Anthony S. Brewer

THE ACTION OF NITRIC OXIDE IN THE PERFUSED HUMAN FETAL-PLACENTAL CIRCULATION

Nitric oxide is thought to be the endogenous endothelium-derived relaxing factor. We investigated the effects of compounds that either generate nitric oxide intracellularly or inhibit its action on the vasculature of the human placental villus. Addition to perfusion medium of methylene blue (10(5) mol/L), which is an inhibitor of activation of guanylate cyclase by nitric oxide, significantly increased perfusion pressure of the fetal-placental circulation over a range of flow rates (1 to 10 ml/min) compared with the perfusion pressures seen in the absence of methylene blue. This suggests basal release of nitric oxide may contribute to maintenance of resting vascular tone. Both glyceryl trinitrate (10(-9) to 5 x 10(6) mol/L) and S-nitroso-N-acetylpenicillamine (10(-8) to 10(-4) mol/L), which generate nitric oxide intracellularly, were able to significantly vasodilate the fetal-placental circulation preconstricted with the thromboxane mimetic U46619 (1 to 5 x 10(-8) mol/L) in a concentration-dependent manner. These compounds had no effect in the absence of the vasoconstrictor. Thus it appears that the placental villus tree has the ability to both generate and respond to nitric oxide.

Attenuation of the vasoconstrictor effects of thromboxane and endothelin by nitric oxide in the human fetal-placental circulation

OBJECTIVE We hypothesized that the endothelial-derived relaxing factor nitric oxide may contribute to low resting vascular tone and may attenuate vasoconstrictor action in the human fetal-placental circulation. STUDY DESIGN Isolated human placental cotyledons were dually perfused in vitro, and the effects of N-monomethyl-L-arginine and N-nitro-L-arginine (3 x 10(-4) mol/L), which are nonmetabolizable analogs of L-arginine, the substrate for nitric oxide synthase, on resting perfusion pressure and on the fetal-placental circulation preconstricted with U46619 (10(-8) mol/L) or endothelin-1 (10(-8) mol/L) were established. Responses before and after inhibition were compared by paired t test. The effects of glyceryl trinitrate (10(-6) mol/L), acetylcholine (10(-4) mol/L), the calcium ionophore A23187 (10(-6) mol/L), and histamine (10(-8) to 10(-4) mol/L) were also determined in the preconstricted fetal-placental circulation. RESULTS Both N-monomethyl-L-arginine and N-nitro-L-arginine (3 x 10(-4) mol/L) increased resting perfusion pressure (p less than 0.06), and N-nitro-L-arginine promptly and significantly increased perfusion pressure in the fetal-placental circulation preconstricted with U46619 (p less than 0.0004) or endothelin-1 (p less than 0.06). Nitric oxide generated by addition of glyceryl trinitrate (10(-6) mol/L) attenuated the vasoconstrictor effects of U46619 (p less than 0.026) or endothelin-1 (p less than 0.01). Neither acetylcholine nor the calcium ionophore A23187 had an effect on the fetal-placental circulation, whereas bradykinin further increased perfusion pressure. Histamine only relaxed the preconstricted preparations at concentrations (10(-6) to 10(-4) mol/L) above those shown to release nitric oxide in other systems. CONCLUSION The stimulus to nitric oxide generation in the fetal-placental circulation may be hydrodynamic. Nitric oxide appears to contribute to maintenance of basal vascular tone and to attenuate the actions of vasoconstrictors in this circulation.

Vasodilator Effects of Parathyroid Hormone, Parathyroid Hormone-Related Protein, and Calcitonin Gene-Related Peptide in the Human Fetal-Placental Circulation:

OBJECTIVE: The purpose of our study was to determine the vasoactivity of calcitonin gene- related peptide (CGRP), parathyroid hormone (PTH), and parathyroid hormone-related protein (PTHrP) in the human fetal-placental circulation in vitro. METHODS: Dually perfused placental cotyledons from term pregnancies were used in this study. RESULTS: Calcitonin gene-related peptide, PTHrP (both 10-10-10-6 mol/L), and PTH (10-8-10-6 mol/L) demonstrated a significant concentration-dependent vasodilator effect (P = .0007, P = .0172, P = .0063, respectively), following preconstriction with a thromboxane mimetic U46619. The CGRP-1 receptor inhibitor CGRP8-37 (10 -6 mol/L) significantly in hibited (P = .0131) the CGRP-induced vasodilator effect, while the nitric oxide synthesis inhibitor n-nitro-l-arginine showed no inhibitory effect. CONCLUSIONS: These results demonstrate the vasodilator effects of CGRP, PTH, and PTHrP in the human fetal-placental circulation. Calcitonin gene-related peptide and PTHrP were of equal potency, and both were approximately 100 times more potent than PTH. This study also suggests the CGRP may exert its vasodilator effect through two classes of receptors in the human placenta and may do so independently of nitric oxide. (J Soc Gynecol Invest 1994;1: 19-24)

The comparative effects of big endothelin-1, endothelin-1, and endothelin-3 in the human fetal-placental circulation

OBJECTIVE This study compared the effects of big endothelin-1, endothelin-1, and endothelin-3 and whether endothelin-converting enzyme was present in the human fetal-placental circulation. STUDY DESIGN Single cotyledons of term placentas were dually perfused in vitro, and increases in fetal-placental perfusion pressure to bolus injections of big endothelin-1, endothelin-1, and endothelin-3 (8 x 10(-10) to 1 x 10(-7) mol/L) were recorded. Responses to big endothelin-1 (10(-7) mol/L) were measured in the same placenta before and after perfusion of the fetal-placental circulation with the neutral metalloprotease inhibitor phosphoramidon (10(-5) mol/L), which acts as an endothelin-converting enzyme inhibitor. All experiments were performed in at least five separate placentas. RESULTS Significant concentration-dependent increases in fetal-placental perfusion pressure were seen with endothelin-1 (p < 0.0005), endothelin-3 (p < 0.0256), and big endothelin-1 (p < 0.0034, analysis of variance). Big endothelin-1 always elicited transient vasodilatation before constriction. Phosphoramidon significantly inhibited the vasoconstrictor effect of big endothelin-1 (p < 0.039, paired t test). CONCLUSION The three endothelins tested are vasoconstrictors, and endothelin-converting enzyme is present in the fetal-placental circulation.

Endothelin-1-induced vasoconstriction is not mediated by thromboxane release and action in the human fetal-placental circulation.

Abstract The vasoconstrictor peptide endothelin-1 (8 x 10- 10 to 1 x 10- 8 mol/L) significantly increased fetal-placental perfusion pressure in vitro in a cumulative manner from 30 ± 2 to 123 ± 25 mm Hg (mean ± SEM, n = 5, p 2 and 6-keto-prostaglandin F 1α in the fetal-placental perfusate revealed a significant reduction in their release ( p p -6 mol/L) nor the thromboxane receptor antagonist SQ29548 (10 -6 mol/L) was able to block the vasoconstrictor actions of endothelin-1. Therefore endothelin-1-induced vasoconstriction in the human fetal-placental circulation does not appear to be mediated by thromboxane release or action. The stimulus to eicosanoid release in the fetal-placental circulation may be hydrodynamic, i.e., flow or shear stress. (AM J OBSTET GYNECOL 1991;165:1717-22.)

Transfer of aspirin across the perfused human placental cotyledon

Pregnancy-induced hypertension is associated with a reduction in prostacyclin synthesis that is relative to normotensive pregnancy, whereas thromboxane A2 synthesis is unchanged or increased. The net effect is a decreased prostacyclin/thromboxane ratio that may result in the reduced fetal-placental blood flow seen in pregnancy-induced hypertension because thromboxane is known to constrict this circulation. Low-dose aspirin (acetylsalicylic acid), which is used to treat pregnancy-induced hypertension, selectively inhibits thromboxane synthesis and therefore may alter fetal-placental blood flow. We have investigated the transfer of acetylsalicylic acid in the perfused human placental cotyledon and its effects on fetal-placental perfusion pressure. Human placental cotyledons were perfused with tissue culture medium 199 plus 5% polyvinylpyrrolidone that was gassed with 95% oxygen/5% carbon dioxide at flow rates of 10 ml/min (maternal) and 4 ml/min (fetal). Acetylsalicylic acid (10(-5) mol/L) was added to the maternal circuit, and cotyledons were perfused for 1 hour with aliquots taken from a closed fetal circuit every 5 minutes. Acetylsalicylic acid was assayed by spectrofluorometry at 306/412 nm. Our data indicate an initial rapid transfer of acetylsalicylic acid during the first 5 minutes into the fetal-placental circulation, the concentration then decreased to a steady state at 0.4 x 10(-5) mol/L. Resting perfusion pressure of both maternal and fetal circulation did not change after the addition of acetylsalicylic acid to maternal perfusate and transfer to the fetal circulation.

The action of two natriuretic peptides (atrial natriuretic peptide and brain natriuretic peptide) in the human placental vasculature

OBJECTIVE Our purpose was to compare the actions of atrial natriuretic peptide and brain natriuretic peptide in the human placental vasculature. STUDY DESIGN Isolated placental cotyledons were dually perfused with fetal perfusion pressure used as an index of vascular response. The effect of angiotensin II (10(-10) to 10(-6) mol/L bolus injection) was established in the absence or presence of atrial natriuretic peptide (10(-8) mol/L) or brain natriuretic peptide (10(-8) mol/L final concentration). The role of nitric oxide as a mediator of natriuretic peptide action was investigated by perfusion of n-nitro-L-arginine (10(-3) mol/L), an inhibitor of nitric oxide synthase. Attenuation of the action of atrial natriuretic peptide by placental peptidases was studied by perfusion with the peptidase inhibitor benzamidine (2 x 10(-2) mol/L). Statistical significance was determined by analysis of variance and paired t test. RESULTS Significant attenuation of vasoconstrictor responses to angiotensin II occurred within both atrial natriuretic peptide and brain natriuretic peptide; however, brain natriuretic peptide was more effective. n-Nitro-L-arginine did not affect the attenuation of angiotensin II-induced vasoconstriction by atrial or brain natriuretic peptides. In the presence of benzamidine atrial natriuretic peptide exerted a significantly greater vasodilator effect. CONCLUSION Brain natriuretic peptide is a more potent vasodilator of the placental vasculature than is atrial natriuretic peptide. The low efficacy of atrial natriuretic peptide may be related to placental peptidases. Nitric oxide does not mediate the action of atrial natriuretic peptide or brain natriuretic peptide.

Selective Vasodilator Effects of Atrial Natriuretic Peptide in the Human Placental Vasculature

Objective: To determine whether atrial natriuretic peptide (ANP) attenuates the vasocon-strictor effects of angiotensin II (AII), a thromboxane mimetic (U46619), and endothelin-1 in the human fetal-placental vasculature and to determine whether nitric oxide (NO) has a role in the vasodilator activity of ANP. Methods: Isolated placental cotyledons were dually perfused, with fetal perfusion pressure used as an index of vascular response. The effects of AII (10-10-10-6 mol/L bolus injection), endothelin-1 (10-7 mol/L bolus), and U46619 (10-9-10-6 mol/L bolus or 10-8 mol/L infusion) were established in the absence or presence of ANP (10-8 mol/L). The role of NO as a mediator of ANP action was investigated by perfusion with n-nitro-L-arginine (NNLA, 10-3 mol/L), an inhibitor of NO synthase. Statistical significance was determined by analysis of variance. Results: Atrial nutriuretic peptide caused significant attenuation of vasoconstrictor responses to AII, but weak attenuation of endothelin-1 and no attenuation of U46619. Use of NNLA did not affect the attenuation of AII-induced vasoconstriction by ANP. Conclusion: Atrial natriuretic peptide is a vasodilator of the fetal-placental vasculature constricted with AII and endothelin-1, but not with U46619. Nitric oxide does not mediate the action of ANP.

Novel endothelial-derived vasoactive factors in the human fetal-placental circulation

Summary The role of novel endothelial-cell derived relaxing and contracting factorshave been investigated in the perfused human placental cotyledon. By using specific inhibitors of synthesis of the relaxing factor, nitric oxide, we have found that nitric oxide synthesis and action appears to contribute to maintenance of basal tone in the human fetal-placental circulation and attenuates the vasoconstrictor actions of thromboxane A 2 and the peptide endothelin-1. Endothelin-1 is a potent long acting vasoconstrictor (8×10 −10 –1×10 −8 M) in the human fetal-placental circulation. The duration of action and potency make it unlikely to have an acute vasoconstrictor role, but rather it may act to contribute towards basal tone or to potentiate the action of other vasoconstrictors. Although it has been suggested that Et-1 releases thromboxane A 2 in the fetal-placental circulation and this mediates the Et-1-induced vasoconstriction, we have shown that Et-1-induced vasoconstriction results in a significant reduction in thromboxane release, probably due to the reduction in flow rate. Further, by using a thromboxane synthase inhibitor, dazoxiben, or a thromboxane receptor antagonist, SQ29548, we have conclusively shown that Et-1 vasoconstriction is not mediated via thromboxane synthesis and action. NO does not appear to be released by the classic EDRF agonists such as acetylcholine, bradykinin, histamine, or calcium ionophores in the fetal-placental circulation. Indeed, bradykinin is a vasoconstrictor in this circulation and the concentrations of histamine necessary to cause vasodilation are in excess of those shown to release EDRF in other systems. Rather the stimulus to NO release in the placenta may be transmural pressure and/or flow/shear stress across endothelium.