Dexamethasone modulates the activity of the eel branchial Na+/K(+)ATPase in both chloride and pavement cells

In fish, gills actively accumulate ions in freshwater (FW) with Na+ absorpt ion taking place at the level of pavement cells, and excrete monovalent ion s, mainly Na+ and Cl-, through the chloride cells in sea water (SW). The Na +/K(+)ATPase plays a crucial role in the functionality of osmoregulatory ce lls and we showed previously that angiotensin II modulates its activity in the eel gill (1). We here show the effects of synthetic steroid dexamethaso ne (DEX) on the activity of Na+/K+ATPase in both gill pavement and chloride cells from FW- and SW-adapted animals. Results showed that in the chloride cells 100 nM DEX provoked a significant increment in the activity of Na+/K (+)ATPase in both SW- and FW-adapted animals. This effect was greatest at 2 hours in SW, and at 6 hours in FW. The increment in the activity of the Na +/K(+)ATPase was dose-dependent in both environmental adaptations, Converse ly, in pavement cells from FW-adapted eels 100 nM DEX decremented the activ ity of Na+/K(+)ATPase (4-fold reduction after 6 hour incubations), while in SW, DEX increased the enzyme activity of about 25% at 2 hours, and of abou t 55% at 6 hours. These results are consistent with the different physiolog ical roles that pavement and chloride cells have in the two different adapt ive conditions.