Characterization of calcium uptake by brush border membrane vesicles of human small intestine.

Calcium uptake was characterized in human duodenal, jejunal, and ileal brush border membrane vesicles. Calcium uptake into human intestinal brush border membrane vesicles represented uptake into intravesicular space as evidenced by studies of osmolality, temperature dependence, calcium ionophore A23187-induced efflux and influx, and lanthanum displacement. Calcium uptake into membrane vesicles was sodium-independent. Negative membrane potential induced by valinomycin and anion substitution studies indicated an electroneutral process. Initial rate of uptake of calcium was linear up to 30 s (Y = 0.11 + 0.02x, r = 0.99). Kinetic parameters were determined from uptake measurements at 7 s, well within the linear phase of uptake. Calcium uptake represented mediated and nonmediated components. These components showed changes along the intestinal tract. Km values of the mediated component increased aborally, being lowest in the duodenum and highest in the terminal ileum. Vmax was highest in the duodenum, followed by, in descending order, the ileum, terminal ileum, and jejunum. The nonmediated component was greatest in the duodenum and decreased aborally. The duodenum appears to have a high-affinity, high-capacity system for the transport of calcium in humans. These studies are the first to characterize calcium transport by brush border membranes of the human small intestine.