Endothelial nitric oxide synthase (NOS) protein and mRNA have been identified and calcium-dependent NOS activity has been measured in human placentae during normal pregnancy. Recently, mRNA and protein for the inducible isoform of NOS have been detected in placentae of women with gestational diabetes. The aim of this study was to determine whether calcium-independent (ciNOS) and/or total (tNOS) NOS activities were increased in placentae obtained after vaginal delivery or Caesarean section from women assigned to the following groups according to standard obstetric criteria: gestational diabetes, diabetes before pregnancy and non-diabetic controls. tNOS and ciNOS were assessed by measuring the conversion of [ 3 H]⌊-arginine to [ 3 H]⌊-citrulline in the three groups. Michaelis–Menten constants ( K m ) and maximum velocities of reaction ( V max ) were calculated using Lineweaver–Burk analysis for tNOS. There were no significant differences in either ciNOS, V max or K m values between any of the three groups (normal, ciNOS 12.7±1.6%, V max 16.6±3.3 pmol⋅min -1 ⋅mg -1 protein, K m 15.30±2.6 µ mol/l; gestational diabetes, ciNOS 15.4±1.4%, V max 14.8±5.2 pmol⋅min -1 ⋅mg -1 protein, K m 10.5±1.7 µ mol/l; diabetes before pregnancy, ciNOS 13.4±1.1%, V max 14.9±3.4 pmol⋅min -1 ⋅mg -1 protein, K m 17.7±2.2 µ mol/l). The presence of macrosomia did not affect tNOS activity in those with diabetes before pregnancy, and glycosylated haemoglobin levels measured between weeks 27 and 39 were not correlated with ciNOS activity. The results from the present study do not provide evidence for increased placental tNOS or ciNOS activities in pregnancies complicated by gestational diabetes or diabetes present before pregnancy.
1. The role of the potent vasodilator nitric oxide in the pathogenesis of pre-eclampsia is unclear. We have tested the hypothesis that placental activity of the enzyme which synthesizes nitric oxide (nitric oxide synthase) is reduced in pre-eclampsia. 2. Placentae were obtained after vaginal delivery or Caesarean section from women who had been assigned to the following groups according to standard obstetric criteria: term non-pre-eclamptic control, term pre-eclamptic, preterm non-pre-eclamptic control and preterm pre-eclamptic. Nitric oxide synthase activity of placental tissue homogenates was assessed by measuring conversion of [ 3 H]l-arginine into [ 3 H]l-citrulline in the presence of NADPH, FAD, tetrahydrobiopterin, calmodulin, CaCl 2 , magnesium acetate and a range of l-arginine concentrations. Michaelis Menton constants (K m ) amd maximum velocities of reaction ( V max ) were calculated using Lineweaver—Burk analysis. 3. V max was significantly reduced in both term and preterm pre-eclamptic placentae compared with placentae from corresponding gestation-matched controls. There were no significant differences in the K m values for nitric oxide synthase between any of the four groups, nor were V max or K m values significantly influenced by mode of delivery. 4. These results provide evidence that human placental nitric oxide synthase activity is significantly reduced in pre-eclampsia. Such a reduction was evident at both term and preterm gestations. Reduced placental nitric oxide synthase activity may have an adverse effect on placental haemodynamic function in pre-eclampsia, and could be involved in the pathogenesis of this important and common obstetric complication.