Stimulation of the Synthesis and Release of Decidual Prolactin by a Placental Polypeptide*

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Abstract
Human decidual explants incubated for 2 h in medium containing placental tissue released 69.1% more PRL (P < 0.05) than decidual explants incubated in control medium alone. Similarly, decidual explants incubated for 1 h in medium containing 5%, 20%, and 50% of a conditioned medium from cultured placental explants (PCM) released 87.5%, 211.5%, and 406.7% more PRL and synthesized 10.3%, 45.0%, and 70.4% more PRL, respectively, than explants incubated in control medium alone (P < 0.01 in each instance). PCM, however, had no measurable effect on the synthesis or release of trichloroacetic acid-precipitable decidual 35S-labeled proteins or on the release of PRL and LH from dispersed rat pituitary cells. Conditioned media from human fibroblasts, human endothelial cells and human parathyroid adenoma cells, human serum, fetal calf serum, and BSA as well as human placental lactogen and hCG had no effect on PRL release. Dialysis or ultrafiltration of PCM using membranes with molecular weight exclusions of 10,000 or delipidation of PCM had no effect on the stimulatory activity. However, trypsin, chymotrypsin, or pronase treatment resulted in a greater than 84% loss of activity. These studies, therefore, suggest that the placenta releases a polypeptide(s) that may be imporatnt in regulation of the synthesis and release of decidual PRL during pregnancy.