Importance of Nitric Oxide in Control of Systemic and Renal Hemodynamics During Normal Pregnancy: Studies in the Rat and Implications for Preeclampsia

Mechanisms of Renal Vasodilation and Hyperfiltration During Pregnancy

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

Emerging role of relaxin in the maternal adaptations to normal pregnancy: implications for preeclampsia

Relaxin is an approximately 6-kilodalton peptide hormone secreted by the corpus luteum, and circulates in the maternal blood during pregnancy. Relaxin administration to awake, chronically instrumented, nonpregnant rats mimics the vasodilatory phenomena of pregnancy. Furthermore, immunoneutralization of relaxin or its elimination from the circulation during midterm pregnancy in awake rats prevents maternal systemic and renal vasodilation, and the increase in global arterial compliance. Human investigation, albeit limited through 2010, also reveals vasodilatory effects of relaxin in the nonpregnant condition and observations consistent with a role for relaxin in gestational renal hyperfiltration. Evidence suggests that the vasodilatory responses of relaxin are mediated by its major receptor, the relaxin/insulin-like family peptide 1 receptor, RFXP1. The molecular mechanisms of relaxin vasodilation depend on the duration of hormone exposure (ie, there are rapid and sustained vasodilatory responses). Newly emerging data support the role of Gα(i/o) protein coupling to phosphatidylinositol-3 kinase/Akt (protein kinase B)-dependent phosphorylation and activation of endothelial nitric oxide synthase in the rapid vasodilatory responses of relaxin. Sustained vasodilatory responses critically depend on vascular endothelial and placental growth factors, and increases in arterial gelatinase(s) activity. Gelatinases hydrolyze big endothelin (ET) at a gly-leu bond to form ET(1-32), which activates the endothelial ET(B)/nitric oxide vasodilatory pathway. Although the relevance of relaxin biology to preeclampsia is largely speculative at this time, there are potential tantalizing links that are discussed in the context of our current understanding of the etiology and pathophysiology of the disease.

Effects of hyperhomocysteinemia on arterial pressure and nitric oxide production in pregnant rats

BACKGROUND

An elevated plasma level of homocysteine (hyperhomocysteinemia) is thought to be an important risk factor for a variety of cardiovascular diseases including preeclampsia. Although clinical studies have reported a two- to threefold elevation in plasma levels of homocysteine in women who developed preeclampsia, the importance of hyperhomocysteinemia in causing endothelial dysfunction and increases in arterial pressure during pregnancy is unknown.

METHODS

Therefore, the purpose of this study was to determine the effects of a two- to threefold elevation in plasma homocysteine levels on arterial pressure, chronic pressure-natriuresis relationship, and endothelial factors during pregnancy in the rat. Homocysteine treatment for 4 weeks increased plasma homocysteine levels in pregnant rats from 7.1 +/- 1.9 to 16.7 +/- 2.3 micromol/l.

RESULTS

Homocysteine treatment decreased urinary nitrate/nitrite levels from 53 +/- 7 vs. 39 +/- 5 (micromol/24 h/kg body weight) in pregnant rats whereas having no effect on urinary excretion of endothelin. Homocysteine treatment had no effect on mean arterial pressure (MAP) in pregnant rats (104 +/- 2 vs. 107 +/- 3 mm Hg) nor on the chronic pressure-natriuresis relationship.

CONCLUSIONS

These results suggest that although hyperhomocysteinemia decreases nitric oxide (NO) production in pregnant rats, hyperhomocysteinemia does not affect MAP, the chronic pressure-natriuresis relationship, or urinary excretion of endothelin in pregnant rats. Moreover, the reported two- to threefold elevation in plasma level of homocysteine in women with preeclampsia is unlikely to contribute to the hypertension of preeclampsia.

Doppler analysis and placental nitric oxide synthase expression during fetal growth restriction

OBJECTIVE

To assess placental nitric oxide (NO) metabolism related to changes in the uteroplacental circulation during fetal growth restriction (FGR).

METHODS

The resistance index (RI) from the uterine arteries and pulsatility index (PI) from the umbilical artery were determined by Doppler analysis in 15 patients with FGR and 12 healthy controls, before elective cesarean section. Inducible (iNOS) and endothelial (eNOS) NO synthase expression were measured in placental samples. Immunohistochemistry was performed for iNOS location in the placenta.

RESULTS

During FGR, we observed a significant elevation of iNOS when compared with controls. Conversely, eNOS did not differ between the two groups. A negative correlation with eNOS (r = -0.85) and a positive correlation with iNOS (r = 0.91) was found correlating to umbilical PI. The iNOS proteins were reduced in syncytiotrophoblast cells and increased in endothelium in the FGR group compared to the controls.

CONCLUSIONS

During FGR, placental iNOS expression is significantly increased; this increase possibly represents an adaptive physiological mechanism for overcoming a fetoplacental circulation deficiency.

Role of relaxin in maternal renal vasodilation of pregnancy

The remarkable hemodynamic changes of normal pregnancy are briefly reviewed. In addition, new findings and current concepts related to the underlying hormonal and molecular mechanisms are presented. Finally, work that is in progress as well as future directions is briefly discussed.

Placental expression of nitric oxide synthase during HELLP syndrome: the correlation with maternal-fetal Doppler velocimetry

BACKGROUND

To correlate Doppler waveform of the uterine and umbilical vessels to placental nitric oxide synthase (NOS) expression in pregnant women with HELLP (hemolysis, elevated liver enzymes, low platelets count) syndrome.

METHODS

mRNA expression of inducible NOS (iNOS) and endothelial NOS (eNOS) was assessed, after cesarean section, in placental samples from 10 women affected by HELLP syndrome and 10 controls. Pulsatility indices on Doppler waveform analysis from uterine and umbilical arteries were measured.

RESULTS

iNOS expression was significantly lower in placenta from women with HELLP syndrome than controls. When comparing the results with Doppler flow measurements, we found a negative correlation between umbilical pulsatility index and eNOS expression (r = -0.91) and a positive correlation with iNOS expression (r = 0.86).

CONCLUSIONS

The reduced iNOS expression in women with HELLP syndrome may indicate the extreme placental dysfunction that is unable to compensate for the endothelial derangement and related hypertension in spite of trying to improve fetoplacental perfusion and the delivery of nutrients to the fetus.

Mechanisms of renal vasodilation and hyperfiltration during pregnancy: current perspectives and potential implications for preeclampsia

A thorough understanding of the renal and cardiovascular adaptations to normal gestation is essential for proper diagnosis and management of hypertensive disorders and renal diseases during pregnancy. Here, we briefly review the renal hemodynamic changes of normal pregnancy. In addition, we present new findings and current concepts related to the underlying hormonal and molecular mechanisms. Finally, we speculate on the potential contribution of these insights from normal pregnancy to the pathogenesis of preeclampsia.

Elevated plasma endothelial microparticles: preeclampsia versus gestational hypertension

OBJECTIVE

Elevated plasma endothelial microparticle levels have been found to be elevated in women with preeclampsia. However, their role in distinguishing preeclampsia from gestational hypertension remains to be elucidated. The objectives of this study were to compare endothelial microparticle levels among patients with preeclampsia, gestational hypertension, and healthy pregnant control subjects and to evaluate the effect of plasma from women with preeclampsia and gestational hypertension on the release of endothelial microparticles by renal microvascular endothelial cells.

STUDY DESIGN

A prospective study was conducted on 52 women with preeclampsia, 20 women with gestational hypertension, and 38 healthy pregnant control subjects. Endothelial microparticles were measured by flow cytometry with fluorescent monoclonal mouse anti-human antibodies against CD31, CD42b, and CD62E.

RESULTS

CD31 + /42b - endothelial microparticle levels were 10497 +/- 5145 counts/microL in women with preeclampsia versus 6768 +/- 1810 counts/microL in women with gestational hypertension ( P < .01). In control subjects, CD31 + /42b - endothelial microparticle levels were 6119 +/- 3592 counts/microL. CD62E + endothelial microparticle levels were 1930 +/- 966 counts/microL in women with preeclampsia versus 822 +/- 150 counts/microL in women with gestational hypertension ( P <.01). In control subjects, CD62E + endothelial microparticle levels were 712 +/- 160 counts/microL. Incubation of renal microvascular endothelial cells with plasma from women with preeclampsia resulted in a rise in CD31 + and CD62E + endothelial microparticle levels as compared with women with gestational hypertension and control subjects.

CONCLUSION

Endothelial microparticle levels are higher in women with preeclampsia than in women with gestational hypertension and control subjects. The measurement of endothelial microparticles may be useful as a diagnostic tool for preeclampsia in pregnant women.

Prolonged inhibition of nitric oxide synthesis in pregnant rats: effects on blood pressure, fetal growth and litter size

METHODS

In order to investigate the effect of chronic inhibition of nitric oxide synthesis along pregnancy, pregnant rats were given drinking water alone (control group) or drinking water containing nonselective nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 15 mg/day/rat equivalent to 50 mg/kg/day; treated group), from postmating days 1 to 18 of pregnancy. On days 1 to 17 of pregnancy, urinary volume, urinary sodium and potassium excretion, and urine protein concentration were measured. Systolic blood pressure (SBP) was recorded daily. On days 6, 11 and 18 of pregnancy the number of sites of implantation, number of embryos, litter size, fetal and placenta weight were determined.

RESULTS

Systolic blood pressure (mmHg) increased (p<0.001) on the 2nd day of administration of L-NAME and remained high throughout the experiment. This treatment increased urinary protein excretion and urine volume (p<0.01), with changes in the sodium and potassium excretion rate along the study. On day 6 of gestation in treated group, the number of implantation sites (0.14+/-0.10) significantly decreased (p<0.05) compared with the control group (1.45+/-0.58), but on day 11 of pregnancy the number of embryos was similar in both groups. By day 18 of pregnancy, L-NAME caused a substantial decrease (p<0.001) in litter weight (6.30+/-0.77 to 12.00+/-0.92 g), weight of placenta (3.17+/-0.22 to 4.74+/-0.21 g) (p<0.001) and litter size (7.95+/-0.59 to 11.95+/-0.45 fetus/litter; p<0.001). Also, treatment with L-NAME caused an important number of fetal resorptions (2.93+/-0.42 No./litter to 0 in control group).

CONCLUSION

Thus, treatment of pregnant rats with L-NAME, has an important effect on systolic blood pressure and on the physiology of reproduction, mainly in the third stage of pregnancy.

L-arginine attenuates hypertension in pregnant rats with reduced uterine perfusion pressure

A chronic reduction in uterine perfusion pressure in the pregnant rat is associated with significant elevations in mean arterial pressure, proteinuria, and reductions in kidney function as is chronic nitric oxide blockade, suggesting that nitric oxide deficiency may contribute to the clinical manifestations of preeclampsia. The purpose of this study was to determine whether supplementation with L-arginine, the precursor for nitric oxide, attenuates the hypertension produced in response to a chronic reduction in uterine perfusion pressure in the pregnant rat. Reduced uterine perfusion was initiated at day 14 of gestation with arterial pressure determined at day 19 of gestation in conscious, chronically instrumented rats. Arterial pressure was significantly elevated in pregnant rats with chronic reductions in uterine perfusion as compared with pregnant control rats (132+/-2 versus 109+/-2 mm Hg, P<0.01, respectively). Treatment with L-arginine (2%) in the drinking water was initiated at day 10 of gestation. l-arginine supplementation resulted in a significant decrease in arterial pressure in both pregnant rats with reduced uterine perfusion pressure (113+/-2 mm Hg treated, P<0.01 versus untreated pregnant with reduced uterine perfusion pressure) and pregnant control (97+/-3 mm Hg treated, P<0.01 versus untreated pregnant) rats. However, supplementation with L-arginine decreased blood pressure by 19 mm Hg in pregnant with reduced uterine perfusion pressure (untreated versus treated) as compared with 12 mm Hg in pregnant (untreated versus treated) rats. Thus, these results suggest that l-arginine supplementation may be beneficial in attenuating the hypertension in preeclampsia.

Functional characteristics of chorionic plate placental arteries from normal pregnant women and women with pre-eclampsia

OBJECTIVES

We aimed to compare placental small artery function from women with pre-eclampsia and normal pregnancy. In particular, we wished to test the hypothesis that these arteries respond differently to an endothelium-dependent vasodilator, to the presence of nitric oxide, and to the presence of cyclic monophosphate nucleotides.

METHODS

A small vessel wire myograph was used to study placental arteries (200 to 550 microm). Contractile function was assessed with vasopressin. Relaxation was assessed with the endothelium-dependent vasodilator, bradykinin, and the endothelium-independent vasodilators sodium nitroprusside (a nitric oxide donor) and papaverine (a phosphodiesterase inhibitor).

RESULTS

The constrictor response to vasopressin did not differ between patient groups (p=0.79; repeated measures ANOVA). For both normal pregnancy and pre-eclampsia, the response of pre-constricted arteries to the endothelium-dependent vasodilator, bradykinin, was minimal. Vasorelaxation to sodium nitroprusside and papaverine was attenuated in pre-eclampsia compared to normal pregnancy (p=0.03 and p<0.001, respectively; repeated measures ANOVA).

CONCLUSIONS

In pre-eclampsia, placental arteries exhibit an attenuated vasodilatory response to nitric oxide.

Enhanced [Ca2+]i in renal arterial smooth muscle cells of pregnant rats with reduced uterine perfusion pressure

Reduction of uterine perfusion pressure (RUPP) during late pregnancy has been suggested to trigger increases in renal vascular resistance and lead to hypertension of pregnancy. We investigated whether the increased renal vascular resistance associated with RUPP in late pregnancy reflects increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and contraction of renal arterial smooth muscle. Single smooth muscle cells were isolated from renal interlobular arteries of normal pregnant Sprague-Dawley rats and a rat model of RUPP during late pregnancy. The cells were loaded with fura 2 and both cell length and [Ca(2+)](i) were measured. In cells of normal pregnant rats incubated in Hanks' solution (1 mM Ca(2+)), ANG II (10(-7) M) caused an initial increase in [Ca(2+)](i) to 414 +/- 13 nM, a maintained increase to 149 +/- 8 nM, and 21 +/- 1% cell contraction. In RUPP rats, the initial ANG II-induced [Ca(2+)](i) (431 +/- 18 nM) was not different from pregnant rats, but both the maintained [Ca(2+)](i) (225 +/- 9 nM) and cell contraction (48 +/- 2%) were increased. Membrane depolarization by 51 mM KCl and the Ca(2+) channel agonist BAY K 8644 (10(-6) M), which stimulate Ca(2+) entry from the extracellular space, caused maintained increases in [Ca(2+)](i) and cell contraction that were greater in RUPP rats than control pregnant rats. In Ca(2+)-free (2 mM EGTA) Hanks' solution, the ANG II- and caffeine (10 mM)-induced [Ca(2+)](i) transient and cell contraction were not different between normal pregnant and RUPP rats, suggesting no difference in Ca(2+) release from the intracellular stores. The enhanced maintained ANG II-, KCl- and BAY K 8644-induced [Ca(2+)](i) and cell contraction in RUPP rats compared with normal pregnant rats suggest enhanced Ca(2+) entry mechanisms of smooth muscle contraction in resistance renal arteries and may explain the increased renal vascular resistance associated with hypertension of pregnancy.

Pregnancy influence on the vascular interactions between nitric oxide and other endothelium-derived mediators in rat kidney

Peripheral vascular resistance and sensitivity to circulating pressor and vasoconstrictor agents are blunted during pregnancy. This has been mainly attributed to an increased production of endothelium-derived mediators. The objective of this work was to evaluate if pregnancy changes the relative participation of nitric oxide (NO) and prostaglandins (PG) in respect to the modulation of the increases in renal perfusion pressure induced by phenylephrine (Phe). Dose-response curves were made with gradually increasing doses of Phe using an isolated kidney preparation in the presence of a NO synthase (NOS) inhibitor (L-NAME, 1 microM), a PG-synthesis inhibitor (indomethacin, 1 microM), both, or neither. Also, renal cyclooxygenase (COX-1 and COX-2) and endothelial NOS (eNOS) expression was determined using PCR. The experiments were done in kidneys from nonpregnant and pregnant rats. Our results showed that the relative participation of renal vasoactive mediators seems to change during pregnancy. We found the presence of a COX-1-dependent vasoconstrictor in the middle of pregnancy that was not found in nonpregnant rats. Our results also suggest that there is increased participation of another renal vasodilator substance, the effect of which is observed when NO or PG synthesis is inhibited during late pregnancy. In addition, an apparent interaction between renal eNOS and COX-1 expression was observed: eNOS expression was diminished, while COX-1 was increased during the 2nd week of pregnancy. In contrast, in kidneys from the 3rd week of pregnancy, the expression of these two enzymes was similar.

Vascular mechanisms of increased arterial pressure in preeclampsia: lessons from animal models

Normal pregnancy is associated with reductions in total vascular resistance and arterial pressure possibly due to enhanced endothelium-dependent vascular relaxation and decreased vascular reactivity to vasoconstrictor agonists. These beneficial hemodynamic and vascular changes do not occur in women who develop preeclampsia; instead, severe increases in vascular resistance and arterial pressure are observed. Although preeclampsia represents a major cause of maternal and fetal morbidity and mortality, the vascular and cellular mechanisms underlying this disorder have not been clearly identified. Studies in hypertensive pregnant women and experimental animal models suggested that reduction in uteroplacental perfusion pressure and the ensuing placental ischemia/hypoxia during late pregnancy may trigger the release of placental factors that initiate a cascade of cellular and molecular events leading to endothelial and vascular smooth muscle cell dysfunction and thereby increased vascular resistance and arterial pressure. The reduction in uterine perfusion pressure and the ensuing placental ischemia are possibly caused by inadequate cytotrophoblast invasion of the uterine spiral arteries. Placental ischemia may promote the release of a variety of biologically active factors, including cytokines such as tumor necrosis factor-alpha and reactive oxygen species. Threshold increases in the plasma levels of placental factors may lead to endothelial cell dysfunction, alterations in the release of vasodilator substances such as nitric oxide (NO), prostacyclin (PGI(2)), and endothelium-derived hyperpolarizing factor, and thereby reductions of the NO-cGMP, PGI(2)-cAMP, and hyperpolarizing factor vascular relaxation pathways. The placental factors may also increase the release of or the vascular reactivity to endothelium-derived contracting factors such as endothelin, thromboxane, and ANG II. These contracting factors could increase intracellular Ca(2+) concentrations ([Ca(2+)](i)) and stimulate Ca(2+)-dependent contraction pathways in vascular smooth muscle. The contracting factors could also increase the activity of vascular protein kinases such as protein kinase C, leading to increased myofilament force sensitivity to [Ca(2+)](i) and enhancement of smooth muscle contraction. The decreased endothelium-dependent mechanisms of vascular relaxation and the enhanced mechanisms of vascular smooth muscle contraction represent plausible causes of the increased vascular resistance and arterial pressure associated with preeclampsia.

TNF-alpha enhances contraction and inhibits endothelial NO-cGMP relaxation in systemic vessels of pregnant rats

Tumor necrosis factor-alpha (TNF-alpha) is elevated in the plasma of preeclamptic women and may have a role in pregnancy-induced hypertension. However, whether the hemodynamic effects of TNF-alpha reflect the direct effects on vascular reactivity is unclear. We tested the hypothesis that TNF-alpha impairs endothelium-dependent relaxation and enhances vascular contraction in systemic vessels of pregnant rats. We measured isometric contraction in aortic strips isolated from virgin and pregnant Sprague-Dawley rats (nontreated vs. treated for 2 h with 10-1,000 pg/ml TNF-alpha). In endothelium-intact vascular strips, TNF-alpha caused greater enhancement of phenylephrine (Phe) contraction in pregnant than virgin rats. TNF-alpha caused significant inhibition of ACh- and bradykinin-induced vascular relaxation and nitrite/nitrate production that were more prominent in pregnant than virgin rats. N(G)-nitro-L-arginine methyl ester [L-NAME, 100 microM, an inhibitor of nitric oxide (NO) synthase] or 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 1 microM, an inhibitor of cGMP production in smooth muscle) inhibited ACh relaxation and enhanced Phe contraction in nontreated but to a lesser extent in TNF-alpha-treated vessels, particularly those of pregnant rats. Endothelium removal enhanced Phe contraction in nontreated but not TNF-alpha-treated vessels, especially those of pregnant rats. Relaxation of Phe contraction with the NO donor sodium nitroprusside was not different between nontreated and TNF-alpha-treated vessels. Thus TNF-alpha enhances vascular contraction and inhibits endothelium-dependent NO-cGMP-mediated vascular relaxation in systemic vessels, particularly those of pregnant rats. The results support a direct role for TNF-alpha as a possible mediator of increased vascular resistance associated with pregnancy-induced hypertension.

Inducible nitric oxide synthase inhibition attenuates renal hemodynamics during pregnancy

Acute, nonselective nitric oxide synthase inhibition in the pregnant rat decreases glomerular filtration rate and renal plasma flow, suggesting a role for nitric oxide in mediating renal vasodilation during pregnancy. As mid-gestation in the rat is associated with a significant increase in renal protein expression of inducible nitric oxide synthase, the aim of this study was to examine the role of inducible nitric oxide synthase in mediating renal hemodynamics changes at mid-gestation in the rat. At day 16 of pregnancy, glomerular filtration rate was significantly higher in pregnant rats compared with virgin rats (3.1 +/- 0.4 versus 2.7 +/- 0.3 mL/min, respectively; P<0.05), as was effective renal plasma flow (13.4 +/- 2.5 versus 10.9 +/- 2.2 mL/min, respectively; P<0.05). Acute administration of the inducible nitric oxide synthase selective inhibitor, AMT hydrochloride (750 nmol/h), markedly attenuated the increase in glomerular filtration rate observed in pregnant rats (2.3 +/- 0.2 mL/min, P<0.01 versus pregnant) without significantly altering glomerular filtration rate in virgin rats (2.1 +/- 0.2 mL/min). Acute AMT administration significantly decreased effective renal plasma flow in pregnant (8.9 +/- 1.8 mL/min, P<0.01 versus pregnant) and virgin rats (7.1 +/- 0.9 mL/min, P<0.05 versus virgin). Acute administration of EIT (380 nmol/h), another inducible nitric oxide synthase selective inhibitor, also attenuated pregnancy-induced increases in glomerular filtration rate (2.1 +/- 0.2, 2.8 +/- 0.3, and 2.3 +/- 0.3 mL/min; virgin, pregnant, and EIT, respectively) and effective renal plasma flow (8.5 +/- 1.1, 13.8 +/- 2.1, and 9.0 +/- 1.1 mL/min; virgin, pregnant, and EIT, respectively). Therefore, these findings suggest that inducible nitric oxide synthase may play an important role in mediating the renal hemodynamic changes that occur during normal pregnancy.

Reduced endothelial NO-cGMP vascular relaxation pathway during TNF-alpha-induced hypertension in pregnant rats

Placental ischemia during pregnancy is thought to release cytokines such as tumor necrosis factor-alpha (TNF-alpha), which may contribute to the increased vascular resistance associated with pregnancy-induced hypertension. We have reported that a chronic twofold elevation in plasma TNF-alpha increases blood pressure in pregnant but not in virgin rats; however, the vascular mechanisms are unclear. We tested the hypothesis that increasing plasma TNF-alpha during pregnancy impairs endothelium-dependent vascular relaxation and enhances vascular reactivity. Active stress was measured in aortic strips of virgin and late-pregnant Sprague-Dawley rats untreated or infused with TNF-alpha (200 ng x kg(-1) x day(-1) for 5 days) to increase plasma level twofold. Phenylephrine (Phe) increased active stress to a maximum of 4.2 +/- 0.4 x 10(3) and 9.9 +/- 0.7 x 10(3) N/m2 in control pregnant and TNF-alpha-infused pregnant rats, respectively. Removal of the endothelium enhanced Phe-induced stress in control but not in TNF-alpha-infused pregnant rats. In endothelium-intact strips, ACh caused greater relaxation of Phe contraction in control than in TNF-alpha-infused pregnant rats. Basal and ACh-induced nitrite/nitrate production was less in TNF-alpha-infused than in control pregnant rats. Pretreatment of vascular strips with 100 microM N(G)-nitro-L-arginine methyl ester, to inhibit nitric oxide (NO) synthase, or 1 microM 1H-[1,2,4]oxadiazolo[4,3-]quinoxalin-1-one, to inhibit cGMP production in smooth muscle, inhibited ACh-induced relaxation and enhanced Phe-induced stress in control but not in TNF-alpha-infused pregnant rats. Phe contraction and ACh relaxation were not significantly different between control and TNF-alpha-infused virgin rats. Thus an endothelium-dependent NO-cGMP-mediated vascular relaxation pathway is inhibited in late-pregnant rats infused with TNF-alpha. The results support a role for TNF-alpha as one possible mediator of the increased vascular resistance associated with pregnancy-induced hypertension.

Preeclampsia: linking placental ischemia with cardiovascular-renal dysfunction

Placental ischemia during preeclampsia is thought to lead to widespread activation/dysfunction of the maternal vascular endothelium. This results in enhanced formation of endothelin and thromboxane and decreased formation of nitric oxide and prostacyclin. These endothelial abnormalities, in turn, cause hypertension by impairing renal pressure natriuresis and increasing total peripheral resistance.

Role of neuronal nitric oxide synthase in mediating renal hemodynamic changes during pregnancy

Renal plasma flow (RPF) and glomerular filtration rate (GFR) are markedly increased during pregnancy. We recently reported that the renal hemodynamic changes observed during pregnancy in rats are associated with enhanced renal protein expression of neuronal nitric oxide synthase (nNOS). The purpose of this study was to determine the role of nNOS in mediating renal hemodynamic changes observed during pregnancy. To achieve this goal, we examined the effects of the nNOS inhibitor 7-nitroindazole (7-NI) on kidney function in normal conscious, chronically instrumented virgin (n = 6) and pregnant rats (n = 9) at day 16 of gestation. Infusion of 7-NI had no effect on RPF (4.7 +/- 0.7 vs. 4.8 +/- 0.9 ml/min), GFR (2.2 +/- 0.2 vs. 2.5 +/- 0.4 ml/min), or mean arterial pressure (MAP; 127 +/- 7 vs. 129 +/- 10 mmHg) in virgin rats. In contrast, 7-NI infused into pregnant rats decreased RPF (8.9 +/- 1.6 vs. 6.5 +/- 1.4 ml/min) and GFR (4.4 +/- 0.7 vs. 3.3 +/- 0.7 ml/min) while having no effect on MAP (123 +/- 4 vs. 123 +/- 3 mmHg). In summary, inhibition of nNOS in pregnant rats at midgestation results in significant decreases in RPF and GFR. nNOS inhibition in virgin rats had no effect on renal hemodynamics. These data suggest that nNOS may play a role in mediating the renal hemodynamic changes that occur during pregnancy.

Reduced uterine perfusion pressure during pregnancy in the rat is associated with increases in arterial pressure and changes in renal nitric oxide

A reduction in nitric oxide (NO) synthesis has been suggested to play a role in pregnancy-induced hypertension. We have recently reported that normal pregnancy in the rat is associated with significant increases in whole-body NO production and renal protein expression of neuronal and inducible NO synthase. The purpose of this study was to determine whether whole-body and renal NO production is reduced in a rat model of pregnancy-induced hypertension produced by chronically reducing uterine perfusion pressure starting at day 14 of gestation. Chronic reductions in uterine perfusion pressure resulted in increases in arterial pressure of 20 to 25 mm Hg, decreases in renal plasma flow (<23%) and glomerular filtration rate (<40%), but no difference in urinary nitrite/nitrate excretion relative to control pregnant rats. In contrast, reductions in uterine perfusion pressure in virgin rats resulted in no significant effects on arterial pressure. Renal endothelial (<4%) and inducible (<11%) NO synthase protein expression did not decrease significantly in the chronically reduced uterine perfusion pressure rats relative to normal pregnant rats; however, significant reductions in neuronal NO synthase were observed (<30%). The results of this study indicate that the reduction in renal hemodynamics and the increase in arterial pressure observed in response to chronic decreases in uterine perfusion pressure in pregnant rats are associated with no change in whole-body NO production and a decrease in renal protein expression of neuronal NO synthase.

[Ca(2+)](i) signaling in renal arterial smooth muscle cells of pregnant rat is enhanced during inhibition of NOS

Vascular resistance and arterial pressure are reduced during normal pregnancy, but dangerously elevated during pregnancy-induced hypertension (PIH), and changes in nitric oxide (NO) synthesis have been hypothesized as one potential cause. In support of this hypothesis, chronic inhibition of NO synthesis in pregnant rats has been shown to cause significant increases in renal vascular resistance and hypertension; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the pregnancy-associated changes in renal vascular resistance reflect changes in contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) of renal arterial smooth muscle. Smooth muscle cells were isolated from renal interlobular arteries of virgin and pregnant Sprague-Dawley rats untreated or treated with the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 4 mg. kg(-1). day(-1) for 5 days), then loaded with fura 2. In cells of virgin rats incubated in Hanks' solution (1 mM Ca(2+)), the basal [Ca(2+)](i) was 86 +/- 6 nM. Phenylephrine (Phe, 10(-5) M) caused a transient increase in [Ca(2+)](i) to 417 +/- 11 nM and maintained an increase to 183 +/- 8 nM and 32 +/- 3% cell contraction. Membrane depolarization by 51 mM KCl, which stimulates Ca(2+) entry from the extracellular space, caused maintained increase in [Ca(2+)](i) to 292 +/- 12 nM and 31 +/- 2% contraction. The maintained Phe- and KCl-induced [Ca(2+)](i) and contractions were reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. Phe- and KCl-induced contraction and [Ca(2+)](i) were not significantly different between untreated and L-NAME-treated virgin rats or between untreated and L-NAME + L-arginine treated pregnant rats. In Ca(2+)-free Hanks', application of Phe or caffeine (10 mM), to stimulate Ca(2+) release from the intracellular stores, caused a transient increase in [Ca(2+)](i) and a small cell contraction that were not significantly different among the different groups. Thus renal interlobular smooth muscle of normal pregnant rats exhibits reduction in [Ca(2+)](i) signaling that involves Ca(2+) entry from the extracellular space but not Ca(2+) release from the intracellular stores. The reduced renal smooth muscle cell contraction and [Ca(2+)](i) in pregnant rats may explain the decreased renal vascular resistance associated with normal pregnancy, whereas the enhanced cell contraction and [Ca(2+)](i) during inhibition of NO synthesis in pregnant rats may, in part, explain the increased renal vascular resistance associated with PIH.

Pregnancy, vascular tone, and maternal hemodynamics: a crucial adaptation

The adaptation of vascular tone in early pregnancy precedes and probably triggers blood volume and cardiac output increase. Because the endothelium is known to regulate vascular smooth muscle action, the role of nitric oxide (NO) in the setting up of adequate uteroplacental and renal blood flow during normal pregnancy was investigated. The persistence of abnormally high uteroplacental resistance is a strong predisposing factor for intrauterine growth retardation and preeclampsia and can be screened by second trimester Doppler assessment of the uterine arteries. Current hypotheses suggested by experimental and clinical data concerning preeclampsia confirm the crucial role played by the endothelium and vascular tone adaptation. The analysis of these data leads to consider apart early-onset preeclampsia affecting pregnancies with growth retarded fetuses and associated with high vascular resistance. Lastly, NO donors seem to significantly decrease the impedance in the uteroplacental circulation and to improve fetoplacental hemodynamics assessed by Doppler measurements, and their therapeutic use in some forms of preeclampsia might be promising.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to summarize the events that regulate vascular tone in pregnancy, specifically the role of nitric oxide and other vasoactive prostaglandins in the regulation of vascular tone and to describe the various hypotheses concerning the mechanism and the mediators responsible for initiating endothelial damage in the various disorders of vascular tone in pregnancy.

Pregnancy-associated reduction in vascular protein kinase C activity rebounds during inhibition of NO synthesis

Vascular reactivity has been shown to be reduced during pregnancy and to be enhanced during chronic inhibition of nitric oxide (NO) synthesis in pregnant rats; however, the cellular mechanisms involved are unclear. The purpose of this study was to investigate whether the pregnancy-induced changes in vascular reactivity are associated with changes in the amount and/or activity of vascular protein kinase C (PKC). Active stress as well as the amount and activity of PKC was measured in deendothelialized thoracic aortic strips from virgin and pregnant rats untreated or treated with the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). In virgin rats, the PKC activator phorbol 12,13-dibutyrate (PDBu, 10(-6) M) and the alpha-adrenergic agonist phenylephrine (Phe, 10(-5) M) caused significant increases in active stress and PKC activity that were inhibited by the PKC inhibitors staurosporine and calphostin C. Western blot analysis in aortic strips of virgin rats showed significant amount of the alpha-PKC isoform. Both PDBu and Phe caused significant translocation of alpha-PKC from the cytosolic to the particulate fraction. Compared with virgin rats, the PDBu- and Phe-stimulated active stress and PKC activity as well as the amount and the PDBu- and Phe-induced translocation of alpha-PKC were significantly reduced in late pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. The PDBu- and Phe-induced changes in active stress and the amount, distribution, and activity of alpha-PKC in virgin rats treated with L-NAME were not significantly different from that in virgin rats, whereas the changes in pregnant rats treated with L-NAME + the NO synthase substrate L-arginine were not significantly different from that in pregnant rats. These results provide evidence that a PKC-mediated contractile pathway in vascular smooth muscle is reduced during pregnancy and significantly enhanced during chronic inhibition of NO synthesis. The results suggest that one possible mechanism of the pregnancy-associated changes in vascular reactivity may involve changes in the amount and activity of the alpha-PKC isoform.

Decreased endothelium-dependent vascular relaxation during reduction of uterine perfusion pressure in pregnant rat

Reduction in uterine perfusion and the ensuing placental ischemia during late pregnancy have been proposed to trigger increases in systemic vascular resistance and pregnancy-induced hypertension; however, the intermediary mechanisms involved are unclear. The purpose of the present study was to test the hypothesis that reduced uterine perfusion pressure during late pregnancy is associated with impaired endothelium-dependent vascular relaxation and, consequently, enhanced systemic vascular reactivity. Active stress was measured in aortic strips isolated from late pregnant Sprague-Dawley rats and a hypertensive pregnant rat model produced through the long-term reduction in uterine perfusion pressure (RUPP). Phenylephrine (Phe, 10(-5) mol/L) caused an increase in active stress to 4.5+/-0.4x10(3) N/m(2) in normal pregnant rats and a larger increase to 9.4+/-0. 7x10(3) N/m(2) in RUPP rats. Removal of the endothelium significantly enhanced Phe-induced stress in pregnant (6.4+/-0. 6x10(3) N/m(2)) but not RUPP (9.95+/-0.95x10(3) N/m(2)) rats. In endothelium-intact strips, acetylcholine (ACh) was more potent in inducing relaxation of Phe contraction in pregnant (ED(50) 0. 1x10(-6) mol/L) than in RUPP (ED(50) 1.2x10(-6) mol/L) rats. Pretreatment of endothelium-intact strips with N(G)-nitro-L-arginine methyl ester(100 micromol/L), to inhibit nitric oxide (NO) synthase, significantly inhibited ACh-induced relaxation and enhanced Phe-induced stress in pregnant (6.2+/-0.5x10(3) N/m(2)) but not RUPP (9.5+/-0.85x10(3) N/m(2)) rats. Pretreatment of endothelium-intact strips with methylene blue (10 micromol/L), to inhibit cGMP production in smooth muscle, also inhibited ACh-induced relaxation and enhanced Phe-induced stress in pregnant (6.9+/-0.65x10(3) N/m(2)) but not RUPP (9.3+/-0.7x10(3) N/m(2)) rats. In endothelium-denuded strips, relaxation of Phe contraction with the exogenous NO donor sodium nitroprusside was not significantly different between pregnant and RUPP rats. These results suggest that an endothelium-dependent relaxation pathway involving the release of NO from endothelial cells and increased cGMP production in smooth muscle is inhibited in systemic vessels of late pregnant rats with reduced uterine perfusion pressure and may in part explain the increased vascular resistance in pregnancy-induced hypertension.

Implication of apolipoprotein E and the L-arginine-nitric oxide system in preeclampsia

OBJECTIVE

During pregnancy, Apolipoprotein (Apo) E is synthesized in the placenta to facilitate the uptake of maternal lipoproteins. Preeclampsia is associated with an abnormal lipid profile. Apo E levels may affect the production of nitric oxide. We investigated whether Apo E variations could be related to the high lipid levels and nitric oxide secretion in preeclamptic women.

METHODS

Blood samples from 15 normotensive women and 12 mild and 23 severe cases of preeclampsia were assayed for standard lipid profile, Apo E, and nitrate. Urine samples were analyzed for nitrate and cyclic GMP.

RESULTS

In women with mild preeclampsia, the triglyceride concentration was significantly higher (p < 0.05) than in normotensive women (3.30 +/- 1.38 versus 2.31 +/- 0.92 g/L) and associated with a higher (p < 0.01) triglyceride/Apo E ratio (0.71; range = 0.40-1.70). In women with severe preeclampsia, the triglyceride/Apo E ratio was similar to normotensive women [0.39 (range = 0.18-1.19) versus 0.41 (range = 0.18-0.79)] associated with a normal triglyceride level and a twofold higher serum nitrate level [36 (range = 1-63 mumol/L) versus 14 (range = 1-37 mumol/L)].

CONCLUSION

The triglyceride/Apo E ratio is significantly higher in mild preeclampsia. In the severe form, this ratio is similar to that of normotensive pregnant women, probably due to a better uptake of triglyceride. Moreover, in the severe form, it is associated with a twofold normal serum nitrate level. Thus, Apo E and the nitric oxide status may be implicated in preeclampsia.

Potentiation of the hypotensive effect of adrenomedullin in pregnant rats

The hypotensive effect of adrenomedullin, a potent vasodilator peptide, was examined in conscious pregnant (6, 13 and 20 days of pregnancy) and non-pregnant rats. The intravenous administration of adrenomedullin (0.01-3.0 nmol/kg) produced a dose-dependent depressor response in pregnant and non-pregnant rats. At low doses (0.01-0.1 nmol/kg), the maximum decrease in blood pressure was significantly higher in pregnant rats (20 days pregnant) than in non-pregnant rats. At high doses, no significant difference was observed between the two groups. Furthermore, the administration of adrenomedullin did not significantly affect the basal mean blood pressure (MBP) at any dose when compared to the non-pregnant group at 6 and 13 days of pregnancy. In the ovariectomized rats, the depressor responses in 17beta-estradiol-treated, progesterone-treated and 17beta-estradiol+progesterone-treated rats were not significantly different from that in the control rats, suggesting that the augmented effect on the depressor response to adrenomedullin in pregnant rats may not be due to hormonal changes during pregnancy. The adrenomedullin receptor mRNA level of the descending thoracic aorta was significantly higher in the late-pregnancy rats (20 days of pregnancy). However, the levels did not show any difference between the early-pregnant rats (6 and 13 days of pregnancy) and the non-pregnant rats. These findings suggested that the changes in the depressor response to adrenomedullin which occur at term in pregnant rats may be mediated by changes of adrenomedullin receptor gene expression rather than by sex hormones.

Ca(2+)-insensitive vascular protein kinase C during pregnancy and NOS inhibition

Pregnancy-induced hypertension is associated with increased vascular resistance; however, the cellular mechanisms involved are unclear. We have previously found that the relation between Ca(2+) entry and the developed force in vascular smooth muscle is altered during normal pregnancy and in a rat model of pregnancy-induced hypertension produced by long-term treatment with the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). The purpose of this study was to investigate whether the pregnancy-associated changes in the vascular Ca(2+) entry-force relation reflect changes in the amount and/or activity of Ca(2+)-insensitive protein kinase C (PKC) isoforms. Active stress and the amount and activity of PKC were measured in deendothelialized aortic strips from nonpregnant and pregnant rats untreated or treated with L-NAME and incubated in Ca(2+)-free (2 mmol/L EGTA) Krebs solution. In nonpregnant rats, the PKC activator phorbol 12,13-dibutyrate (PDBu, 10(-6) mol/L) and the alpha-adrenergic agonist phenylephrine (Phe, 10(-5) mol/L) caused significant, maintained increases in active stress and PKC activity that were inhibited by the PKC inhibitors staurosporine and calphostin C. Western blots in aortic strips of nonpregnant rats revealed the Ca(2+)-insensitive delta-PKC and zeta-PKC isoforms. Both PDBu and Phe caused translocation of delta-PKC from the cytosolic to the particulate fraction. Compared with nonpregnant rats, the amount of delta-PKC and zeta-PKC and the PDBu-stimulated and Phe-stimulated stress, PKC activity and translocation of delta-PKC were significantly reduced in late pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. The PDBu-induced and Phe-induced responses in nonpregnant rats treated with L-NAME were not significantly different from nonpregnant rats, whereas the responses in pregnant rats treated with L-NAME+L-arginine were not significantly different from pregnant rats. These results provide evidence that a signaling pathway in vascular smooth muscle possibly involving the Ca(2+)-insensitive delta-PKC and zeta-PKC isoforms is reduced in late pregnancy and enhanced during long-term inhibition of nitric oxide synthesis. The changes in the amount and activity of vascular PKC isoforms may, in part, explain the changes in vascular resistance during normal pregnancy and pregnancy-induced hypertension.

Nitric oxide synthase activity and Doppler parameters in the fetoplacental and uteroplacental circulation in preeclampsia

OBJECTIVE

We investigated the hypothesis that changes in blood flow in the uteroplacental and fetoplacental circulation in preeclampsia are associated with an abnormality of placental or uterine placental bed nitric oxide (NO) synthesis.

METHODS

We measured pulsatility indices on Doppler waveform analysis from uterine and umbilical arteries in 20 patients with preeclampsia and 14 healthy pregnant controls before elective cesarean section. During cesarean section, biopsies from the uterine placental bed and the placenta were taken and the nitric oxide synthase (NOS) activity was measured by the [3H] L-arginine-[3H] L-citrulline conversion assay in these samples.

RESULTS

The NOS activity was significantly lower in the uterine placental bed in comparison to the placental tissue (p < 0.01). Placental NOS activity was similar between patients with preeclampsia and healthy controls and in the groups with either a pathological or a normal Doppler flow in the umbilical artery. In the uterine placental bed however, NOS activity from patients with preeclampsia was significantly lower (p < 0.01), whereas the blood flow resistance in the uterine arteries was elevated (p < 0.01) in comparison to healthy controls.

CONCLUSIONS

Our data show that pathological Doppler waveforms in the uterine arteries of patients with preeclampsia are paralleled by diminished NOS activity in the uterine placental bed. Therefore, the compromised NO production in the uterine placental bed may play an important role in the impaired uteroplacental blood flow and potentially in some pathological features of preeclampsia such as intervillous thrombosis formation and fetal growth retardation.

Stimulated mechanisms of Ca2+ entry into vascular smooth muscle during NO synthesis inhibition in pregnant rats

We have previously found that the vascular responsiveness to alpha1-adrenergic agonists is reduced in pregnant rats and enhanced in a rat model of pregnancy-induced hypertension produced by chronic treatment of pregnant rats with the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). The purpose of this study was to investigate whether the observed changes in vascular reactivity during normal pregnancy and during pregnancy-induced hypertension reflect changes in the mechanisms of Ca2+ entry into vascular smooth muscle. 45Ca2+ influx and active stress during alpha1-adrenergic stimulation by phenylephrine and membrane depolarization by 96 mM KCl were measured in deendothelialized aortic strips isolated from virgin and pregnant Sprague-Dawley rats untreated or treated with 1 mg/day L-NAME for 4-6 days and incubated in Krebs solution containing increasing concentrations of extracellular Ca2+ ([Ca2+]e). In all groups of rats, both phenylephrine and 96 mM KCl caused [Ca2+]e-dependent increases in active stress and 45Ca2+ influx. The phenylephrine- and 96 mM KCl-induced active stress and Ca2+ influx were significantly reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. The phenylephrine-induced Ca2+ influx-stress relationship was significantly greater than that induced by 96 mM KCl in pregnant rats treated with L-NAME. The phenylephrine-induced Ca2+ influx-stress relationship was reduced in pregnant rats but enhanced in pregnant rats treated with L-NAME. Chronic treatment with L-NAME had minimal effect on active stress, Ca2+ influx, and the Ca2+ influx-stress relationship in virgin rats. These results provide evidence that the mechanisms of Ca2+ entry into vascular smooth muscle are inhibited during pregnancy but enhanced during inhibition of NO synthesis in late pregnancy. The enhancement of the phenylephrine-induced Ca2+ influx-stress relationship in pregnant rats treated with L-NAME suggests activation of other contractile mechanisms in addition to stimulation of Ca2+ entry. These mechanisms appear to be inhibited during normal pregnancy.

Differential expression of renal nitric oxide synthase isoforms during pregnancy in rats

Alterations in nitric oxide (NO) production have been suggested to play a role in mediating changes in renal function during normal pregnancy and in pregnancy-induced hypertension. Although NO production is enhanced during normal pregnancy, the mechanisms for the increase are unknown. The purpose of this study was to determine whether the elevation in NO production during pregnancy is associated with increases in renal expression of endothelial (eNOS), inducible (iNOS), and neuronal (nNOS) nitric oxide synthases. To achieve this goal we examined systemic and renal hemodynamics, urinary excretion of nitrate/nitrite, and renal protein expression of the three NOS isoforms in prepregnant rats, pregnant rats at days 6, 13, and 19 of gestation and at day 4 postpartum. Mean arterial pressure decreased by 14% in late pregnancy whereas the glomerular filtration rate and renal plasma flow increased by 21% and 24%, respectively, in mid pregnancy. Excretion of nitrate/nitrite increased throughout pregnancy with a 3.4-fold increase present at day 19 (12.2+/-0.7 to 41.1+/-1.3 micromol/24 h). Renal eNOS protein expression decreased by 39% during pregnancy with the lowest level resulting at day 19 and returning to virgin levels by day 4 post partum. In contrast, renal iNOS and nNOS protein expression increased 31% and 25%, respectively, with highest expression occurring for both at day 13 of pregnancy. These data suggest that the increased NO production and renal hemodynamics associated with pregnancy in rats may be caused by the upregulation of iNOS and nNOS in the kidney.

Renal cortical Na+-K+-ATPase activity and abundance is decreased in normal pregnant rats

During late pregnancy, the rat undergoes massive plasma volume expansion due to cumulative renal sodium retention. In the present study, conducted in virgin, mid- (days 11-13), and late-pregnant (days 18-20) rats, we measured both Na+-K+-ATPase activity (by coupled enzyme assay) and abundance of the alpha-subunits of the Na+-K+-ATPase (by Western and slot blot analyses) in renal cortex, medulla, and brain stem. Unexpectedly, Na+-K+-ATPase in renal cortex, in both stages of pregnancy, is reduced versus the virgin, consistent with our finding of a reduced quantity of the alpha1-subunit. In renal medulla, there is a small rise in activity at midterm, but there is no difference in either activity or abundance of the alpha1-subunit in late pregnancy, when renal Na retention is maximal. In brain stem, where only alpha2- and alpha3-subunits are evident, pregnancy has no impact on enzyme activity or abundance of either isoform. In conclusion, the outcome of these experiments was unexpected in that we did not observe increased renal Na+-K+-ATPase activity in late pregnancy in the rat. In fact, in renal cortex, Na+-K+-ATPase activity and abundance are reduced. Whatever promotes net sodium retention in pregnancy must be capable of overwhelming this and several other strong natriuretic signals.

Effects of transdermal nitroglycerin on impedance to flow in the uterine, umbilical, and fetal middle cerebral arteries in pregnancies complicated by preeclampsia and intrauterine growth retardation

OBJECTIVE

We studied whether transdermal nitroglycerin, a donor of nitric oxide, affects uterine, umbilical, and fetal cerebral blood flow in pregnancies complicated by preeclampsia and impaired uteroplacental blood flow.

STUDY DESIGN

Seventeen patients with preeclampsia were treated with a nitroglycerin patch, 10 mg per 24 hours, for three consecutive days between 28 and 36 weeks' gestation. The uterine, umbilical, and fetal middle cerebral artery pulsatility index and resistance index were assessed by color Doppler ultrasonography before the start of treatment, daily during treatment, and on the first 2 days after the removal of the last patch.

RESULTS

The nitroglycerin patch caused a significant fall in the mean uterine pulsatility index and resistance index that reached its maximum (18% +/- 4% and 17% +/- 3%, respectively, from baseline) on the last treatment day. After the removal of the last patch, uterine pulsatility index and resistance index rose to the pretreatment value within 12 hours. No significant changes in umbilical or middle cerebral artery pulsatility index and resistance index were observed during treatment. Maternal mean arterial pressure fell from 122 +/- 8 to 117 +/- 7 mm Hg (P = .05).

CONCLUSION

Transdermal administration of nitroglycerin may offer a potential for treatment for patients with preeclampsia who have increased uteroplacental impedance.

Enhanced vascular reactivity during inhibition of nitric oxide synthesis in pregnant rats

Pregnancy-induced hypertension has been suggested to be mediated by several mechanisms, including reduced nitric oxide (NO) synthesis. In this study, the effects of chronic treatment with the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) on blood pressure and the underlying changes in vascular reactivity were investigated in virgin and late-pregnancy Sprague-Dawley rats. The systolic blood pressure was 120+/-2, 124+/-5, 116+/-4, and 171+/-5 mm Hg in untreated virgin, virgin treated with L-NAME, untreated pregnant, and pregnant treated with L-NAME rats, respectively. The rats were killed, and the thoracic aorta was cut into strips for measurement of active stress in response to alpha1-adrenergic stimulation with phenylephrine and membrane depolarization by high KCl. In pregnant rats, the maximal active stress to phenylephrine (0.31+/-0.03 x 10(4) N/m2) and the high-KCl-induced active stress (0.55+/-0.09 x 10(4) N/m2) were smaller than those in virgin rats. By contrast, in the L-NAME-treated pregnant rats, the maximal phenylephrine-induced active stress (0.76+/-0.1 x 10(4) N/m2) was greater than that in virgin rats (0.52+/-0.1 x 10(4) N/m2), whereas the high-KCl-induced active stress (1.08+/-0.14 x 10(4) N/m2) was indistinguishable from that in virgin rats (1.03+/-0.14 x 10(4) N/m2). Treatment with L-NAME did not affect the phenylephrine-releasable Ca2+ stores in pregnant rats and had minimal effect on active stress in virgin rats. Thus, reduction of NO synthesis during late pregnancy is associated with a significant increase in blood pressure and vascular responsiveness to alpha-adrenergic stimulation, which can possibly be explained in part by enhanced Ca2+ entry from extracellular space. However, other mechanisms such as increased myofilament force sensitivity to Ca2+ and/or activation of a completely Ca2+-independent mechanism cannot be excluded.

Reduced sensitivity of the renal circulation to angiotensin II in pregnant rats

The renal circulation undergoes significant changes during pregnancy and pregnancy-induced hypertension. Although numerous studies indicate that the pressor response to angiotensin II (Ang II) is reduced during pregnancy, it is unclear as to whether this altered sensitivity to Ang II occurs in the renal circulation. The first aim of this study was to determine whether the renal vascular responsiveness to exogenous Ang II is altered in the midterm pregnant rat. All rats were pretreated with an intravenous infusion of the converting-enzyme inhibitor captopril (20 microg x kg(-1) x min(-1)) to block endogenous Ang II formation. Following a control period, Ang II was infused at a dose of 10 ng x kg(-1) x min(-1) for 50 minutes into the renal arteries via a suprarenal aortic catheter. In anesthetized virgin rats, Ang II markedly decreased renal plasma flow (RPF) by 39% (5.0+/-0.4 to 3.1+/-0.4 mL/min), glomerular filtration rate (GFR) by 39% (1.9+/-0.1 to 1.16+/-0.2 mL/min), and urine flow by 47% (22.1+/-5.6 to 12.3+/-4.8 microL/min). In contrast, Ang II had no significant effect on RPF, GFR, and urine flow in the anesthetized pregnant rats. Since nitric oxide (NO) has been previously reported to modulate the renal vascular actions of Ang II in normal animals and NO synthesis is thought to be elevated in pregnancy, this study examined the role of NO in the attenuated renal response to Ang II. In pregnant rats pretreated with L-NAME, the arterial pressure was higher and RPF was lower than in the control pregnant rats. However, the renal response to Ang II in the L-NAME-pretreated pregnant rats was similar to control pregnant rats. These data indicate that the renal circulation has a reduced sensitivity to Ang II during pregnancy. We also found that NO synthesis inhibition does not alter the attenuated renal response to Ang II in the anesthetized pregnant rats.