Molecular Pharmacology, Vol 18, 476-482, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Changes in the Fluidity of Myocardial Membranes during Hibernation: Relationship to Myocardial Adenosinetriphosphatase Activity

¹ Department of Pharmacology, University of Alberta, Edmonton, Canada
² Department of Pediatrics, Flinders Medical Centre, Bedford Park, South Australia, Australia
³ C.S.I.R.O. Plant Physiology Unit, Division of Food Research and School of Biological Sciences, Macquarie University, N.S.W., Australia

The activity of both myocardial ouabain-sensitive (Na⁺ + K⁺)-ATPase and ouabain-insensitive Mg²⁺-ATPase is reduced in the hearts of winter-hibernating ground squirrels compared to the activity in the hearts of awake-active animals sacrificed during the summer. (Na⁺ + K⁺)-ATPase preparations from the nonhibernating animals show non-linear Arrhenius kinetics, with a marked increase in the Arrhenius activation energy (E_a) below 16°C. During hibernation this pattern of temperature dependence probably does not change significantly, but the level of enzyme activity below 16°C is now too low to measure with confidence. Conversely, even the reduced level of myocardial Mg²⁺-ATPase from the hearts of animals killed during hibernation continues to display the linear Arrhenius kinetics that were observed in these myocardial preparations from active summer animals. Biophysical studies of the whole myocardial membrane preparations by differential scanning calorimetry and examination of the total lipid extract from these membranes by electron spin resonance spectroscopy after labeling of the lipids with 16 NS suggest that the myocardial membrane is in a more fluid state when the animals are in hibernation. However, only a small percentage of the lipids of the membranes is involved in the thermal transitions observed, and the lack of coincidence of the temperature for these transitions and the parameters of activation energy for both myocardial adenosinetriphosphatases indicates that the activities of (Na⁺ + K⁺)-ATPase and Mg²⁺-ATPase are insulated against this change in lipid structure—possibly by a domain of boundary lipids possessing thermal properties different from those of the bulk phase. During hibernation there is a significant increase in the 18² 6 fatty acid (linoleic) content of myocardial membranes.

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