Roles of three branchial Na+-K+-ATPase α-subunit isoforms in freshwater adaptation, seawater acclimation, and active ammonia excretion in Anabas testudineus

Ip YK, Loong AM, Kuah JS, Sim EW, Chen XL, Wong WP, Lam SH, Delgado IL, Wilson JM, Chew SF. Roles of three branchial Na+-K+-ATPase alpha-subunit isoforms in freshwater adaptation, seawater acclimation, and active ammonia excretion in Anabas testudineus. Am J Physiol Regul Integr Comp Physiol 303: R112-R125, 2012. First published May 23, 2012; doi:10.1152/ajpregu.00618.2011.-Three Na+-K+-ATPase (nka) alpha-subunit isoforms, nka alpha 1a, nka alpha 1b, and nka alpha 1c, were identified from gills of the freshwater climbing perch Anabas testudineus. The cDNA sequences of nka alpha 1a and nka alpha 1b consisted of 3,069 bp, coding for 1,023 amino acids, whereas nka alpha 1c was shorter by 22 nucleotides at the 5' end. In freshwater, the quantity of nka alpha 1c mRNA transcripts present in the gills was the highest followed by nka alpha 1a and nka alpha 1b that was almost undetectable. The mRNA expression of nka alpha 1a was downregulated in the gills of fish acclimated to seawater, indicating that it could be involved in branchial Na+ absorption in a hypoosmotic environment. By contrast, seawater acclimation led to an upregulation of the mRNA expression of nka alpha 1b and to a lesser extent nka alpha 1c, indicating that they could be essential for ion secretion in a hyperosmotic environment. More importantly, ammonia exposure led to a significant upregulation of the mRNA expression of nka alpha 1c, which might be involved in active ammonia excretion. Both seawater acclimation and ammonia exposure led to significant increases in the protein abundance and changes in the kinetic properties of branchial Na+-K+-ATPase (Nka), but they involved two different types of Nka-immunoreactive cells. Since there was a decrease in the effectiveness of NH4+ to substitute for K+ to activate branchial Nka from fish exposed to ammonia, Nka probably functioned to remove excess Na+ and to transport K+ instead of NH4+ into the cell to maintain intracellular Na+ and K+ homeostasis during active ammonia excretion.