ION MICROSCOPIC IMAGING OF CALCIUM DURING 1,25-DIHYDROXYVITAMIN D-MEDIATED INTESTINAL-ABSORPTION

A combination of ion microscopic and conventional radionuclide techniq ues was employed to investigate the temporal-spatial dynamics of 1,25- dihydroxyvitamin D-3 intestinal calcium (Ca) absorption. At varying ti mes following the administration of a single intravenous dose of 1,25( OH)(2)D-3 to vitamin D-deficient chicks, transepithelial transport and tissue retention of Ca were quantitated in vivo, using the ligated du odenal loop technique and Ca-47 as the tracer. The localization of Ca in the intestinal tissue during absorption was monitored by ion micros copy, using the stable Ca isotope, Ca-44, as the absorbed species. The re was little transepithelial absorption of Ca in the vitamin D-defici ent animals despite a substantial tissue accumulation of luminally der ived Ca, the latter localizing predominantly in the brush border regio n of the enterocyte, as shown by the Ca-44-ion microscopic images. The early (30 min-1 h) response to 1,25(OH)(2)D-3 was an increased tissue uptake of luminal Ca-47, which also primarily associated with the bru sh border region, again as shown by ion microscopy. At 2-4 h after the 1,25(OH)(2)D-3 dose, there was a progressive redistribution of Ca fro m the brush border region throughout the cytoplasm and into the lamina propria. At 8-16 h, Ca-47 absorption was maximal and Ca-44 was sparse ly distributed in the intestinal tissue. Ca-47 absorption gradually de clined and reached pre-dose levels by 72 h. At this time, tissue Ca-44 was again largely limited to the brush border region. These results p rovide support for the multiple actions of 1,25(OH)(2)D-3 on the intes tinal Ca absorption process. The ion microscopic images provided uniqu e information on the specific time-dependent changes in the tissue loc alization of Ca during the process of its intestinal absorption as aff ected by 1,25(OH)(2)D-3.