Calcium binding activity by chick intestinal brush-border membrane vesicles

Uptake of Ca2+ by vesicle preparation of chick intestinal brush-border membranes was rapid and extensive. With tracer quantities of Ca2+ uptake was complete in 10 min whereas with 2.0 mM Ca2+ maximum uptake by the vesicles occurred after one hour incubation. The maximum concentration of Ca2+ found in the vesicles was four times greater than the external Ca2+ concentration showing that the majority of the Ca2+ was membrane bound. The Ca2+ taken up by the vesicles was probably bound to the vesicle's interior since it was not replaced by exposure of loaded vesicles to La3+ (5 mM). The uptake of Ca2+ by the vesicles at different Ca2+ concentrations was analyzed and a high affinity Ca2+ binding site was found with an association constant for Ca2+ of 5 x 10(-5) M. More of the sites were found in the duodenum than the ileum and vitamin D increases the number of these sites.