Large intestinal epithelium and short-chain fatty acids. Expected, unexpected, unexplained findings

Short-chain fatty acids (SCFA) are present in hindgut contents in high concentrations. SCFA are generated and also absorbed rapidly in the large intestine. Absorption results from diffusion of the undissociated and lipid soluble form or in exchange for bicarbonate. The controversial concepts concerning the extent of diffusion or the exchange for bicarbonate are partly due to contradictory findings and unequal mechanisms in different species and in different segments of the large intestine as well as in the different methods used. An almost neutral pH microclimate at the luminal surface is of importance for absorption of SCFA. The apical membranes of the epithelial cells in caecum and in proximal colon of guinea pigs are an substantial barrier for the diffusion of SCFA. After withdrawal of butyrate for only one hour from the perfusion or incubation solutions a massive apoptosis had been observed during the in situ perfusion of segments of guinea pig large intestine and also in in vitro studies with isolated epithelia in Ussing-chambers. In vitro apoptotic bodies and cells are emitted at the epithelial surface. However, in vivo induced by butyrate deprivation resident macrophages were tightly clustered below the surface epithelium. Aided by cytoplasmatic projections these macrophages phagocytose and transport apoptotic material from the epithelial intercellular spaces into their bodies. Obviously macrophages can be overloaded by this massive apoptosis, and some of the undigested apoptotic fragments are emitted into the lamina propria. Formation of a colitis ulcerosa may originate from these released undigested apoptotic bodies. Furthermore hints indicate aetiological interrelations between deprivation of butyrate and colon cancer. Butyrate-paradox denotes the contrarian effects on apoptosis and cell proliferation after addition or deprivation of butyrate in cultures of large intestinal tumour-cell-lines in comparison with the healthy, intact epithelium.