The location of calcium during its transport by the small intestine of the chick.

During the in vivo absorption of Ca by chicks, cholecalciferol (vitamin D3) increased the translocation of Ca by the mucosal cells of the small intestine and this was accompanied by a greater "turnover" of Ca within these cells. The location of Ca within mucosal cells was studied by differential centrifugation after conventional homogenation of the cells in isotonic sucrose medium at 0 degrees. Most of the Ca was in mitochondria with less than 2% of Ca being in the soluble phase. However, this finding may be misleading since there may be a redistribution of Ca at 0 degrees, with the mitochondria taking up Ca which in vivo would be located in other parts of the cell. When cells were homogenized in the presence of inhibitors of Ca uptake by mitochondria or were homogenized at 22 degrees-28 degrees, 15-30% of the Ca was found in the soluble cytosol fraction. The uptake of Ca by mitochondria was compared in sucrose media and in this media containing dialysed cell cytosol. The non-dialysable substances in the cytosol influenced uptake and release of Ca by mitochondria so that more Ca would be in the soluble phase in the presence of cell cytosol. Dialysed cell cytosol from cholecalciferol-pretreated chicks had greater effect than cytosol from rachitic chicks and this may partially account for the increased translocation brought about by cholecalciferol.