Immunolocalization of Na+/K+-ATPase in branchial epithelium of chum salmon fry during seawater and freshwater acclimation

Immunolocalization of the -subunit of Na+/K+-ATPase was examined in the gill epithelium of chum salmon (Oncorhynchus keta) fry during acclimation to brackish water (25 salinity) and reintroduction to fresh water. In freshwater fish, strong immunoreactivity was associated with the large spherical cells located on the free surface of the primary lamellae, especially in those found at the base of the secondary lamellae, and with the large spherical cells located on the secondary lamellae.The large spherical cells located near the central venous sinus at the base of the secondary lamellae and in the interlamellar regions, however, showed little or no immunoreactivity. When freshwater fish were acclimated to brackish water, immunoreactivity developed in the large spherical cells near the central venous sinus concomitant with an increase in the hypo-osmoregulatory ability of the fish. In contrast, reintroduction from brackish water to fresh water caused the disappearance of the immunoreactivity in the large spherical cells near the central venous sinus and a reduction in hypo-osmoregulatory ability. During acclimation to brackish water and reintroduction to fresh water, the hypo-osmoregulatory ability of the fish did not correlate with the total number of large spherical cells located on the primary lamellae but was closely correlated with the number of large spherical cells showing strong immunoreactivity for Na+/K+-ATPase. We conjecture that these immunopositive large spherical cells are mature differentiated chloride cells, whereas the immunonegative large spherical cells are young developing chloride cells. The development of immunoreactivity for Na+/K+-ATPase in young chloride cells may be one of the most important factors in the development of hypo-osmoregulatory ability by chum salmon fry.