Dose- and Age-Dependent Effects of Prolactin (PRL) on Luteinizing Hormone- and PRL-Binding Sites in Rat Ley dig Cell Homogenates*

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Abstract
Various doses of ovine PRL (oPRL; NIH-P-S12) were injected into immature (22 days old) and adult (82 days old) Sprague-Dawley rats at 1400 h for 2 days to determine the effects of PRL on hCG- and hPRL-binding sites in Leydig cell homogenates. Since hCG and LH bind to the same receptor site, labeled hCG was used for the assessment of LH-binding sites. From a total of 30–60 immature rats for each treatment group, testes from 6 rats were pooled for homogenization and preparation of 27,000 × g Leydig cell homogenates. From a total of 35 adult rats, testes from single rats were pooled for each 27,000 × g preparation. Binding characteristics for hCG and hPRL were determined for each homogenate pool, viz. 5–10 immature and 5–8 adult pools per treatment group. In immature rats, 1, 2.5, 5, 10, or 20 μg oPRL sc/day for 2 days resulted in an increase in hCG-binding sites to 291, 318, 876, 642, and 591 fmol hCG/mg protein compared to 212 fmol hCG/mg protein in saline injected controls. Receptor affinity remained unchanged. These increases in hCG-binding sites were accompanied by a decrease in hPRLbinding sites, at all doses of PRL, to 37–67% of the control binding sites. Treatment of immature rats with 1 μg human GH resulted in a 3-fold increase in hCG-binding sites concomitant with a 68% decrease in hPRL-binding sites. In adult rats, 30 μg oPRL sc/day for 2 days resulted in a 1.3- fold increase in hCG-binding sites, whereas, the 5-μg dosage was without effect. The administration of 100 fig oPRL/day resulted in a decrease to 55% capacity of control hCG-binding sites. When 50 fig oPRL were injected on day 82 and 100 μg oPRL were injected on day 83, a striking decrease to 30% of control hCGbinding sites was observed. Treatment with 5, 30, or 100 μg oPRL for 2 days resulted in a decrease in hPRL sites to 53%, 30%, or 10% of control. The injection of 50 jug on day 82, followed by 100 μg oPRL on day 83, resulted in a reduction of hPRLbinding sites to 7% of control. These dose- and age-related effects of oPRL on binding sites for hCG and hPRL occurred in the absence of significant changes in serum rat LH, rat FSH, and rat PRL values. There was no change in specific binding sites for FSH in Sertoli cell homogenates of oPRL-treated rats. These results indicate that PRL regulates both its own and LH-hCGbinding sites in the Leydig cells of the rat testis. Both positive and negative modulation of the LH-hCG receptor by oPRL appear to be dose and age dependent.

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