Mechanisms of Ca2+ uptake in freshwater and seawater-acclimated killifish, Fundulus heteroclitus, and their response to acute salinity transfer
Killifish (Fundulus heteroclitus) has been extensively used as a model for ion regulation by euryhaline fishes. Na+ and Cl− dynamics have been well studied in killifish, but few studies have addressed that of Ca2+. Therefore, this study aimed to characterize Ca2+ fluxes in freshwater (FW) and seawater (SW)-acclimated killifish, their response to salinity transfer, and to elucidate the mechanisms of Ca2+ influx in FW and SW. SW killifish displayed a significantly higher Ca2+ influx rate than that of FW fish, while Ca2+ efflux rates were comparable in both salinities. Ca2+ influx was saturable in FW (Km = 78 ± 19 µmol/L; Jmax = 53 ± 3 nmol/g/h) and influx by SW killifish was linear up to 7 mmol/L Ca2+. In SW-acclimated fish, 36% of Ca2+ influx was attributed to “intestinal Ca2+ intake”, likely caused by drinking, whereas intestinal Ca2+ intake in FW contributed to < 2% of total. Throughout the study, results suggested that “cation competition” in SW modulates Ca2+ influx. Therefore, we hypothesized that SW-acclimated fish actually have a higher affinity Ca2+ influx system than FW-acclimated fish but that it is competitively inhibited by competing SW cations. In agreement with this cation competition hypothesis, we demonstrated for the first time that “extra-intestinal” Ca2+ influx was inhibited by Mg2+ in both FW and SW-acclimated killifish. Following acute salinity transfer, extra-intestinal Ca2+ influx was rapidly regulated within 12–24 h, similar to Na+ and Cl−. Ca2+ influx in FW was inhibited by La3+, an epithelial Ca2+ channel blocker, whereas La3+ had no significant effect in SW.
KeywordsIon regulation Salinity Epithelial Ca2+ channel (ECaC) Cation competition Osmoregulation Drinking
Special thanks are given to Linda Diao who conducted many of the experiments and analyses at McMaster University, to Sunita Nadella who also helped with experiments and analyses, and to Marina Giacomin who assisted with experiments at UBC. Constructive comments from four anonymous reviewers improved the MS. Funded by a NSERC Discovery Grant to CMW. CMW was supported by the Canada Research Chairs Program. KVB was supported by a NSF Post-Doctoral Fellowship (DBI-1306452) and by a NSERC Discovery Grant to CMW. AMZ is supported by a NSERC Post-doctoral fellowship.
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