Prostacyclin rather than nitric oxide lowers human umbilical artery tone in vitro

Implications of Endothelial Cell-Mediated Dysfunctions in Vasomotor Tone Regulation

Cardiovascular diseases (CVD) constitute the major cause of death worldwide and show a higher prevalence in the adult population. The human umbilical cord consistsof two arteries and one vein, both composed of three tunics. The tunica intima, lined with endothelial cells, regulates vascular tone through the production/release of vasoregulatory substances. These substances can be vasoactive factors released by endothelial cells (ECs) that cause vasodilation (NO, PGI2, EDHF, and Bradykinin) or vasoconstriction (ET1, TXA2, and Ang II) depending on the cell type (ECs or SMC) that reacts to the stimulus. Vascular studies using ECs are important for the analysis of cardiovascular diseases since endothelial dysfunction is an important CVD risk factor. In this paper, we will address the morphological characteristics of the human umbilical cord and its component vessels. the constitution of the vascular endothelium, and the evolution of human umbilical cord-derived endothelial cells when isolated. Moreover, the role played by the endothelium in the vasomotor tone regulation, and how it may be associated with the existence of CVD, were discussed.

The role of nitric oxide in female reproduction

The cardiovascular system undergoes profound changes during pregnancy. Maternal intravascular volume begins to increase in the first trimester rising an average of 45% by term.1Cardiac output increases similarly2and is redistributed to organs whose functions are crucial for a successful pregnancy. In the guinea pig, uterine artery (UA) blood flow increases 3500%, while mesenteric and renal artery blood flows increase only 90% and 10% respectively.3Blood flow to the trunk actually diminishes. The mechanism underlying this redistribution is unknown. Coupled with the rise in cardiac output is a decrease in the systemic pressor response to angiotensin II (AII), norepinephrine(NE), and epinephrine.4–8There is also a decrease in the contraction response among some but not all vascular beds. For example, contraction of UA to NE and thromboxane is characteristically reduced by pregnancy, whereas the response of the carotid artery is unaltered8–10Since pregnancy does not alter neuroeffector mechanisms of NE such as release, receptor sensitivity, and accumulation11, changes in sympathetic control during pregnancy must be dependent on alterations at sites other than the neuroeffector junction. We have hypothesized that the mechanisms which alter vascular reactivity during pregnancy also mediate the redistribution of maternal cardiac output.9We have further hypothesized that many of these mechanisms involve endothelium-dependent factors which are modulated by sex hormones.

Diaspirin cross-linked hemoglobin does not alter isolated human umbilical artery or vein tone

Diaspirin cross-linked hemoglobin (DCLHbTM; Baxter Healthcare Corp., Round Lake, IL, USA) is undergoing clinical trials as a blood substitute. Administration of DCLHb is associated with an increase of mean arterial pressure in vivo and contraction of selected adult isolated blood vessels of from certain species in vitro. The mechanisms of these pressor effects may be due to scavenging of the endothelium derived relaxing factor, nitric oxide (NO), by hemoglobin. Unlike adult blood vessels, prostacyclin (PGI2) rather than EDNO is the important relaxing agent in human umbilical vessels. In this study, we examined if DCLHb had vasoconstrictor effects on isolated human umbilical vessels. Human umbilical veins and arteries were excised, cut into rings and placed in organ chambers filled with 25 ml Krebs-Ringer solution (37 degrees C). 5-hydroxytryptamine (5-HT, 0.01-10 microM) increased the tension of human umbilical arteries (HUA, from 0.4 +/- 0.2 g to 2.6 +/- 0.4g) and veins (HUV, from 0.8 +/- 0.4g to 2.5 +/- 0.4g) in a dose-dependent manner. DCLHb (0.01-10 microM) did not have a significant effect on HUA and HUV. Substance P (1 microM, via prostanoid synthesis) and nitroglycerin (NG, 1 microM) but not acetylcholine (ACh, 1 microM) caused relaxation of both HUA and HUV. The NO synthase inhibitor L-NA did not have significant effects on HUA and HUV. DCLHb did not alter 5-HT preconstricted tension of HUA and HUV. The basal cGMP contents of HUA and HUV were low. These results support our previous finding that DCLHb-induced vasoconstriction in isolated vessels is dependent primarily on the binding of NO by hemoglobin.

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.

Forearm vascular responses to serotonin are not mediated by products of cyclooxygenase

The involvement of cyclooxygenase-derived substances in the forearm arteriolar responses to serotonin was determined in 10 healthy male volunteers. Serotonin was infused in five incremental doses (0.003-30 micrograms min-1) for 2 min each into the brachial artery of the non-dominant forearm. Forearm blood flow (FBF) responses to each dose were measured by strain-gauge venous occlusion plethysmography. The cyclooxygenase inhibitor indomethacin (100 mg) was administered orally following the serotonin infusions. After 90 min the serotonin infusions were repeated and FBF responses measured. Serotonin caused a biphasic response with vasodilatation occurring at low doses (0.003-3 micrograms min-1, P < 0.01) and vasoconstriction occurring at the highest dose (P < 0.01). There was no significant difference in vascular response to serotonin following cyclooxygenase inhibition. In conclusion, cyclooxygenase products are not involved in the forearm vascular responses to serotonin in healthy human subjects.

Effect of diadenosine phosphates on human umbilical vessels: novel platelet-derived vasoconstrictors

The responses of human isolated umbilical artery to the novel platelet-derived mediators, diadenosine phosphates, P1,P5-di(adenosine-5')pentaphosphate (AP5A) and P1,P6-di(adenosine-5')hexaphosphate (AP6A) were studied in vitro. AP5A (1 nM-300 microM; n = 7) and AP6A (1 nM-30 microM; n = 5) produced transient concentration-dependent contractions. Responses to AP5A were unaffected by co-incubation with indomethacin (10 microM; n = 4), NGmonomethyl-L-arginine (L-NMMA; 300 microM; n = 4) and theophylline (1 microM; n = 5). These results demonstrate that diadenosine phosphates have constrictor effects on human blood vessels in vitro and are consistent with a role for these platelet-derived mediators in the regulation of blood vessel tone.

Umbilical arteries of babies born to cigarette smokers generate less prostacyclin and contain less arginine and citrulline compared with those of babies born to control subjects

OBJECTIVE

The hypothesis of this study was that umbilical arteries of babies born to smoking mothers produce less nitric oxide and prostacyclin than do those of nonsmoking mothers.

STUDY DESIGN

L-Arginine, L-citrulline, L-cysteine, and prostacyclin were measured in the umbilical arteries of 11 babies born to smoking mothers and 16 infants born to nonsmoking controls. The concentrations in the two groups were compared with the modified t test.

RESULTS

The generation of prostacyclin was reduced in the umbilical arteries of infants of smoking mothers. Similarly, L-arginine and L-citrulline, but not L-cysteine levels, in these arteries were suppressed compared with those of the nonsmoking controls.

CONCLUSION

Along with the known direct vasoconstrictive effect of nicotine, nitric oxide and prostacyclin deficiency may affect the uteroplacental blood flow and contribute to the impaired fetal nutrition and increased perinatal mortality of babies born to women who smoke.

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.

Excretion of prostacyclin and thromboxane metabolites before, during, and after pregnancy-induced hypertension

The aim of the present study was to assess whether changes in prostacyclin (PGI2) and thromboxane (TXA2) generation precede the manifestation of pregnancy-induced hypertension (PH). The metabolites 6-oxo-PGF1 alpha and TXB2 were measured in the urine of 69 randomly selected pregnant women from 16-20 weeks of gestation (wg) until delivery and more than 6 weeks postpartum. Between 16-20 and 21-24 wg 6-oxo-PGF1 alpha excretion did not change in patients who later developed PIH (n = 6) but increased significantly in the control group (n = 63). In contrast, a marked rise in TXB2 excretion was found in the PIH group but not in controls. Thereafter significant differences between both groups persisted from 25 wg until delivery. The 6-oxo-PGF1 alpha/TXB2 ratio was below the 10th percentile from 21-24 wg until delivery in patients with developing PIH. The excretion of both metabolites was substantially lower in the non-pregnant state without any difference between patient groups. These results show an altered urinary excretion of both 6-oxo-PGF1 alpha and TXB2 preceding the onset of the disease. A pathophysiological role of PGI2 deficiency and increased TXA2 formation in PIH appears substantiated.

Inhibition of nitric oxide synthesis in rats during pregnancy produces signs similar to those of preeclampsia

OBJECTIVES

Preeclampsia is associated with hypertension, fetal growth retardation, and proteinuria. We hypothesized that impaired vascular nitric oxide synthesis during pregnancy may be an important causal factor in preeclampsia.

STUDY DESIGN

An inhibitor of nitric oxide synthase, L-nitro-arginine methyl ester, or a nitric oxide donor, nitroglycerin, was infused subcutaneously to rats at a constant rate from day 17 of gestation. Systolic blood pressure, day of spontaneous delivery, weight, and mortality rate of pups were recorded.

RESULTS

Systolic blood pressures in rats infused with L-nitro-arginine methyl ester at daily doses of both 25 and 50 mg were significantly elevated compared with controls. This treatment also caused a substantial decrease in the weight of pups, with an increase in mortality rate, without affecting the gestational length. These effects were dose dependent. Nitroglycerin infusion, on the other hand, affected neither the weight and mortality rate of the pups nor the length of gestation.

CONCLUSIONS

Infusion of an inhibitor of nitric oxide synthesis during pregnancy causes hypertension and fetal growth retardation, without affecting gestational length. These signs are similar to those of preeclampsia and indicate that an alteration in nitric oxide synthesis may be one of the factors responsible for this disorder. Treatment with nitric oxide inhibitors may be used in an animal model for preeclampsia, to test various therapeutic strategies.