Early stimulation of alkaline phosphatase activity in response to 1 alpha, 25-dihydroxycholecalciferol.

Alkaline phosphatase activity (APA) stimulation in response to 1,25-dihydroxycholecalciferol (1,25 (OH)₂D₃) has been studied in vitamin-D-deficient rat intestinal brush borders prepared from $\text{ex}$-$\text{vivo}$-perfused duodeno-jejunal segments. Basal APA in intestines perfused with ethanol remained constant throughout the experiments. APA was significantly increased when intestines were perfused with 1,25 (OH)₂D₃ (3 nM) for 30, 45 or 60 min. A dose-effect response was observed when 1,25 (OH)₂D₃ increased in the perfusion medium. The maximal alkaline phosphatase activity after a 45-min perfusion (2404 ± 379 m^TU/mg prot.) was observed when 1,25 (OH)₂D₃ concentration was 6 nM. Cholecalciferol had no effect in this system.