Abstract
To determine which region of the intestinal villus was primarily responsible for calcium uptake and whether cells from the different regions of the villus differed in their response to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], we studied cells eluted from the duodenal villus in a sequential fashion at various times after vitamin D-deficient chicks had received 1,25-(OH)2D3. The elution scheme employed removes cells from the villus tip first and cells from the villus base last, as was documented by the distribution of alkaline phosphatase activity, sucrase activity, and cytosolic calcium-binding protein (CaBP) in the eluted fractions. Brush border membrane vesicles (BBMV) were prepared from different fractions of the villus. Calcium uptake was greatest in BBMV from cells eluted from the villus tip and least in those from the villus base. The distribution of calcium uptake and alkaline phosphatase activity in the same BBMV were parallel. After 1,25-(OH)2D3 treatment, cytosolic CaBP was observed in the cells from the villus base by 4 h and in all fractions by 8 h; at all times (from 4-24 h), cells from the villus base contained more cytosolic CaBP than did cells from the villus tip. Alkaline phosphatase activity in BBMV was stimulated in all fractions by 4 h; at all times, alkaline phosphatase activity was greatest in BBMV from cells of the villus tip. In contrast, calcium uptake by BBMV was stimulated 2 h after 1,25-(OH)2D3 administration only in cells from the villus tip and was not stimulated even by 24 h in cells from the villus base. These results indicate that the cellular response to 1,25-(OH)2D3 depends on the location of the cell on the villus and that 1,25-(OH)2D3-stimulated calcium flux across the brush border can be dissociated from 1,25-(OH)2D3-stimulated alkaline phosphatase activity and CaBP production.
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