Development and hormone regulation of androgen receptor levels in the sexually dimorphic larynx of Xenopus laevis

Development of the sexually dimorphic larynx in African clawed frogs is controlled by secretion of androgenic steroids (D. Sassoon and D. Kelley, 1986, Amer. J. Anat 177, 457-472). Adult laryngeal muscle shows high levels of androgen binding relative to other skeletal muscles and binding activity in males is three times that in females (N. Segil, L. Silverman, and D. Kelley, 1987, Gen. Comp. Endocrinol. 66, 95-101). To determine when androgen sensitivity and sex differences arise, we assayed [³H]dihydrotestosterone (DHT) binding activity in larynges from metamorphic and postmetamorphic male and female frogs. Scatchard analyses indicate that DHT binds to a saturable component with high affinity. At metamorphosis, male and female juveniles have average binding levels of 262 and 269 fmoles/mg protein, respectively, approximately 7 to 20 times their adult values. At 3 months postmetamorphosis (PM), sexually dimorphic binding levels are observed. Binding activity declines gradually in females from metamorphosis to 9 months PM. In males, levels of binding activity remain high throughout the first 6 months PM and then decrease to near adult levels by 9 months PM. Administration of exogenous DHT to 3 months PM juveniles decreases average binding activity from 180 (male) or 74 fmoles/mg (female) to 33.5 fmoles/mg in both sexes. Testosterone has a less pronounced effect on binding activity in males than DHT and is ineffective in females. We conclude that sexually dimorphic adult levels of androgen binding in larynx arise by differential decrease from initially high, sexually monomorphic levels and that high titers of circulating androgens normally present by 6 months PM in males are responsible for the marked decrease in binding activity observed during laryngeal development.