Discordant effects of glucocorticoids on active and passive transport of calcium in the rat duodenum.

The effect of glucocorticoids on duodenal active and passive calcium transport was studied by the in situ intestinal loop technique, using 2.0 mM and 50 mM concentrations of calcium. The administration of prednisolone resulted in a significant increase of fluid and sodium absorption. The final luminal calcium concentration was increased in the prednisolone-treated group. Under conditions where active transport predominates (2 mM), glucocorticoids decreased lumen-to-plasma and increased plasma-to-lumen calcium flux, resulting in a decrease of net calcium absorption. Under conditions where passive transport predominates (50 mM), glucocorticoids did not decrease calcium absorption in the duodenum. These data suggest that under conditions where passive transport predominates, glucocorticoids indirectly enhance calcium absorption by an effect on water movement. Water movement enhances calcium absorption by concentrating unabsorbed calcium within the intestinal lumen. The increase in passive diffusion of calcium in the duodenum compensates for the inhibition of the active transport of calcium by glucocorticoids.