Developmental Changes in Testicular Inhibin and Androgen-Binding Protein during Sexual Maturation in the Cynomolgus Monkey, Macaca fascicularis*

Inhibin was measured by RIA in testicular extracts and plasma of cynomolgus monkeys during four stages of sexual maturation. Immunoactive inhibin levels were compared to those of another Sertoli cell secreted protein, androgenbinding protein (ABP). ABP steroid-binding (bioactive) activity was measured in testes and epididymal segments using the radiolabeled ligand [3H]dihydrotestosterone (DHT). Testicular immunoreactive inhibin concentrations were maximal in late prepubertal monkeys, 2.5–3.5 yr old, while the total testicular content of inhibin progressively increased with age into adulthood. Bioactive testicular ABP concentrations were maximal during the pubertal period of the cynomolgus monkey (3.5–4.0 yr old), while the total ABP content of the testes also increased with sexual maturation. Mean (±SE) plasma concentrations of inhibin and testosterone (T) in adults, 6-8 yr old (17.72 ± 3.5 μl inhibin equivalents/ml and 7.07 ± 2.45 ng/ml T, respectively), were significantly higher (P < 0.05 and P < 0.001, respectively) than those in early prepubertal, juvenile monkeys, aged 1.5–2.5 yr (5.85 ± 2.1 μU inhibin equivalents/ml and 0.27 ± 0.02 ng/ml T). The increased plasma levels of inhibin and T in adults were associated, respectively, with the increased inhibin and androgen contents of the testes in these same animals. The developmental changes in testicular steady state mRNA concentrations for the inhibin α-, βA-, and βB-subunits as well as ABP were examined during sexual maturation by Northern blot analysis using heterologous human cDNA probes. Densitometric analysis of the autoradiograms revealed that the inhibin a-subunit mRNA concentrations were higher than those of inhibin-βA and -βB and ABP mRNA during all stages of pubertal development. Although the relative concentrations of each inhibin subunit mRNA were decreased in the adult animals relative to those in the juvenile monkeys, the total amount of steady state mRNA for the subunits was greater than that in the immature animals. A similar situation existed for the ABP mRNA. These results demonstrate that 1) while the concentrations of inhibin and ABP protein and mRNA are decreased in adult animals, the total content increases with age; 2) plasma inhibin levels in the primate increase during puberty, as observed in man; 3) increased inhibin production is associated with increased testicular content of inhibin mRNAs; and 4) the changes in relative concentrations of the testicular inhibin α-, βA-, and βA-subunit mRNAs as well as ABP mRNA during sexual development are consistent with the hypothesis that the expression of the genes that code for these peptides is independently regulated. (Endocrinology126: 2858–2867, 1990)