Cold acclimation of carp from 30°C to 10°C causes a restructuring of liver microsomal phospholipids characterised by increased proportions of monounsaturated fatty acid in phosphatidylcholine (PC)

poikilothermic organisms is the restructuring of cellular membranes in response to changing environmental temperature. Many poikilotherms have been shown to adjust the compositions of their cellular membranes to preserve physical properties such as membrane fluidity within a relatively tight physiological range. One mechanism to achieve this is to increase the proportion of unsaturated fatty acids in membrane phospholipids, thereby causing a disordering of the hydrocarbon interior of the membrane, which compensates for the cold-induced ordering (Cossins, 1994; Hazel, 1995). Maintenance of membrane structure is critical for survival, and the molecular composition of membrane phospholipids appears to be rapidly modified at different temperatures to keep the physical properties of the membrane relatively constant. Trueman et al. (2000) reported that endoplasmic reticulum phospholipids from carp liver showed substantial changes in fatty acid composition, with linear decreases in the proportion of saturates with temperature over a cooling regime of 30°C to 10°C. The plasma membrane of trout hepatocytes has also been shown to be capable of a continuous lipid restructuring and turnover, responding rapidly upon cold exposure by adjusting membrane phospholipid composition (Williams and Hazel, 1994, 1995; Hazel and Williams, 1990). Traditional methods of analysing the phospholipid components of biological membranes have tended to concentrate upon their component fatty acids, rather than examining the biologically relevant molecules in their entirety, thereby oversimplifying the processes occurring. Changes in lipid composition during temperature-induced membrane restructuring are extremely complex. The structure of membrane phospholipids is itself complicated, the physical properties of each molecular species being dependent upon headgroup, fatty acid chain length, unsaturation of individual fatty acids and the positional distribution of the two fatty acids within the molecule (Stubbs and Smith, 1984; Hazel and Williams, 1990). Inclusion of the first unsaturated bond causes the greatest change in the physical properties of any fatty acid, with subsequent unsaturation having progressively less effect (Coolbear et al., 1983; Stubbs and Smith, 1984). 3989 The Journal of Experimental Biology 205, 3989–3997 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4408