title: Diurnal rhythm and effects of feeding, exercise and recombinant equine growth hormone on serum insulin concentrations in the horse.
abstract: BACKGROUND
  As growth hormone increases lean body mass, it could be a therapy for obese horses. However, growth hormone use induces hyperinsulinaemia in some species, so further investigation is warranted.
  
  
  OBJECTIVE
  To investigate the effects of feeding, exercise and growth hormone therapy on basal insulin concentrations in healthy horses.
  
  
  METHODS
  In vivo experimental study.
  
  
  METHODS
  Blood samples were obtained every 30 min from 12 geldings over 24 h, to establish basal serum insulin concentrations, before they underwent a 3-week exercise programme. Horses were allocated into 2 groups and exercised for another 4 weeks. Group A received daily i.m. injections of recombinant equine growth hormone; 5 mg/day for 5 days, then 12.5 mg/day for 16 days. Blood samples were taken daily before feeding. Insulin vs. time area under curve of Groups A and B were compared using a Student's unpaired t test.
  
  
  RESULTS
  Horses demonstrated insulin peaks within 2 h of feeding of 577 ± 108.3 pmol/l at 09.30 h and 342.4 ± 75.7 pmol/l at 17.30 h, despite receiving the same meal. The nadir was between midnight and 07.30 h. Exercise had no effect on basal insulin concentrations prior to equine growth hormone administrations. The equine growth hormone injections increased serum insulin concentrations (P = 0.01) within Group A, from 44.4 ± 15.3 pmol/l initially to 320.9 ± 238.2 pmol/l by Day 12. Exogenous growth hormone caused variable hyperinsulinaemia, which was alleviated once equine growth hormone administration ceased.
  
  
  CONCLUSIONS
  Single serum samples taken prior to the morning meal provide basal insulin concentrations. Exercise did not change basal insulin concentrations. However, equine growth hormone injections increased basal insulin concentrations, which were not ameliorated by exercise.
  
  
  CONCLUSIONS
  This therapy is not recommended to address obesity in insulin-resistant equids.
authors: School of Animal & Veterinary Sciences, Charles Sturt University, New South Wales, Australia.
authors: G K
authors: GK
authors: Noble
authors: M N
authors: MN
authors: Sillence
mesh_terms: D000822:Animal Husbandry / Q000379:methods*; D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D013006:Growth Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D006736:Horses / Q000378:metabolism*; D007328:Insulin / Q000097:blood*; D008297:Male; D010805:Physical Conditioning, Animal / Q000502:physiology*
chemical_list: D007328:Insulin; D013006:Growth Hormone
keywords: diurnal rhythm; horse; hyperinsulinaemia; laminitis; metabolism; weight loss