Alteration of the microbiota with vancomycin and high-fiber diet affects short-chain fatty acid/free fatty acid receptor signaling in rat caecum

Microbial short-chain fatty acids (SCFA) regulate intestinal functions via free-fatty acid (FFA) receptors type 2 and 3. Though the caecum is the most important fermentation chamber in many species, it is unknown whether this signaling system is modulated in dependence on the rate of fermentation within the lumen of this part of the large intestine. Thus, we asked the question whether alteration of the microbiota composition by antibiotic treatment or high-fiber diet affects the SCFA/FFA signaling using rat caecum as model system. SCFA concentrations and microbiota were analyzed in caecal samples from untreated rats, following vancomycin treatment, or after feeding with a high-fiber diet. Oral and aboral caecal segments were harvested for Ussing chamber experiments paralleled by Ca²⁺ imaging experiments with Fura-2 loaded crypts, immunofluorescence, and qPCR. Vancomycin treatment reduced total SCFA concentrations in the caecal content, whereas the high-fiber diet increased the concentration of acetate, but reduced that of propionate and butyrate. Propionate-induced anion secretion was abolished in the vancomycin group, whereas it nearly doubled in the high-fiber group. These effects could not be explained by changes in the expression of FFA2 receptor or in Ca²⁺ signaling evoked by FFA2 receptor activation. Parallel changes in ion secretion evoked by carbachol suggest that alterations in cholinergic signaling might be responsible for the observed changes in epithelial ion transport. Additionally, mucosal mast cell and enterochromaffin cell density increased after vancomycin and high-fiber diet, respectively. This study emphasizes the complex interactions between the microbiota and the caecal epithelium focusing on SCFA/FFA signaling.