Modulation of sulfate renal transport by alterations in cell membrane fluidity

Changes in membrane fluidity have been shown to alter the sodium-dependent renal transport of glucose and phosphate; however, this has not been examin ed for sodium/sulfate cotransport in the renal proximal tubule. Sodium/sulf ate cotransport regulates the homeostasis of sulfate in mammals. The object ive of this study was to investigate the influence of alterations of membra ne fluidity on sodium-coupled sulfate transport in the Madin-Darby canine k idney cells, which have been stably transfected with sodium/sulfate cotrans porter (NaSi-1) cDNA (MDCK-SI). Preincubation of cells with 0.2 mM choleste rol significantly decreased the V-max for sodium/sulfate cotransport (13.69 +/- 1.11 vs 10.15 +/- 1.17 nmol/mg protein/5 min, mean +/- SD, n = 4, p < 0.01) with no significant alteration in K-m. The addition of benzyl alcohol (20 mM) to cells increased the V-max of sulfate uptake by 20% (11.97 +/- 0 .91 vs 14.35 +/- 0.56 nmol/mg protein/5 min, mean +/- SD, n = 3, p < 0.05) with no significant change in Km. Membrane fluidity, as measured by the flu orescence polarization of 1,6-diphenyl 1,3,5-hexatriene (DPH), was signific antly increased in MDCK-Si cells treated with 20 mM benzyl alcohol and decr eased in the cells preincubated with 0.2 mM cholesterol, compared with cont rol cells. Our results suggest that alterations in membrane fluidity that m ay occur as a result of disease states, aging, and pregnancy may play an im portant role in the modulation of renal sodium/sulfate cotransport.