Effect of Potassium-Chelating Resins on Phosphorus Absorption

a K P, Ca and K are expressed in mg/l00 ml. *p < 0.05; **p < O.Ol, versus baseline. Dear Sir, Calcium-containing resins are frequently used to prevent the inordinate increase of plasma potassium in dialysis patients. The mechanism of action of these resins, which are insoluble and nonabsorbable, is the exchange of one cation for another [1, 2]. The resins used orally for the routine prevention of potassium increase usually contain calcium for exchanging with potassium [2]. Sodium-containing resins are also available, but are less frequently prescribed on a routine basis [1, 2]. The occupancy of the cationic site of the resin by K+ displaces and releases Ca2+ from the resin and the Ca2+ released from the resin appears not to be absorbed in significant amounts [2]. Therefore, the possibility exists for this Ca2+ to combine with phosphorus within the intestine, to produce low-solubility Ca2+ phosphate compounds. We tested this hypothesis in two ways. First, we mixed different amounts of Ca resin (calcium polystyrene sulfonate, Resin-Ca®, Rubió, Barcelona, Spain) with a fixed quantity of P04K2H solution (5.8 mg/l0 ml). According to the information provided by the manufacturer, each gram of resin contained approximately 70-90 mg of Ca element. As shown in table 1, the phosphorus and potassium content of the solution decreased proportionally to the amount of the resin included in the mixture. Although the results shown in table 1 were obtained after a 20-min incubation at room temperature, the observed effect was almost complete at 5 min of contact between the resin and the P04HK solution [data not shown, p < 0.05 with respect to the baseline]. To address the putative effect of the resin on phosphorus absoφtion, we administered the resin orally to a group of normal male Wistar rats, on a diet with a known amount of phosphorus (4.9 mg/g diet, Panlab, Barcelona, Spain), with the resin mixed into the powdered rat chow. The amount of food and