Fluidity and composition of brush border and basolateral membranes from rat kidney

The physical state of rat renal brush border and basolateral membranes was examined using the technique of electron spin resonance. As estimated from the order parameters of both 5- and 12-doxyl spin-labeled phosphatidylcholine, the brush border membrane was significantly more ordered than the basolateral membrane. To confirm these findings, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene was studied. Fluorescence anisotropy at 25 and 37 degrees C was significantly greater in the brush border membrane when compared with the basolateral membrane. The relative contribution of the lipid component to this difference was examined using multilamellar liposomes prepared from lipid extracts of these membranes. Fluorescence anisotropy in the brush border membrane lipids was significantly greater than that in the basolateral membrane lipids at both 25 and 37 degrees C. Compositional determinants of fluidity were examined in both membranes. Factors that may contribute to the more fluid state of the basolateral membrane include 1) a greater lipid-to-protein ratio; 2) a greater ratio of phosphatidylcholine to sphingomyelin; and 3) a tendency toward shorter-length fatty acyl chains.