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Mechanisms of cold tolerance in poikilothermic animals
ABSTRACTS,
23rd ANNUAL
species of membranes, there is now strong evidence which suggests that subtle rearrangements of the lipid acyl chains on the polar lipid molecules may play an important role in membrane acclimation and modulating membrane properties. This is supported by physical studies which have confirmed that the properties of a given lipid are determined by the length and degree of unsaturation of acyl chains, but also by theil position (sn- I or sn-2) and pairing on the molecule. In addition, the insight gained from these types of analyses have also led to an improved understanding of plant lipid metabolism. Employing this approach to investigate lipid metabolism in Dunulielia. for example, it has been established that the endoplasmic reticulum responds rapidly to a shift in temperature by altering its lipid species composition and by supplying lipids to the chloroplast, the metabolic activities of which have been inhibited by low temperature. Related studies have also demonstrated that membranes not directly involved in the mainstream of lipid metabolism (such as the plasma membrane) can have considerable lipid retailoring capabilities. Thus, any given cellular membrane type may possess its own complement of enzymes (phospholipase A, fatty acyl CoA synthetase, and acyl transferase) permitting lipid retailoring. These various aspects of membrane acclimation point out the great complexity that exists with respect to lipid structure and function and the intracellular dynamics of lipid metabolism. 49. Mechanisms of Cold Tolerance in Poikilothertnic Animals. .I. R. HAZEL (Department of Zoology, Arizona State University, Tempe. Arizona 85287). Poikilothermic animals exposed to temperatures below the melting point of their body fluids display one of two strategies for survival under such extreme conditions: (I) they may supercool: or (2) they may tolerate freezing of their body fluids. Supercooling capacity is frequently augmented by the production of antifreeze (polar fishes) or thermal hysteresis agents (insects), whereas freezing tolerance in insects is dependent upon ice-nucleating agents which initiate extracellular ice formation at relatively high subzero temperatures. High concentrations of low molecular weight cryoprotectants (glycerol, trehalose, glucose) may be found in both freeze-tolerant (insects and anurans) and freeze-intolerant (primarily insects) species. Animals that remain active at cold, but not subfreezing, temperatures are faced with the problem of maintaining nearly normal levels of physiological function at reduced temperatures. The remodeling of membrane structure is perhaps the most ubiquitous cellular response to temperature change and permits the physical properties of membrane lipids to be matched to the prevailing environmental temperature. Such “homeoviscous” adaptation of membrane struc-
MEETING
563
ture is a complex process involving alterations in both the molecular species profiles of membrane phosphatides and phospholipid head group composition. For example, in plasma membranes of rainbow trout hepatocytes. acclimation to 5°C resulted in a loss of short chain (cC34) mono- (34[0.1]) and dienoic (30[0,2]) species of phosphatidylcholine (PC) and the accumulation of long chain (K38) tetra- and hexaenoic (primarily 38[0,6]) species. Phospholipase A,, as the first and rate-limiting step in a deacylationi reacylation cycle, may be a key enzyme in mediating changes in molecular species composition as suggested by increased activity of the microsomal enzyme in cold-acclimated trout and the substrate specificity of the enzyme. In gill membranes, changes in phospholipid head group composition (evident as a decline in the ratio of PCiPhosphatidylethanolamine [PE]) occurred rapidly (within 24 hours) in response to cold exposure and preceded changes in acyl chain unsaturation. Furthermore, the magnitude of the changes in headgroup porportions observed during adaptation to cold exceeded the magnitude of the differences between fully acclimated fish, indicating that head-group modulation may represent a rapid response to temperature change which is subsequently supplanted by more slowly developing adaptations. In accordance with this hypothesis. activity of the deltafive desaturase in hepatic microsomes of rainbow trout did not change significantly for at least 6 days following temperature change. suggesting that, in animals, adjustments in the ability to produce polyunsaturated fatty acids occur slowly. Plasma membrane compositions for skeletal muscle of the Sonoran Desert fish, Agosiu c~hr~so~trsrrr,subjected to diurnal temperature cycles provide further evidence in support of headgroup modulation as a short term adaptive mechanism-proportions of PE declined from more than IO%,in the cool of the morning to less than I% in the heat of the afternoon. In summary, changes in temperature appear to activate a variety of metabolic responses which contribute to membrane remodeling at different times during the adaptation process. 50. Cellrtlar Adupftrtions of Hibernating Murnmrrls to Hyporhermia. J. S. WILLIS (Department of Physiology and Biophysics, University of Illinois, Urbana. Illinois 61801). As whole organisms most mammals have a poor tolerance for hypothermia. But their cells may have a capacity for a far wider cold tolerance, which may be expressed in peripheral tissues. sporadically in core tissue and in cultured cells. Against this background the cold resistance of cells of deep hibernators may be seen as the extreme of a continuum and is complicated by the consideration that the voluntary hypothermia of hibernation is probably in most cases a metabolic adaptation to forestall starvation. Hence attempts to
23rd ANNUAL
species of membranes, there is now strong evidence which suggests that subtle rearrangements of the lipid acyl chains on the polar lipid molecules may play an important role in membrane acclimation and modulating membrane properties. This is supported by physical studies which have confirmed that the properties of a given lipid are determined by the length and degree of unsaturation of acyl chains, but also by theil position (sn- I or sn-2) and pairing on the molecule. In addition, the insight gained from these types of analyses have also led to an improved understanding of plant lipid metabolism. Employing this approach to investigate lipid metabolism in Dunulielia. for example, it has been established that the endoplasmic reticulum responds rapidly to a shift in temperature by altering its lipid species composition and by supplying lipids to the chloroplast, the metabolic activities of which have been inhibited by low temperature. Related studies have also demonstrated that membranes not directly involved in the mainstream of lipid metabolism (such as the plasma membrane) can have considerable lipid retailoring capabilities. Thus, any given cellular membrane type may possess its own complement of enzymes (phospholipase A, fatty acyl CoA synthetase, and acyl transferase) permitting lipid retailoring. These various aspects of membrane acclimation point out the great complexity that exists with respect to lipid structure and function and the intracellular dynamics of lipid metabolism. 49. Mechanisms of Cold Tolerance in Poikilothertnic Animals. .I. R. HAZEL (Department of Zoology, Arizona State University, Tempe. Arizona 85287). Poikilothermic animals exposed to temperatures below the melting point of their body fluids display one of two strategies for survival under such extreme conditions: (I) they may supercool: or (2) they may tolerate freezing of their body fluids. Supercooling capacity is frequently augmented by the production of antifreeze (polar fishes) or thermal hysteresis agents (insects), whereas freezing tolerance in insects is dependent upon ice-nucleating agents which initiate extracellular ice formation at relatively high subzero temperatures. High concentrations of low molecular weight cryoprotectants (glycerol, trehalose, glucose) may be found in both freeze-tolerant (insects and anurans) and freeze-intolerant (primarily insects) species. Animals that remain active at cold, but not subfreezing, temperatures are faced with the problem of maintaining nearly normal levels of physiological function at reduced temperatures. The remodeling of membrane structure is perhaps the most ubiquitous cellular response to temperature change and permits the physical properties of membrane lipids to be matched to the prevailing environmental temperature. Such “homeoviscous” adaptation of membrane struc-
MEETING
563
ture is a complex process involving alterations in both the molecular species profiles of membrane phosphatides and phospholipid head group composition. For example, in plasma membranes of rainbow trout hepatocytes. acclimation to 5°C resulted in a loss of short chain (cC34) mono- (34[0.1]) and dienoic (30[0,2]) species of phosphatidylcholine (PC) and the accumulation of long chain (K38) tetra- and hexaenoic (primarily 38[0,6]) species. Phospholipase A,, as the first and rate-limiting step in a deacylationi reacylation cycle, may be a key enzyme in mediating changes in molecular species composition as suggested by increased activity of the microsomal enzyme in cold-acclimated trout and the substrate specificity of the enzyme. In gill membranes, changes in phospholipid head group composition (evident as a decline in the ratio of PCiPhosphatidylethanolamine [PE]) occurred rapidly (within 24 hours) in response to cold exposure and preceded changes in acyl chain unsaturation. Furthermore, the magnitude of the changes in headgroup porportions observed during adaptation to cold exceeded the magnitude of the differences between fully acclimated fish, indicating that head-group modulation may represent a rapid response to temperature change which is subsequently supplanted by more slowly developing adaptations. In accordance with this hypothesis. activity of the deltafive desaturase in hepatic microsomes of rainbow trout did not change significantly for at least 6 days following temperature change. suggesting that, in animals, adjustments in the ability to produce polyunsaturated fatty acids occur slowly. Plasma membrane compositions for skeletal muscle of the Sonoran Desert fish, Agosiu c~hr~so~trsrrr,subjected to diurnal temperature cycles provide further evidence in support of headgroup modulation as a short term adaptive mechanism-proportions of PE declined from more than IO%,in the cool of the morning to less than I% in the heat of the afternoon. In summary, changes in temperature appear to activate a variety of metabolic responses which contribute to membrane remodeling at different times during the adaptation process. 50. Cellrtlar Adupftrtions of Hibernating Murnmrrls to Hyporhermia. J. S. WILLIS (Department of Physiology and Biophysics, University of Illinois, Urbana. Illinois 61801). As whole organisms most mammals have a poor tolerance for hypothermia. But their cells may have a capacity for a far wider cold tolerance, which may be expressed in peripheral tissues. sporadically in core tissue and in cultured cells. Against this background the cold resistance of cells of deep hibernators may be seen as the extreme of a continuum and is complicated by the consideration that the voluntary hypothermia of hibernation is probably in most cases a metabolic adaptation to forestall starvation. Hence attempts to