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Hepatocytes from 5- or 20 degrees C-acclimated rainbow trout (Salmo gairdneri) were incubated with [1-14C]oleic, -linoleic, or -linolenic acid. Both acclimation groups demonstrated greater incorporation of derivatives from linolenic and linoleic acids into phospholipids when assayed at 5 and 20 degrees C; few derivatives of oleic acid were formed. Cells from cold-acclimated trout, when assayed at 5 degrees C with linolenic acid, incorporated a large proportion of radioactivity into free fatty acids. Analysis of each lipid fraction revealed a relatively specific incorporation of certain fatty acids. For example, "dead end" elongation products of the three substrates were preferentially incorporated into neutral lipids, while delta 6 desaturation products of the three acids were retained in the free fatty acid fraction. Twenty-carbon acid derivatives of linoleic and linolenic acids were directed into the phospholipid fraction. Incorporation of the delta 5 desaturation products was temperature sensitive in cells from cold-acclimated but not warm-acclimated trout. The results suggest that selectivity of incorporation of specific fatty acids into phospholipids may be of importance in restructuring membranes of poikilotherms during thermal adaptation.

Fatty acid binding proteins (FABP) constitute a family of small, cytosolic carriers of hydrophobic ligands. These proteins are thought to be important for lipid trafficking toward specific metabolic pathways, and are potentially important for the establishment of characteristic lipid compositions of neural tissue. In the embryonic chick retina and brain, FABP resembles the heart subtype, as determined by protein characterization and immunoblot studies. In this paper, the developmental expression and cellular localization of chick retinal FABP were examined. Results of immunoblot analysis suggest that FABP is maximally expressed around embryonic day 9 (E9) and declines thereafter. In adult retinas, FABP is barely detectable on a Western blot. Immunohistochemical staining of the retina shows light labeling on day E6 and a more intense staining throughout the retina on day E9. As the retina differentiates, labeling becomes increasingly localized. By day E l 8 subpopulations of ganglion cells and photoreceptor inner segments are stained, as are all photoreceptor cell bodies, most of the inner nuclear layer, and the nerve fiber layer. Staining is decreased in older retinas such that in adult animals, only light staining of the photoreceptor cell bodies is visible. The decrease in relative amount of FABP in the retina after day E9 suggests a role for FABP in the early stages of retinal differentiation. Localization in the retina is consistent with this hypothesis, as label becomes more restricted to those cells undergoing maturation at a particular developmental age. Thus, in young embryos (E6-E9), FABP immunolabeling is apparent throughout the retina, and transiently localizes at different ages (E12-E 15) to plexiform and nuclear layers. Near hatching (E18-E21), the photoreceptors are in the final stages of maturation, and are the principal cells immunoreactive for FABP. In the adult retina, FABP lightly labels only the photoreceptor cells bodies; thus, we conclude that chick retinal FABP is not involved in outer segment membrane homeostasis. Instead, FABP may serve to sequester fatty acids needed for the period of neurite outgrowth which occurs as the retina ends its mitotic cycles and begins the process of synaptogenesis. 0 1994 Wiley-Liss, Inc.

Rainbow trout (Salmogairdneri), acclimated to 5°C or 20"C, were reacclimated to water of the opposite temperature. The time course of alterations in fatty acid composition of microsomes from gills and liver was monitored over a 28-day period. Two to six days were required for maximum changes in fatty acid levels with acclimation to 20"C, whereas 10-17 days were required in the opposite direction. Fatty acids from gill microsomes had lower U/S (ratio of unsaturates to saturates) and UI (unsaturation index) and a shorter average chain length than liver microsomes; these differences were preserved with acclimation. In both tissues the U/S, UI, and chain length increased with cold adaptation and decreased with warm adaptation. In gill microsomes, changes in UI and U/S were due primarily to altered levels of saturated fatty acids and 20:4 (n-6) and 205 (11-31. The latter two acids, products of the A5 desaturation step in PUFA biosynthesis, also contributed to the changes in unsaturation in liver microsomes, but to a lesser extent. In liver, adjustments in unsaturation indexes were initially mediated by large but transient changes in levels of saturates and monoenes, whereas changes in polyunsaturated fatty acids of the n-6 and n-3 families were delayed 3-6 days. Thus, changes in fatty acid composition that accompany acclimation to a different temperature can be resolved into those metabolic reactions (e.g., A5 desaturation) that are of major importance and, in the liver, into temporally more than one mechanism (e.g., altering monoenes and PUFA at different times).

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