A mechanism for branchial acid excretion in marine fish: identification of multiple Na+/H+ antiporter (NHE) isoforms in gills of two seawater teleosts

Abstract

Both Na+/H+ exchange and the electrogenic extrusion of H+ via an H+-ATPase have been postulated to drive acid excretion across the branchial epithelium of fishes. While the H+-ATPase/Na+ channel system appears to be the predominant mechanism in some freshwater species, it may play a reduced role in seawater and brackish-water animals, where high external Na+ concentrations may thermodynamically favor Na+/H+ exchange driven by a Na+/H+ antiporter (NHE). In this study, we used molecular and immunological methods to assess the role of NHE isoforms in the branchial epithelium of the marine long-horned sculpin (Myoxocephalus octodecimspinosus) and the euryhaline killifish (Fundulus heteroclitus).Northern blot analysis of RNA probed with the human NHE-1 BamHI fragment suggested the presence of homologous gill NHE mRNA in sculpin. RT-PCR on gill RNA isolated from sculpin recovering from metabolic acidosis provided evidence for two distinct NHE isoforms; one with 76 % amino acid homology to mammalian NHE-2, and another 92 % homologous to trout erythrocytic beta-NHE. Killifish also have transcripts with 91 % homology to beta-NHE. Immunological detection using monoclonal antibodies for mammalian NHE-1 revealed a protein antigenically similar to this isoform in the gills of both species. Metabolic acidosis caused an approximately 30-fold decrease in expression of the NHE-1-like protein in sculpin. We speculate that beta-NHE in the gills plays the intracellular ‘housekeeping’ roles described for mammalian NHE-1. During systemic acidosis, apical gill NHE-2 (which is sensitive to external amiloride and low [Na+]) in parallel with a dramatic suppression of basolateral NHE-1 activity enhances net capdelta H+ transfers to the water.

The extensive transgenerational transcriptomic effects of ocean acidification on the olfactory epithelium of a marine fish are associated with a better viral resistance

“…OA alone or coupled with ocean warming poses globalscale threats to marine life ranging from the invisible scale of microbes to ecosystem levels [6][7][8][9]. Given their efficient capacity for maintaining acid-base homeostasis, fishes have long been considered to be robust organisms capable of tolerating OA [10][11][12][13]. However, reported indirect impacts of OA include both inter-and intraspecific changes in sensory behaviours [14], which result from directly altered neurosensory systems [15][16][17][18][19][20][21][22][23] that ultimately affect reproduction, fitness and mortality [24][25][26][27].…”

Immunochemical analysis of the vacuolar proton-ATPase B-subunit in the gills of a euryhaline stingray (Dasyatis sabina): effects of salinity and relation to Na+/K+-ATPase

“…Experiments on rainbow trout have corroborated this hypothesis by demonstrating that gill V-H + -ATPase activity, immunoreactivity and mRNA expression all increase after exposure to environmental hypercapnia (Lin and Randall, 1993;Lin et al, 1994;Sullivan et al, 1995;Sullivan et al, 1996;Perry et al, 2000). Studies on branchial V-H + -ATPase in a true marine teleost have yet to be published, but the transporter's role in acid-base regulation of seawater teleosts is assumed to be minimal, given the favorable Na + gradient for Na + /H + exchangers (see Claiborne, 1998;Claiborne et al, 1999).…”

Role of the Basolateral Na+/H+ Exchanger-2 (NHE2) in Ionocytes of Seawater- Acclimated Medaka (Oryzias latipes)

“…It is generally accepted that sodium (Na + ) and hydrogen (H + ) exchangers (NHEs) are the major transporters of acid (H + ) secretion by ionocytes ( Hwang and Chou, 2013 ; Liu et al, 2013 ). Several NHE isoforms (NHE1, NHE2, NHE3, NHE5, NHE6, NHE7, and NHE8) were found in fishes, ( Hu et al, 2013 ; Yan and Hwang, 2019 ); and NHE1, NHE2, and NHE3 were found in the gills of fishes ( Claiborne et al, 1999 , 2002 ; Edwards et al, 2005 ). In freshwater (FW) fishes, NHE3 at the apical membrane was demonstrated to be the major isoform for acid secretion by ionocytes ( Yan et al, 2007 ; Inokuchi et al, 2008 ; Wu et al, 2010 ).…”

“…Compared to NHE3, however, the localization and function of NHE2 in ionocytes of SW fish are still uncertain. Several studies showed that messenger (m)RNA and protein expressions of NHE2 were stimulated by acidic water or infusion of HCl, suggesting that NHE2 is involved in acid secretion by gills ( Claiborne et al, 1999 ; Tresguerres et al, 2005 ; Catches et al, 2006 ).…”