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 NH 4 + excretion, a scanning ion‐selective electrode technique (SIET) was applied to measure Na + and NH 4 + 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‐ NH 4 + induced a larger inward Na + gradient. When measuring the ionic gradients, raising the external NH 4 + level (5 mM) simultaneously blocked NH 4 + excretion and Na + uptake; in contrast, raising the external Na + level (10 mM) simultaneously enhanced Na + uptake and NH 4 + excretion. The addition of MOPS buffer (5 mM) which is known to block NH 4 + excretion also suppressed Na + uptake. These results showed that Na + uptake and NH 4 + excretion by larval skin are associated when ambient Na + level is low. Knockdown of Rhcg1 translation with morpholino‐oligonucleotides decreased both NH 4 + 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 NH 4 + 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.