Evidence for the Secretion of Decidual Luteotropin: A Prolactin-Like Hormone Produced by Rat Decidual Cells*

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Abstract
Rat decidual tissue contains a PRL-like hormone named decidual luteotropin. We have recently revealed some of its physiological and biochemical characteristics. However, because rat decidual tissue contains specific binding sites for PRL, it was important to demonstrate that the hormone found in the tissue is not locally stored and structurally transformed PRL but a hormone actively synthesized by the rat decidual tissue. Decidual explants or decidual cells obtained from day 9 pseudopregnant rats were incubated for different times under either static conditions or continuously perifused with medium at a rate of 1 ml/h. Levels of decidual luteotropin were measured by a specific radioreceptor assay using luteal membranes as source of receptors and [125I]iodo-ovine(o)PRl as a tracer. In the static incubation, no proof of hormone production was obtained; levels of decidual luteotropin in medium and tissue or cells at the end of the incubation were similar to levels found in either cells or tissue before incubation. In sharp contrast, decidual cells perifused with media secreted large amounts of hormone. This may suggest that an inhibitor of decidual luteotropin production was being removed from the culture by the perifusion. For the first 4 h of perifusion, no hormone was produced. However by the fifth hour, cells began to actively release decidual luteotropin. Secretion of the hormone increased with time and reached maximal values between 7-15 h of perifusion. During the 15 h of perifusion, decidual cells released approximately 1000 times more decidual luteotropin than the amount they originally contained. A dose-response increase in hormone secretion was obtained with increased concentrations of decidual cells. The net amount of decidual luteotropin released into the medium over an 18-h period was appoximately 6.5 .mu.g/30 .times. 106 cells, 3.5 .mu.g/10 .times. 106 cells, and 0.5 .mu.g/2 .times. 106 cells. A similar profile of decidual luteotropin release was obtained when decidual explants were perifused. However, in contrast to decidual cells which secreted no hormone for the first 4 h of culture, decidual explants immediately began to release decidual luteotropin in the medium. The secretion of decidual luteotropin in vitro was inhibited 75% by the protein synthesis inhibitor, cycloheximide. In summary, results of this investigation demonstrate for the first time that rat decidual cells secrete in vitro a PRL-like hormone, decidual luteotropin.