Acid secretion by mitochondrion-rich cells of medaka (Oryzias latipes) acclimated to acidic freshwater

In the present study, medaka embryos were exposed to acidified freshwater (pH 5) to investigate the mechanism of acid secretion by mitochondrion-rich (MR) cells in embryonic skin. With double or triple in situ hybridization/ immunocytochemistry, the Na ⁺/H ⁺ exchanger 3 (NHE3) and H ⁺-ATPase were localized in two distinct subtypes of MR cells. NHE3 was expressed in apical membranes of a major proportion of MR cells, whereas H ⁺-ATPase was expressed in basolateral membranes of a much smaller proportion of MR cells. Gill mRNA levels of NHE3 and H ⁺-ATPase and the two subtypes of MR cells in yolk sac skin were increased by acid acclimation; however, the mRNA level of NHE3 was remarkably higher than that of H ⁺-ATPase. A scanning ion-selective electrode technique was used to measure H ⁺, Na ⁺, and NH4 ⁺ transport by individual MR cells in larval skin. Results showed that Na ⁺ uptake and NH4 ⁺ excretion by MR cells increased after acid acclimation. These findings suggested that the NHE3/Rh glycoprotein-mediated Na ⁺ uptake/NH4 ⁺ excretion mechanism plays a critical role in acidic equivalent (H ⁺/NH4 ⁺) excretion by MR cells of the freshwater medaka.