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

Immunolocalization of the alpha-subunit of Na+/K+-ATPase was examined in th e gill epithelium of chum salmon (Oncorhynchus keta) fry during acclimation to brackish water (25 parts per thousand salinity) and reintroduction to f resh water. In freshwater fish, strong immunoreactivity was associated with the large spherical cells located on the free surface of the primary lamel lae, especially in those found at the base of the secondary lamellae, and w ith the large spherical cells located on the secondary lamellae. The large spherical cells located near the central venous sinus at the base of the se condary lamellae and in the interlamellar regions, however, showed little o r no immunoreactivity, When freshwater fish were acclimated to brackish wat er, immunoreactivity developed in the large spherical cells near the centra l venous sinus concomitant with an increase in the hypo-osmoregulatory abil ity of the fish, In contrast, reintroduction from brackish water to fresh w ater caused the disappearance of the immunoreactivity in the large spherica l cells near the central venous sinus and a reduction in hypo-osmoregulator y 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 b ut was closely correlated with the number of large spherical cells showing strong immunoreactivity for Na+/K+-ATPase. We conjecture that these immunop ositive large spherical cells are mature differentiated chloride cells, whe reas the immunonegative large spherical cells are young developing chloride cells. The development of immunoreactivity for Na+/K+-ATPase in young chlo ride cells may be one of the most important factors in the development of h ypo-osmoregulatory ability by chum salmon fry.