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Abstract
In the first experiment, the objective was to evaluate plasma LH, FSH and testosterone in bulls treated with GnRH analogs. Prepubertal bulls received a small or large dose of GnRH agonist (azagly nafarelin), GnRH antagonist (cetrorelix) or vehicle for 28 d. Serial blood samples were collected at different times during the treatment period. Agonist-treated bulls had reduced testosterone pulse frequency and increased mean and basal concentrations of testosterone as well as increased basal concentrations of LH, decreased frequency and amplitude of LH pulses and a slight increase of mean FSH concentration. Treatment with cetrorelix slightly reduced amplitude of LH pulses. The greater plasma testosterone concentrations in bulls with prolonged treatment with GnRH agonist may result from functional changes induced by enhanced basal concentrations of LH during the treatment period.
In the second experiment, the hormonal response to chronic treatment with GnRH analogs was compared in rains and bulls. Rams and bulls were treated with GnRH agonist, GnRH antagonist, or vehicle for 28 d. Hormonal patterns and response to GnRH were evaluated during and post treatment. Agonist treatment inhibited response to GnRH, suppressed LH pulses, and increased basal LH in rams and bulls which was associated with a dramatic (bulls) or slight (rams) increase of testosterone. Antagonist treatment suppressed the response to GnRH and LH pulses in bulls, but not in rams. The GnRH agonist treatment suppressed LH pulses and increased basal LH and T in both species, although the increase was more dramatic in bulls. There are still effects of treatment with either analog at Day 40 post-treatment.
