Proton-facilitated ammonia excretion by ionocytes of medaka (Oryzias latipes) acclimated to seawater

The proton-facilitated ammonia excretion is critical for a fish's ability to excrete ammonia in freshwater. However, it remains unclear whether that mechanism is also critical for ammonia excretion in seawater (SW). Using a scanning ion-selective electrode technique (SIET) to measure H⁺ gradients, an acidic boundary layer was detected at the yolk-sac surface of SW-acclimated medaka (Oryzias latipes) larvae. The H⁺ gradient detected at the surface of ionocytes was higher than that of keratinocytes in the yolk sac. Treatment with Tricine buffer or EIPA (a NHE inhibitor) reduced the H⁺ gradient and ammonia excretion of larvae. In situ hybridization and immunochemistry showed that slc9a2 (NHE2) and slc9a3 (NHE3) were expressed in the same SW-type ionocytes. A real-time PCR analysis showed that transfer to SW downregulated branchial mRNA expressions of slc9a3 and Rhesus glycoproteins (rhcg1, rhcg2, and rhbg) but upregulated that of slc9a2. However, slc9a3, rhcg1, rhcg2, and rhbg expressions were induced by high ammonia in SW. This study suggests that SW-type ionocytes play a role in acid and ammonia excretion and that the Na⁺/H⁺ exchanger and Rh glycoproteins are involved in the proton-facilitated ammonia excretion mechanism.