Effects of environmental Ca²⁺ levels on branchial chloride cell morphology in freshwater-adapted killifish Fundulus heteroclitus

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To examine the involvement of chloride cells in the uptake of Ca²⁺ in freshwater (FW) killifish, chloride cell morphology was compared in fish acclimated to different defined FW environments with Ca²⁺ concentrations of either 0.1 mM, 0.5 mM, or 2.5 mM. Numerous chloride cells were detected in whole-mount preparations of the gill filaments, which were stained with an antiserum specific for Na⁺, K⁺-ATPase. Chloride cells, located mostly on the afferent-vascular edge of the filaments, were larger at lower Ca²⁺ concentrations. Electron microscopic observations showed that in the 0.1 mM and 0.5 mM Ca²⁺ experimental groups, the apical membrane of chloride cells were flat or slightly projecting and equipped with numerous microvilli. In the 2.5 mM Ca²⁺ group, some chloride cells formed an apical pit, whereas other cells were similar to those observed in the 0.1 mM and 0.5 mM Ca²⁺ groups. Plasma osmolality decreased with decreasing ambient Ca²⁺ concentration, suggesting that environmental Ca²⁺ affects the permeability of the body surfaces. Gill Na⁺, K⁺-ATPase activity in the 0.1 mM and 0.5 mM Ca²⁺ groups were significantly higher than that in the 2.5 mM Ca²⁺ group, implying the involvement of the Na⁺-Ca²⁺ exchanger in Ca²⁺ uptake in the gills. These findings suggest that chloride cells function as the site for Ca²⁺ uptake in killifish acclimated to low Ca²⁺ environments.