Transport of monovalent anions by isolated small intestine of the rat

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Abstract
Monovalent anions could be divided into three groups, according to relative rates of absorption along the small intestine. Group 1 (chloride, bromide, iodide, nitrate and thiocyanate) moved at approximately equal rates in both the ileal and the jejunal regions. Group 2 (bicarbonate, acetate, propionate and phosphate) moved preferentially in the jejunum. Group 3 (iodate) did not move in either region. Radioisotope flux measurements indicated that the low rate of transport of group 2 in the ileum was owing to decreased permeability. All anions tested invariably moved down the electrochemical potential gradient. There is a correlation between the three anion groupings and their physicochemical properties suggesting that group 1 anions move preferentially along aqueous channels, and group 2 anions diffuse through lipid regions in the undissociated, acidic form. In this respect, both the mucous coating of the mucosal cells and the preferential release of hydrogen ions on the mucosal side of the jejunum, as a result of lactic-acid production, have an essential function in the absorption of group 2 anions.