How mammals regulate their body temperature
Austrian researchers have made some unexpected findings that show how mammals can make significant and speedy changes to the lipid composition of their cell membranes. The team, from the Research Institute of Wildlife Ecology at the University of Veterinary Medicine, Vienna, made the findings after investigating changes in the fatty-acid composition of cell membranes in wild-living marmots - squirrel like creatures typically found living in mountainous regions.
As typical hibernators, alpine marmots lower their body temperatures to close to the ambient temperature throughout most of the winter. Even when their hibernation is interrupted, their body temperature stays a few degrees below typical summer levels.
The study, published in the journal Public Library of Science (PLoS) ONE, found that the amount of so-called 'n-6' polyunsaturated fatty acids (those with the final double bond at the sixth position) in the membranes was found to increase dramatically before the start of hibernation, most likely to prepare the body and the heart for working at incredibly low temperatures.
The team also found that as well as increasing dramatically, this transition takes place extremely quickly. Further, once spring begins, the process is reversed and the animals return to living at high-body temperatures.
Animal cell membranes are a bilayer of phospholipids (charged fat molecules) made up to various degrees of fatty acids that must be acquired from the diet. Essential polyunsaturated fatty acids have been shown to be important in resistance to a variety of diseases and in coping with changes in body temperature. Furthermore, mammals are unlikely candidates for extensive temperature-induced alteration, known to occur in fish or reptiles, because they typically maintain high and rather constant body temperatures.
The fatty acids incorporated in the membranes probably stem from the marmots' white adipose tissue. However fatty acids are not simply taken from the fat stores at random; n-6 polyunsaturated fatty acids are transported preferentially, although the mechanism by which they do this remains a mystery to scientists.
Previously, it was widely believed that mammals were unable to alter the proportions of essential fatty acids in their cell membranes except by changing their diets. However, these findings move forward the debate by showing that the changes in marmots cannot be related to immediate dietary influence; during and immediately after hibernation marmots are unable to eat anything because their food is under a thick layer of snow
This study strongly implies that animals have specific ways of transporting individual groups of fatty acids around the body, and given that the animals hibernate underground and are isolated from any external signals, these changes must be controlled by an endogenous clock as part of an annual cycle.
Lead researcher Walter Arnold explains that the findings show it is unlikely that the mechanisms are specific to animals that hibernate, as all mammals, including humans, lower their body temperature to a certain extent during the winter months.
'The humble marmot could revolutionise our way of thinking about fatty acid metabolism. The idea that changes in the essential fatty acid content of membranes can only be made via the diet is clearly too simple.'
Arnold also adds that 'the incidence of heart attacks in humans, well known to increase when membranes contain a high n-6 to n-3 ratio, peaks at the end of winter.' The study therefore implies that there is a chance this high incidence of heart attacks could be linked to a seasonal peak of n-6 polyunsaturated fatty acid concentrations in heart muscle.
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Document Reference: Arnold, W., et al. (2011) Diet-Independent Remodeling of Cellular Membranes Precedes Seasonally Changing Body Temperature in a Hibernator. PLoS ONE 6(4): e18641. DOI:10.1371/journal.pone.0018641.
Subject Index: Life Sciences; Scientific Research