Endothelin inhibits basal and stimulated release of prolactin by human decidual cells

Recent studies have shown that the basal release of PRL from anterior pituitary cells is inhibited by endothelin-1 (ET-1) and ET-3. To determine whether ET also regulates the synthesis and release of PRL by decidual cells, we examined the effects of ET on the synthesis and release of PRL from an enriched fraction of human decidual cells prepared by isopycnic centrifugation of enzymatically dispersed term decidual tissue. Exposure of decidual cells to ET-1 (10(-7) M) for 96 h caused a progressive decrease in basal PRL synthesis and release beginning 24 h after exposure with half-maximal inhibition occurring at an ET-1 concentration of 5 x 10(-9) M. Between 72-96 h of culture, ET-1-exposed cells synthesized 37.2 +/- 2.7% (SEM) and released 32.3 +/- 1.3% less PRL than control cells (P < 0.01). ET-1-exposed cells incubated with [35S]methionine between 72-96 h also released less [35S] PRL than control cells. Sarafotoxin S6C, an ETB receptor agonist, also inhibited basal PRL release, whereas BQ-123, an ETA receptor antagonist, had no effect on basal or on ET-1-mediated inhibition of PRL release. ET-1 also markedly inhibited the stimulation of PRL synthesis and release in response to insulin and insulin-like growth factor-1 (IGF-1). Cells exposed to insulin (100 ng/ml) and IGF-1 (50 ng/ml) alone released 61.2 +/- 3.6% and 40.0 +/- 3.8% more PRL, respectively, than control cells between 72-96 h of exposure. However, cells exposed simultaneously to insulin and ET-1 (10(-7) M) released only 17.1 +/- 3.5% more PRL than control cells during the same time period, and cells exposed simultaneously to IGF-1 and ET-1 released only 4.1 +/- 1.8% more PRL than controls during the same time interval (P vs. insulin or IGF-1 alone < 0.001 in each instance). Both basal and insulin- and IGF-1-stimulated PRL release were also inhibited by the ET isotypes ET-2 and ET-3, and by the ET-1 precursor, big ET. Since macrophages and decidual cells synthesize ET, and decidual tissue contains specific receptors for ET, the inhibitory action of ET on basal and stimulated PRL release may result from an autocrine and/or paracrine effect and appears to be mediated through the ETB receptor.