Sodium-hydrogen exchangers in C. elegans and their potential role in hypodermal H+ excretion, Na+ uptake, ammonia excretion and acid-base balance
O'Donnell, Michael J.
Quijada-Rodriguez, Alex R.
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Cation/proton exchangers of the Cation proton antiporter 1 (CPA1) subfamily (NHEs, SLC 9) play an important role in many physiological processes, including cell volume regulation, acid-base homeostasis and ammonia excretion. The soil nematode Caenorhabditis elegans expresses nine paralogues (NHX-1 to -9) of these transporters, which all contain the highly conserved amiloride binding motif. The current study was undertaken to investigate the role of the cation/proton exchanger in hypodermal Na+ and H+ fluxes as well in ammonia excretion processes. Measurements using the scanning ion-selective electrode technique (SIET) showed that the hypodermis promotes H+ secretion as well as a Na+ uptake, indicating the participation of a cation/proton exchanger. Interestingly, while both fluxes, as well as whole body ammonia excretion rates were partially inhibited by amiloride (100 µmol l-1), no effect was observed on the H+ efflux and ammonia excretion rates when animals were exposed to 100 µmol l-1 EIPA, suggesting participation of apical Na+ channels in Na+ uptake, but probably not NHX transporters. In response to stress induced by starvation or exposure to 1 mmol l-1 NH4Cl (HEA), pH = 5.5, pH = 8.0), body pH stayed fairly constant, with changes of mRNA expression levels detected in intestinal NHX-2 and hypodermal NHX-3. Excretory cell NHX-9 was altered by exposure to pH 5.5 or 8.0 but not by HEA. In conclusion, the study suggest that apical localized EIPA-sensitive sodium/hydrogen exchangers do play a role in ammonia excretion and Na+ uptake in the hypodermis of C. elegans.