LIPID-COMPOSITION AND MICROSOMAL ATPASE ACTIVITIES IN GILLS AND KIDNEYS OF WARM-ACCLIMATED AND COLD-ACCLIMATED SEA BASS (DICENTRARCHUS-LABRAX L)

The response to cold of gill and kidney membrane lipid composition and microsomal (Na+ + K+)-ATPase, Na+-ATPase and Mg2+-ATPase activities in reared sea bass (Dicentrarchus labrax L.) was investigated. Fish acclimation was carried out according to the seasonal cycle from August to March. No cold-promoted increase in fatty acid unsaturation was shown in gill and kidney polar lipids and in total lipids of mitochondria and microsomes. In both tissues the (Na+ + K+)-ATPase exhibited positive compensation for cold acclimation whereas the Na+-ATPase displayed negative compensation. The Mg2+-ATPase showed no compensation in the gills and positive compensation in the kidneys. During cold acclimation the break in the Arrhenius plot of the (Na+ + K+)-ATPase decreased, whereas breaks of both the Na+-ATPase and the Mg2+-ATPase activities remained unchanged. The results indicate that the sea bass does not adopt membrane unsaturation as a cold-facing strategy. The cold-promoted enhancement of (Na+ + K+)-ATPase activity in osmoregulatory tissues may be advantageous to maintain efficient osmoregulation under thermodynamically unfavourable conditions.