Different mechanisms of Na + uptake and ammonia excretion by the gill and yolk sac epithelium of early life stage rainbow trout.

In rainbow trout, the dominant site of Na+ uptake ( J Na,in ) and ammonia excretion ( J amm ) shifts from the skin to the gills over development. Post-hatch (PH; 7 days post-hatch) larvae utilize the yolk sac skin for physiological exchange, whereas by complete yolk sac absorption (CYA; 30 days post-hatch), the gill is the dominant site. At the gills, J Na,in and J amm occur via loose Na+ /NH4 + exchange, but this exchange has not been examined in the skin of larval trout. Based on previous work, we hypothesized that, contrary to the gill model, J Na,in by the yolk sac skin of PH trout occurs independently of J amm Following a 12 h exposure to high environmental ammonia (HEA; 0.5 mmol l-1 NH4 HCO3 ; 600 µmol l-1 Na+ ; pH 8), J amm by the gills of CYA trout and the yolk sac skin of PH larvae, which were isolated using divided chambers, increased significantly. However, this was coupled to an increase in J Na,in across the gills only, supporting our hypothesis. Moreover, gene expression of proteins involved in J Na,in [Na+ /H+ -exchanger-2 (NHE2) and H+ -ATPase] increased in response to HEA only in the CYA gills. We further identified expression of the apical Rhesus (Rh) proteins Rhcg2 in putative pavement cells and Rhcg1 (co-localized with apical NHE2 and NHE3b and Na+ /K+ -ATPase) in putative peanut lectin agglutinin-positive (PNA+ ) ionocytes in gill sections. Similar Na+ /K+ -ATPase-positive cells expressing Rhcg1 and NHE3b, but not NHE2, were identified in the yolk sac epithelium. Overall, our findings suggest that the mechanisms of J Na,in and J amm by the dominant exchange epithelium at two distinct stages of early development are fundamentally different.