Ion microscopic imaging of calcium during 1,25-dihydroxyvitamin D-mediated intestinal absorption.

A combination of ion microscopic and conventional radionuclide techniques was employed to investigate the temporal-spatial dynamics of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-stimulated intestinal calcium (Ca) absorption. At varying times following the administration of a single intravenous dose of 1,25(OH)2D3 to vitamin D-deficient chicks, transepithelial transport and tissue retention of Ca were quantitated in vivo, using the ligated duodenal loop technique and 47Ca as the tracer. The localization of Ca in the intestinal tissue during absorption was monitored by ion microscopy, using the stable Ca isotope, 44Ca, as the absorbed species. There was little transepithelial absorption of Ca in the vitamin D-deficient animals despite a substantial tissue accumulation of luminally derived Ca, the latter localizing predominantly in the brush border region of the enterocyte, as shown by the 44Ca-ion microscopic images. The early (30 min-1 h) response to 1,25(OH)2D3 was an increased tissue uptake of luminal 47Ca, which also primarily associated with the brush border region, again as shown by ion microscopy. At 2-4 h after the 1,25(OH)2D3 dose, there was a progressive redistribution of Ca from the brush border region throughout the cytoplasm and into the lamina propria. At 8-16 h, 47Ca absorption was maximal and 44Ca was sparsely distributed in the intestinal tissue. 47Ca absorption gradually declined and reached pre-dose levels by 72 h. At this time, tissue 44Ca was again largely limited to the brush border region. These results provide support for the multiple actions of 1,25(OH)2D3 on the intestinal Ca absorption process. The ion microscopic images provided unique information on the specific time-dependent changes in the tissue localization of Ca during the process of its intestinal absorption as affected by 1,25(OH)2D3.