Effects of phospholipid or cholesterol enrichment of rat intestinal brush border membrane on membrane order and transport of calcium.

Calcium uptake by brush border membrane vesicles from rat small intestine measured under initial rate conditions comprises both saturable and nonsaturable components. Because the brush border is a lipid bilayer and may be sensitive to changes in membrane lipid, vesicles were treated with liposomes to enrich phospholipid (PL) or cholesterol (C) content above that of the control (Reference) vesicle. The effects of the changes in lipid composition on membrane fluidity were determined from fluorescence anisotropy (r) of diphenylhexatriene. Compared with Reference vesicles, liposome-treated vesicles showed decreased Vmax for saturable and KD for nonsaturable uptakes. Liposome treatment changed vesicle phospholipid composition compared with Reference vesicles. Liposome-treated vesicles had similar phospholipid composition but differed in greater cholesterol content of C- compared with PL-vesicles. Mean Vmax and KD were lower in C- than PL-vesicles, but the difference did not reach statistical significance, although fluidity was significantly lower in C- than PL-vesicles. The mechanism of inhibition of saturable calcium uptake in PL- and C-vesicles was uncompetitive. Thus, lipid composition is crucial for determining calcium uptake: any change from native lipid composition decreased transport. Fluidity, measured by the conventional probe diphenylhexatriene, did not correlate with calcium uptake by Reference compared with liposome-treated vesicles.