Rhcg1 and NHE3b are involved in ammonium‐dependent sodium uptake by zebrafish larvae acclimated to low‐sodium water

To investigate whether Na+ uptake by zebrafish is dependent on NH4 + excretion, a scanning ion‐selective electrode technique (SIET) was applied to measure Na+ and NH4 + gradients at the yolk‐sac surface of zebrafish larvae. Low‐Na+ acclimation induced an inward Na+ gradient (uptake), and a combination of low‐Na+ and high‐ NH4 + induced a larger inward Na+ gradient. When measuring the ionic gradients, raising the external NH4 + level (5 mM) simultaneously blocked NH4 + excretion and Na+ uptake; in contrast, raising the external Na+ level (10 mM) simultaneously enhanced Na+ uptake and NH4 + excretion. The addition of MOPS buffer (5 mM) which is known to block NH4 + excretion also suppressed Na+ uptake. These results showed that Na+ uptake and NH4 + excretion by larval skin are associated when ambient Na+ level is low. Knockdown of Rhcg1 translation with morpholino‐oligonucleotides decreased both NH4 + excretion and Na+ uptake by the skin and Na+ content of whole larvae. Knockdown of nhe3b translation or inhibitor (EIPA) treatment also decreased both the NH4 + excretion and Na+ uptake. This study provides loss‐of‐function evidence for the involvement of Rhcg1 and NHE3b in the ammonium‐dependent Na+ uptake mechanism in zebrafish larvae subjected to low‐Na+ water.

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