The gill arch of the mullet, Mugil cephalus. II. Modification in surface ultrastructure and Na,K‐ATPase content during adaptation to various salinities

Abstract

AbstractThe effect of salinity changes on the surface ultrastructure and Na, K‐ATPase content of the gill filament of mullent was determined using scanning electron microscopy and 3H‐ouabain binding. The number of ouabain binding sites/mg wet weight of tissue increased with increasing salinity. Mullet adapted to fresh water, or to salinities of 10, 26 and 45 g salt/liter exhibited ouabain binding capacities of about 1, 2, 4 and 5 pmoles ouabain bound/mg wet weight of tissue. Ouabain binding was strongly effected by ATP and KCI, whereas bound 3H‐ouabain was uneffected by the addition of unlabelled ouabain. Similarly, epithelial pores on the filament surfaces exhibited ultrastructural features which were indicative of the various salinities to which the fish were adapted. In fresh water, the pores were broad (average diameter 3‐6 μ) and shallow, and a cellular mass protruded from the pores above the epithelial surface. In 10 g salt/liter, the pores averaged 1–5 μ in diameter, were slightly deeper, and contained obvious, protruding cellular extensions. In 45 g salt/liter, the pores were narrower (average diameter 1–3 μ), considerably deeper, and exhibited fewer, less obvious cellular extensions in their interior. Because the pores likely represent the apical pits of ion secreting chloride cells and because the Na, K‐ATPase has been localized in chloride cells, these observations likely represent two different manifestations of the same process, adaption of chloride cells for osmotic regulation.