garnered from the cited references is
presented in Fig. 6.6. Although passive
diffusion of the undissociated acid does
occur, uptake of the anion predominates.
Anion uptake is coupled with exchange of
intracellular bicarbonate with luminal
anion. The VFA anions compete with
chloride directly on the same exchange
permease, or the competition is indirect for
the same pool of intracellular bicarbonate
and the presence of a separate luminal
chloride/intracellular bicarbonate exchanger.
The anions of VFAs are weaker bases than
bicarbonate and the exchange acidifies the
cytosol of the epithelial cell. Na+/H+
exchangers in both the apical and basolateral
membranes diminish the change in cytosolic
pH. The apical exchanger enhances Na+140 R.W. Russell and S.A. Gahr
Fig. 6.6.Epithelial transport of VFAs. Stratified rumen epithelium shown (anatomy based on Steven and
Marshall, 1970). Caecal and colonic epithelia are only one cell thick.