Dining on the dead in the deep: Active NH4+ excretion via Na+/H+(NH4+) exchange in the highly ammonia tolerant Pacific hagfish, Eptatretus stoutii

Abstract Aim Pacific hagfish are exceptionally tolerant to high environmental ammonia (HEA). Here, we elucidated a cellular mechanism that enables hagfish to actively excrete ammonia against steep ammonia gradients expected to be found inside a decomposing whale carcass. Methods Hagfish were exposed to varying concentrations of HEA in the presence or absence of environmental Na + , while plasma ammonia levels were tracked. 14 C‐methylammonium was used as a proxy for NH 4 + to measure efflux in whole animals and in isolated gill pouches; the latter allowed us to assess the effects of amiloride specifically on Na + /H + exchangers (NHEs) in gill cells. Western blotting and immunohistochemistry were utilized to evaluate the abundance and sub‐cellular localization of Rhesus glycoprotein (Rh) channels in the response to HEA. Results Hagfish actively excreted NH 4 + against steep inwardly directed E NH4 + (Δ E NH4 + ~ 35 mV) and pNH 3 (ΔpNH 3 ~ 2000 μtorr) gradients. Active NH 4 + excretion and plasma ammonia hypo‐regulation were contingent on the presence of environmental Na + , indicating a Na + /NH 4 + exchange mechanism. Active NH 4 + excretion across isolated gill pouches was amiloride‐sensitive. Exposure to HEA resulted in decreased abundance of Rh channels in the apical membrane of gill ionocytes. Conclusions During HEA exposure, hagfish can actively excrete ammonia against a steep concentration gradient using apical NHEs energized by Na + ‐K + ‐ATPase in gill ionocytes. Additionally, apical Rh channels are removed from the apical membrane, presumably to reduce ammonia loading from the environment. We suggest that this mechanism allows hagfish to maintain tolerable ammonia levels while feeding inside decomposing carrion, allowing them to exploit nutrient‐rich food‐falls.