LOCALIZATION AND REGULATION BY VITAMIN-D OF CALCIUM-TRANSPORT PROTEINS IN RABBIT CORTICAL COLLECTING SYSTEM

The 1 alpha,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3]-induced expressio n of Na+/Ca2+ exchanger, Ca2+-adenosinetriphosphatase (Ca2+-ATPase), a nd calbindin-D-28k was investigated in the rabbit distal nephron. Immu nocytochemical studies in rabbit kidney sections revealed colocalizati on of the three Ca2+ transport proteins in the majority of cells in th e distal nephron, including connecting tubules and cortical collecting ducts. Subsequently, rabbit connecting and cortical collecting tubule cells were immunodissected and cultured on permeable supports. Immuno cytochemical analysis of the cultured cells by confocal microscopy rev ealed that Na+/Ca2+ exchanger and Ca2+-ATPase were present at the baso lateral membrane, whereas calbindin-D-28k was evenly distributed throu ghout the cytosol. Concomitant with an increase in Ca2+ transport, 1,2 5(OH)(2)D-3 increased calbindin-D-28k protein and RNA content two- to threefold, as determined by Northern and Western blotting. By contrast , neither Na+/Ca2+ exchanger nor Ca2+-ATPase RNA or protein content wa s noticeably altered. Our findings suggest that 1,25(OH)(2)D-3 stimula tion of transcellular Ca2+ transport in primary cultures of rabbit cor tical collecting system cells involves an increase in the gene express ion of calbindin-D-28k but not of Na+/Ca2+ exchanger and Ca2+-ATPase.