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

Abstract

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.

Aquaporin 4 as a NH3 Channel

“…NH 4 ϩ is readily transported by a range of K ϩ transporting mechanisms (7)(8)(9)(10)(11)(12)(13)(14)(15)(16), whereas NH 3 , probably due to its resemblance to water, appears to cross cell membranes by facilitated diffusion through a range of aquaporins, among which we here propose that AQP4 is featured. The cerebral glutamate-glutamine cycle encompasses vesicular release of glutamate from the presynapse with subsequent astrocytic uptake, amidation of glutamate to glutamine (a process requiring free ammonia), shuttling of glutamine to the neuronal structures, and its hydrolyzation to glutamate and ammonia.…”

“…At physiological pH, its pK a of 9.25 dictates that ϳ1.5% of the ammonia is found as NH 3 , whereas the rest exists as NH 4 ϩ , the latter of which absolutely requires membrane transporters or channels to cross the plasma membrane. Although dedicated NH 4 ϩ transport proteins have been identified (6), NH 4 ϩ is, due to its resemblance to K ϩ , in addition transported by a range of K ϩ transporters and channels, such as the Na ϩ /K ϩ -ATPase, the Na ϩ /K ϩ /2Cl Ϫ cotransporter, the K ϩ /Cl Ϫ cotransporter, and inwardly rectifying, voltage-, and Ca 2ϩ -activated K ϩ channels (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). The permeability of NH 4 ϩ through K ϩ channels usually amounts to ϳ10 -20% of the permeability of K ϩ (7).…”

Functional Association between K+-Cl- Cotransporter-4 and H+,K+-ATPase in the Apical Canalicular Membrane of Gastric Parietal Cells

“…Interestingly it has been reported that KCC1, KCC2, and KCC3 are inhibited at pH Ͻ7.5 when these are expressed in Xenopus oocytes, whereas KCC4 is activated at acidic pH (44). This finding suggests that KCC4 may be specialized to operate in an acidic environment.…”

Purinergic Stimulation Induces Ca2+-dependent Activation of Na+-K+-2Cl- Cotransporter in Human Nasal Epithelia

“…Characterization of the Basolateral NH 4 ϩ -transporting Mechanisms-To date, several transporters are known to transport NH 4 ϩ , in particular via K ϩ binding sites (19). Therefore, we characterized the basolateral NH 4 ϩ -transporting activities of NHNE cells.…”