Ammonia first? The transition from cutaneous to branchial ammonia excretion in developing rainbow trout is not altered by exposure to chronically high NaCl

Larval rainbow trout (Oncorhynchus mykiss) were reared from hatch under control ([Na+]=0.60 mmol l(-1)) or high NaCl ([Na+]=60 mmol l(-1)) conditions to elucidate the driving force for the ontogeny of branchial Na+/NH4(+) exchange, one of the earliest gill functions. We hypothesized that if Na+ uptake is the driving force, then in high NaCl there would be a delay in the skin-to-gill shift in ammonia excretion (J(amm)) and/or an elevation in whole-body total ammonia (T-amm). In both groups, however, the skin-to-gill shift for J(amm), determined using divided chambers, occurred at the same time (13 days post-hatch; dph) and whole-body T-amm was unchanged. Moreover, high NaCl larvae displayed elevated whole-body [Na+] relative to controls by 18 dph, suggesting that maintaining branchial J(amm) occurs at the expense of Na+ balance. Overall, these results support the 'ammonia hypothesis', which posits that ammonia excretion, probably as Na+/NH4(+) exchange, is the primary function of the early fish gill.