Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
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Many small mammals living in northern latitudes cope with the restricted food supplies and extreme cold temperatures of winter by retreating into subterranean burrows and using the physiological strategy of hibernation. The golden-mantled ground squirrel, a medium sized (approximately 0.1-0.4 kg in mass) member of the family Rodentia, is one such mammal. Once situated in a dry, protected hibernaculum, this animal enters an extended period of torpor (weeks to months) during which body temperature drops from the normal euthermic temperature of 37°C to approximately 5°C. Body functions, including cardiac output, heart rate, respiratory rate and oxygen consumption, are greatly slowed with the lowered body temperature characteristic of hibernation.

Figure 1. Golden-mantled ground squirrel Spermophilus lateralis. The Audubon Society Field Guide to North American Mammals. Whitaker, J.O. Alfred A. Knopf, New York 1980.

Figure 2. Thirteen-line ground squirrel. Spermophilus tridecumlineatus. The Audubon Society Field Guide to North American Mammals. Whitaker, J.O. Alfred A. Knopf, New York 1980.

The regulation of membrane ion channels and ion pumps and the transmembrane gradients of ions that they control, including Na⁺ and K⁺ gradients, is a key factor in achieving the energy conservation that results in metabolic rate depression (1,2,3).The sodium-potassium activated adenosine triphosphatase (Na⁺K⁺-ATPase; EC 3.6.1.37) is a highly conserved plasma membrane enzyme essential for cellular homeostasis. The enzyme helps to maintain the low Na⁺ and high K⁺ concentrations of the intracellular milieu. Na⁺K⁺-ATPase is also central in the control of cell volume and has a major function in the recovery of the membrane potential and ionic gradients. To carry out its various functions, the Na⁺K⁺-ATPase ion pump utilizes cytosolic ATP as a substrate and is one of the single greatest users of cellular energy, particularly in endotherms, being responsible for 5-40% of steady state energy consumption depending on the cell type (4). Cellular homeostasis depends on the maintenance of adequate ATP levels and a near-continuous balance between ATP-generating and ATP-utilizing processes. Since overall net metabolic rate falls dramatically during hibernation (usually < 5% of euthermic resting rate) and since ATP-generating mechanisms are known to be suppressed during hibernation (e.g. oxygen consumption is reduced, carbohydrate catabolism is suppressed), then it follows that ATP-utilizing processes must also be suppressed during hibernation. We hypothesized, therefore, that regulated suppression of the activity of the cellular Na⁺K⁺-ATPase ion pumps would have to occur in order to achieve a stable state of torpor. In the present study, we describe experiments that quantified the maximal activities of the Na⁺K⁺-ATPase in organs of the euthermic and hibernating golden-mantled ground squirrels and that assessed the role of reversible protein phosphorylation in the regulation of the enzyme during hibernation.

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