Intracellular pH regulation in guinea-pig caecal and colonic enterocytes during and after loading with short-chain fatty acids and ammonia

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--HCO3- 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 NH3 concentrations. The major regulation mechanisms of pH(i) are located in the basolateral membrane of the enterocytes.