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.