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 and⁴⁷Ca 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 the⁴⁴Ca-ion microscopic images. The early (30 min-1 h) response to 1,25(OH)₂D₃ was an increased tissue uptake of luminal⁴⁷Ca, which also primarily associated with the brush border region, again as shown by ion microscopy. At 2–4 h after the 1,25(OH)₂)D₃ 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 and⁴⁴Ca was sparsely distributed in the intestinal tissue.⁴⁷Ca absorption gradually declined and reached pre-dose levels by 72 h. At this time, tissue⁴⁴Ca 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