Liu ST, Tsung L, Horng JL, Lin LY. Proton-facilitated ammonia excretion by ionocytes of medaka (Oryzias latipes) acclimated to seawater. Am J Physiol Regul Integr Comp Physiol 305: R242-R251, 2013. First published May 15, 2013 doi:10.1152/ajpregu.00047.2013.-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 ϩ gradients, an acidic boundary layer was detected at the yolk-sac surface of SW-acclimated medaka (Oryzias latipes) larvae. The H ϩ 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 ϩ 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 ϩ /H ϩ exchanger and Rh glycoproteins are involved in the proton-facilitated ammonia excretion mechanism. mitochondrion-rich cell; gill; fish; embryos; skin AMMONIA IS A NITROGENOUS PRODUCT of amino acid metabolism. Ammonia excretion in fish is largely accomplished by their gill epithelium, which has a large surface area and a small diffusion distance. Ammonia exists as two distinct chemical species, dissolved ammonia gas (NH 3 ) and ammonium ions (NH 4 ϩ ). The conventional term, "ammonia," used in this article refers to both NH 3 and NH 4 ϩ . In freshwater (FW) fish, it is generally accepted that ammonia excretion occurs by diffusion of nonionic NH 3 down a favorable gradient across the gill epithelium (7,11,35). The excreted NH 3 can be trapped via acid secretion into the unstirred layer of water on gill surfaces. This "acidtrapping" or "proton-facilitated" mechanism maintains a favorable NH 3 gradient across the gill epithelium (35). Although phospholipid membranes are permeable to nonionic NH 3 , the existence of NH 3 channels in cell membranes provides an efficient and regulated pathway. In recent years, Rhesus glycoproteins (Rh proteins) in cell membranes of the gill and skin epithelium were found to facilitate ammonia excretion in fishes (11,12,39). The human Rh antigen on blood cells has long been associated with blood typing; however, the role of Rh proteins in ammonia transport of erythrocytes and nonerythrocytes was discovered in the past decade. The human erythroid RhAG and its nonerythroid homologues, RhBG and RhCG, were demonstrated to function as ammonia transporters or channels (31, 32). In teleosts, Nakada et al. (24) (3...