TRANSPORT OF CHOLINE BY MADIN-DARBY CANINE KIDNEY-CELLS

Choline is an essential precursor for the synthesis of phosphatidylcholine, the most abundant phospholipid classes in renal cells, as well as for the synthesis of the osmolyte glycerophosphorylcholine. The characteristics of choline uptake in the renal epithelial cell line MDCK were investigated. In the range of physiological concentrations, choline entered MDCK cells, grown as a monolayer on solid support, via a specific sodium-independent transport system (apparent K-m = 43 mu M, apparent V-max = 284 pmol/mg protein per 5 min). Cell ATP depletion, addition of KCI to the medium to reduce the cell membrane potential, and hemicholinium-3 (HC-3) inhibited choline uptake. Specific binding of [H-3]HC-3 was detected on the apical membrane of cells grown on plastic dishes, whereas it occurred only on the basolateral domain of cells grown on permeant support. When growing cells on filter, choline uptake from the basolateral side was 10-times the apical uptake. This suggests that the choline carrier present at the apical domain of cells grown on solid support is either inactivated or no longer targeted to the apical but to the basolateral membrane of MDCK cells grown on filter.