AJP - Gastrointestinal and Liver Physiology, Vol 250, Issue 3 323-G330, Copyright © 1986 by American Physiological Society
Sodium-dependent transport of Pi by an established intestinal epithelial cell line (CaCo-2)
I. Mohrmann, M. Mohrmann, J. Biber and H. Murer
The uptake of inorganic phosphate (Pi) was analyzed in monolayers and in
apical membrane vesicles (AMV) of the established intestinal cell line
CaCo-2. AMV, prepared by a MgCl2 precipitation technique, were enriched
approximately 10-fold in alkaline phosphatase activity. Pi uptake into
intact cells as well as into AMV was specifically dependent on the presence
of sodium. In the presence of high sodium concentrations, the apparent Km
for Pi was 214 +/- 17 mumol/l in monolayers and 300 +/- 19.7 mumol/l in
AMV. Increasing the sodium concentration increased the apparent affinity of
the transport system for Pi but hardly affected the maximal velocity
(Vmax). At 0.1 mmol/l Pi and pH 7.4, the apparent Km for sodium was
approximately 70 mmol/l in intact cells as well as in AMV. The results
obtained in both systems suggested the involvement of two sodium ions and
one phosphate ion in the transport process. Advancing
confluence--independently of the age of the monolayers--reduced
sodium-dependent uptake of Pi significantly by a decrease in Vmax, whereas
the apparent Km for Pi remained unchanged. It is concluded that the apical
membrane of CaCo-2 cells contains a sodium-dependent transport system for
Pi.
