Apical sodium uptake in toad kidney epithelial cell line A6.

The characteristics of the apical entry pathway for sodium into the cultured toad kidney epithelial cell line A6 are studied. Unidirectional apical sodium fluxes were determined by measuring the uptake of 22Na into confluent A6 epithelia growing in filter-bottomed cups. Apical sodium uptake was found to be a saturable function of sodium concentration with a Michaelis constant of 18 mM and a maximum velocity of 2.5 nmol X min-1 X cm-2. Amiloride competitively inhibits this sodium entry pathway with an inhibitor dissociation constant of 5 X 10(-8) M. Incubation of the epithelium with 10(-7) M aldosterone leads to a threefold increase in apical sodium uptake after 4 h. Both the aldosterone-stimulated and base-line sodium fluxes are completely inhibited by 10(-4) M amiloride. The similarity of these results to those from other tissues such as toad bladder and frog skin indicate that the A6 cells provide a useful model system for studying the apical entry pathway for sodium in tight epithelia.