Ammonium transport and pH regulation by K+-Cl- cotransporters

The Na+- K+- Cl- cotransporters (NKCCs), which belong to the cation-Cl- cotransporter (CCC) family, are able to translocate NH4+ across cell membranes. In this study, we have used the oocyte expression system to determine whether the K+-Cl- cotransporters (KCCs) can also transport NH4+ and whether they play a role in pH regulation. Our results demonstrate that all of the CCCs examined (NKCC1, NKCC2, KCC1, KCC3, and KCC4) can promote NH4+ translocation, presumably through binding of the ion at the K+ site. Moreover, kinetic studies for both NKCCs and KCCs suggest that NH4+ is an excellent surrogate of Rb+ or K+ and that NH4+ transport and cellular acidification resulting from CCC activity are relevant physiologically. In this study, we have also found that CCCs are strongly and differentially affected by changes in intracellular pH ( independently of intracellular [NH4+]). Indeed, NKCC2, KCC1, KCC2, and KCC3 are inhibited at intracellular pH <7.5, whereas KCC4 is activated. These results indicate that certain CCC isoforms may be specialized to operate in acidic environments. CCC-mediated NH4+ transport could bear great physiological implication given the ubiquitous distribution of these carriers.