ABSORPTION OF DISSOCIATED AND UNDISSOCIATED SHORT‐CHAIN FATTY ACIDS ACROSS THE COLONIC EPITHELIUM OF GUINEA‐PIG

Diffusion potentials for Na + , Cl - , acetate (Ac − ), propionate (Pr − ) and butyrate (Bu - ) across the isolated epithelium of the proximal and the distal colon of guinea−pig were measured with flowing electrodes in Ussing chambers. Permeabilities of the anions are expressed in relation to the permeability of sodium. Unidirectional fluxes of Na + and short‐chain fatty acids (SCFAs) were estimated. Fluxes of SCFA anions and of undissociated SCFAs were calculated from relative permeabilities and flux values. In the proximal colon of guinea‐pig 50% of SCFAs were transported in the dissociated and 50% in the undissociated form; transport rates of the three SCFAs were similar. In the distal colon SCFA transport was based primarily on diffusion of the free acids; the ratios of fluxes were: HAc/Ac ‐ = 2·4; HPr/Pr − = 6·9; HBu/Bu − = 9·3, and transport rates increased with chain length of the SCFA.