Duodenal active calcium transport in female rats increases with serum calcitriol concentrations, but reaches a plateau far below maximal calcitriol levels

To determine the relationship between serum calcitriol concentration and duodenal active calcium (Ca) transport, a wide range of circulating calcitriol concentrations (18‐950 pg/ml) was obtained by feeding nonmated, lactating, and weaned rats vitamin D‐sufficient diets containing 0.04, 0.06, 0.1, or 0.4% Ca. Ca transport was measured in vitro with the everted gut sac technique using both the proximal (D‐1) and distal (D‐2) duodenal halves. The ratio of serosal [Ca]/mucosal [Ca] (S/M) as well as the amount of Ca transported was calculated. The S/M ratio correlated with the serum calcitriol concentration over the range 18–90 pg/ml with slopes for the regression lines of 0.066 ± 0.010 ( R 2 = 0.64, n = 27) for D‐1 and of 0.036 ± 0.005 ( R 2 = 0.73, n = 24) for D‐2. The regression lines are significantly different from zero ( p < 0.001) and from each other ( p < 0.01). For D‐1, a plateau of the S/M ratio of 7–9 appeared to be reached at a calcitriol concentration of approximately 90 pg/ml, and the plateau was maintained over the range 90–900 pg/ml calcitriol. For D‐2, a plateau of the S/M ratio of 4–6 appeared to be reached at 200–300 pg/ml calcitriol. Calculating the amount of Ca transported per 10 cm per hour revealed a pattern similar to that of the S/M ratio. When net Ca absorption was determined from balance studies over 4 days for rats on a 0.04% Ca diet, maximal absorption (mg/day) was already observed at a serum calcitriol concentration of 60–70 pg/ml ( n = 14). We conclude that active Ca transport correlates with serum calcitriol concentrations, but that the transport capacity quickly reaches a maximal value, which is maintained over a 10‐fold higher range of serum calcitriol concentrations. (J Bone Miner Res 1996;11:1640–1645)