Ammonia exposure affects the mRNA and protein expression levels of certain Rhesus glycoproteins in the gills of climbing perch.

The freshwater climbing perch, , is an obligate air-breathing and euryhaline teleost capable of active ammonia excretion and tolerant of high concentrations of environmental ammonia. As Rhesus glycoproteins (RhGP/Rhgp) are known to transport ammonia, this study aimed to obtain the complete cDNA coding sequences of various isoforms from the gills of , and to determine their mRNA and protein expression levels during 6 days of exposure to 100 mmol l NHCl. The subcellular localization of Rhgp isoforms in the branchial epithelium was also examined in order to elucidate the type of ionocyte involved in active ammonia excretion. Four (, , and ) had been identified from the gills of They had conserved amino acid residues for NH binding, NH deprotonation, channel gating and lining of the vestibules. Despite inwardly directed NH and NH gradients, there were significant increases in the mRNA expression levels of the four branchial in at certain time points during 6 days of ammonia exposure, with significant increases in the protein abundances of Rhag and Rhcg2 on day 6. Immunofluorescence microscopy revealed a type of ammonia-inducible Na/K-ATPase α1c-immunoreactive ionocyte with apical Rhag and basolateral Rhcg2 in the gills of fish exposed to ammonia for 6 days. Hence, active ammonia excretion may involve NH entering the ionocyte through the basolateral Rhcg2 and being excreted through the apical Rhag, driven by a transapical membrane electrical potential generated by the apical cystic fibrosis transmembrane conductance regulator Cl channel, as suggested previously.