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 5×10^–5 M. More of these sites were found in the duodenum than the ileum and vitamin D increases the number of these sites.