AUTHOR=Chen Xiu L. , Zhang Biyan , Chng You R. , Ong Jasmine L. Y. , Chew Shit F. , Wong Wai P. , Lam Siew H. , Ip Yuen K. 

TITLE=Na+/H+ Exchanger 3 Is Expressed in Two Distinct Types of Ionocyte, and Probably Augments Ammonia Excretion in One of Them, in the Gills of the Climbing Perch Exposed to Seawater

JOURNAL=Frontiers in Physiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2017.00880

DOI=10.3389/fphys.2017.00880

ISSN=1664-042X

ABSTRACT=<p>The freshwater climbing perch, <italic>Anabas testudineus</italic>, is an euryhaline teleost and an obligate air-breather with the ability to actively excrete ammonia. Members of the Na<sup>+</sup>/H<sup>+</sup> exchanger (NHE) family help maintain intracellular pH homeostasis and ionic balance through the electroneutral exchange of Na<sup>+</sup> and H<sup>+</sup>. This study aimed to obtain, from the gills of <italic>A. testudineus</italic>, the full cDNA coding sequence of <italic>nhe3</italic>, and to determine the effects of exposure to seawater or 100 mmol l<sup>−1</sup> of NH<sub>4</sub>Cl in fresh water on its mRNA and protein expression levels. Efforts were also made to elucidate the type of ionocyte that Nhe3 was associated with in the branchial epithelium of <italic>A. testudineus</italic>. The transcript level and protein abundance of <italic>nhe3</italic>/Nhe3 were very low in the gills of freshwater <italic>A. testudineus</italic>, but they increased significantly in the gills of fish acclimated to seawater. In the gills of fish exposed to seawater, Nhe3 was expressed in two distinct types of seawater-inducible Na<sup>+</sup>/K<sup>+</sup>-ATPase (Nka)-immunoreactive ionocytes. In Nkaα1b-immunoreactive ionocytes, Nhe3 had an apical localization. As these ionocytes also expressed apical Rhcg1 and basolateral Rhcg2, which are known to transport ammonia, they probably participated in proton-facilitated ammonia excretion in <italic>A. testudineus</italic> during seawater acclimation. In Nkaα1c-immunoreactive ionocytes, Nhe3 was atypically expressed in the basolateral membrane, and its physiological function is uncertain. For <italic>A. testudineus</italic> exposed to NH<sub>4</sub>Cl in fresh water, the transcript and protein expression levels of <italic>nhe3</italic>/Nhe3 remained low. In conclusion, the branchial Nhe3 of <italic>A. testudineus</italic> plays a greater physiological role in passive ammonia transport and acid-base balance during seawater acclimation than in active ammonia excretion during environmental ammonia exposure.</p>