NITRIC-OXIDE METABOLITE CONCENTRATIONS IN MATERNAL PLASMA DECREASE DURING PARTURITION - POSSIBLE TRANSIENT DOWN-REGULATION OF NITRIC-OXIDE SYNTHESIS

To elucidate the possible involvement of nitric oxide (NO) in parturit ion, we measured the maternal plasma concentrations of the NO metaboli tes, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and guanosine 3',5'-cyclic phosphate (cGMP) in pregnant women at vari ous gestational ages including those at vaginal and elective Caesarean deliveries. The plasma cGMP and NO metabolite concentrations at vagin al delivery were significantly lower than those of the pregnant women in the third trimester of pregnancy. These concentrations remained low until 4 h after delivery but returned 24 h after delivery to values s imilar to those of the non-pregnant women. Such suppressions of plasma cGMP and NO metabolite concentrations were not observed in the women who underwent elective Caesarean section before the onset of labour. M oreover, no significant changes were observed in the plasma ANP and BN P concentrations at the time of vaginal and Caesarean deliveries, exce pt that a slight but significant elevation of the plasma ANP concentra tion was observed 1 h after Caesarean delivery. In conclusion, the pla sma concentrations of cGMP and NO metabolites significantly decreased at vaginal delivery but not at Caesarean delivery. These changes were independent of the plasma ANP and BNP concentrations, suggesting the p ossible down-regulation of maternal NO synthesis during parturition.