Nitric oxide inhibits corticotropin-releasing hormone exocytosis but not synthesis by cultured human trophoblasts.

Nitric oxide (NO) plays an important role in many cell-cell signaling systems, but its mechanism of action is variable. We have previously reported that NO reduces secretion of the peptide hormone, CRH, from cultured placental cells and the perfused placenta. Because placental CRH production seems linked to human parturition, we wished to explore the mechanism of action of NO in this setting in more detail. We report here that in the placenta, NO specifically inhibited CRH exocytosis, not synthesis, and that endogenous NO affects this process. Cytotrophoblasts were prepared from term human placentas and cultured as monolayers. CRH immunoreactivity in the cell supernatants and cell extracts were measured by RIA. CRH messenger RNA was determined by Northern blot analysis. Sodium nitroprusside (SNP; 1-100 mumol/L) and S-nitroso-N-acetyl-penicillamine (SNAP; 1-100 mumol/L), NO donors, significantly reduced basal CRH concentration in the media, while increasing the concentration of CRH in the cells (P < 0.01), suggesting that exocytosis of CRH was inhibited. These effects could be attenuated by the NO scavenger hemoglobin (20 micrograms/mL). KCl (45 mmol/L), which causes exocytosis by depolarizing the cell membrane, increased CRH release by 2- to 3-fold, and this was inhibited by SNP. Basal release of CRH was augmented by the NO synthase competitive inhibitor N omega-L-arginine methyl ester (1 mmol/L; P < 0.01) and the guanylate cyclase inhibitor, LY83583 (1 mumol/L; P < 0.01). The inhibitory effect of SNP was also blocked by LY83583. CRH messenger RNA content did not change when the placental cells were incubated with SNP, N omega-L-arginine methyl ester, and LY83583 for 6 and 24 h, and this was consistent with studies showing that total CRH immunoreactivity (cells plus media) did not change in the presence of SNP. These studies indicate that exogenous NO inhibits CRH exocytosis, rather than biosynthesis, by human trophoblasts and that endogenous NO has tonic inhibitory effects on CRH release by these cells. The inhibitory effect of NO on basal and stimulated CRH release by placental trophoblasts seems to be a guanylate cyclase-mediated inhibition of exocytosis.