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The interaction of 125I-labeled human follitropin (hFSH), human lutropin (hLH), and human choriogonadotropin (hCG) with a particulate fraction prepared from the testes of adult men was investigated. The hormone specific binding was maximal at pH 7.5 and 34 degrees C in the presence of 10 mM MgCl2. The FSH receptor was relatively more stable than the LH receptor (t1/2 at 34 degrees C of 14 and 4 h, respectively). The dissociation constants (Kd) calculated for hLH or hCG were very similar (approximately 1.0 X 10(-10) - 1.4 X 10(-10) M). The affinity of FSH and LH to their receptors did not appear to change with age, as shown from analysis of testes from 17- to 80-year-old men. The number of FSH receptors was greater than the number of LH receptors in these tissues. The hFSH receptor did not show any preference to binding of the homologous hormone, because FSH from nonprimates could displace 125I-labeled hFSH, but the LH receptor showed a marked preference for hLH or hCG as the ligand.

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The interaction of 125I-labelled hFSH with primate testicular tissue from 4 species of adult monkeys (Macaca mulatta, M. nemestrina, M. fascicularis and Papio cynocephalus) was investigated. 125I-labelled hFSH binding to a particulate fraction (P1, 40 000 g) of frozen testes was highly specific and saturable. Displacement curves generated using the P1 fraction of testes from the 4 species and 125I-labelled hFSH and unlabelled FSH were very similar. The binding of FSH to the monkey testicular receptor was not species specific because purified FSH from heterologous species such as horse, sheep, pig and rat were very effective in competing with 125I-labelled hFSH for binding. The equine FSH was about 10 times more active than hFSH in this respect. Similarly, 125I-labelled ovine FSH bound as well as labelled hFSH to the testes fractions of all 4 monkey species. In marked contrast to the high binding of 125I-labelled hFSH, binding of 125I-labelled hCG with rhesus monkey testis homogenates and P1 fractions was very low. The FSH receptor in the adult rhesus monkey testis was present in much larger quantity than the LH receptor and was more readily detectable. Our studies show that frozen primate testis can be utilized for investigating testicular-FSH interactions.

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