Functional roles of Na(+)/K(+)-ATPase in active ammonia excretion and seawater acclimation in the giant mudskipper, Periophthalmodon schlosseri.

Shit F Chew,Seow W Ong,Sock P Lam,Wei L Wee,Yuen K Ip,Kum C Hiong,Wai P Wong

doi:10.3389/fphys.2014.00158

Shit F Chew, Seow W Ong + Show 5 more

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https://doi.org/10.3389/fphys.2014.00158

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Abstract

The giant mudskipper, Periophthalmodon schlosseri, is an amphibious fish that builds burrows in the mudflats. It can actively excrete ammonia through its gills, and tolerate high environmental ammonia. This study aimed to examine the effects of seawater (salinity 30; SW) acclimation and/or environmental ammonia exposure on the kinetic properties of Na+/K+-ATPase (Nka) from, and mRNA expression and protein abundance of nka/Nka α–subunit isoforms in, the gills of P. schlosseri pre-acclimated to slightly brackish water (salinity 3; SBW). Our results revealed that the Nka from the gills of P. schlosseri pre-acclimated to SBW for 2 weeks had substantially higher affinity to (or lower Km for) K+ than NH+4, and its affinity to NH+4 decreased significantly after 6-days exposure to 75 mmol l−1 NH4Cl in SBW. Hence, Nka transported K+ selectively to maintain intracellular K+ homeostasis, instead of transporting NH+4 from the blood into ionocytes during active NH+4 excretion as previously suggested. Two nkaα isoforms, nkaα1 and nkaα3, were cloned and sequenced from the gills of P. schlosseri. Their deduced amino acid sequences had K+ binding sites identical to that of Nkaα1c from Anabas testudineus, indicating that they could effectively differentiate K+ from NH+4. Six days of exposure to 75 mmol l−1 NH4Cl in SBW, or to SW with or without 50 mmol l−1 NH4Cl led to significant increases in Nka activities in the gills of P. schlosseri. However, a significant increase in the comprehensive Nkaα protein abundance was observed only in the gills of fish exposed to 50 mmol l−1 NH4Cl in SW. Hence, post-translational modification could be an important activity modulator of branchial Nka in P. schlosseri. The fast modulation of Nka activity and concurrent expressions of two branchial nkaα isoforms could in part contribute to the ability of P. schlosseri to survive abrupt transfer between SBW and SW or abrupt exposure to ammonia.

Highlights

Majority of fully aquatic fishes are ammonotelic, excreting more than 50% of their nitrogenous waste as ammonia-N through their gills
Exposure of P. schlosseri to SW or to ammonia in slightly brackish water (SBW) or SW resulted in significant increases in the Vsat of branchial Nka, with Na+ and K+/NH+4 as substrates, indicating that branchial Nka was essential for salt excretion in a hyperosmotic medium and active NH+4 excretion in a medium containing high concentrations of ammonia
1fo0rmNmHo+4l .l−T1akainndg the plasma 1 mmol l−1, K+ and NH+4 respectively, it concentrations as would mean that branchial Nka would be saturated with K+, and it is rheipghlalcyemimepnrtoobfabKle+.thMatorberainmcphoiarltaNnktlay,waoftuelrd6t-rdaanysspoexrtpoNsHur+4e in to 75 mmol l−1 icantly to 23

Summary

Introduction

Majority of fully aquatic fishes are ammonotelic, excreting more than 50% of their nitrogenous waste as ammonia-N through their gills. Air-breathing fishes would experience difficulties in excreting ammonia during emersion or exposure to puddles of water with high concentrations of ammonia. Air-breathing fishes adopt various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure (Ip et al, 2001a, 2004a,b; Chew et al, 2006; Ip and Chew, 2010; Chew and Ip, 2014). Three air-breathing tropical fishes, Periophthalmodon schlosseri (Randall et al, 1999; Chew et al, 2003, 2007), Anabas testudineus (Tay et al, 2006; Ip et al, 2012a,b; Loong et al, 2012) and Clarias gariepinus; (Ip et al, 2004c) are capable of active ammonia excretion through their gills

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