DUODENAL ACTIVE CALCIUM-TRANSPORT IN FEMALE RATS INCREASES WITH SERUMCALCITRIOL CONCENTRATIONS, BUT REACHES A PLATEAU FAR BELOW MAXIMAL CALCITRIOL LEVELS

To determine the relationship between serum calcitriol concentration a nd duodenal active calcium (Ca) transport, a nide range of circulating calcitriol concentrations (18-950 pg/ml) was obtained by feeding nonm ated, lactating, and weaned rats vitamin D-sufficient diets containing 0.01, 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) a s well as the amount of Ca transported was calculated. The S/M ratio c orrelated 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) fo r D-2. The regression lines are significantly different from zero (p < 0.001) and from each other (p < 0.01!, For D-l, a plateau of the S/M ratio of 7-9 appeared to be reached at a calcitriol concentration of a pproximately 90 pg/ml, and the plateau was maintained over the range 9 0-900 pg/ml calcitriol, For D-2, a plateau of the S/M ratio of 4-6 app eared to be reached at 200-300 pg/ml calcitriol, Calculating the amoun t of Ca transported per 10 cm per hour revealed a pattern similar to t hat of the S/M ratio. When net Ca absorption was determined from balan ce studies over 4 days for rats on a 0.04% Ca diet, maximal absorption (mg/day) aas already observed at a serum calcitriol concentration of 60-70 pg/ml (ii = 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 h igher range of serum calcitriol concentrations.