Changes in luteinizing hormone bioactivity associated with gonadotrophin pulses in the cycling mare

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.