title: Changes in luteinizing hormone bioactivity associated with gonadotrophin pulses in the cycling mare.
abstract: Equine plasma luteinizing hormone (LH) possesses both biological (in vitro bioassay, B) and immunological (radioimmunoassay, I) activities and the ratio of B:I varies with stage of the oestrous cycle. To estimate the contribution made by pituitary secretion and peripheral metabolism to changes in the B:I ratio, pituitary venous effluent and circulating plasma from 5 dioestrous and 2 oestrous mares were analyzed using both an in vitro bioassay and a radioimmunoassay. During dioestrus, LH was released in a pulsatile fashion with a frequency of 1.4 pulses/24 h and a pulse duration of 20-40 min (centrally) or 2-4 h (peripherally). Between pulses, further secretion of LH from the pituitary was undetectable. During spontaneous pulses, the B:I ratio increased as much as 2- to 3-fold and remained elevated for at least 1 h. A low dose of 10 ng/kg bodyweight (bwt) of the GnRH agonist Buserelin provoked similar changes, unless the exogenous stimulus was close to an endogenous LH discharge. A high dose (50 micrograms) of Buserelin reduced the B:I ratio significantly, in spite of a massive LH release. Samples taken from mares during oestrus showed constantly elevated B:I ratios, a consequence of much greater LH pulse frequency.
authors.affiliation: Department of Veterinary Clinical Sciences, University of Melbourne, Werribee, Victoria, Australia.
authors.forename: P
authors.initials: P
authors.lastname: Pantke
authors.forename: J
authors.initials: J
authors.lastname: Hyland
authors.forename: D B
authors.initials: DB
authors.lastname: Galloway
authors.forename: A A
authors.initials: AA
authors.lastname: MacLean
authors.forename: H O
authors.initials: HO
authors.lastname: Hoppen
mesh_terms: D000818:Animals; D004971:Estrus / Q000502:physiology*; D005260:Female; D006064:Gonadotropins, Equine / Q000097:blood* / Q000276:immunology / Q000378:metabolism; D006736:Horses / Q000502:physiology*; D007986:Luteinizing Hormone / Q000097:blood* / Q000276:immunology / Q000378:metabolism; D010902:Pituitary Gland / Q000502:physiology*
chemical_list: D006064:Gonadotropins, Equine; D007986:Luteinizing Hormone