Isfahan University of Technology
Isfahan, Iran
Advanced
Digestive Physiology
(part 5)
By: A. Riasi
(PhD in Animal Nutrition & Physiology)
http://riasi.iut.ac.ir
Absorption of ammonia

An optimal growth rate of ruminal microbes requires
ammonia concentrations between 3.5 and 6 mmol/l

A diet rich in rapidly degradable proteins can easily raise
ruminal ammonia concentrations up to 20 mmol/l.

35-65% of ammonia is absorbed across rumen epithelium.

About 10% of ammonia flows into the omasum and can be
absorbed there.
Absorption of ammonia

The majority of ammonia is present in the form of
NH4+ at physiological rumen pH.

Quinidine sensitive NH4+ and K+ permeable
channels have been demonstrated in isolated RECs
using the patch clamp technique.
Absorption of ammonia
Effect of luminal pH on Na+ and Cl- absorption in

rumen and omasum.

At pH 6.4: ammonia raises ruminal Na+ absorption

A predominant absorption of NH4+ at pH 6.4 would provide protons
to the cell interior and could thereby stimulate apical NHE activity

At pH 6.9: ammonia
has no effect on ruminal Na+
absorption

At pH 7.4: ammonia depresses Na+ absorption

A predominant absorption of NH3 at a more alkaline pH will,
conversely, increase intracellular pH and thereby reduce NHE activity.
Absorption of ammonia
 An
increase in luminal NH3 concentration enhanced
ruminal Cl- absorption (Abdoun et al., 2005).

A diet rich in nitrogen seems to change the relative
permeabilities of rumen and omasum for NH3 and
NH4+ resulting in altered ammonia dependent effects
on the absorption of other ions.
Absorption of ammonia
Ammonia interferes with ruminal Mg2+ absorption

with effects on both the electrogenic and the
electroneutral component.

An electrogenic uptake of NH4+ should depolarize the apical
membrane potential and thereby reduce the driving force for
electrogenic uptake of Mg2+.

At the same time absorption of NH4+ should increase the
intracellular provision of protons and thus increase a vH+ATPase dependent stimulation of Mg2+ absorption and/or an
uptake via Mg2+/H+ exchange.
Absorption and secretion of chloride
Absorption and secretion of chloride

The
concentration
of
chloride
within
the
reticulorumen varies between 10 and 40 mmol/l.
In vitro – in the absence of electrochemical gradients

chloride is absorbed across rumen epithelia.

Chloride absorption depends on:

The presence of sodium

An active Na+ /K+ -ATPase

The presence of bicarbonate
Absorption and secretion of chloride
 An
increase in luminal Cl- concentration resulted in
an increase in surface pH.

It has shown that isolated RECs express mRNA for
the anion transporters PAT, DRA and AE2.

All of which might contribute to Cl- transport across rumen
epithelium

These anion transporters have been shown to exchange Cland HCO3- in other tissues
Absorption and secretion of chloride

A Cl- dependent base (HCO3-) extrusion should
increase the intracellular availability of protons.

For this reason Cl- has stimulatory effects on the
electroneutral absorption of Na+, Mg2+ and Ca2+ across
rumen epithelium.
Absorption and secretion of chloride
 Effects

of Cl- on electrogenic cation absorption:
Apical Cl- channels might mediate a Cl- dependent change
in membrane potential and thereby increase the electric
driving force for cation uptake.

In contrast to this expectation, an increase in luminal Clconcentration
did
not
hyperpolarize,
but
instead
depolarized the apical membrane of rumen epithelium
Absorption and secretion of chloride
 Net
electroneutral transport of NaCl requires:

an apical uptake pathway via NHE

an anion exchanger
 As
potassium is recirculated, the basolateral efflux of
Na+ would result in an increasingly negative cytosol
unless Cl-, too, leaves the tissue electrogenically.
Absorption and secretion of chloride
 Accordingly,
studies with isolated RECs have shown
the expression of a large conductance anion channel.

This channel was shown to be permeable not only to Cl-, but
also to HCO3- and the anions of SCFA and it has been
proposed that Cl- efflux occurs through this channel, coupled
via its charge to the efflux of Na+ via the Na+/K+-ATPase.
Absorption and secretion of chloride
 Chloride
transport rates measured across isolated
epithelia from reticulum and omasum are higher than
those across the rumen.
 In
the absence of electrochemical gradients, both
epithelia exhibit a small net Cl- absorption.
Absorption and secretion of bicarbonate
 Bicarbonate
in rumen had two sources:

Salivary production of HCO3-

Bicarbonate secreted by the rumen epithelium.


Bicarbonate contributing to:
An optimally buffered environment within the reticulorumen
in general

In the microclimate adjacent to the transporting layer of cells
in particular
Absorption and secretion of bicarbonate
 Removing

patways of bicarbonate in forestomachs:
A Part of bicarbonate is converted to CO2 in the course of
acid buffering.

The remaining bicarbonate passes to the omasum and is
absorbed there in exchange for chloride.
Absorption and secretion of bicarbonate
 Changing
from hay to concentrate diets:

Raised net bicarbonate secretion

Riased net SCFA absorption.
Absorption and secretion of bicarbonate

Presence of powerful buffers with a high pK value
require:

The secretion of large amounts of HCl by the abomasum to
reduce pH to a level at which gastric enzymes to work.

Formation of gas and accumulation of Cl should lead to a distension of
the organ with possible subsequent displacement.
Absorption and secretion of bicarbonate

The transport of bicarbonate across the omasum has
been systematically studied.

It clearly depends on the transepithelial and transmembranal
concentration gradients for Cl- and HCO3-.
Absorption and secretion of bicarbonate
 Interestingly
Cl- and HCO3- do not seem to interfere
directly with the transport of SCFA across the
omasum.

Suggesting that the anion exchangers expressed by this
epithelium show very low affinity to the larger and more polar
anions of SCFA.
Absorption and secretion of bicarbonate

In summary, the omasum shows a considerable
absorptive capacity for bicarbonate.

Resulting in a net absorption of this anion from the
forestomachs.
Absorption and secretion of bicarbonate

High individual variations in basal and gradient
induced HCO3- absorption may explain why
ruminants
on
the
same
diet
show
different
susceptibility to abomasal gas accumulation and
displacement.
Absorption of SCFAs
Absorption of SCFAs

Microbial fermentation of carbohydrates delivers 4
to 6 mol SCFA per kg of organic matter.

The majority of these SCFA are absorbed across the
forestomachs and cover a substantial part of the animals
energy requirements.
Absorption of SCFAs

The major fraction of SCFA crosses the forestomach
epithelia unmetabolized.

SCFA, in particular butyrate, are also used by the epithelial
cells themselves for cell metabolism, which could support
active transport of other electrolytes.
Absorption of SCFAs

Two possible mechanisms of SCFA uptake across
the apical membrane:

Diffusion of the protonated form through the lipid bilayer

Exchange of the dissociated SCFA- for bicarbonate via a
transport protein.
Absorption of SCFAs

Higher absorption affinity of butyrate attributed to:

A higher lipophility

A higher metabolic breakdown, thereby ensuring a permanent
butyrate gradient across the apical membrane.
Absorption of SCFAs

The absorptive flux of propionate across sheep
rumen epithelium could be stimulated by:

A decrease in pH in line with uptake of undissociated acid via
lipid diffusion or via a specific transport protein

A reduction in mucosal Cl- concentration in line with uptake
via anion exchange
Absorption of SCFAs

Acetate is present in the forestomachs in the highest
amounts under most feeding conditions.

A decrease in pH was able to stimulate the uptake of
acetate into rumen epithelia.

In the presence of bicarbonate, however, the same decrease in
pH induced an additional gradient for bicarbonate across the
apical membrane and resulted in a much greater stimulation of
acetate uptake.
Absorption of SCFAs

The capacity of the omasum for the absorption of
SCFA may be of particular importance in animals on
high concentrate diets.

Both an increase in DM intake and increases in the uptake of
concentrate increase the absolute amounts of SCFA flowing
out of the rumen into the omasum.
Absorption of SCFAs

Additionally, SCFA can also be produced by
ongoing fermentation in the omasum itself and might
contribute about 15% of total SCFA production.
Absorption of SCFAs

This suggested the predominant absorption of
acetate in its undissociated form across omasum
rather than by anion exchange mechanisms.

Possibly, the anion exchangers expressed by the
omasum show poor binding with the larger and more
polar SCFA anions than those expressed by the
rumen.
Absorption of SCFAs

In summary, influx of SCFA into the omasum is
thought to occur primarily via lipid diffusion,

Possibly enhanced by an apical microclimate that is acidified
both by NHE and by the absorption of bicarbonate via an
anion exchanger.
 Within
the cytosol, HSCFA taken up dissociate. The
basolateral efflux may occur as the anion via a large
conductance anion channel.
Absorption of SCFAs

Absorption of SCFA in the omasum removes a
potent buffer substance from the ingesta, prevents
possible disturbances of the integrity of the abomasal
epithelium and prevents inhibition of abomasal
motility.
Concluding remarks

Forestomach epithelia exhibit powerful absorptive
and secretory mechanisms with mutual interactions
between the transport rates of the different ions.

This interdependence changes with the animals
needs and with the specific localization within the
forestomachs.
Concluding remarks

New transport pathways have recently been added to
our picture of epithelial transport across rumen and
omasum:

An apical NSCC, a basolateral anion conductance

An apical H-ATPase

Differently expressed anion exchangers

A monocarboxylate transporter
Concluding remarks

In terms of regulation, specific transport rates have
been shown to be regulated by an apical pH
microclimate in the stratum corneum or by the
concentration of intracellular Mg ions.
Concluding remarks

Although the ruminal anion exchangers show
considerable binding to SCFA-, those of the omasum
display a high preference for Cl- and HCO3-, with the
direction of transport depending strictly on the
gradients present.
Partial model depicting for SCFA absorption
Adapted from Aschenbach et al. (2010)
Secretion of urea into the rumen

The large amounts of urea are transferred from the
plasma into the gut of mammalian species, with the
quantities being particularly high in ruminants.

In camels, the transfer rate is so high that no excretion of
urea into the urine is observed.
Secretion of urea into the rumen

In
the
ruminant,
urea
gastrointestinal tract via:

The rumen (~70%)

The colon (25%)

The saliva (usually below 5%)
primarily
enters
the
Secretion of urea into the rumen

The fraction of urea transferred into the gut or
excreted in the urine is regulated by dietary intake.

In response to an increase in the fermentational
production of CO2, the urea flux into the rumen can
thus increase significantly.
Secretion of urea into the rumen

Similar effects are seen after an exposure to SCFA.

Transport of urea is mediated by a transport protein.

Many studies support the expression of UT-B by the
rumen.
Secretion of urea into the rumen

In a recent study reported that:

Urea flux rates are not correlated with the flux rates for
mannitol.

Urea transport is modulated by maneuvers hat have an
impact on cytosolic pH.
Secretion of urea into the rumen

Urea secreted into the rumen and not used for
microbial protein synthesis is rapidly degraded to
CO2 and NH3.

The subsequent resynthesis of urea from ammonia requires
energy, this would appear to be an energetically wasteful
arrangement.
Secretion of urea into the rumen

Urea secretion is inversely correlated to the
concentration of ruminal ammonia.

Up to 10 mol/day of urea can be recycled to the
rumen in the bovine species, an amount that greatly
exceeds the requirements of microbial populations.

Urea not utilized will be converted to CO2 and
2NH3.
Secretion of urea into the rumen

The permeability of the rumen to urea drops when
ruminal pH is either very high or very low.

If carbohydrate intake is low, the decrease in urea
transport seen at high values of pH will preserve
energy.

If carbohydrate intake is high, but nitrogen intake is
insufficient, urea nitrogen will fill the gap.
Secretion of urea into the rumen