Prolactin (PRL) secretion and PRL mRNA expression in human myometrial explant cultures was inhibited by the addition of human placental conditioned medium (HPCM). After 3 days treatment with 5% HPCM PRL secretion was reduced to 24% of control values (P < 0.001). The effect persisted even after removal of progesterone, which suppresses myometrial PRL production, from the HPCM. In search for additional regulatory substances present in the uteroplacental unit, we tested a number of growth factors and cytokines known to affect pituitary PRL secretion. Treatment with endothelin-3 (ET-3) at a dose of 10(-6) M was found to increase PRL release by 20% over 3 days (P < 0.05) whereas interleukin-6 (IL-6) showed no effect on PRL levels. Epidermal growth factor, vasoactive intestinal peptide and interferon-alpha (IFN-alpha) were inhibitory. Interleukin-4 (IL-4) was the most potent inhibitor of PRL expression in myometrial tissue, causing a reduction of secreted PRL to less than 50% of controls after 3 days at a dose of > or = 5 ng/ml (P < 0.001) and a concomitant reduction of PRL mRNA levels. These results demonstrate a modulation of PRL expression in the myometrium by locally present factors.