SIEVING COEFFICIENTS FOR SMALL SOLUTES DURING EXPERIMENTAL PERITONEAL-DIALYSIS IN RATS

A single 4-hour experimental peritoneal dialysis was performed in 6 no rmal Sprague-Dawley rats to investigate sieving coefficients (S) for s mall solutes during peritoneal dialysis. A modified 3.86% Dianeal solu tion with approximately the same concentrations of urea, sodium, and p otassium as in the rat plasma (isochratic solution) was used to avoid diffusion of investigated solutes and to achieve sufficient ultrafiltr ation. As a control, a 4-hour peritoneal dialysis in 7 normal Sprague- Dawley rats was performed using the conventional 3.86% Dianeal solutio n. The infusion volume was 30 ml. A dilution of radioiodinated serum a lbumin was used to determine the intraperitoneal dialysate volume. S w as calculated (1) from the mass and volume balances for the initial 30 min of the exchange with the isochratic solution (S-I, isochratic met hod) and (2) by using a membrane model based on the thermodynamic theo ry of mass transport (S-M). The diffusive mass transport coefficient K -BD for the solutes investigated was estimated using the membrane mode l. The S-I values for urea, sodium, and potassium were similar with th e isochratic solution. For urea and sodium, the S values were within t he physiological range 0-1,whereas the S values for glucose were close to 0 and for potassium were negative. S-M for glucose, urea, and sodi um using the conventional solution did not differ from the values obta ined with the isochratic solution; however, S-M for potassium was sign ificantly lower than with the isochratic solution. S-I and S-M for pot assium and sodium were correlated. The K-BD values for glucose, urea, and sodium using the isochratic solution did not differ from the value s obtained with the conventional solution, whereas the K-BD values for potassium were significantly higher with the isochratic solution as c ompared with the conventional. solution. We conclude that the net siev ing coefficients S-I and S-M for urea and sodium were lower than unity in the rats dialyzed with the two solutions and did not differ from t he previously reported S values measured in continuous ambulatory peri toneal dialysis patients with the isochratic solution. However, the tr ansport of potassium was abnormal with the isochratic solution, sugges ting mechanisms other than passive diffusive and convective potassium transport.