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Rapidly alternating photoperiods disrupt central and peripheral rhythmicity and decrease plasma glucose, but do not

Tamara J Varcoe1, Kathryn L Gatford2, Athena Voultsios2

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Disrupting circadian rhythms in sheep altered clock gene expression in skeletal muscle but minimally impacted glucose homeostasis. This suggests species-specific differences in circadian regulation of metabolism.

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Area of Science:

  • Chronobiology
  • Metabolic Physiology
  • Animal Science

Background:

  • Circadian rhythm disruption in rodents affects glucose metabolism and adiposity.
  • The impact of circadian disruption on metabolic function in large diurnal animals remains less understood.

Purpose of the Study:

  • To investigate the effects of circadian rhythm disruption on metabolic function in sheep, a large diurnal animal.
  • To assess central and peripheral rhythmicity and their relationship with metabolic homeostasis under altered photoperiods.

Main Methods:

  • Adult ewes were exposed to a control photoperiod followed by rapidly alternating photoperiods (RAPs).
  • Central rhythms (melatonin, core body temperature) and peripheral rhythms (skeletal muscle clock and metabolic gene expression) were measured.
  • Metabolic homeostasis was assessed via glucose tolerance tests and 24-hour glucose and insulin profiles.

Main Results:

  • Melatonin and core body temperature rhythms resynchronized quickly after photoperiod shifts.
  • Skeletal muscle exhibited high-amplitude clock gene (Bmal1, Clock, Nr1d1, Cry2, Per3) mRNA rhythms, phase-advanced after RAPs.
  • Metabolic genes (Pparα, Pgc1α, Nampt) showed constitutive expression; nocturnal glucose concentrations were reduced, but insulin, glucose tolerance, and secretion remained unaltered.

Conclusions:

  • Ovine skeletal muscle clock gene expression oscillates and responds to photoperiod changes.
  • Unlike rodents, key metabolic genes linking circadian and metabolic clocks were arrhythmic in sheep skeletal muscle.
  • Circadian disruption had minimal impact on glucose homeostasis in sheep, potentially due to digestive system differences.