Salt secretion is linked to acid-base regulation of ionocytes in seawater-acclimated medaka: new insights into the salt-secreting mechanism

Abstract

Ionocytes in the skin and gills of seawater (SW) teleosts are responsible for both salt and acid secretion. However, the mechanism through which ionocytes secrete acid is still unclear. Here, we hypothesized that apical Na+/H+ exchangers (NHE2/3), carbonic anhydrase (CA2-like), and basolateral HCO3−/Cl− exchanger (AE1) are involved in acid secretion. In addition, the hypothesized involvement of basolateral AE1 suggested that acid secretion may be linked to Cl− secretion by ionocytes. The scanning ion-selective electrode technique (SIET) was used to measure H+ and Cl− secretion by ionocytes in the skin of medaka larvae acclimated to SW. Treatment with inhibitors of NHE, CA, and AE suppressed both H+ and Cl− secretion by ionocytes. Short-term exposure to hypercapnic SW stimulated both H+ and Cl− secretion. mRNA of CA2-like and AE1 were localized to ionocytes in the skin. Branchial mRNA levels of NKCC1a, CA2-like, and AE1a increased together with the salinity to which fish were acclimated. In addition, both AE1a and AE1b mRNA increased in fish acclimated to acidified (pH 7) SW; NKCC1a mRNA increased in fish acclimated to pH 9 SW. This study reveals the mechanism of H+ secretion by ionocytes, and refines our understanding of the well-established mechanism of Cl− secretion by ionocytes of SW fish.

Ion uptake pathways in European sea bass Dicentrarchus labrax

“…Despite its very low expression, sea bass nhe2c is about 28 times more expressed in SW than in FW suggesting its role in acid secretion as reported previously for several nhe2/3 paralogs in SW (Chen et al, 2017;Claiborne et al, 1999;Liu et al, 2016). Contrary to trout nhe2, sea bass nhe2b and nhe2c…”

“…3 killifish cuboidal cells, Na + uptake seems not to be coupled to active proton excretion in this species (Katoh et al, 2003). There is also evidence that NHE2 and NHE3 are expressed in gill ionocytes of teleosts maintained in seawater (SW), playing critical roles in acid secretion (Chen et al, 2017;Claiborne et al, 1999;Liu et al, 2016;Weihrauch et al, 2009). In many teleost species, nhe3 mRNA expression is high in FW compared to SW (Bollinger et al, 2016;Hiroi et al, 2008;Inokuchi et al, 2008;Watanabe et al, 2008) except in the climbing perch Anabas testudineus, where the opposite pattern has been shown with increased nhe3 mRNA and NHE3 protein expression in SW (Chen et al, 2017).…”

Functional classification of gill ionocytes and spatiotemporal changes in their distribution after transfer from seawater to fresh water in Japanese seabass

“…In SW fish, NHE in the apical membrane of ionocytes is thought to be the major transporter for acid secretion (Claiborne et al, 2002). In medaka gills, mRNA levels of NHE2 and NHE3 were upregulated by acidified SW ( pH 7), suggesting that NHE was involved in acid secretion by SW-type ionocytes (Liu et al, 2016). Moreover, applying the scanning ion-selective electrode technique (SIET), Liu et al (2013Liu et al ( , 2016 have shown that treatment with an inhibitor of NHE suppressed both H + and Cl − secretion at ionocytes of SWacclimated medaka larvae.…”

“…In medaka gills, mRNA levels of NHE2 and NHE3 were upregulated by acidified SW ( pH 7), suggesting that NHE was involved in acid secretion by SW-type ionocytes (Liu et al, 2016). Moreover, applying the scanning ion-selective electrode technique (SIET), Liu et al (2013Liu et al ( , 2016 have shown that treatment with an inhibitor of NHE suppressed both H + and Cl − secretion at ionocytes of SWacclimated medaka larvae. These findings indicate that NHE2 and NHE3 play critical roles in acid secretion, and that the acid secretion is linked to Cl − secretion in SW.…”

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

“…In fishes, ionic regulation and acid-base balance are intricately linked because of the tight coupling of apical Na + /H + exchange through Nhe3 in branchial ionocytes (Evans et al, 2005 ; Perry and Gilmour, 2006 ). Ionocytes in fish gills respond to changes in pH and salinity of the external environments through the concurrent activation of acid-base regulatory and osmoregulatory mechanisms, and Nhe3 is implicated as the relevant transporter (Furukawa et al, 2011 ; Liu et al, 2016 ). Furthermore, acid-base regulation, ammonia excretion, and osmoregulation have been shown to be closely linked in the seawater-inducible ionocytes of medaka acclimated to seawater (Liu et al, 2013 , 2016 ).…”