The metabolic adjustments responsible for the "homeoviscous adaptation" of membrane lipid composition in fish are examined with special reference to the rainbow trout, Salmo gairdneri. The percentage of fatty acid lipogenesis attributable to unsaturates was elevated after an acute drop in temperature but declined with continued cold exposure (i.e., cold acclimation). In contrast, selected desaturation reactions [particularly those involved in the production of polyunsaturated fatty acids (PUFA) of the n-3 and/or n-6 families] proceeded more rapidly in cold-than in warm-acclimated trout. Different time courses for the change in monoene and PUFA levels of hepatic microsomal membranes during thermal acclimation suggest that the various desaturase enzymes contribute to the acclimatory response at different times. Certain fatty acids, particularly the delta 5-desaturation products of the n-3 (20:5 delta 5,8,11,14,17) and n-6 (20:4 delta 5,8,11,14) series, were preferentially incorporated into phospholipids at cold temperatures and by cold-acclimated trout, due in part to the direct effect of temperature on the substrate preferences of the phospho- and acyltransferase enzymes of de novo phospholipid biosynthesis; however, chain length rather than degree of unsaturation per se may determine the temperature-dependent pattern of fatty acid incorporation. Both acute and chronic cold exposure elevated the incorporation of PUFA into phosphatidylserine (PS), suggesting that the conversion of PS to phosphatidylethanolamine (PE) may be activated at cold temperatures. The rate of homeoviscous adaptation appears to be limited by the rate of membrane lipid turnover, which although generally positively correlated with acclimation temperature, did vary depending on the phospholipid moiety and tissue considered. Finally the direct acylation of lysophospholipids formed during the process of membrane turnover may contribute to both rapid and acclimatory adjustments in membrane lipid composition.

Rainbow trout, Salmo gairdneri, were fed pelleted food ad libitum for 35 and 50 days. Lipid deposits were determined in the viscera, liver and muscle (epaxial and hypaxial). Most of the lipid accumulated in the viscera, but the lipid content of liver and epaxial muscle also increased. Hypaxial muscle lipid content remained the same throughout the feeding period. Upon starving the fish for 27 and 48 days following 50 days of feeding, visceral lipid contributed most to energy metabolism among the depots investigated. Muscle also contributed a considerable share while the absolute amount derived from the liver was much smaller.Patterns of fatty acid mobilization during starvation were also investigated. Saturated acids were preferentially mobilized from the viscera, resulting in a rise in the percentage of monoenes and polyunsaturates. In liver, the percentage of saturates remained relatively constant whereas the percentage of monoenes declined and polyunsaturates increased. In muscle, a substantial increase in saturates was caused by a decline in monoenes; polyunsaturate content remained constant. As a result of these shifts in relative fatty acid composition the U/S (unsaturates to saturates) ratio rose in viscera and liver and decreased in muscle. The UI (unsaturation index) responded in essentially the same way. Possible explanations for preferential fatty acid utilization are discussed in terms of energetics and structural relationships.

The phospholipid composition of plasma membranes from the kidney of rainbow trout, Salmo gairdneri, was determined over a period of 21 days as fish were acclimating between temperatures of 5 and 20 degrees C. Proportions of phosphatidylethanolamine (PE) were significantly higher (29.03 vs. 23.26%) in membranes of 5 degrees C- than 20 degrees C-acclimated trout, whereas levels of phosphatidylcholine (PC) were similar. During acclimation, changes in headgroup composition were restricted primarily to PE and PC, with proportions of the former increasing and the latter decreasing on cold exposure. Headgroup composition changed rapidly as indicated by variations in PC/PE, which dropped from 1.71 +/- 0.13 to 0.78 +/- 0.11 within 8 h of cold acclimation and rose from 1.31 +/- 0.15 to 2.0 +/- 0.13 by the 2nd day of warm acclimation. Differences between warm- and cold-acclimating fish in the proportion of both PC and PE attained during the acclimation time course exceeded the magnitude of the differences between fully acclimated fish. Because of differences in molecular geometry and physical properties between PC and PE, the observed changes in headgroup composition may result in thermal compensation of membrane function, and the more rapid response to cold as opposed to warm stress suggests a primary role in adaptation to cold temperatures. However, headgroup modulation is most likely to contribute to homeoviscous adaptation only during the initial stages of the acclimatory response.

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.

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Since fish are poikilothermic, changes in temperature may perturb hormonal activation of cell function. To test this hypothesis, and determine the extent to which hormonal responses are thermally compensated, the effect of temperature on epinephrine-stimulated glucose release in perfused trout liver was studied. Thermally acclimated (5 and 20°C) rainbow trout (Oncorhynchus mykiss) were responsive to epinephrine (0.5×10-6 mol l-1) at both 5 and 20°C. Metoprolol (beta2 antagonist) and propranolol (beta1and2) decreased the response significantly (to 1.4 % and 8.4 % of stimulated values, respectively) while phentolamine (alpha1and2) was without effect, implying the response is beta2-mediated. Both basal (86 and 19 micromole g-1 liver h-1 in 5 and 20°C trout, respectively) and epinephrine-stimulated (210 and 168 micromole g-1 h-1) rates of glucose release were higher (2.4-fold higher for epinephrine-stimulated and 8.8-fold for basal) in 5°C- than in 20°C-acclimated fish, regardless of perfusion temperature. Although the dose-response curve for epinephrine was markedly temperature-dependent, cold- and warm-acclimated fish were affected in different ways. Cold-acclimated fish (5°C) were less responsive to epinephrine when perfused at 5°C (ED50 6.8×10-9 mol l-1) than when perfused at 20°C (ED50 8.2×10-10 mol l-1); in contrast, warm-acclimated fish (20°C) were less responsive to epinephrine when perfused at 20°C (ED50 4.6×10-7 mol l-1) than at 5°C (ED50 6.6×10-9 mol l-1). These results are interpreted as being indicative of adaptations to maintain the capacity for hepatic glucose mobilization at low temperature.

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