Adult male northern elephant seals maintain high rates of glucose production during extended breeding fasts

Abstract

Many species undergo natural fasts as part of their life histories. Extended fasting is associated with increased β-oxidation of fatty acids and reduced oxidation of glucose to minimize commitment of body protein to gluconeogenesis. However, the metabolic strategies used to sustain extended fasts simultaneous with high rates of energy expenditure are not well understood. Studies in fasting adult female and weanling northern elephant seals (NES) have revealed high rates of endogenous glucose production (EGP) under constraints of high nutrient demand for lactation or development but relatively low rates of metabolism. These studies revealed low rates of glucose oxidation and high rates of glucose recycling through the Cori cycle. We measured rates of glucose flux in fasting adult male NES to assess how significantly longer fasting durations, higher metabolic rates, and greater rates of muscular activity affect glucose kinetics. We measured glucose turnover in 18 adult males using the clearance of [6-H3] glucose during breeding and molting. Adult male NES maintain high rates of EGP across extended fasts. EGP greatly exceeded estimated needs for glucose-dependent tissues, varied directly with plasma insulin and lactate concentrations, and was inversely related to plasma ketoacid concentrations. Together, these findings suggest that high rates of glucose production and recycling during breeding maintain high blood glucose levels to support glucose-dependent tissues while minimizing production of ketoacids and commitment of protein stores to glucose production.

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Abbreviations

βHBA:

Betahydroxybutyrate

CNS:

Central nervous system

EGP:

Endogenous glucose production

NES:

Northern elephant seal

RIA:

Radioimmunoassay

TCA:

Tricarboxylic acid

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Acknowledgements

The research was supported by NSF Grant IOS-0818018. Any opinions and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. We thank Año Nuevo State Park and the UC Natural Reserve System for logistical support.

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Correspondence to Daniel E. Crocker.

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Communicated by I.D. Hume.

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Crocker, D.E., Wenzel, B.K., Champagne, C.D. et al. Adult male northern elephant seals maintain high rates of glucose production during extended breeding fasts. J Comp Physiol B 187, 1183–1192 (2017). https://doi.org/10.1007/s00360-017-1098-1

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Keywords

  • Fasting
  • Gluconeogenesis
  • Pinniped