THE ABSORPTION OF VOLATILE FATTY ACIDS FROM THE RUMEN II. THE INFLUENCE OF pH ON ABSORPTION

In a previous paper the relative rates of absorption of acetic acid and propionic acid through the rumen wall were discussed (Gray, 1947a). While the manuscript was being prepared, Danielli, Hitchcock, Marshall & Phillipson (1945) described a series of experiments from which it was evident that the lower homologues of the fatty acids are absorbed through the rumen wall at different rates when the rumen contents are acid, and that the relative velocities of absorption fall in the series butyric > propionic > acetic. The observations made in this laboratory (Gray, 1947 a) independently and by different methods were in agreement with these conclusions. There is therefore no reason to doubt that the higher homologues are absorbed preferentially when the rumen contents are acid. Danielli and his colleagues (Danielli et al. 1945), however, concluded that the relative rates of absorption were reversed when the medium was slightly alkaline, and experimental data were presented which, it was claimed, proved that the relative velocities of absorption from a mixture of fatty acids at pH 7-5.fell in the series acetic > propionic > butyric. These latter experimental data were unconvincing. The experiments conducted in this laboratory which are described here lend no support to the claim, or to the general theory in which Danielli and his colleagues suggested that diffusion from acid solutions may proceed through both the intercellular substance and the limiting membranes of the cells themselves, and that only the former pathway is traversed when the solution is slightly alkaline. There is no satisfactory evidence to support the contention that fatty acids diffuse through the rumen wall from a slightly alkaline medium.