Calcium Uptake by Intestinal Brush Border Membrane Vesicles Comparison with In Vivo Calcium Transport

In prior studies, we examined kinetics of steady state in vivo transepithelial calcium transport in rat and hamster. The present studies related calcium uptake by the brush border to in vivo transport. We measured calcium uptake by brush border mem-brane vesicles from the two species. In the rat, our prior in vivo studies had shown that (a) calcium transport was mediated, (b) no nonmediated component was detectable, and (c) V,, was 2.5 times greater in proximal than distal small intestine. In brush border membrane vesicles from the rat, V., for the saturable component of calcium uptake was again 2.5 times greater in proximal than distal intestine. Contrasting with in vivo studies, a major nonsaturable component was present in vesicles from proximal and distal small intestine. In the hamster, our previous in vivo studies had shown (1) both mediated and nonmediated components ofcalcium transport, (2) greater nonmediated trans-port in proximal than distal small intestines, and (3) V.,, for calcium transport twice as great in distal as in proximal small intestine. In the present study with brush border membrane ves-icles from hamster, V,,,. for saturable calcium transport was again twice as great in distal as in proximal small intestine. However, nonsaturable calcium transport rates relative to sat-urable rates were much greater with vesicles than in in vivo stud-ies, and were greater in vesicles from distal than proximal small intestine. Since rates of saturable calcium uptake by brush border membrane vesicles parallel corresponding in vivo mediated transport rates, we conclude that the segmental rates of calcium transport in rat and hamster could be determined by brush border function