SPHINGOMYELIN METABOLISM IS LINKED TO SALT TRANSPORT IN THE GILLS OF EURYHALINE FISH

By in vivo and in vitro studies of L-(3-H-3)serine and [9, 10(n)-H-3]p almitic acid incorporation into phospholipids, we show a change in the renewal of the ceramide moiety of sphingomyelin in the gills of euryh aline fish (sea bass and eels) when the animals were subjected to abru pt alterations in environmental salinity. In vivo, decrease of the sal inity from sea water (salinity 3.7%) to diluted sea water (salinity 1% ) induced an increase of label incorporation into gill sphingomyelin. The same was true when gills from sea water-adapted sea bass or sea wa ter-adapted eels were incubated in diluted sea water. A decrease in fr ee ceramides synthesis was also observed in the gills of sea water-ada pted sea bass when the salinity of the incubation medium was reduced. Direct inhibition of Na+/K+-ATPase activity with ouabain decreased the sphingomyelin synthesis in the gills of sea bass during in vitro incu bation in diluted sea water, whereas treatment with furosemide stimula ted sphingomyelin synthesis in the same gills incubated in sea water. These findings indicate that changes in Na+ fluxes modify the sphingom yelin turnover and control the production of free ceramides and sphing osine in gill cells of euryhaline fish. In view of the well-known effe cts of these sphingomyelin degradation products on isolated tumor cell differentiation, we suggest that they play a very important role in m odulating chloride cell distribution and metabolism of fish gills duri ng abrupt changes in environmental salinity.