Evidence of short-loop inhibition of decidual prolactin synthesis by decidual proteins. Part I.

Induction of uterine endometrial prolactin synthesis is dependent on progesterone-induced decidualization of stromal cells. These decidual cells are not dependent on progesterone for continued prolactin synthesis. The factors modifying decidual prolactin synthesis remain largely unknown. To test the hypothesis that a decidual protein is the major modulator of new prolactin synthesis, decidua were cultured within dialysis membranes allowing the accumulation of proteins greater than 12,000 molecular weight in a metabolically neutral environment, and the rate of new synthesis was compared with prolactin synthesis from samples cultured in 10 times the available volume for protein distribution. The rate of new prolactin synthesis at 48-hour intervals up to 144 hours was compared. Initial and postculture decidual prolactin content was obtained and was found not to vary significantly between groups (0.05 less than p less than 0.10). At 48 hours significant suppression of decidual prolactin synthesis was apparent (p less than 0.05) within the dialysis membranes. As prolactin concentration increased during in vitro culture this suppression was enhanced (p less than 0.005). Gel chromatography and immunoprecipitation of iodine 125-labeled prolactin added at time 0 revealed no significant degradation of the 125I-labeled prolactin and maintenance of its immunoactivity even at 144 hours. This confirms that the plateauing of prolactin concentration within the dialysis membranes is due to suppression of new synthesis rather than metabolism of previously synthesized prolactin.