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

Abstract

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 + /NH 4 + 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 NH 4 HCO 3 ; 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 -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.

Ammonia and urea handling by early life stages of fishes

“…Nakada et al (2007b) showed that the apical Rh glycoprotein Rhcg1 (see Box 1) was expressed in the yolk sac epithelium of zebrafish, and that J amm and the number of Rhcg1-expressing cells on the yolk sac increased in conjunction over development. The expression levels of Rh genes in zebrafish embryos and larvae also increased in parallel with J amm over development (Braun et al, 2009a), and this same phenomenon has been observed in early life stages of trout (Hung et al, 2008;Zimmer et al, 2014b). Additionally, many studies have demonstrated strong evidence of a role for Rh glycoproteins in J amm by larval zebrafish using morpholino gene knockdown (Braun et al, 2009a;Kumai and Perry, 2011;Shih et al, 2008Shih et al, , 2012Shih et al, , 2013.…”

“…In rainbow trout larvae reared in neutral waters with sufficient Na + (>0.5 mmol l −1 ), J amm and J Na,in increased over development (Fu et al, 2010;Zimmer and Wood, 2015;Zimmer et al, 2014b), and these processes were tightly coupled to one another at the gill, but not the skin (Zimmer et al, 2014b). Cutaneous mechanisms of J amm and J Na,in by larval trout are thus similar to those of zebrafish, where indirect Na + /NH 4 + exchange is absent under comparable conditions.…”

“…Thus, embryonic and larval fishes produce a substantial ammonia load as a by-product of an amino acid-based metabolism. This is reflected by developmental increases in whole-body ammonia content (Braun et al, 2009a;Chadwick and Wright, 1999;Essex-Fraser et al, 2005;Finn et al, 1991;Fyhn and Serigstad, 1987;Kharbuli et al, 2006;Rice and Stokes, 1974;Rønnestad et al, 1992a,b;Wright et al, 1995a;Zimmer et al, 2014b) and ammonia excretion rate (Braun et al, 2009a;Chadwick and Wright, 1999;Essex-Fraser et al, 2005;Finn et al, 1991;Kharbuli et al, 2006;Rønnestad et al, 1992a;Wright et al, 1995a;Zimmer et al, 2014b).…”

“…Early in development, the skin comprises the majority of functional surface area (Rombough, 1999;Rombough and Moroz, 1990;Wells and Pinder, 1996a) and has higher ionocyte density than the gill (Katoh et al, 2000;Rombough, 1999). In fact, the skin of larval fishes accounts for the majority of both gas exchange and ion regulation following hatching (Fu et al, 2010;Wells and Pinder, 1996b;Zimmer et al, 2014b), and the majority of J amm also occurs via the skin in rainbow trout post-hatching (Zimmer and Wood, 2015;Zimmer et al, 2014b) (Fig. 4A).…”

“…Dashed vertical lines represent the skin-to-gill transition for each process. Data and calculations from Zimmer et al (2014b).…”