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.