Na uptake in larval zebrafish (Danio rerio) is coordinated by three mechanisms: Na/H-exchanger 3b (NHE3b) expressed in H-ATPase-rich (HR) cells， an unidentified Na channel coupled to electrogenic H-ATPase expressed in HR cells， and Na-Cl-cotransporter (NCC) expressed in NCC cells. Recently， acid-sensing ion channels (ASICs) were proposed to be the putative Na channel involved in H-ATPase-mediated Na uptake in adult zebrafish and rainbow trout. In the present study， we hypothesized that ASICs also play this role in Na uptake in larval zebrafish. In support of this hypothesis， immunohistochemical analyses revealed that ASIC4b was expressed in HR cells on the yolk sac skin at 4 days post-fertilization ... More
Na uptake in larval zebrafish (Danio rerio) is coordinated by three mechanisms: Na/H-exchanger 3b (NHE3b) expressed in H-ATPase-rich (HR) cells， an unidentified Na channel coupled to electrogenic H-ATPase expressed in HR cells， and Na-Cl-cotransporter (NCC) expressed in NCC cells. Recently， acid-sensing ion channels (ASICs) were proposed to be the putative Na channel involved in H-ATPase-mediated Na uptake in adult zebrafish and rainbow trout. In the present study， we hypothesized that ASICs also play this role in Na uptake in larval zebrafish. In support of this hypothesis， immunohistochemical analyses revealed that ASIC4b was expressed in HR cells on the yolk sac skin at 4 days post-fertilization (dpf). However， neither treatment with the ASIC-specific blocker 4，6-diamidino-2-phenylindole (DAPI) nor morpholino knockdown of ASIC4b reduced Na uptake in circumneutral conditions at 4 dpf. However， because ASIC4b knockdown led to significant increases in the mRNA expression of nhe3b and ncc and a significant increase in HR cell density， it is possible that Na influx was sustained by increased participation of non-ASIC4b pathways. Moreover， when fish were reared in acidic water (pH = 4)， ASIC4b knockdown led to a stimulation of Na uptake at 3 and 4 dpf， results which also were inconsistent with an essential role for ASIC-mediated Na uptake， even under conditions known to constrain Na uptake via NHE3b. Thus， while ASIC4b clearly is expressed in HR cells， the current functional experiments cannot confirm its involvement in Na uptake in larval zebrafish.
