Plasma LH and FSH concentrations in prepubertal beef heifers before and in response to repeated injections of low doses of Gn-RH

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Abstract
Blood samples were taken at 15-min intervals from prepubertal beef heifers, 6 aged 4 mo. and 6 aged 10 mo., to determine plasma LH [luteinizing hormone] and FSH concentrations. Two animals of each age group were then subjected to 9 consecutive injections of 0.5, 2.0 or 5.0 .mu.g GnRH [gonadotropin releasing hormone] at 2-h intervals. LH episodes were evident in all animals before treatment, and in 11/12 heifers occurred at a rate of between 1 and 4 per 24 h. One 10-mo. old heifer, later detected in estrus, displayed 10 episodes per 24 h. Mean FSH concentrations did not differ significnatly between age groups. In all animals, GnRH injections induced an episodic pattern of LH secretion which was significantly different between GnRH doses, but not between age groups. A distinct episode-like response in FSH concentrations occurred to each GnRH injection in only 1 heifer. The prepubertal period apparently is associated with a lack of pituitary stimulation, rather than an inability to respond to GnRH.