Sphingomyelin metabolism is linked to salt transport in the gills of euryhaline fish.

By in vivo and in vitro studies of L-(3-3H)serine and [9,10(n)-3H]palmitic acid incorporation into phospholipids, we show a change in the renewal of the ceramide moiety of sphingomyelin in the gills of euryhaline fish (sea bass and eels) when the animals were subjected to abrupt alterations in environmental salinity. In vivo, decrease of the salinity 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 water-adapted eels were incubated in diluted sea water. A decrease in free ceramides synthesis was also observed in the gills of sea water-adapted 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 incubation in diluted sea water, whereas treatment with furosemide stimulated sphingomyelin synthesis in the same gills incubated in sea water. These findings indicate that changes in Na+ fluxes modify the sphingomyelin turnover and control the production of free ceramides and sphingosine in gill cells of euryhaline fish. In view of the well-known effects of these sphingomyelin degradation products on isolated tumor cell differentiation, we suggest that they play a very important role in modulating chloride cell distribution and metabolism of fish gills during abrupt changes in environmental salinity.