Alterations of duodenal vitamin D-dependent calcium-binding protein content and calcium uptake in brush border membrane vesicles in aged Wistar rats: role of 1,25-dihydroxyvitamin D3.

Previously we reported that uptake of Ca2+ in cells isolated from rat duodenum declined in senescence. In this paper we examined the possible mechanisms for this age-related defect. Duodenal vitamin D-dependent calcium-binding protein decreased steadily from 3-12 months (mo), followed by a minimal decline at 24 mo. On the contrary, Ca2+ uptake was not different in 3-, 6-, and 12-mo-old rats. A significant decline of Ca2+ uptake was observed at 24 mo. ATP contents in duodenal cells from 6- and 24-mo-old rats were not different. This suggests that the metabolic status of the duodenal cells was not the cause of the change in Ca2+ uptake. Ca2+ uptake activity was significantly lower in brush border membrane vesicles isolated from 24-mo-old rats than in those from 6-mo-old rats. The decrease in Ca2+ uptake activity in old rats was not due to a change in the Ca2(+)-binding capacity of the membranes. Kinetic analysis shows that the Vmax, the apparent maximum uptake capacity of membrane vesicles, decreased in senescent rats, whereas the Km, the apparent affinity to Ca2+, was unchanged. Since duodenal Ca2+ influx at the brush border was regulated by 1,25-dihydroxy-vitamin D3 [1,25-(OH)2D3], we tested the effect of 1,25-(OH)2D3 administration on the uptake activity in isolated membrane vesicles. After 1,25-(OH)2D3 treatment, Ca2+ uptake activity in brush border membranes prepared from senescent rats was only slightly lower than that in membranes from adult rats. We conclude that the decline in the influx of Ca2+ at the brush border membrane was the main cause of the decrease in duodenal Ca2+ uptake activity in aging. This defect was probably due to the low serum 1,25-(OH)2D3 concentration and not the result of impaired response to 1,25-(OH)2D3.