Evidence for nitric oxide acting as a luteolytic factor in the human corpus luteum

The aims of the present study were to characterize the expression and cellu lar localization of isoforms of nitric oxide synthase (NOS) in the human co rpus luteum (CL) and to determine the effects of nitric oxide (NO) on CL st eroidogenesis, Immunoblotting analyses revealed that endothelial MOS (eNOS) is the most abundant isoform in human CL with highest values during the la te luteal phase. Immunoreactive eNOS was localized predominantely in the th eca lutein layer, being particularly abundant in endothelial cells, but wit h positive staining also in some steroidogenic cells. Immunoreactive induci ble NOS (iNOS) was also detected, but to lesser degree, and did not display apparent phase-specific changes. The effect of NO on CL steroid synthesis was examined using human chorionic gonadotrophin (HCG)-stimulated dispersed CL cells cultured in vitro, Progesterone production was significantly decr eased (P < 0.05) by the MO donor spermine NONOate (10(-5) mol/l) in cells o f the late, but not mid-, luteal phase. To investigate a potential link bet ween NO and the local prostaglandins (PG), concentrations of PGF(2 alpha) a nd PGE(2) were measured in culture medium. NO significantly increased (P < 0.05) concentrations of both PGF(2 alpha) and PGE(2) during the late luteal phase, It is concluded that NO may be luteolytic in the human CL of menstr uation.