A series of studies was performed to more clearly evaluate gonadal function in llamas. Luteinizing hormone (LH) and progesterone (P₄) concentrations were compared following natural or hormone-induced ovulation. Luteal regression was evaluated following administration of the prostaglandin F₂α analog cloprostenol. The interaction of age, season and castration on testosterone (T) and LH concentrations in the male llama were examined in peripubertal and adult animals.
Study I. Progesterone increased (P < 0.05) following treatment with 1 or 5μg GnRH/kg ; 1.25, 6.25 or 31.25 IU hCG/kg; vasectomized or fertile mating. Peak P₄ concentrations were found on d 8 (4.4 ± 0.4 ng/ml, mean ± SEM) post-treatment with return to basal concentrations (< 0.5 ng/ml) by d 12 after all treatments except fertile matings. No treatment differences in time course of rise and decline, magnitude released or peak concentrations of P₄ were noted. Release of LH was examined following treatment with 1μg GnRH/kg and breeding to a vasectomized or a fertile male. While the magnitude and duration of the LH peak (4.3 ± 0.6 ng/ml) was similar for all 3 treatments, GnRH caused LH to peak approximately 1 h sooner than mating.
Study II. Six llamas treated with cloprostenol aborted and underwent a 2 phase decline in P₄: 1) an initial rapid phase to half of pretreatment levels within 1 h of treatment and 2) a slower phase to basal levels (< 0.5 ng/ml) over the succeeding 48 h.
Study III. In 5 peripubertal males studied at 2 mo intervals from 8 to 20 mo of age, 24 h mean T concentrations increased with age, except for a decline at 14 mo. Basal LH and response to GnRH did not change as a function of age while response of T to GnRH increased with age. Although all animals were producing sperm at 20 mo of age, 3 of 5 had preputial-penile adhesions. In 6 adult male llamas, season had no effect on basal T or response to GnRH or on testicular dimensions.
Study IV. Although 24 h basal LH concentrations rose (0.7 ± 0.1 to 1.2 ± 0.1 ng/ml) 1 mo following castration in 4 male llamas, response to GnRH did not differ pre-or post-castration. While mean T in hourly samples for 12 h following 0.1, 1.0 or 5.0 μg GnRH/kg BW in 4 male llamas increased relative to the 12 h control period, there was no difference in T concentrations between dosages of GnRH. After castration, T in male llamas decreased sharply by 15 min and was undetectable by 3 h, exhibiting a 2phase clearance pattern.
Llamas appear to have similar responses within the hypothalamo- hyophysealgonadal axis to other species, but with a lower "setpoint". Basal LH is low in the llama and small increases of LH are capable of causing ovulation and stimulating T production.
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Previous issue date: 1996-03-11