A Comparison of Moment to Moment and Diurnal Changes in Circulating Inhibin and Testosterone Concentrations in Male Rhesus Monkeys (Macaca mulatto)*

Abstract

The secretion of inhibin by the testis was studied in the rhesus monkey, a species which exhibits marked episodic and diurnal patterns of testosterone (T) secretion. Inhibin and T were measured by RIA in blood samples drawn every 20 min for 24 h from 5 adult male monkeys. The molecular size of circulating inhibin, estimated by gel chromatography, was approximately 31 kDA. Plasma inhibin levels were undetectable in long term castrates. T was secreted episodically at a frequency of 6.0 +/- 0.9 pulses/24 h. The computer algorithm also identified 4.6 +/- 0.8 inhibin pulses/24 h. Of 30 T pulses among the 5 animals, however, only 7 coincided with low amplitude inhibin secretory bursts. Each animal demonstrated a significant diurnal periodicity in T secretion, with mean maximum concentrations at 0108 h (range, 2100-0640 h). By contrast, there was no significant diurnal rhythm for inhibin in any of the animals. The pulsatile administration of GnRH (0.1 micrograms/min, iv, for 3 min every 3 h) was used to activate the pituitary testicular axis in 6 juvenile monkeys. After 5 weeks of GnRH priming, a pulse of GnRH produced an immediate 4-fold rise in serum LH concentrations, followed within 30-50 min by a 5-fold increase in circulating T levels. FSH levels rose 50%. During the 3-h GnRH interpulse interval, however, there was no change in serum inhibin levels. Two GnRH-treated juvenile monkeys underwent bilateral orchidectomy. In each animal, circulating inhibin levels declined rapidly, with estimated first phase half-lives of 23 and 32 min, respectively. In conclusion, circulating inhibin concentrations in male rhesus monkeys exhibit neither the prominent moment to moment changes nor the circadian pattern characteristic of T secretion in this species. The relatively constant inhibin levels cannot be explained by prolonged metabolic clearance. The data are consistent with the proposal that most of the inhibin in the circulation is released across the apical surface of Sertoli cells into the seminiferous tubular fluid with passage into the rete testis from which it is continuously absorbed. The intermittent LH signal, by contrast, appears to make a minor contribution to the release of inhibin from the primate testis into the circulation.