Differentiation of sex steroid-binding protein in adult rhesus monkeys

Abstract

Several species of primates have sex differences in sex steroid-binding protein (SBP), female adults having higher serum binding capacities (micrograms of dihydrotestosterone bound per deciliter) than male adults, e.g., male humans, 1.28 ± 0.4; human females, 2.86 ± 0.9; Macaca nemestrina male animals, 5.62 ± 1.24; Macaca nemestrina female animals, 11.07 ± 1.85 (means ± standard deviations). SBP correlates inversely with metabolic clearance rates of testosterone (T). The sex difference was identified in rhesus monkeys, six per group, evaluated 4 years after postpubertal castration: male animals 3.95 ± 1.14; female animals 5.85 ± 0.98 (p < 0.05). Estradiol-17β (E2) pellets producing physiologic levels of E2 in female monkeys obliterated the sex difference by increasing SBP in male animals. After withdrawal of E2, physiologic levels of T in male monkeys produced a marked decrease in SBP levels (p < 0.01), and the sex difference reappeared; castrated female animals and prenatally androgenized female animals responded similarly to T (2.81 ± 0.81 and 2.64 ± 0.49, respectively). Both values were greater (p < 0.05) than that of the male group (2.02 ± 0.33). These data suggest that the sex steroid milieu influences the binding capacity of SBP for potent androgens in adulthood but that the differentiation of the SBP sex in rhesus monkeys is determined by factors other than prenatal androgen exposure.