Sieving coefficients for small solutes during experimental peritoneal dialysis in rats

A single 4-hour experimental peritoneal dialysis was performed in 6 normal Sprague-Dawley rats to investigate sieving coefficients (S) for small solutes during peritoneal dialysis. A modified 3.86% Dianeal solution with approximately the same concentrations of urea, sodium, and potassium as in the rat plasma (isochratic solution) was used to avoid diffusion of investigated solutes and to achieve sufficient ultrafiltration. As a control, a 4-hour peritoneal dialysis in 7 normal Sprague-Dawley rats was performed using the conventional 3.86% Dianeal solution. The infusion volume was 30 ml. A dilution of radioiodinated serum albumin was used to determine the intraperitoneal dialysate volume. S was calculated (1) from the mass and volume balances for the initial 30 min of the exchange with the isochratic solution (SI, isochratic method) and (2) by using a membrane model based on the thermodynamic theory of mass transport (SM). The diffusive mass transport coefficient KBD for the solutes investigated was estimated using the membrane model. The SI values for urea, sodium, and potassium were similar with the isochratic solution. For urea and sodium, the S values were within the physiological range 0-1, whereas the S values for glucose were close to 0 and for potassium were negative. SM for glucose, urea, and sodium using the conventional solution did not differ from the values obtained with the isochratic solution; however, SM for potassium was significantly lower than with the isochratic solution. SI and SM for potassium and sodium were correlated. The KBD values for glucose, urea, and sodium using the isochratic solution did not differ from the values obtained with the conventional solution, whereas the KBD values for potassium were significantly higher with the isochratic solution as compared with the conventional solution. We conclude that the net sieving coefficients SI and SM for urea and sodium were lower than unity in the rats dialyzed with the two solutions and did not differ from the previously reported S values measured in continuous ambulatory peritoneal dialysis patients with the isochratic solution. However, the transport of potassium was abnormal with the isochratic solution, suggesting mechanisms other than passive diffusive and convective potassium transport.