Prolactin Receptors in Luteinized Rat Ovaries: Unmasking of Specific Binding Sites with Detergent Treatment*

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Abstract
Luteinized rat ovaries were analyzed for lactogenbinding sites as a potential source of receptors for purification. Parallel studies were carried out in lactating rabbit mammary preparations for comparison of binding parameters in these two target tissues. Quantitative binding analysis was performed with [125I]iodo-hGH in particulate fractions, detergent-treated particles, and soluble preparations. In competitive binding studies, human GH (hGH) and ovine PRL (oPRL) were equipotent, whereas human placental lactogen was 20% and ovine GH (oGH) 1–3% as effective as oPRL in both particulate preparations. hGH was 6-fold more effective than oPRL in displacing [125I]iodohGH from the solubilized ovarian receptors, but only 16% as effective in the solubilized mammary receptors. Human placental lactogen and oGH retained the same potencies relative to oPRL in both solubilized ovary and mammary receptors as those seen in the particulate fractions. The 1–5% potency of oGH relative to oPRL can be accounted for by oPRL contamination rather than the presence of somatogen binding sites. Scatchard analysis revealed a high affinity binding site with a Ka of 5 × 109 M-1 in all ovary preparations with binding capacities (femtomoles per mg protein) of 1853 ± 589 for particulate, untreated; 3372 ± 2998 for particulate, detergent-treated; and 11420 ± 1564 for solubilized receptors (P < 0.01, solubilized vs. particulate, untreated). Mammary receptors showed a high affinity site with a Ka of 7 × 109 M-1 for particulate, untreated; 15 × 1099 M-1 for particulate, detergent-treated; and 4 × 109 M-1 for solubilized receptors (differences not significant). The corresponding binding capacities (femtomoles per mg protein) were 458 ± 174, 480 ± 162, and 2801 ± 775 (P < 0.05, solubilized vs. particulate, untreated). Luteinized rat ovaries are a richer source of lactogen receptors than lactating rabbit mammary gland (4-fold/mg protein) and are thus of value for receptor purification and related studies. The 7- to 8-fold increase in binding capacity seen with detergent treatment in both of these tissues may represent nascent receptors unavailable for hormone binding.