Effect of vitamin D on transepithelial phosphate transport in chick intestine.

Abstract

The transport of Pi by everted gut sacs prepared from chick small intestine was investigated. Vitamin D increased the concentration ratio Pi across all intestinal segments, with the greatest response occurring in the jejunum. In this segment, Pi uptake was by a saturable process, with a Vmax of 0.32 and 0.13 .mu.mol/g tissue per min and a Km of 0.19 and 0.20 mM in the presence or absence of vitamin D, respectively. Only the Vmax was significantly affected by vitamin D. The transport of Pi was not dependent on the presence of C in the mucosal solution, and absorbed Pi does not appear to enter the total mucosal pool of Pi. Influx and efflux of Pi across the serosal boundary were observed to be vitamin D-independent. Anaerobic conditions depressed mucosal Pi influx, pointing to a vitamin D-dependent, energyrequiring Pi system at the brush border. Based on current observations and on thermodynamic considerations, Pi probably enters the mucosal tissue by vitamin D-dependent uphill transport and traverses the epithelial layer by a path distinct from endogenous Pi. The transfer of Pi from tissue to serosal compartment appears to be by diffusion (possibly facilitated) and nonvitamin D dependent. If an active exit pump exists at the basal-lateral membrane, it was undetected under the present conditions.