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In-vitro the transport of HCO3- in the mucosal–serosal direction (absorption), Jms, was studied across the isolated epithelium of sheep omasum. The conventional Ussing chamber technique was combined with the pH-stat method. The mucosal side of the tissues was incubated with a buffer solution of 50 mmol∙l-1 HCO3- and gassed with 10% CO2. The buffer solution of the serosal side was HCO3- and buffer free. The pH (7.4) of the serosal side was kept constant by titration of H2SO4. The amount of titrated H2SO4 was considered as amount of transported HCO3-.
a) The current model of anion transport across the omasal epithelium with two anion exchangers in series in the apical and basolateral membrane was confirmed. The omasal epithelium has the capability to transport HCO3- in both directions according the applied ion gradients. However, the flux from mucosal to serosal side was significantly larger than the transport in the reversed direction.
b) The transport of HCO3- depends on the activity of Na/H exchanger (NHE3) in the apical membrane. Inhibition of NHE3 by mucosal amiloride (1 mmol∙l-1) or reduction of mucosal Na+ concentrations reduced HCO3- transport significantly. It is concluded that the NHE3 mediated extrusion of H+ is of predominant importance for regulation of the intracellular pH, pHi, and hence HCO3- transport.
c) The manipulation of pHi by transport of SCFA (uptake of the undissociated SCFA in the mucosal-serosal direction) reduced HCO3- transport. However, this effect was only of minor importance in tissues from concentrate-fed sheep.
d) Inhibition of carbo-anhydrase by carboxyzolamide (0.1 mmol∙l-1) caused a significant decrease of HCO3- transport.
e) The suggested anion exchanger of HCO3- transport was examined by using levetiracetam, a Na+-coupled Cl-/HCO3 exchanger inhibitor. The addition of levetiracetam (1 mmol∙l-1) failed to produce a reduction of the HCO3- transport rates.
f) It is concluded that the transport of HCO3- is mediated by two anion exchangers in series and that the regulation of pHi is of paramount importance of undisturbed HCO3- transport. There is evidence for adaptation of the omasal epithelium to diet.
g) It is proposed that the absorption of HCO3- should be included in the discussion of the pathogenesis of displaced abomasum.