OBJECTIVE: It has previously been demonstrated that a lining of surface-active phospholipid (SAPL) is reversibly bound (adsorbed) to normal peritoneal mesothelium.The lining acts as a boundary lubricant and release (anti-stick) agent preserving mechanical Integrity of the epithelial surface. In a review of clinical trials on the use of SAPL (akin to "surfactant") to restore ultrafiltration (UF) in patients on peritoneal dialysis (PD), speculation is that, by adsorption, the SAPL lining might also be Imparting the semipermeability vital for UF. DESIGN: To evaluate the hypothesis, SAPL harvested from the spent dialysate of 5 patients with normal UF was deposited onto a porous, inert medium, and the resulting 7 "membranes" were clamped in an Ussing chamber used as an osmometer. RESULTS: With every "membrane," a clinical concentration of glucose (2.5%) was able to induce a statistically significant osmotic pressure when dialyzed against saline. We also demonstrated how synthetic SAPL--in the form of dipalmitoyl phosphatidylcholine (DPPC) and an admixture of DPPC with phosphatidylglycerol (PG) called artificial lung-expanding compound (ALEC)--imparts greater osmotic pressure In proportion to an Increasing glucose gradient. Our findings prove that human peritoneal SAPL has the physical capability to Impart semipermeability when adsorbed to a surface. COMMENT: As a lipid lining, adsorbed SAPL could also explain the high permeability of the natural membrane to lipophilic substances in PD. Administered as a very fine powder or as a solution in a lipid solvent, ALEC offers a potential treatment for restoring UF, if applied during the interdialytic Interval. In various physical forms, ALEC and DPPC have both been widely used for two decades with complete safety in the treatment of respiratory distress syndrome in newborns. The question of formulation of exogenous SAPL In restoring UF Is discussed as a complex physicochemical compromise between the higher surface activity of saturated phosphatidylcholine and its lower solubility in water.

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