Intestinal calcium transport in systemic disease. Studies with brush-border membrane vesicles.

Recent experiments utilizing brush-border membrane vesicles (BBMV) from small intestine to examine calcium uptake processes by the enterocyte are reviewed. Results demonstrate that (a) uptake must be examined in the time frame approaching initial rate, and (b) kinetics of saturable and nonsaturable processes be defined. Saturable uptake of calcium is carrier mediated, since it is inhibited competitively by strontium and noncompetitively by magnesium. Intravesicular strontium (but not magnesium), doubles Vmax for saturable uptake of calcium from the medium (countertransport), consistent with a mobile carrier. Since membrane lipid composition is expected to be an important determinant of function of a mobile carrier in a lipid bilayer, BBMV were treated with liposomes to change phospholipid and cholesterol content. Any change from native lipid composition decreased Vmax, but in treated BBMV, Vmax correlated with BBMV cholesterol content and fluidity. Since nonsaturable calcium uptake is extensively inhibited by both strontium and magnesium, electrostatic binding of calcium to the membrane is a major component of nonsaturable uptake. The remaining uninhibitable component of nonsaturable uptake is diffusion down the calcium concentration gradient. These concepts are applied to considering calcium uptake in disease and to the relationship of uptake to transenterocyte calcium movement.