Binding of radioactive rat follicle-stimulating hormone (FSH) to a particulate fraction of testicular homogenate of Japanese quail cockerels increased progressively with age when the cockerels were reared under long-day photoperiods from the day of hatch. The binding per unit weight of tissue (density of binding) showed a rapid increase from Day 23 to 29. It showed a decrease during the period between Days 29 and 36 when active spermatogenesis took place. The binding per testes (total binding) increased during the period from Day 23 to 36, Both density of binding and total binding remained low in cockerels reared under short-day photoperiods. Injections of testosterone to short-day cockerels at 1.0 mg/day for 3 days increased the density of binding 1.3-fold, but did not increase the testicular weight significantly. Injections of NIH-FSH-S12 at 16 μg/day for 3 days did not increase the density of binding significantly but increased the testicular weight 1.7-fold. The total binding was increased 1.8-fold by this treatment. Combined administration of FSH and testosterone increased the testicular weight 2.3-fold, the density of binding 1.7-fold, and the total binding 3.9-fold. The Scatchard plot analysis of the binding suggested that the hormonal treatment increased the number of binding sites but did not influence the affinity of binding. Histological examination of the testis indicated that the increase in testicular weight by hormonal treatment was mainly due to hypertrophy of the Sertoli cells. These results suggest that FSH induces, by acting synergistically with testosterone, its own receptors in the testis and consequently increases the sensitivity of the testis to FSH. This illustrates a kind of self-potentiating action of FSH. This self-potentiation mechanism, in addition to the synergism between FSH and testosterone, most likely enables the extremely rapid increase in testicular weight in photoperiodically stimulated birds.
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