Regulation of Testicular Prolactin and Luteinizing Hormone Receptors in Golden Hamsters*

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Abstract
Iodinated ovine PRL (oPRL) was used to detect the presence of PRL receptors in hamster testes. Binding to these receptors was specific for PRL and was saturable and incompletely reversible. Testicular PRL receptors possessed a single class of independent, high affinity binding sites (Ka = 2.4 × 1010m−1). The ability of PRL to regulate testicular PRL and LH receptors was measured in experiments wherein mature hamsters were injected daily for 10 days with 600 μg bromocriptine, 600 μg bromocriptine plus 250 μg oPRL, or injection vehicles. Bromocriptine injections, which caused significant reductions in serum PRL, produced commensurate decreases in concentrations (femtomoles per mg protein) of testicular LH and PRL receptors, without affecting binding of PRL to adrenal or liver tissue preparations. Decreases in testicular PRL binding were not always statistically significant, but were consistently measured. The administration of oPRL with bromocriptine significantly increased both concentration and total content (femtomoles per paired testes) of testicular LH receptors and PRL-binding sites above those measured in bromocriptine-injected and control hamsters. Liver PRL-binding sites were, likewise, increased above values measured in controls and bromocriptineinjected hamsters by this latter treatment, which had no significant effect on adrenal PRL-binding sites. Neither bromocriptine nor bromocriptine plus oPRL had any effect on the affinity (Ka) of testicular PRL receptors for labeled oPRL. Serum testosterone and FSH concentrations were unaltered by treatment, thereby suggesting that effects on receptors did not occur secondary to changes in these hormones, but, rather, represent direct effects of PRL. An additional experiment in prepubertal male hamsters (aged 14–30 days) demonstrated that bromocriptine injections for 17 days significantly reduced the total content of testicular PRL receptors. In vitro experiments eliminated the possibility of direct effects of bromocriptine on testicular receptors. The data demonstrate the possibility that PRL is required for maintenance of normal concentrations of testicular LH/hCG and PRL receptors in adult and immature golden hamsters. An increase in putative serum anti-oPRL antibodies was measured in oPRL-injected hamsters. Since such antibodies could contribute, to an unknown degree, to PRL binding measured in tissue preparations, the present data indicate only that increased testicular binding occurs subsequent to injection of heterologous PRL, without demonstrating that this binding is indeed due to increased PRL receptors. (Endocrinology114: 594, 1984)