Ammonia excretion by the skin of zebrafish (Danio rerio) larvae

The mechanism of ammonia excretion in freshwater teleosts is not well understood. In this study, scanning ion-selective electrode technique was applied to measure H⁺ and NH₄⁺ fluxes in specific cells on the skin of zebrafish larvae. NH₄⁺ extrusion was relatively high in H⁺ pump-rich cells, which were identified as the H⁺-secreting ionocyte in zebrafish. Minor NH ₄⁺ extrusion was also detected in keratinocytes and other types of ionocytes in larval skin. NH₄⁺ extrusion from the skin was tightly linked to acid secretion. Increases in the external pH and buffer concentration (5 mM MOPS) diminished H⁺ and NH ₄⁺ gradients at the larval surface. Moreover, coupled decreases in NH₄⁺ and H⁺ extrusion were found in larvae treated with an H⁺-pump inhibitor (bafilomycin A1) or H⁺-pump gene (atp6v1a) knockdown. Knockdown of Rhcg1 with morpholino-oligonucleotides also decreased NH₄⁺ excretion. This study demonstrates ammonia excretion in epithelial cells of larval skin through an acid-trapping mechanism, and it provides direct evidence for the involvement of the H⁺ pump and an Rh glycoprotein (Rhcg1) in ammonia excretion.