Gill arch of the mullet, Mugil cephalus III. Rate of response to salinity change.

The rate of response of gill filament epithelium to salinity changes was determined by scanning electron microscopy and biochemical techniques. An increase in Na-K-ATPase content, as measured by [3H]ouabain binding, was detected at 3 h and continued to about 24 h after mullet were transferred from a salinity of 1-4.5%. In contrast, filaments from mullet transferred to freshwater exhibited a decrease in ouabain-binding sites. Similarly, scanning electron microscopy revealed that ultrastructural modifications occurred in epithelial pores of the filament surface in response to salinity changes. These modifications, which included changes in pore diameter and depth and in the number of cellular extensions in the pore's interior, were evident as early as 6 h and were mostly completed by 24 h after the salinity change. Because epithelial pores likely represent apical pits of chloride cells and because the Na-K-ATPase is localized within chloride cells, the present observations likely represent different manifestations of the very rapid response of chloride cells to osmotic stress, and indicate the close association of structure with function.