Androgen-binding levels in a sexually dimorphic muscle of Xenopus laevis.

The larynx of adult South African clawed frogs, Xenopus laevis, is larger in males than in females and hypertrophies in adult females and juveniles in response to androgen. Sexual dimorphism and androgen sensitivity suggest that the larynx is a testosterone target tissue. Saturation analysis of androgen (R1881) binding in laryngeal cytosol revealed an approximately threefold quantitative difference between male and female androgen-binding levels (36.4 vs 11.5 fm/mg protein). By contrast, as measured by one-point assay, androgen-binding levels in thigh muscle of either males or females were between 0 and 4 fm/mg protein with no apparent sex difference. Competition studies indicated that dihydrotestosterone was the most effective competitor for R1881 binding activity in the larynx. Saturation analysis showed the binding activity to be saturable and of high affinity (apparent Kd 0.46 nM in the male and 0.38 nM in the female). After 1 month of testosterone treatment, female binding levels averaged 16.6 fm/mg protein with a Kd of 0.49 nM, within the range for normal females. In males castrated for 4 months, binding levels were 52 fm/mg protein. After 1 year of castration, binding levels were 25 fm/mg protein. We conclude that laryngeal muscle is an androgen target tissue with sexually dimorphic levels of binding in adults.