Circulating Inhibin α Concentrations in Infant, Prepubertal, and Adult Male Rhesus Monkeys (Macaca mulatta) and in Juvenile Males during Premature Initiation of Puberty with Pulsatile Gonadotropin-Releasing Hormone Treatment*

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Abstract
Circulating inhibin .alpha. concentrations were determined in infant, juvenile, and adult male rhesus monkeys with a RIA employing antisera to a synthetic fragment of the .alpha.-subunit of porcine inhibin. Binding of tracer, [DSer1,Nle5]human inhibin .alpha.(1-25)-Gly-125I-Tyr, to antibody was inhibited by standard, [DSer1,Nle5]human inhibin .alpha.(1-25)-Gly-Tyr, and by plasma from adult male monkeys in a parallel fashion. Castration in adults resulted in a 5-fold decline in the levels of immunoreactivity in plasma. Mean (.+-. SE) plasma inhibin .alpha. concentrations in infants and adults (322.9 .+-. 51.9 and 460.1 .+-. 43.9 pg/ml, respectively) were significantly higher (P < 0.05) than those in juveniles (191.3 .+-. 28.3 pg/ml). Moreover, initiation of puberty in juvenile males, 13-18 months of age, with a chronic (10- to 12-week) intermittent iv infusion of GnRH (0.1 .mu.g/min for 3 min every 3 h) resulted in a progressive rise in circulating inhibin .alpha. that plateaued, after 5 weeks of pituitary stimulation, at concentrations (343.9 .+-. 38.2 pg/ml) comparable to those of infants and adults and twice those observed before initiation of the pulsatile infusion of GnRH. Circulating FSH concentrations increased during the first week of GnRH stimulation from 2.7 .+-. 0.1 ng/ml before treatment to 6.0 .+-. 1.2 ng/ml, where they remained for the duration of the experiment. Testosterone secretion during the initiation of precocious puberty occurred in discrete episodes that were robustly correlated with GnRH-induced LH discharges. In contrast, changes in circulating inhibin .alpha. concentrations over the 3-h interval between GnRH pulses were unremarkable. Activation of Sertoli and Leydig cells during initiation of puberty in the juvenile males, as reflected by circulating inhibin .alpha. and testosterone concentrations, respectively, occurred with similar time courses. At the time of orchidectomy, 10-12 weeks after initiation of GnRH treatment, testicular tissue was prepared for histological examination. In spite of a 2-fold gain in testicular weight and in hypertrophy of Sertoli cells in association with GnRH stimulation, maturation of the germinal epithelium did not progress past prophase I spermatocytes, and the number of these latter cells was meager. These findings indicate that the testis of the infant primate, like that of the adult, secretes significant amounts of inhibin, and that the quiescent Sertoli cell of the juvenile males may be readily provoked by appropriate gonadotropin stimulation into producing inhibin. The results also fail to provide evidence for the view that changes in circulating inhibin concentrations are robustly related, in an inverse fashion, to maturation of the germinal epithelium.