Heterologous radioreceptor assays, commonly using ovine prolactin, may generate inconsistent results since prolactin (PRL) from one species may be recognized as growth hormone in another. Homologous radioreceptor assays (RRA) are most similar to the in vivo hormone-tissue receptor environment; however, lactogenic homologous RRAs have been reported only for mouse hepatic membranes. In this study, an assay system was developed to investigate homologous binding for porcine PRL in porcine uterine tissue. The pig does not produce a decidual PRL or a placental lactogen; yet, PRL affects uterine physiology during reproductively important events. Optimal binding conditions established for porcine PRL homologous RRA include 150 micrograms membrane, 45,000 cpm labeled porcine PRL and 500 microliters sodium phosphate buffer pH 7.6, incubated at 25 degrees C for 24 h. Binding of porcine PRL tracer is very low (less than 3%); however, when tissue is treated with the chaotropic agent, MgCl2 (4 M), binding increases from 3 to 28%. Dissociation kinetics show a rate of 3.79 X 10(-6)/s initially, and then 1.63 X 10(-6)/s. Competition for labeled PRL on binding sites with unlabeled porcine PRL results in 80% displacement with unlabeled porcine prolactin (NSB) of 7% at 1000 ng. Affinity constant generated from homologous inhibition assays is 0.326 X 10(8) M-1. Porcine growth hormone (GH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH) do not displace porcine PRL tracer. These data describe a lactogenic homologous RRA for porcine endometrial membranes similar to that previously reported for murine hepatic tissue. Homologous RRAs may allow elucidation of PRL receptor characteristics with more similarity to the in vivo hormone-receptor milieu.
