Defecation and the fate of dietary sodium in the common killifish (Fundulus heteroclitus macrolepidotus Walbaum, 1792)

While uptake of Na ⁺ from the water is well-characterized in fish, little is known about the uptake of Na ⁺ from the diet. A method involving radiolabeling of the diet with [ ²²Na ⁺] and [ ³H] PEG-4000 has been developed to study this process, and to separate the systemic efflux of absorbed [ ²²Na ⁺] via gills and kidney from the potential efflux of [ ²²Na ⁺] into the water via defecation. Killifish were acclimated to 10% sea water, a typical salinity for this estuarine fish where they are hyperosmotic to the environment. The fish were fed a single meal (0.81% ration, containing 5.09μmol total Na ⁺gbody weight ^-1) of re-pelleted food labeled with both radiotracers. Effluxes into the water were monitored for 48h. Sharp increases in [ ³H] PEG-4000 appearance provided a clear definition of defecation events, which started at about 7h post-feeding and finished by 16h, with the midpoint at 11.5h. In contrast, the evolution of [ ²²Na ⁺]-radioactivity in the water showed a smooth curvilinear relationship starting at 3h, with a gradually declining slope through 48h. There was no efflux of [ ²²Na ⁺] associated with defecation events, showing that all [ ²²Na ⁺] was quickly absorbed from the meal. By 48h, about 50% of the consumed [ ²²Na ⁺] had been excreted to the external water by systemic efflux. The efflux relationship was best described by a two-phase exponential relationship, with a breakpoint at about 13h post-feeding. The first phase (rate constant 0.0309h ^-1) corresponded to branchial efflux of dietary [ ²²Na ⁺] equilibrated with the exchangeable whole body Na ⁺ pool, while the second phase was much slower (0.0114h ^-1). These results emphasize the importance of dietary Na ⁺ when the killifish is hyper-regulating, and provide a method applicable to a wide range of conditions, ions, and species.