Gestational changes in endothelium-dependent vasorelaxation in human umbilical artery

Effect of Transdermal Nitroglycerine on Doppler Velocity Waveforms of the Uterine, Umbilical and Fetal Middle Cerebral Arteries in Patients with Chronic Placental Insufficiency: A Prospective RCT

INTRODUCTION

Increase in Nitric Oxide (NO) may be important in vascular adaptation needed to accommodate increased uteroplacental blood flow as pregnancy advances. Hence, in certain conditions like Pregnancy Induced Hypertension (PIH) and Fetal Growth Restriction (FGR), NO donors may play an effective role in increasing uteroplacental perfusion. Transdermal route appears to be a safe and effective route.

AIM

To evaluate the effect of nitroglycerine patch on Doppler velocity waveforms of the uterine, umbilical and fetal middle cerebral arteries in patients with chronic placental insufficiency.

MATERIALS AND METHODS

A prospective randomized controlled clinical trial was conducted on eighty consecutive pregnant women with FGR with or without PIH and having evidence of altered waveform velocimetry in uterine, umbilical and fetal middle cerebral artery. They were divided into two groups- study and control group. Transdermal nitroglycerine patch (10 mg per 24 hours) was applied in study group for three consecutive days. Changes in various Doppler indices were noted after three days of patch application and compared between the two groups. Analysis was carried out using SPSS (Statistical Package for Social Studies) for Windows version 20.0 and online GraphPad software (Prism 5 for Windows) version 5.01.

RESULTS

A significant fall in the systolic and diastolic ratio (S/D), Pulsatility Index (PI) and Resistivity Index (RI) of the uterine (3.07±0.52, 1.04±0.14 and 0.54±0.10 respectively, p<0.001) and umbilical artery (3.73±3.30, 1.18±0.21and 0.64±0.07 respectively, p<0.001) was noted after three days of patch application. No such significant change was observed in the middle cerebral artery indices.

CONCLUSION

The therapeutic approach of NO donor administration via transdermal route in pregnant patients with chronic placental insufficiency, apparently improved both maternal and fetoplacental haemodynamics, thus may help in improving perinatal outcome.

Augmented dilation to nitric oxide in uterine arteries from rats with type 2 diabetes: implications for vascular adaptations to pregnancy

Pre-existing diabetes increases the risk of maternal and fetal complications during pregnancy, which may be due to underlying maternal vascular dysfunction and impaired blood supply to the uteroplacental unit. Endothelial dysfunction and reduced vascular smooth muscle responsiveness to nitric oxide (NO) are common vascular impairments in type 2 diabetes (T2D). We hypothesized that uterine arteries from diabetic rats would have reduced vascular smooth muscle sensitivity to NO compared with nondiabetic rats due to impairment in the NO/soluble guanylate cyclase (sGC)/cGMP signaling pathway. Uterine arteries from pregnant Goto-Kakizaki (GK; model of T2D) and Wistar (nondiabetic) rats were studied in a wire myograph. GK nonpregnant uterine arteries had reduced responses to ACh and sodium nitroprusside (SNP) but increased responses to propylamine propylamine NONOate and greater sensitivity to sildenafil compared with Wistar nonpregnant arteries. In late pregnancy, Wistar rats had reduced uterine vascular smooth muscle responsiveness to SNP, but GK rats failed to show this adaptation and had reduced expression of sGC compared with the nonpregnant state. GK rats had a smaller litter size (13.9 ± 0.48 vs. 9.8 ± 0.75; P < 0.05) and a greater number of resorptions compared with Wistar controls (0.8 ± 0.76% vs. 19.9 ± 6.06%; P < 0.05). These results suggest that uterine arteries from rats with T2D show reduced sensitivity of uterine vascular smooth muscle sGC to NO. During pregnancy, the GK uterine vascular smooth muscle fails to show relaxation responses similar to those of arteries from nondiabetic rats.

In preeclampsia endogenous cardiotonic steroids induce vascular fibrosis and impair relaxation of umbilical arteries

BACKGROUND

Marinobufagenin (MBG), a bufadienolide cardiotonic steroid, induces cardiovascular fibrosis. Because levels of MBG in preeclampsia are increased, and anti-MBG monoclonal antibody reduces blood pressure (BP) in a rat model of preeclampsia, we hypothesized that in preeclampsia, elevated MBG levels would be associated with the development of fibrosis in feto-placental circulation and with impairment of vascular relaxation.

METHOD

We studied 16 patients with preeclampsia (systolic BP=150±4 mmHg; 28±2 years, 37±1 weeks gestational age) and 14 gestational age-matched normal pregnant women (systolic BP=112±2 mmHg).

RESULTS

Preeclampsia was associated with a rise in plasma and placental levels of MBG. In preeclamptic umbilical arteries, the expression of Fli-1, a transcription factor and a negative regulator of fibrosis, was significantly reduced (P<0.001), whereas procollagen-1 expression was increased (P<0.01). As compared to control vessels, isolated rings of umbilical arteries from patients with preeclampsia demonstrated unaltered responsiveness to endothelin-1 (EC50=2.2 and 3.2 nmol/l, respectively), but exhibited an impaired response to the relaxant effect of sodium nitroprusside (EC50=1.5 vs. 32.4 nmol/l, P<.001) following endothelin-1-induced constriction. Ex-vivo treatment of normal umbilical arteries explants with 1 and 10 nmol/l MBG for 24 h mimicked the effects of preeclampsia, specifically suppressed Fli-1 and increased collagen-1 expression while impairing vasorelaxation.

CONCLUSION

Our results indicate that in preeclampsia, elevated levels of MBG induce vascular fibrosis via a Fli-1-dependent mechanism which leads to an impairment of vasorelaxation, and suggest that MBG represents a potential target for therapy of this syndrome.

Efficacy of different vasodilators on human umbilical arterial smooth muscle under normal and reduced oxygen conditions

The ability of the smooth muscle of the human umbilical artery to relax may vary under physiological and pathological conditions. We investigated the responsiveness of that preparation to relaxation, as well as the influence of reduced oxygen condition on these responses. Rings of human umbilical arteries from full-term Caesarian deliveries were suspended in modified Krebs-Henseleit solutions bubbled with a gas mixture of 95% O2:5% CO2 (normal oxygen condition) or 2.5% O2:8% CO2 balanced with N2 (reduced oxygen condition). These rings were contracted with potassium chloride, serotonin or the thromboxane A2 mimetic U46619, before being exposed to either the nitrovasodilator sodium nitroprusside, the potassium channel opener levcromakalim or the calcium channel antagonist amlodipine. While sodium nitroprusside elicited relaxation in this blood vessel, the maximal relaxation to the nitric oxide donor was significantly smaller than that induced by levcromakalim and amlodipine. The nature of the constrictor agent used, or changes of oxygen conditions did not significantly affect the relaxation profile of this human blood vessel. These data suggest that the smooth muscle of the human umbilical artery may be less responsive to vasodilators that act via the nitric oxide pathway. Moreover, vascular responses of umbilical arterial smooth muscle to relaxing agents do not alter under hypoxic or different vasoconstricting conditions.

Umbilical vein and middle cerebral artery blood flow response to partial occlusion by external compression of the umbilical vein (pressure test)

OBJECTIVES

We studied the responses of flow in the umbilical vein and of the circulation in the proximal segment of the middle cerebral artery (MCA) during partial external compression of the umbilical vein.

METHODS

Partial occlusion of the umbilical vein was achieved by the pressure test, consisting of applying external pressure on the maternal abdominal wall for 1-2 s.

RESULTS

During 60 pressure tests in 45 fetuses with nuchal cords, we observed complete cessation of the umbilical vein flow, while blood flow in the umbilical arteries was preserved, and proximal MCA flow was moderately or markedly decreased. Blood flow in other segments of the MCA was unchanged. This effect, which was more pronounced before the 32nd week of gestation, was also observed in association with spontaneous fetal movements.

CONCLUSION

The pressure test decreases flow in the proximal MCA, and interrupts umbilical vein blood flow. Further analysis of this response suggested the presence of associated vasoconstriction of the proximal MCA.

Characterization of changes in mechanical responses to histamine in omental resistance arteries in pre-eclampsia

Changes in the effect of histamine on the smooth muscle of resistance arteries in pre-eclampsia were investigated by measuring isometric contractions in endothelium-denuded strips of omental resistance arteries from pre-eclamptic and normotensive pregnant women (pregnancy-term matched). Histamine (0.03 -1 microM) caused concentration-dependent relaxation of the contraction induced by 9, 11-epithio-11,12-methano-thromboxane A(2) (STA(2)) in strips from both groups. Sensitivity (for pre-eclampsia: pD(2)=6.66+/-0.04, n=5 and for normotensive pregnant women: pD(2)=7.07+/-0.03, n=10, P<0.001) was lower and the maximum response (90.6+/-0.6% vs 95.5+/-1.1%, P<0.05) was smaller in strips from pre-eclamptic women. Although 8-bromoadenosine-3', 5'-cyclic monophosphorothioate (Sp-isomer: Sp-8-Br-cAMPS, 0.1 - 0.3 mM), a phosphodiesterase (PDE)-resistant activator of adenosine-3',5'-cyclic monophosphate (cyclic AMP)-dependent protein kinase, concentration-dependently attenuated the contraction induced by STA(2) in strips from both groups, the sensitivity (for pre-eclampsia: pD(2)=3.68+/-0.04, n=5 and for normotensive pregnant women: 3.94+/-0.09, n=7, P:=0.02) was lower and the maximum response (64.2+/-2.4% vs 74.9+/-4.4%, P:<0.05) was smaller in pre-eclampsia. In beta-escin-skinned strips, the pD(2) value for the contraction-inducing effect of Ca(2+) did not differ significantly between the two groups (for pre-eclampsia, n=6; for normotensive pregnant women, n=6). Thus, omental resistance arteries from human subjects with pre-eclampsia showed (i) a weaker H(2)-receptor-mediated relaxation to histamine and (ii) a weaker cyclic AMP-analogue-induced relaxation, suggesting that the reduced action of histamine may be partly due to a decreased effect of cyclic AMP.

The effect of chronic nitric oxide synthase inhibition on blood pressure and heart rate in unrestrained pregnant rats as recorded by radiotelemetry

OBJECTIVES

The objective of this study was to determine the effect of chronic nitric oxide synthase inhibition on heart rate and intravascular blood pressure in unrestrained pregnant rats as recorded by radiotelemetry.

STUDY DESIGN

Heart rate and systolic and diastolic blood pressures were monitored beginning with day 6 of pregnancy and until 1 week post partum with a radiotelemetric device. On day 10 of pregnancy osmotic minipumps were implanted subcutaneously and loaded to continuously deliver N(G)-nitro-L -arginine methyl ester (50 mg/d per rat, n = 6 animals) or vehicle (control group, n = 6 animals).

RESULTS

Blood pressure in the animals treated with N(G)-nitro-L -arginine methyl ester significantly increased compared with that in the control group and heart rate significantly decreased immediately after nitric oxide synthase blockade. Blood pressure then trended downward as gestation progressed, until the difference between the control group and the group treated with N(G)-nitro-L -arginine methyl ester became nonsignificant after day 17. Refractoriness to nitric oxide synthase blockade was especially evident in the diastolic pressure. Systolic, diastolic, and mean blood pressures in the rats treated with N(G)-nitro-L -arginine methyl ester were again significantly higher than those in the control group immediately after delivery and remained so despite a lower heart rate until the experiment was ended on postpartum day 6.

CONCLUSIONS

Radiotelemetry can be used to monitor heart rate and intra-arterial blood pressure in unstressed, unrestrained animals. Chronic inhibition of nitric oxide does not cause sustained hypertension throughout pregnancy. Nitric oxide does not appear to be the only factor responsible for the vascular changes in pregnancy. The factors responsible for the refractoriness to nitric oxide synthase blockade are specific to pregnancy and disappear immediately after delivery.

Heme oxygenase and nitric oxide synthase in the placenta of the guinea-pig during gestation

Nitric oxide (NO) and carbon monoxide (CO) are novel gaseous chemical messengers that play key roles in cell function and cell-cell communication in many organ systems, including the cardiovascular system. Although the presence of NO synthase (NOS) in the placenta and its role in the regulation of fetoplacental and uteroplacental blood flow are well established, little is known about placental expression and activity of heme oxygenase (HO), the enzyme that catalyses the oxidation of heme to CO, biliverdin and iron, during gestation. The objectives of this study were to elucidate the localization of HO-1 and HO-2 isoforms relative to NOS III protein, and to determine the enzymatic activity of HO in the placenta of the guinea-pig during gestation. Placentae were obtained from pregnant guinea-pigs at gestational day (GD) 34, 50, 62 and full term (term, about GD 68). Immunohistochemical localization of HO-1, HO-2 and NOS III protein was conducted using selective polyclonal antibodies. HO activity was determined by using a gas chromatographic method to measure the rate of formation of CO from heme. Faint staining for HO-1 was observed in the adventitial layer of larger fetal blood vessels of the placenta at GD 34. The intensity of this staining was higher at GD 50 and GD 62, and decreased at full term. Similar areas in serial sections of placentae obtained at these selected times during gestation exhibited lower staining intensity when incubated with anti-HO-2 antiserum. Placental HO activity was significantly increased (P<0.05) at GD 62 compared with GD 34, GD 50 and full term. NOS III (endothelial constitutive NOS) staining was highest at GD 34, decreasing thereafter, and was localized mostly to trophoblast lining maternal channels. The data demonstrate that, in the guinea-pig, placental HO and NOS differ in tissue localization during the second half of gestation, with expression of HO protein and its catalytic activity being higher during near-term pregnancy. In a preliminary immunohistochemical investigation of the full-term human placenta, HO-1 protein was localized primarily in the adventitial region of fetal blood vessels of stem chorionic villi. In view of the vasodilator action of CO and NO, the HO and NOS systems may play key roles in the regulation of placental haemodynamics.

Major vasodilator role for nitric oxide in the gastrointestinal circulation of the mid-gestation fetal lamb

As nitric oxide (NO) may be a particularly important vasodilator in early life, we investigated its role in the regulation of the gastrointestinal (GI) circulation at mid-gestation. Cardiac output and GI blood flow were measured by the radioactive microsphere technique in eight chronically instrumented and unanesthetized mid-gestation fetal sheep. Mean arterial pressure (MAP), heart rate, blood flow, oxygen delivery, and vascular resistance were determined before and after infusion of the specific NO synthase inhibitor, Nomega-nitro-L-arginine (L-NNA) at doses of 10 and 25 mg/kg. In response to L-NNA infusion, MAP increased (p < 0.01) and combined ventricular output decreased (p < 0.001). GI blood flow and oxygen delivery decreased and vascular resistance increased in the stomach and all segments of the small and large intestine (all p < 0.001). The greatest reduction in blood flow was in the small intestine (p < 0.01) and the basal differential pattern of small intestinal blood flow exceeding large intestinal flow was completely abolished. These changes were much greater than those previously described in late-gestation fetuses. Our results suggest that, at mid-gestation, NO plays a major role in the regulation of blood flow and vascular tone across all segments of the fetal GI tract, with its effects being more pronounced than later in development.

The effect of nitric oxide on uterine and umbilical artery flow velocity waveform in pre-eclampsia

We compared the flow velocity waveforms of uterine and umbilical arteries in normotensive and pre-eclamptic patients at mid-gestation. In a randomised controlled trial we tested the effects of isosorbide dinitrate (ISDN, nitric oxide donor) patch therapy on the flow velocity waveform of pre-eclamptic patients at mid-gestation. The resistance indices (RI) of human uterine and umbilical arteries were higher in pre-eclamptic patients compared to the normotensive patients. ISDN patch therapy significantly reduced the increased RI values of the umbilical artery in pre-eclamptic patients without any change in systemic blood pressures, but the RI values of the uterine artery were not significantly attenuated. The change of the umbilical artery might be due to the improvement of end-diastolic flow velocity. These results suggest that the feto-placental circulation in pre-eclampsia, perhaps due to the disturbance of the endothelium-dependent vaso-relaxation system, and that ISDN therapy may improve the impaired endothelium dependent nitric oxide system.

Systemic and fetal-maternal nitric oxide synthesis in normal pregnancy and pre-eclampsia

OBJECTIVE

To investigate systemic and fetal-placental nitric oxide synthesis by biochemical and molecular biology means in normal human pregnancy and pre-eclampsia.

DESIGN AND PARTICIPANTS

Three groups of women were studied: healthy pregnant women (n = 8), pregnant women with pre-eclampsia (n = 8), and age-matched nonpregnant controls (n = 8). Pre-eclamptic patients were treated with nifedipine (30-60 mg/day) for severe hypertension. Systemic nitric oxide synthesis was assessed in normal pregnant women at weeks 18-21, 29-32 and 38-39 and in pre-eclamptic women on admission to the hospital (29-32 weeks, 30 on average), before the morning nifedipine administration. Nonpregnant women were studied twice at four-week intervals as controls. The pattern of nitric oxide biosynthesis in fetal-placental circulation was studied in normal and pre-eclamptic women at the delivery.

SETTING

Mario Negri Institute for Pharmacological Research, Bergamo, and the Division of Obstetrics and Gynaecology of the University of Brescia.

MAIN OUTCOME MEASURES

Plasma cGMP levels and platelet nitric oxide synthesis, assessed by measuring the conversion of [3H]L-arginine to [3H]L-citrulline as well as intracellular cGMP, were evaluated. Constitutive nitric oxide synthase (EC-NOS) gene expression by Northern blot analysis and nitric oxide release by the conversion of [3H]L-arginine to [3H]L-citrulline were assessed in umbilical vein endothelial cells (HUVEC) and in placenta. Inducible nitric oxide synthase activity was also evaluated in HUVEC exposed to tumour necrosis factor alpha (TNF alpha) and in placenta homogenates incubated in calcium free medium.

RESULTS

Plasma cGMP was higher in both normal pregnant and pre-eclamptic women than in nonpregnant controls. In normal pregnancy cGMP rose as early as 18-21 weeks and remained elevated throughout pregnancy. [3H]L-citrulline production and intracellular cGMP were comparable in platelets from all women. EC-NOS gene expression and nitric oxide synthesis were identical in HUVEC and placenta from normal pregnant and pre-eclamptic women.

CONCLUSIONS

Systemic levels of CGMP, the nitric oxide second messenger, are increased in normal pregnancy. Excessive nitric oxide production does not derive from platelets. Pre-eclampsia is not associated with changes in fetal-placental nitric oxide synthesis.

Comparison of nitric oxide and prostacyclin in endothelium-dependent vasorelaxation of human umbilical artery at midgestation

OBJECTIVE

We intended to compare the relative importance of nitric oxide and prostacyclin as endothelium-derived vasorelaxing factors released by histamine in human umbilical artery at the midstage (18 to 22 weeks) of gestation.

STUDY DESIGN

By use of very thin muscle strips, which allows rapid diffusional access of applied drugs (in a few seconds), isometric tensions were recorded in response to histamine. The histamine-induced contractions and the relaxing effect of histamine on the potassium chloride (39 mmol/L) contractions were studied in relation to the existence of endothelium, L-NG-nitro arginine, and indomethacin. The relaxing effects of glyceryl trinitrate and prostacyclin on the potassium chloride contractions were also examined.

RESULTS

The contractile responses to histamine were more sensitive and the relation tensions of histamine contractions, compared with the 39 mmol/L K(+)-induced contractions, were large in endothelium-denuded strips than in endothelium-intact strips. Histamine contractions were enhanced in endothelium-intact strips in the presence of 10 mumol/L L-NG-nitro arginine (competitive inhibitor of nitric oxide synthase) but not in the presence of 10 mumol/L indomethacin (cyclooxygenase inhibitor). Histamine produced a concentration-dependent relaxation during the maintained contraction induced by 39 mmol/L K+. These histamine-induced relaxations were completely blocked by L-NG-nitro arginine but not by indomethacin. Glyceryl trinitrate and prostacyclin relaxed the sustained contractions induced by 39 mmol/L K+ in a dose-dependent manner; however, the degree of relaxation by glyceryl trinitrate was more prominent than that by prostacyclin.

CONCLUSION

These results suggest that nitric oxide is more potent than prostacyclin as a vasorelaxing substance released from the endothelium and that nitric oxide has an important role for controlling fetoplacental circulation at midgestation.

Nitric oxide synthase in human placenta and umbilical cord from normal, intrauterine growth-retarded and pre-eclamptic pregnancies

1. It has been suggested that a deficiency of nitric oxide (NO) may explain many of the pathophysiological features of pre-eclampsia (PE) and intra-uterine (foetal) growth retardation (IUGR). To elucidate further the role of NO in the pathophysiology of pregnancy we have determined the relative amount and activity of NO synthase (NOS) in first trimester and normal-term placental tissues, as well as in the placenta and umbilical cord in pregnancies complicated by PE and IUGR, using NG-nitro-L-[2,3,4,5(-3)H]-arginine ([3H]-L-NOARG) binding, quantitative in vitro autoradiography, [3H]-arginine to [3H]-citrulline conversion and Western blotting. 2. Specific, high affinity (KD = 38 nM) [3H]-L-NOARG binding was demonstrated in the villous trophoblast of normal-term placentae. Binding was calcium-independent, stereoselective and exhibited a rank order of inhibition by NOS inhibitors and substrate (L-NOARG > or = L-NMMA > or = 7-NI > L-NAME > L-Arg > or = L-NIO > ADMA). 3. [3H]-L-NOARG binding density and NOS activity were both significantly greater in placental tissues from first trimester and PE or IUGR complicated pregnancies compared to normal-term placentae. 4. Western blotting, using an endothelial NOS peptide antiserum, demonstrated a approximately 140 KDa protein band in placental extracts and indicated that the amount of immunoreactive material was significantly greater in first trimester compared to normal-term placentae. 5. Specific [3H]-L-NOARG binding was also localized to the endothelial lining of umbilical arteries and veins, binding density being greater in the artery than the vein. [3H]-L-NOARG binding to the umbilical artery endothelium was significantly lower in PE and IUGR complicated pregnancies compared to normal-term controls. 6. The role of trophoblast-derived NO in human placental pathophysiology remains to be established, but differences in the amount of placental [3H]-L-NOARG binding, NOS activity and immunoreactive material indicate that expression of NOS in the villous trophoblast falls during pregnancy. Conversely, the apparent reduction in NOS in the umbilical artery endothelium in PE and IUGR complicated pregnancies may be indicative of endothelial dysfunction.

Role of nitric oxide on vasorelaxation in human umbilical artery

OBJECTIVE

Endothelium-derived relaxing factor (or nitric oxide) is thought to play an important role in control of blood flow in umbilical blood vessels at midgestation compared with term. Previous studies suggest that histamine releases endothelium-derived relaxing factor from umbilical arteries. In this study we intended to clarify the mechanism by which histamine releases endothelium-derived relaxing factor and causes vasorelaxation in human umbilical artery at the midstage (18 to 22 weeks) of gestation.

STUDY DESIGN

By means of very thin muscle strips that allow rapid diffusional access of applied drugs (in a few seconds), contractile properties of human umbilical artery were examined. Isometric tensions were measured in response to potassium chloride (39 mmol/L) or caffeine and inhibitory effects of histamine, A23187, glyceryl trinitrate, and 8-bromo-cyclic guanosine monophosphate on these contractions were also examined.

RESULTS

Histamine (0.01 to 0.1 mumol/L) did not inhibit 39 mmol/L K(+)-induced contractions of tissues taken at the terminal (38 to 41 weeks) stage of gestation. However, at midgestation histamine (0.01 to 0.1 mumol/L), A23187 (10 mumol/L), and 8-bromo-cyclic guanosine monophosphate (membrane-permeable analog of cyclic guanosine monophosphate, 0.1 mmol/L) inhibited 39 mmol/L K(+)-induced contractions. The inhibitory effects of histamine were antagonized by mepyramine (an H1 antagonist), L-NG-nitro arginine, methylene blue, and Ca++ depletion of the extracellular space but not by cimetidine (an H2 antagonist). Caffeine produced contractions both in the presence and absence of extracellular Ca++ possibly because of the release of Ca++ from intracellular storage sites. Glyceryl trinitrate and 8-bromo-cyclic guanosine monophosphate reduced the caffeine-induced contractions in Ca(++)-free solution. In addition, 10 mumol/L cyclic guanosine monophosphate did not attenuate the Ca++ sensitivity for contractile elements.

CONCLUSION

These results suggest that (1) histamine coupled to the histamine H1 receptor increases intracellular Ca++ concentration to stimulate nitric oxide synthase in human umbilical endothelial cells, (2) nitric oxide from endothelial cells activates guanylate cyclase to produce cyclic guanosine monophosphate in the umbilical smooth muscle cells, and (3) cyclic guanosine monophosphate relaxes the umbilical tissues, perhaps as a result of the activation of a Ca++ extrusion system.

Nitric oxide and human umbilical vessels: pharmacological and immunohistochemical studies

Human umbilical vessels are devoid of nerves and therefore endothelial cells may play an important role in the control of fetoplacental blood flow. In this study we examined the pharmacological effects of various substances, known to produce endothelial-mediated vasodilation in many blood vessels, on the human umbilical artery and vein from legal terminations [mean gestational age, 15 (8-17) weeks; n = 12] and normal term vaginal deliveries [mean gestational age, 39 (38-41) weeks; n = 12]. Acetylcholine, adenosine 5'-triphosphate, the calcium ionophore A23187 and substance P had no effect on raised vascular tone, whereas sodium nitroprusside relaxed 5-hydroxytryptamine (5-HT) preconstricted, umbilical artery and vein from both early and late pregnancy. L-NG-Nitroarginine methyl ester (L-NAME) had no effect on basal tone or on high tone, after it was raised by 5-HT. Localization of nitric oxide synthase [NOS, type I (neuronal)] was examined in the same umbilical vessels using electron immunocytochemistry. No NOS-immunoreactive endothelial cells were observed in the umbilical vessels taken during early pregnancy. However, the percentage of NOS-immunoreactive endothelial cells in umbilical artery and vein from late pregnancy was 3 and 10 per cent, respectively. These results suggest that nitric oxide contributes little, if any, to the local control of umbilical blood flow throughout pregnancy, despite the presence of NOS-immunoreactivity in a subpopulation of endothelial cells in late pregnancy.