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

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.