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

Shih TH, Horng JL, Liu ST, Hwang PP, Lin LY. Rhcg1 and NHE3b are involved in ammonium-dependent sodium uptake by zebrafish larvae acclimated to low-sodium water. Am J Physiol Regul Integr Comp Physiol 302: R84-R93, 2012. First published October 12, 2011; doi:10.1152/ajpregu.00318.2011.-To investigate whether Na+ uptake by zebrafish is dependent on NH4+ excretion, a scanning ion-selective electrode technique 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) 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 (5-ethylisopropyl amiloride) 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.