Abstract

Introduction

Materials & Methods

Results

Discussion & Conclusion

References

Discussion
Board

Estivation is a dormancy induced by dry environmental conditions and is common among animals inhabiting arid regions of the Earth. Many frog and toad species estivate (1) and are often active for only a few weeks of the rainy season each year. The spadefoot toad, Scaphiopus couchii (Baird), of the American southwest typically spends about 9-10 months of the year buried under the desert soil (2,3). During this time its metabolic rate drops to ~20 % of the resting rate of awake toads (4) and animals rely mainly on oxidization of stored lipid reserves for energy production. As the soil dries out over time, the toad's body also loses water. Up to 60% of total body water (47-50 % of body mass) may be lost after several months (2). To help retard water loss, body fluid osmolality is increased by as much as 300 mM by the accumulation of urea, a by-product of protein catabolism (3).

Metabolic rate depression is a critical component of estivation (1). Part of the reduction of metabolic rate can be traced to the cessation of digestion and skeletal muscle movements as well as strong suppressions of heart and breathing rates, but overall a coordinated suppression of the rates of all cellular processes, both ATP-consuming and ATP-producing, is needed so that stable long term dormancy is achieved. Thus, one of the components of estivation may be a rearrangement of the metabolic potential of cells and organs, altering the activities of enzymes so that those in necessary metabolic pathways are sustained or enhanced whereas those in pathways that are little used during dormancy are suppressed. Furthermore, the sensitivity of metabolism to extracellular stimuli might be altered during dormancy by targeting signal transduction pathways with changes in the activities of their protein kinase and protein phosphatase components.

The present study assesses the metabolic potential of toad organs during estivation with a broad survey of the maximal activities of many enzymes of intermediary metabolism, comparing control (awake, nonestivating) toads with estivating individuals. We also assessed estivation-induced changes to cyclic AMP-dependent protein kinase (PKA), calcium and phospholipid dependent protein kinase C (PKC), and protein phosphatases, to determine if metabolic adjustments during dormancy included changes to the reversible phosphorylation controls over enzymes.

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