TRANSPORT OF CHOLINE BY MADIN-DARBY CANINE KIDNEY-CELLS

Choline is an essential precursor for the synthesis of phosphatidylcho line, the most abundant phospholipid classes in renal cells, as well a s for the synthesis of the osmolyte glycerophosphorylcholine. The char acteristics of choline uptake in the renal epithelial cell line MDCK w ere investigated. In the range of physiological concentrations, cholin e entered MDCK cells, grown as a monolayer on solid support, via a spe cific sodium-independent transport system (apparent K-m = 43 mu M, app arent V-max = 284 pmol/mg protein per 5 min). Cell ATP depletion, addi tion of KCI to the medium to reduce the cell membrane potential, and h emicholinium-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 gr own on permeant support. When growing cells on filter, choline uptake from the basolateral side was 10-times the apical uptake. This suggest s that the choline carrier present at the apical domain of cells grown on solid support is either inactivated or no longer targeted to the a pical but to the basolateral membrane of MDCK cells grown on filter.