2. By: A. Riasi (PhD in Animal Nutrition & Physiology) http://riasi.iut.ac.ir Advanced Digestive Physiology (part 4) Isfahan University of Technology Isfahan, Iran

3.  Forestomach fermentation occur at nearly neutral pH.  Two factors prepare the optimal fermentation environment: Ample secretion of saliva Absorption from the forestomach (specially ruminal) wall Absorption from the forestomach wall Adapted from: Leonhard-Marek et al. 2010. Animal, Page:1 of 20

4.  Efficient absorptive mechanisms across forestomach epithelia are essential to allow: An early recycling of essential electrolytes Water recycling from the gastrointestinal cavity to the blood Absorption from the forestomach wall

5.  Fermentation releases additional cations and anions Short chain fatty acids (SCFAs - ) NH4 +  Rumen also is responsible for absorption of some minerals: Mg 2+ Ca 2+, Pi and K + (if delivered in high amounts) Absorption from the forestomach wall

6.  The transport mechanisms generally seem to be the same in the rumen and omasum. Omasom has a greater efficiency of absorption:  Highly specialized anatomy of the omasum  The electrophysiological properties of this organ A major difference between the two organs is the handling of Cl - and HCO3 -.  In the rumen, Cl - is absorbed and HCO3 - is secreted, while the movements of these anions are reversed in the omasum Absorption from the forestomach wall

7. The 4 cellular layers of the stratified epithelium: stratum corneum (SC), stratum granulosum (SG), stratum spinosum (SS) and stratum basale (SB), Scale bar = 20 µm Absorption from the forestomach wall

8.  About 80% of the total amount of sodium secreted with saliva has been absorbed by the forestomachs.  All forestomach epithelia have ability to absorb Na +, that is mediated by: Primary active transport Secondary active transport Absorption of sodium

9.  Na + transport across the rumen, reticulum and omasum have two components: Electroneutral Electrogenic Absorption of sodium

10.  Na + transport is mainly via the electroneutral pathway: At high Na + concentrations Low pH  The electrogenic pathway for Na + transport: At low Na + concentrations Alkaline pH. Absorption of sodium

11. Adapted from: Leonhard-Marek et al. 2010. Animal, Page:1 of 20

12.  Sodium transport proteins have assymetric distribution at the apical and basolateral of epithelial cells.  The driving force for active transport across the forestomachs is generated by the basolaterally localized Na + /K + ATPase. Absorption of sodium

13.  Immunohistochemical staining have shown The α-subunit of the transport protein was most prominent at the basal membrane stratum basale cells (SB). Stratum spinosum (SS) and stratum granulosum (SG) have lower concentration of transport protein, respectively. Absorption of sodium

14.  Electroneutral absorption of Na + is mainly performed via Na + /H + exchange (NHE). High doses of amiloride had the inhibitory effect on Na + absorption across ruminal, reticulular and omasal epithelia.  Nine isoforms of NHE have been identified in different tissues (Lam et al., 2009). Absorption of sodium

15.  Experiments with isolated sheep rumen epithelial cells have shown that the specific inhibitors: HOE 694 of NHE-1 S3226 of NHE-3  The mRNA of NHE1, NHE2, NHE3 and NHE8 isoforms are identified in bovine rumen epithelium. NHE1 are prominent in stratum granulosum and the exchanger is decreased in SS and SB. Absorption of sodium

16.  Functionally, at least, expression of NHE isoforms by sheep forestomachs resembles that found in other gastrointestinal epithelia: NHE3 localized apically NHE1 is basolaterally expressed Absorption of sodium

17.  Ruminal electroneutral Na + absorption via NHE can be stimulated by: acidifying intracellular pH:  protons being provided by dissociation of protonated SCFA (HSCFA)  NH4  Conversion of CO2 Absorption of sodium

18.  Conversely, electroneutral Na + absorption is minimized by: Removal of stimulatory anions:  Cl -  HCO3 -  SCFA - Absorption of sodium

19.  Na + has a counterion accompanying function in the absorption of SCFA - from the rumen.  In omasum, in addition to NHE an apically localized Na + Cl - cotransport is presented. Absorption of sodium

20.  At alkaline pH or high ruminal NH3, an uptake via NHE is low. Absorption of Na + must be maintained by additional pathways under these conditions to prevent an alkalinization of the cytosol. Absorption of sodium

21.  Ruminal electrogenic Na + transport differs from that of classical Na + absorbing epithelia It is not blocked by low doses of amiloride It varies with the luminal concentration of divalent cations  In further experiments with isolated rumen epithelia, a divalent sensitive current and conductance could also be shown. Absorption of sodium

22.  The Na + current blocked by:  Extracellular Ca 2+ and Mg 2+ ions  Intracellular Mg 2+ ions Absorption of sodium

23.  Conductance increased and inward rectification decreased when internal Mg 2+ was removed, This allowing more influx of Na + at higher potentials. Absorption of sodium

24.  Elevation of cAMP opened the conductance, via: Effects on a basolateral Na + /Mg 2+ exchanger Direct mechanisms on the channel Absorption of sodium

25.  Increase of dietary K intake results in an increased Na + absorption from the rumen This keeps the sum of Na + and K + concentrations within the rumen almost constant. Absorption of sodium

26.  Recent structural studies have given evidence for the expression of the channels transient receptor potential vanilloid channel TRPV6 and TRPM7 in rumen epithelium.  Given the decreasing Na + uptake via NHE at alkaline pH, the effect of pH on electrogenic Na + transport was especially interesting. Absorption of sodium

27.  A higher mucosal pH decreases the stimulatory effects of SCFA and CO2 on ruminal NHE and enhances the inhibitory effects of ammonia on NHE. Higher pH also reduces the concentration of free Ca 2+ and Mg 2+ ions in rumen fluid (Johnson and Aubrey Jones, 1989). Absorption of sodium

28.  An increase in luminal Cl - induces a visible increase in rumen surface pH in the presence as well as in the absence of bicarbonate, pointing to an action via apical Cl - /HCO3 - or Cl - /OH - exchange.  Direct alkalinization of mucosal pH, this manoeuvre had impacts on Isc depending on the mucosal presence or absence of Ca 2+ and Mg 2+. Absorption of sodium

29.  As for luminal pH, the effects of hormones and neurotransmitters acting via cAMP also seem to have different effects on electroneutral and electrogenic Na transport respectively. Absorption of sodium

30.  While an intracellular increase in cAMP or the addition of prostaglandins decreased Na absorption via NHE, cAMP stimulated the transepithelial Na + current via a stimulation in Mg 2+ /Na + exchange and a subsequent decrease in intracellular Mg 2+. Absorption of sodium

31. Absorption of magnesium Adapted from: Leonhard-Marek et al. 2010. Animal, Page:1 of 20

32.  The forestomachs are the main site of magnesium (Mg 2+ ) absorption in ruminants  At least two mechanisms ensure an efficient Mg 2+ uptake into rumen epithelium cells (RECs): Electrogenic transport Electroneutral transport Absorption of magnesium

33.  In electrogenic patway the uptake of Mg 2+ depends on the potential difference across the apical cell membrane.  Two factors have negative effect on Mg 2+ absorption: High ruminal K + concentrations NH4 + absorption Absorption of magnesium

34.  The potassium insensitive Mg 2+ absorption occur in the presence of high ruminal K + concentration. This is due to a second electroneutral mechanism of Mg 2+ absorption. Absorption of magnesium

35.  Different anions may stimulate Mg 2+ uptake in the rumen: Cl -, CO2/HCO3 -, SCFA  A vacuolar type H + -ATPase (vH + -ATPase) has been found in RECs. Absorption of magnesium

36.  The extrusion of Mg 2+ across the basolateral membrane to the blood side is a secondary active process Reducing the Na + concentration on the basolateral side of rumen epithelia decreases transepithelial Mg 2+ absorption The Mg 2+ efflux from Mg 2+ -loaded RECs increases with the extracellular Na + concentration Absorption of magnesium

37.  A rapid efflux of K + from the gastrointestinal tract of the ruminant should be undesirable.  To gain a more complete understanding of the role that the rumen plays in K + metabolism, recirculation of K + via saliva has to be taken into account. Absorption of potassium

38.  The salivary and the ruminal concentration of K + may rise in two situations: In Na –depleted animal Under stimulation by aldosterone  In these conditions, K + replaces with Na + in saliva, and not surprisingly, perhaps, a sizable reabsorption of K + from the rumen can be observed. Absorption of potassium

39.  It is reported that ruminal K + concentrations through the range of 35 to 100mmol/l, linearly increase its absorption.  Warner and Stacy (1972) calculated positive K + absorption rates only when ruminal concentration exceeded 100mmol/l. Absorption of potassium

40.  Although the efflux of K + from the rumen is passive, in that it follows the electrochemical gradient. Thus the permeability of the rumen to the K + ion is clearly regulated in a complex fashion that is yet poorly understood. Absorption of potassium

41.  The use of different blockers has revealed the contribution of both apical and basolateral K + channels to ruminal K + transport. Apical K + channels could be blocked by quinidine or verapamil Basolateral K + channels were sensitive to inhibition by barium. Absorption of potassium

42.  The K + -dependent depolarization of membrane potential decreases ruminal Mg 2+ absorption.  The increase in the active, electrogenic transport of Na + across the tissue under high K + conditions also has implications for the transport of K + : As the transepithelial potential across the rumen increases, the gradient driving the efflux of K + decreases. Absorption of potassium

43.  The small intestine is regarded as the main absorption site for Ca 2+ in monogastric animals.  In ruminants Ca 2+ absorbed from all gastrointestinal regions.  The mechanisms of gastrointestinal Ca 2+ absorption are of great interest in high yielding dairy cows. Absorption of calcium

44.  Balance studies show that preintestinal Ca 2+ absorption gains significance at higher Ca 2+ intake.  Some studies have shown that a high preintestinal Ca 2+ absorption is usually accompanied by: a low intestinal Ca 2+ absorption even by intestinal Ca 2+ secretion Absorption of calcium

45.  In a meta-analysis Lean et al. (2006) have shown that prevention of milk fever seems to be possible not only at low Ca intake (0.5% of DM), but also at high Ca intake (2% of DM). which may indicate that at high Ca 2+ intake, absorption of Ca 2+ from the forestomachs could help to cover increased Ca 2+ requirement at parturition. Absorption of calcium

46.  Ruminal and omasal Ca 2+ absorption are at least partly active and involve: Electrogenic pathway Electroneutral pathway  Active ruminal Ca 2+ absorption can be increased: In the presence of SCFAs By increasing the amounts of luminal chloride Absorption of calcium

47.  The stimulating effects of SCFA have suggested that Ca 2+ /H + antiporters contribute to apical Ca 2+ uptake. The presence of a ruminal vH + -ATPase might be additionally explained as a stimulation of other Ca 2+ transport proteins by an apical pH microclimate. Absorption of calcium

48.  Ca 2+ channels might thus represent the functional basis for an electrogenic Ca 2+ absorption across forestomach epithelia.  In the rumen, the Ca 2+ channel agonist Bay K 8644 was able to increase Ca 2+ absorption, which could indicate the presence of L-type Ca 2+ channels in rumen epithelium. Absorption of calcium

49.  Both the epithelial TRPV-type Ca 2+ channels and L- type Ca 2+ channels increase their conductance with alkaline pH Therfore an increased uptake of Cl - or SCFA - in exchange for bicarbonate could stimulate a luminal Ca 2+ conductance via an increase in surface pH. Absorption of calcium

50.  Transepithelial Ca 2+ absorption depends on the activity of the basolateral Na + /K + -ATPase and on the presence of sodium suggesting the contribution of Na + /Ca 2+ exchangers (NCXs) to basolateral Ca 2+ extrusion. Absorption of calcium