Sieving and reflection coefficients for sodium salts and glucose during peritoneal dialysis in rats

Abstract

The two-part studies reported herein address peritoneal membrane ultrafiltrate (UF) characteristics during peritoneal dialysis exchanges in rats. In the studies of part 1, the sieving coefficients for sodium, chloride, and total solutes during hydrostatic UF after instillation of rat serum into the peritoneal cavity of rats were calculated. Thirty-six rats were divided into six groups (N = 6) according to the following peritoneal dialysis exchange cycle times: 60, 120, 180, 240, 480, and 960 min. Thirty milliliters of pooled rat serum were infused i.p. with the animal being con-scious except during infusion and drainage. The study showed in the early phase of exchanges, when oncotic and osmotic pressure gradients were absent, net UF presumably due to capillary hydrostatic pressure and sodium sieving during such UF. Sieving coefficients for sodium (0.72), chloride (0.77) and total solutes (0.73) were determined by using standard formulae. In the second part of these studies, the kinetics of fluid movement after the instillation of 5% dextrose solution into the peritoneal cavity of rats were analyzed. A very low UF rate was observed early in the exchange when the glucose gradient between the dialysis solution and blood was at its peak. The UF rate gradually increased as the sodium entered the dialysis solution from the blood. At the time of low UF rate with high glucose gradient, presumably the osmotic pressure generated by the glucose in the dialysis solution was countered by the osmotic pressure of solutes in plasma, i.e., sodium and its onions. Under such conditions, the ratio of concentration gradients between the blood and dialysis solution of sodium salt and glucose reflects the ratio of reflection coefficients of corresponding solutes. The calculated ratio of reflection coefficients for sodium salts and glucose for the peritoneal membrane in our study was 0.85. The reflection coefficient for glucose was derived to be 0.32, approximately similar to previously reported values. These studies showed that solute sieving could occur during what was believed to be hydrostatic UF during peritoneal dialysis exchanges and thus is not a phenomenon unique to osmotic pressure UF. During these simple studies, approximate sieving and reflection coefficients for various solutes that are present in the blood and dialysis solution were calculated. The calculated values were similar to previously reported values derived during complex mathematical models.