Cation uptake by exchange resin in vitro and the colon as a sodium-conserving organ.

Cation-exchange resins have not fulfilled early expectations. Those in clinical use do not retain much sodium in the feces when dietary sodium is much restricted. Comparison of the sodium and potassium bound by a resin, in equilibrium with presumably normal electrolytes as found in the terminal ileum, with the amounts of those ions in the feces, permits an overall evaluation of two major factors operating in the gastrointestinal tract. These are a decreased sodium concentration in the terminal ileal contents when sodium intake is restricted and an absorptive power of the colon sufficient to detach sodium from the resin. T has been a consistent observation that cation-exchange resins will retain much more sodium in the feces of experimental animals or patients who are consuming diets with a liberal sodium content than when dietary sodium is much restricted. When subjects have been maintained on diets with unrestricted sodium intake, a carboxylic cation-exchange resin similar to the one used in this study has been found to retain in the feces 0.5 to 2.5 mEq. of sodium and 0.4 to 2.9 mEq. of potassium per gram of resin.' When diets containing between 500 and 800 mg. of sodium per day were consumed the resin has bound, in the feces, 0.26 to 0.80 mEq. of sodium and 0.27 to 2.32 mEq. of potassium per gram of resin.2 The reason for these differences in the amount of sodium fixed by the resin in the feces when subjects are taking liberal and low sodium diets has been obscure. There is, normally , a large amount of sodium in the secretions of the gastrointestinal tract, to which the resin is exposed. It has been suggested that "endogenous" sodium does not combine with cation-exchange resin in the intestinal tract and that the sodium removed in the feces by the resin is "exogenous" sodium of dietary origin.3 It is difficult to understand why the resin should react differently with sodium of different From the General Medical Service and the Research Laboratory, Veterans Administration Center , Martinsburg, W. Va. 32 origins. It has seemed necessary to seek a better explanation for the seemingly different behavior of the resin to "exogenous" and "endogenous" sodium. The uptake of cations by exchange resins depends upon the following factors4: (a) concentration of ions, (b) pH of the medium, (c) the relative affinity of the resin for different cations and (d) the concentration of resin. …