Are membrane lipids involved in osmoregulation? Studies in vivo on the European eel, Anguilla anguilla, after reduced ambient salinity

Eel gill lipids were labelled in vivo with (³²P) phosphate and (¹⁴C) acetate as precursors added to the water in the incubation tank. We compared the transfer of fish from brackish water (BW) to fresh water (FW) and also the transfer from sea water (SW) to FW, with the corresponding transfer from FW to demineralised FW (soft fresh water, SFW). Results show a common (³²P) phosphatidylethanolamine (PE) dominated phospholipid incorporation pattern at steady state, whatever environmental salinity the eels are adapted to, be it SW, BW, FW or finally after about a week in SFW. A deviation from any established steady state, by lowering the environmental salinity, leads to a temporary loss of the (³²P) PE dominated pattern and this applies equally, whether fish are transferred from a hyper/iso- to a hypo-osmotic medium, or remain in a hypo-osmotic medium. After about 1 week in the transfer media, the original (³²P) PE dominated phospholipid pattern is restored. The concomitant incorporation of (¹⁴C) acetate into eel gill phospholipids is not affected by the induced environmental changes. It shows a (¹⁴C) phosphatidylcholine dominated incorporation pattern throughout.