VFA absorption is very active in the stomach. Difference between
gastric vein blood and arterial blood is 0
35 mM for acetic acid. Propionic acid and
.
acid
were
when
noticeable
butyric
present only
quantities of these acids were measured in the gastric contents. The absorption of VFA in the small intestine appeared
negligible (ileal vein o.i
3 and artery 0
15 mM) to the point that it may be masked
.
by metabolism of the walls.
Caecum and colon.
In rats VFA absorption occured to a considerable extent
and colon. The arteriovenous differences for these two organs
only
were
in the caecum
determined in
with the concentration of the digestive contents. The arteriovenous
differences were proportional (r > 8)
97 to the respective concentrations of VFA
.
0
in the caecal contents (fig. i). This good correlation indicates that the blood flow is
proportional to the caecal volume. As in the caecum, there was considerable VFA
absorption particularly of butyric acid by the wall of the colon (table 7
).
comparison
Ketogenesis b
v
the
caecum
and colon walls.
Since the caecal contents contained no ketone bodies the arteriovenous diffefound for D-hydroxybutyrate and acetoacetate resulted from meta3
bolism of VFA (especially butyrate) by the caecal wall. The quantities of acetorences
and 3
-hydroxybutyrate produced were quite similar. Together they
represented 12 p. 100 of the butyric acid absorbed at the level of the
caecum (table !).
Total absorption
acetate
Diurnal variations and
influence of fasting.
proportions of VFA were absorbed by the portal and caecal veins,
the
indicating
important role of the caecum.
The greatest reduction in absorption occured during the first 24 hours of starThe
same
vation, thereafter, it occured more slowly. The portal arteriovenous differences varied
with the concentration of VFA in the caecum contents : the absorption of butyric
acid was particularly reduced (table 8 and fig. i). The arterial levels of acetic acid
was
nous
modified very little, which indicates that acetic acid
origin (A!rmsorr and WHITE, i962).
Influence of caecumectomy.
This operation permitted
can
be
equally
of
endoge-
estimation of the part due to the caecum in the
vein.
Caecal ablation was very rapid and it was
absorption by
portal
to
after
one
take
about
minute, as for the controls. The acetic acid
possible
samples
levels in the portal vein dropped from 0
21 mM and those of butyric acid
.
51 to 0
.
mnI
in
the
became
difficult
to
Since it was absent from artedetect.
8
.
0
(
controls)
rial blood and little metabolised by the caecum wall, propionic acid in portal blood
permitted a correct estimation of the absorption by the caecum : o.2r mM in the
control, but reduced to 0
07 mM in operated animals. Thus, it can be estimated
.
that the caecum absorbed 2/3 of the VFA.
When rats had been caecumectomized for 2
i days, absorption was much reduced
and did not equal that of the colon in normal animals since fermentations in the colon
were depressed in animals which had undergone a caecumectomy (table 6).
total
an
the
DISCUSSION
Stomach
In the rat, the presence of an important non secretory region with an
wall is extremely favorable for the development of fermentations and
the content of the cul-de-sac has a longer transit time.
epithelial
probably
In the stomach, lactic acid is found in greater quantity than acetic acid :
mM of acetic acid compared with 141
1 of lactic acid. The development of a
.
microflora in the stomach may depend on the heterogeneity of the gastric contents
as confirmed by pH levels, which were sufficiently high to permit development of
an acidophile flora. The fermentations were no doubt greatest in certain small parts
of the stomach and were facilitated by the presence of complex coarsely ground
rations. The concentrations of both acetic and lactic acids were much greater in
the cul-de-sac zone than in the secretory zone. In the rabbit, H
ENNING and
IRD (
H
1972 a) showed diurnal differences in the division of acetic acid between he
fundic and cardiac zones. The extent of coprophagy in our animals was not determined but BARN!s et al. )
. It is possible
1957 estimated that it could reach 50 p. 100
(
that extensive coprophagy may increase fermentation for the ingested faeces provide
a source of VFA, of substrates and of microbes. The reingested faeces, found mainly
in the cul-de-sac, did not become homogenised with the gastric contents immediately. They can therefore, constitute fermentation centres in which the pH,
substrates and microorganisms differed from those of the gastric liquid.
6.o
5
Small intestine
In the small intestine, the sudden increase in pH caused by the pancreatic and
intestinal secretions and the antiseptic effect of the bile salts stopped the acidophilic
fermentations. SMITH and T
AYLOR (i
j8) discovered that VFA could be transported
9
across the duodenum mucosa of the rat even against a concentration gradient.
The extent of this transport was probably not very great since VFA production
in situ was negligible. There was no significant increase in acetic acid in the ileal
vein (
13 mM in the vein, compared with o.1 mM in the artery) which indicates
.
0
that the small amount of acetic acid absorbed is completely metabolised by the
intestinal wall.
Caecum
Types of fermentations.
The large volume of the caecum and its remarkable constancy of weight for a
given diet show that transit is efficiently regulated in this part of the alimentary
canal. The stable conditions in this organ are very favorable for the development
of a flora adapted to the substrates coming from the small.intestine. As a result, VFA
concentrations tended to remain rather constant throughout the day and over long
periods when the animals were kept in stable conditions. These observations suggest
that mechanisms may exist which regulate the still little studied caecal production
of VFA in monogastrics. It is not impossible that high concentrations of VFA
are capable of inhibiting bacterial metabolism. In addition, interactions between
different types of bacteria (fusiform and coliform bacilli) may be induced by the
VFA (LEE and GE
, 2).
L
E
MM
ETE
(F
97 However this is a point of some controversy R
i
and AB
AMS, I9!2).
R
Germ-free animals show a considerable increase in caecal volume accompanied
by atrophy of mucosa and musculosa (AsArto, 19
). The role of VFA, known
9
6
to have an effect on the morphogenesis of rumen epithelium (SANDER et al., 1959
),
have not been yet studied.
It is of interest to note that the constancy of the caecal volume and caecal
VFA concentration may permit a rapid estimation of the extent of bacterial digestion
with a given diet. A similar relationship has been demonstrated between production
of VFA in the rumen and either concentration (L
RETT 19
B
ENG and ,
66) or pool
size estimated by concentration of VFA and rumen volume (W
ESTON and HoGAN,
The
concentrations
found
in
in
contents
of rats on the
caecal
the
the
g68).
I
morning
described diet (acetic acid : 90
mM
and butyacid
I20 mM ; propionic
50
40
ric acid : 20
from
and
those
A
M
NOHARAN
YANG,
o mM)differ
3
reported by
who
found
a
of
acid
than
of
MrcK!t,s!v )
butyric
greater proportion
1970
(
propionic
acid. Lower concentrations are often found in the rumen and the caecum of ruminants (P
ACKSON 19
J
TT, MY!RS and ,
ACKE
66) and proportions of V FA vary simiRAS and KA
F
R
, 19
Y
larly in the caecum and in the rumen (O
).
9
6
,E
RSKOV
Metabolism
of VFA by
the walls
of the alimentary canal.
1972 b, c) showed in vivo with
(
IRD
ENN and H
H
G
N
i
rabbit that the caecal wall
transforms only a small fraction of radioactive butyrate into ketone bodies (about
ORBE
13 p. I
oo). These workers found greater ketogenesis in vitro, and similarly W
TAZ )
T
O
and M
1970 noted considerable ketogenesis from butyric acid in rat in vitro.
(
Our results obtained by arteriovenous differences after a short anaesthesia (
2 mn)
in the same conditions as for the VFA show that there was little ketogenesis in the
caecal wall. Transformation of 12 p. I
oo of butyric acid gives almost the totality
of the ketone bodies produced. Although high concentrations of VFA exist in the
colon content we found that there was no ketogenesis by the walls. These results
IRD (
NNING and H
agree with those of HE
1972 c) in the rabbit, which indicated that
the reduction in activities of enzymes of ketogenesis in the colon is correlated with
the distance from the caecum. However, it is probable that in rats this reduction
is more marked. In contrast with ketogenesis, G
IRD (
ENNIN and H
H
1972 b) found
that there was no regular reduction in the oxydation of butyrate to CO, along the
colon and it was not correlated with the concentration of VFA in the lumen.
Absorption.
The arteriovenous differences do not give quantitative measurements of the
absorption of different acids since the rate of blood flow and the extent of metabolism
in the wall were not known. The close parallel between VFA concentrations in the
caecal contents and the arteriovenous differences in rats shows that absorption occurs
in the direction of the concentration gradient and is not affected by metabolic
inhibitors ,
RS JeCKSOIV and PACK!TT, Ig6!). Classic results (DANIE
E
Y
(M
, 1945)
LLI
showed that the free forms of VFA traverse lipoproteic membranes of rumen
epithelium more easily than the dissociated forms.
The decrease of pH probably increases VFA absorption, at least in the rumen
(AAFJES, 7
g.
I
)
6 However in the caecum the VFA are always absorbed, although
they are entirely in ionized form (pH about 7
), in fasting animals for instance. In
5
.
addition, a pH increase does not cause any modification of the proportion of the
VFA absorbed, these proportions are the same as in the caecal contents. No completely satisfactory mechanism for VFA transport in the caecum of rats has been described. The most precise investigations concern jejunal absorption in the rat (SMITH
and TAYLOR, 1958 ; Jecxsorr, 1974). Some observations suggest involvement of
sodium transport in absorption of VFA in ionized form, as this sodium transport in
R
R
A
Z
IO,
E MiLLER, ENGEI,particularly important in the hind-gut (F,DMONDS, ig7q ; N
A75).
H
9
1
RDT,
No clear differences exist between the absorption of acetic, propionic and butyric
acids could be observed. ETT,
RS J
E
Y
M
ACK ,
P
ACKSON 66)
19 showed that in sheep
(
acetic acid was more utilised by the caecal wall than was propionic acid. In the rat,
acetic acid metabolism by the caecal wall is probably small in absolute values ; the
metabolism of 12 p. 100 of the butyric acid to ketone bodies shows that it penetrates the caecal wall most rapidly (if we assume the ketogenesis is only from buty-
rate).
The concentrations found in the caecal vein represent about 2 p. 100 of those
in the caecal contents although M
OTTAZ and Woxsx )
1972 cited figures of about
(
from
in
.
100
This
could
result
differences
the rate of blood flow during
4 p.
divergence
the sampling. The same parallel between the concentrations in the caecum and in
IRD (
the caecal vein was observed by H
ENNING and H
1972 c) in the rabbit.
In the colon in the absence of ketogenesis butyric acid is still absorbed the most
rapidly and acetic the most slowly. This suggest that the mechanisms of absorption
are different in the caecum and the colon.
The
influence of fasting.
The reductions in concentration which we measured are significant only if the
reduction in caecal volume is taken into account. YANG, M
ANOHARAN and MiCKELSEN
1970 found that during fasting the reduction in the quantity of VFA present in
(
)
the caecum could be described by exponential functions. The presence of residual
amounts of VFA in the caecum of animals subjected to a prolonged starvation poses
the problem of the origin of the substrates used for these fermentations. They may
arise from degradation of endogenous mucoproteins, passage of various molecules
from the blood into the alimentary canal and also recycling of various substrates
after coprophagy. As in those fed normally, in fasted rats the relativeabsorption
of the VFA depends on the concentration within very wide limits, despite an increase
of the
pH.
Colon
Caecumectomy, in spite of a dilation of the colon and slowing of the transit,
resulted in very low levels of VFA in this organ. Modifications of colic fermentations
suggest that caecum VFA have effects on the nature of the colon flora. In addition,
there may be considerable lysis of bacteria in the Rat’s colon as in Rabbit’s, which
may be due to a lytic compound (B
ONNAFOUS and RAy!rAUn, 19
68).
Im!oytayace of
VFA
as an
energy
source.
The VFA appear to provide a regular source of energy which reduces the sudden
fluctuations in the supply of nutrients after feeds. In addition the nutrient source
is equilibriated since VFA provide both glucogenic and lipogenic precursors. It is
difficult to establish exact balance accounts since the rate of flow and fluctuations
of the portal vein were not determined. VFA concentrations in the portal vein is far
from negligible in rats compared in similar conditions with ruminants : I to 3 mM of
acetic acid in ruminants compared with 0
.8 mM in rats (A
5 to 0
.
SON, HILL and
NNI
I,!WIS, 1957 ; BENSADOU
DINE and RF
A
L
PA
, S
N
, ig6z ; CooK and MILLER, 1965).
ID
Comparison of these figures must take into account possible difference in the metabolism of acetic acid by the rumen and caecum walls.
R and E
A
H
O
N
ISEN
M
L
CK )
YANG, MA
.7 p. ioo the contri1970 estimated at 4
(
bution of VFA to the digested energy in the rat with a diet comprising mainly
ground maize and soya-bean meal : in this diet certain carbohydrates of soya-bean
are probably the main substrates for caecal fermentations. The importance of VFA
AN S
OEST and YOUNG 6
depends on the diet since KEYS, V
19 found that lucerne
(
)
9
hemicellulose could have greater than 40 p. ioo digestibility when this species made
0 p. 100 of the diet. The energetic importance of VFA is much greater in rumiup 5
nant (ANNISO!r and ,
RMSTRONG 1970
A
) but the extent of caecal fermentations are
certainly comparable for rat and ruminant in which the fermentative capacities are
limited
Tlac role
RSKOV
(0
et al., ig7o).
of fermentation.
In rats the development of fermentations appears to be linked with the presence
of easily fermented carbohydrates as the period of transit is relatively short. The
action of the flora increases the energy available to the animal since the poorly digestible carbohydrates are transformed into VFA. These VFA are almost completely
absorbed in the colon and faecal looses are small.
In addition, the presence in the caecum of highly fermentable substrates such
as starch results in increased faecal loss of nitrogen of bacterial origin in both rat
and sheep (MASON and E
SKOV
et al., zg
ALM ig
P
,
R
73
; 0R
o). The origin of the nitro7
gen incorporated into bacterial cells has not been determined. It is possible that
part of the excretion of urea occurs in the caecum, reducing the extent of urinary
excretion of nitrogen. The exchanges between the pools of free amino acids in the
plasma and in the caecal and colon contents are poorly understood. Since the concentration of free amino acids in the colon and the caecum is weak (CoMS! and PION,
ig66), extensive exchanges between the plasma and digestive contents could occur
only if there are mechanisms for active or facilitated transport.
More complete balance sheets of caecal fermentation need to be established and
the influence of quantity and type of substrate determined including that of poorly
digestible proteins which escape enzymatic attack (membrane or tannined proteins,
hexosamines).
Requ pour publication
en
ddcembre 1975.
SUMMARY
The
partition and absorption of volatile fatty acids (VFA) have been studied in the rat
purified diet comprising mainly a mixture of cereals and soya-bean meal.
. The concentration of acetic and lactic acid was compared in non secretory (cul-de-sac)
2
and secretory regions of the stomach. The greatest concentrations of both acids were found in
the cul-de-sac region where the pH was highest (pH 5
).
0
.
. Concentration and absorption of VFA were negligible in the small intestine.
3
i.
using
a non
High VFA concentrations were found in the caecal contents, while lactic acid concentrations were lower. The volume and concentrations of the caecum contents varied little during the
4 h of fasting.
day and decreased rapidly during the first z
The concentrations of VFA, particularly that of butyric acid, were lower in the proximal
colon than in the caecum. Caecumectomy caused a considerable decrease in VFA concentra
tions in the colon.
6. Arteriovenous differences in VFA and ketone bodies were studied for the caecal and colun
veins in rat. A close parallel between VFA concentration in the caecal contents and the arteriovenous differences was observed. In rat, ketogenesis was weak in the caecal wall (
12 p. 100
of butyric acid absorption) and negligible in the colon wall. VFA are absorbed entirely in ionized
form when caecal pH attains 7
5 in fasted rats. Special mechanisms for caecal absorption should
.
therefore be considered, perhaps linked with the large sodium transport in the caecum.
The relatively high VFA concentrations measured in the portal vein suggested that
they are important in the digestive processes.
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!VILLIAmsoN