title: Cardiorespiratory and endocrine effects of endogenous opioid antagonism by naloxone in ponies anaesthetised with halothane.
abstract: Halothane depresses cardiorespiratory function and activates the pituitary-adrenal axis, increasing beta endorphin. In horses, beta endorphin may enhance the anaesthetic-associated cardiorespiratory depression and mortality risk. The authors studied endogenous opioid effects on cardiorespiratory function and pituitary-adrenal activity in halothane-anaesthetised ponies by investigating opioid antagonism by naloxone. Six ponies were anaesthetised three times (crossover design). Anaesthesia was induced with thiopentone and maintained with 1.2 per cent halothane for 2 hours. Immediately after induction, naloxone was administered either intravenously (0.5 mg kg(-1)bolus then 0.25 mg kg(-1)hour(-1)for 2 hours) or intrathecally (0.5 mg) or was replaced by saline as control. Pulse and respiratory rates, arterial blood gases, cardiac output and plasma cortisol and adrenocorticotrophic hormone (ACTH) concentrations were measured. All groups developed cardiorespiratory depression (40 per cent decrease in cardiac output) and plasma cortisol increased. Plasma ACTH concentration was higher in ponies treated with intrathecal naloxone. Endogenous opioids may inhibit ACTH secretion, attenuating the stress response to halothane anaesthesia in equidae.
authors.affiliation: Department of Veterinary Surgery and Anaesthesiology, Faculty of Veterinary Medicine and Animal Science (FMVZ), Unesp, Botucatu, Sao Paulo, 18618-000, Brazil.
authors.forename: S P
authors.initials: SP
authors.lastname: Luna
authors.forename: P M
authors.initials: PM
authors.lastname: Taylor
mesh_terms: D000324:Adrenocorticotropic Hormone / Q000097:blood; D000769:Anesthesia, Inhalation / Q000009:adverse effects / Q000662:veterinary*; D018685:Anesthetics, Inhalation / Q000008:administration & dosage / Q000009:adverse effects / Q000494:pharmacology*; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D001831:Body Temperature / Q000187:drug effects; D002320:Cardiovascular Physiological Phenomena / Q000187:drug effects; D018592:Cross-Over Studies; D004562:Electrocardiography / Q000187:drug effects / Q000662:veterinary; D005260:Female; D006221:Halothane / Q000008:administration & dosage / Q000009:adverse effects / Q000494:pharmacology*; D006339:Heart Rate / Q000187:drug effects; D006736:Horses / Q000502:physiology*; D006854:Hydrocortisone / Q000097:blood; D008297:Male; D009270:Naloxone / Q000008:administration & dosage / Q000009:adverse effects / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D018847:Opioid Peptides / Q000037:antagonists & inhibitors*; D010092:Oximetry / Q000662:veterinary; D010913:Pituitary-Adrenal System / Q000187:drug effects / Q000502:physiology; D012143:Respiratory Physiological Phenomena / Q000187:drug effects
chemical_list: D018685:Anesthetics, Inhalation; D009292:Narcotic Antagonists; D018847:Opioid Peptides; D009270:Naloxone; D000324:Adrenocorticotropic Hormone; D006221:Halothane; D006854:Hydrocortisone