Effects of a low-Ca2+environment on branchial chloride cell morphology in chum salmon fry and immunolocalization of Ca2+-ATPase in chloride cells

Abstract

To clarify the involvement of branchial chloride cells in Ca2+uptake in fresh water (FW), chloride-cell morphology was compared in chum salmon (Oncorhynchus keta) fry acclimated to defined FWs with different Ca2+concentrations (0, 0.1, and 0.5 mM). Using immunocytochemical staining with an antiserum specific for Na+,K+-ATPase, chloride cells were detected in both filament and lamellar epithelia. The numbers and sizes of chloride cells in the lamellar epithelia were greater in the low-Ca2+groups (0 and 0.1 mM Ca2+) than in the normal-Ca2+groups (0.5 mM Ca2+and normal FW), whereas filament chloride cells were not affected in number or size by the environmental Ca2+concentration. Electron-microscope observations also revealed that enlarged lamellar chloride cells were more frequently observed in the 0 mM Ca2+group than in the 0.5 mM Ca2+group. To obtain morphological evidence for Ca2+uptake through the branchial epithelia, cellular localization of Ca2+-ATPase was examined with a monoclonal antibody specific for human erythrocyte Ca2+-ATPase. Ca2+-ATPase immunoreactivity was detected in Na+,K+-ATPase-immunoreactive chloride cells in both filament and lamellar epithelia. Using electron-microscope immunocytochemistry, Ca2+-ATPase was found to be localized in the tubular system, which is continuous with the basolateral membrane of chloride cells. These findings indicate that chloride cells in the lamellar epithelia activated by a low Ca2+concentration may constitute the extra Ca2+and NaCl uptake capacity required to maintain homeostasis in soft water.