Intestinal absorption of calcium and phosphorus.

The intestinal absorption of calcium and phosphorus has received considerable attention in recent years. The evidence has clearly indicated that calcium is absorbed by two processes: active transport and diffusion. Vitamin D appears to affect both processes, and has a significant effect at the brush border of the intestinal cell. Vitamin D also affects the release of calcium from the intestinal cell to the lamina propria. Several proposed models to account for the transmural movement of calcium are discussed. The active transport of phosphate is under the control of vitamin D and is located at the brush border region of the intestinal cell. This transport system, like several others, appears to be sodium-dependent and inhibited by ouabain. In-transit phosphate does not mix with the cellular phosphate pool, implying that phosphate is moving through the cell as a distinct packet or through specialized channels, or possibly a phosphorylated derivative. Emphasized in the presentation is current knowledge of the transport mechanisms and macromolecular changes that potentially account for the stimulatory effect of vitamin D on calcium and phosphate transport.