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Uptake, handling and excretion of Na+ and Cl− from the diet in vivo in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus, an agastric teleost

SUMMARY A radiotracer approach using diets labelled with 22Na+, 36Cl− and [14C]polyethylene-4000 (PEG-4000) was employed to investigate the role of intestinal uptake from the food in ion homeostasis in the killifish Fundulus heteroclitus. This euryhaline teleost lacks both a stomach and the capacity for Cl− uptake at the gills in freshwater. PEG-4000 appearance in the water was minimal up to 10–11 h post-feeding, indicating the virtual absence of Na+ and Cl− loss in the faeces up until this time. Rapid uptake of dietary Na+ and Cl− occurred and more than 88% of 22Na+ and 36Cl− were absorbed in the intestine by 3 h post-feeding; excretion rates of Na+ and Cl− originating from the food were greatest during this period. Uptake and excretion of Cl− from the diet was fivefold to sixfold greater than that of Na+ in freshwater, and approximately threefold greater in seawater. Excretions of dietary Na+ and Cl− by seawater-acclimated killifish were far greater than by freshwater-acclimated killifish in this time frame, reflecting the much greater branchial efflux rates and turnover rates of the internal exchangeable pools. At both 3 and 9 h post-feeding, the largest fraction of dietary Na+ was found in the carcass of freshwater-acclimated fish, followed by the external water, and finally the digestive tract. However, in seawater-acclimated fish, more was excreted to the water, and less was retained in the carcass. For Cl−, which was taken up and excreted more rapidly than Na+, the majority of the dietary load had moved to the external water by 9 h in both freshwater and seawater animals. After 7 days training on a low-salt natural diet (live Lumbriculus variegatus worms; 31.5 μmol Na+ g−1 wet mass) versus a high-salt synthetic pellet diet (911 μmol Na+ g−1 dry food mass), freshwater killifish exhibited a lower absolute excretion rate of Na+ from the low-salt diet, but relative uptake from the intestine and retention in the carcass were virtually identical from the two diets. Seawater killifish excreted relatively more Na+ from the low-salt diet. Overall, our results emphasize the importance of dietary Na+ and Cl− in the electrolyte economy of the killifish, particularly in freshwater, and especially for Cl−.