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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

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

¹Department of Life Science, National Taiwan Normal University, Taipei, Taiwan, Republic of China; and ²Institute of Cellular and Organismic Biology, Academia Sinica, Taipei, Taiwan, Republic of China

Submitted 14 May 2008 ; accepted in final form 22 September 2008

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 NH₄⁺ 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 NH₄⁺ 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.

H⁺-adenosine 5'-triphosphatase; ionocytes; Rhesus glycoprotein; ammonium; knockdown; ion-selective electrode

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