Effects of 1,25(OH)2D3 on enterocyte basolateral membrane Ca transport in rats

One, twenty-five dihydroxyvitamin D3 [1,25(OH)2D3], commonly known as calcitriol, stimulates intestinal Ca absorption through increased activity of a cellular transport process. To determine whether transcellular Ca transport involves energy-dependent Ca efflux across enterocyte plasma membrane in vitamin D-sufficient rats, in vitro bidirectional Ca fluxes were measured under short-circuited conditions across proximal duodenum from rats fed diets adequate in vitamin D and containing a normal Ca diet (NCD), a low Ca diet (LCD), or fed NCD and injected with 50 ng of 1,25(OH)2D3 daily for 4 days before study. LCD or 1,25(OH)2D3 increased Ca net flux [Jnet, mucosal-to-serosal flux minus the serosal-to-mucosal flux] by increasing Ca mucosal-to-serosal flux (Jm----s) (mean +/- SE, NCD vs. LCD vs. 1,25(OH)2D3, 16 +/- 4 vs. 179 +/- 18 vs. 82 +/- 21 nmol.cm-2. h-1, P less than 0.0001). Initial ATP-dependent Ca uptake rates by duodenal basolateral membrane vesicles (BLMV) was greater in vesicles from rats fed NCD compared with LCD and not different from NCD injected with 1,25(OH)2D3. These studies suggest that in vitamin D-replete animals, 1,25(OH)2D3 increases epithelial Ca Jm----s by mechanisms that do not involve ATP-dependent BLM Ca efflux. ATP-dependent Ca exit from the cell under these conditions may play a role in intracellular Ca homeostasis rather than Ca absorption.