Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
Board

For many small mammals the key to winter survival is hibernation. The little brown bat, Myotis lucifugus is one of these and in the northern parts of its range spends many months of the year in hibernation with bouts of continuous torpor that can last up to 90 days between arousals (1,2). During periods of hibernation, metabolic rate is strongly suppressed, often to as little as 1-4 % of the resting rate at euthermic body temperature, and core body temperature can fall as low as 2-5°C (3). Metabolic regulation during torpor serves two roles: one, to achieve a coordinated suppression of the rates of multiple cellular processes (both energy-producing and energy-utilizing) in order that homeostasis is re-established at a much lower net rate of ATP turnover; and two, to implement specific metabolic adjustments that are needed to support life at low body temperature. The latter, for example, includes a switch to a primary reliance on lipid oxidation to fuel metabolism during torpor.

One critical control mechanism is proving to be reversible protein phosphorylation. For example, in several species, entry into hibernation is accompanied by changes in the phosphorylation state of regulatory enzymes of glycogenolysis (phosphorylase, phosphofructokinase, pyruvate kinase) and of carbohydrate oxidation (pyruvate dehydrogenase) that lower their activity states (4-7). Reversible phosphorylation controls are also likely to regulate the suppression of numerous other enzymes and functional proteins to create the state of torpor. Indeed, hibernation-dependent changes in the activities and kinetic properties of pyruvate kinase and lactate dehydrogenase have been described in skeletal muscle and other tissues of M. lucifugus (8,9) although no assessments of possible reversible phosphorylation controls were made in these studies.

Given that widespread protein phosphorylation is critical to metabolic depression in hibernators, protein kinases clearly become a central instrument for regulating and coordinating metabolic arrest. Adenosine 3'-5'-cyclic monophosphate dependent protein kinase (PKA) is the best known of the protein kinases and is intimately linked with the regulation of intermediary energy metabolism in animals. Hence, an important role for PKA in managing the transitions to and from the hibernating state can be envisioned. The action of PKA in mediating changes in the activity states of enzymes and functional proteins in hibernator tissues may be facilitated by adaptive changes to the enzyme itself or by effects of high versus low temperature on enzyme properties. To analyze this proposal, the catalytic subunit of PKA from bat skeletal muscle was purified and characterized. An analysis of enzyme properties at high and low assay temperatures, in comparison with the enzyme from a nonhibernating species, was used to evaluate the regulation of the enzyme with respect to metabolic transitions between hibernating and euthermic states.

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