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Absorption of short-chain fatty acids (SCFA) and ammonia implies considerable fluxes of protons across the epithelium of the large intestine. Efficient regulation of intracellular pH (pH(i)) is therefore essential in these cells. The aim of the present study was to examine the effects of SCFA and of ammonia on pH(i), on pH(i) regulation and to characterize the mechanisms involved in pH(i) regulation in surface enterocytes of the guinea-pig caecal and colonic mucosa. Intact epithelia from the caecum and the distal colon were mounted in a microperfusion chamber. pH(i) was measured by fluorescence microscopy using 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Addition of SCFA or ammonia to the serosal side changed the enterocyte pH(i) markedly, whereby ammonia caused larger changes in pH(i) than SCFA. In contrast, addition of SCFA to the mucosal solution had no effect on pH(i) and ammonia increased pH(i) only slightly. Basolaterally located pH(i) regulation mechanisms, Na(+)-H(+) exchange and Cl(-)-HCO(3)(-) exchange, are involved mainly in returning pH(i) to normal values. It is concluded that, due to apparently lower permeability of the apical membranes, the caecal and colonic epithelium is protected against pH(i) disturbances caused by the naturally high luminal SCFA and NH(3) concentrations. The major regulation mechanisms of pH(i) are located in the basolateral membrane of the enterocytes.

SUMMARYThe study was carried out in northern Kenya in 1984–87. Forestomach volumes and digesta retention times were measured using Cr-EDTA or Co-EDTA as fluid markers and Ce-labelled particles or Cr-mordanted particles as paniculate phase markers.Mean retention times of fluid and of particles were longer in the dry season than in the green season in all four animal species. The increase of particle mean retention time, as a percentage of the values in the green season, was highest in sheep (46%), followed by cattle (27%), goats (22%) and camels(18%). Forestomach volumes were also greater in the dry than in the green season; the increase was again highest in sheep (55%), followed by cattle (31%), goats (29%) and camels (28%). Outflow rates of fluid from the forestomach and the selectivity factor, by which small particles were retained longer in the forestomach than fluid, did not differ significantly between the seasons.It is suggested that the increase of forestomach volumes is an effective adaptation to dry-season pasture conditions. It enables the animals to retain feed particles longer in the forestomach and so improve fibre digestion when feed quality is low. Cattle and sheep, which depend on a poor quality diet, improve fibre digestion in this way in the dry season more effectively than camels and goats. Camels and goats, on the other hand, were able to select a diet of such quality, even in the dry season, that their need to augment fibre digestion was reduced.