Attempts to restore PRL receptors (PRL-R) in liver membranes of hypophysectomized rats with injections of PRL have so far been only partly successful. In the present study, bovine PRL (bPRL) was mixed with polyvinylpyrrolidone (PVP) in order to sustain PRL blood levels with a single injection a day. PRL-R were measured by displacement of the binding of [l25I]ovine PRL (lactoperoxidase oxidation) to a 5000 × g particulate fraction of liver by unlabeled ovine PRL. The number of binding sites was calculated by Scatchard analysis and expressed as femtomoles per mg protein. PRL-R were 85 ± 12 fmol/mg in normal intact female rats. Seven days posthypophysectomy, PRL-R were undetectable. Daily injections of bPRL with PVP for 10 days fully restored PRL-R (117 ± 32 fmol/mg). No significant change in PRL-R was noted when bPRL was injected with bovine GH (bGH; 120 ± 23 fmol/mg), bovine LH (bLH; 84 ± 14 fmol/mg), bGH plus bLH (90 ± 12 fmol/mg), or estrogens (79 ± 12 fmol/mg). Daily injections of bGH or bLH, alone or in combination, or estrogens with PVP failed to restore PRL-R. These results demonstrate a direct role of PRL in stimulating the production of its own receptors when a sustained blood level of the hormone is achieved.
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