Assessment of mammary lactogenic receptor changes in pregnant rabbits

Lactogenic receptor was purified from rabbit mammary tissue and used to generate an antiserum in goats. The purified lactogenic receptor material bound lactogenic hormones specifically and reversibly. Antiserum generated in a goat bound a labeled human growth hormone/receptor complex; this was displaced by nonlabeled solubilized receptor preparations. This was used as a radioimmunoassay and was able to detect 0.037 fmol of lactogenic receptor. The specificity of the radioimmunoassay for lactogenic receptor was supported by three lines of evidence; first, the ligand used in the radioimmunoassay was an iodine 125-labeled human growth hormone/receptor combination; therefore, only membrane protein with structural homology to the protein which bound 125I-labeled human growth hormone competed for binding to the antiserum; second, depletion of radioreceptor binding sites by affinity chromatography with ovine prolactin as the fixed ligand was detected; third, an increase in breast lactogenic receptor during pregnancy was detected by both radioreceptor assay and the radioimmunoassay. We found a progressive increase in lactogenic receptors by radioimmunoassay which corresponded to parallel increases by radioreceptor assay in rabbit mammary tissue during pregnancy.