Investigation of the Ba2+-sensitive NH4+ transport pathways in the apical cell membrane of primary cultured rabbit MTAL cells.

BACKGROUND: Several apical ammonium (NH(4)(+)/NH(3)) transport pathways have been described in medullary thick ascending limb (MTAL) cells. The exact nature and importance of some of these pathways remain controversial.

METHODS: Ammonium transport in primary cultured rabbit MTAL cells was investigated by measuring intracellular pH (pH(i)).

RESULTS: To create physiological conditions, experiments were performed in the symmetrical presence of NH(4)Cl, which acidified the cells to pH(i) 6.89. When blockers of apical NH(4)(+) transport were used, the cells alkalinized due to a decreased NH(4)(+) loading. The following values (pH units) were observed: bumetanide, +0.05; verapamil, +0.04; Ba(2+) and Cs(+), +0.19; tertiapin, +0.09. Tetraethylammonium had no effect. Depolarizing the cells by increasing the K(+) concentration alkalinized the cells by 0.16 pH units. Because NH(4)(+) might enter through nonspecific channels, ammonium pulse experiments were performed: an NH(4)Cl pulse acidified controls as well as depolarized cells. In contrast, when Ba(2+), Cs(+) or tertiapin were present, an NH(4)Cl pulse alkalinized the cells. The pharmacological profile of this apical NH(4)(+) transport pathway correlates with the renal outer medullary K(+) (ROMK) channel. Indirect immunofluorescence showed the presence of the ROMK protein.

CONCLUSION: In these MTAL cells the Ba(2+)-sensitive component of NH(4)(+) transport is predominant and consists of permeation of NH(4)(+) through an apical ROMK-related channel.