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

Guidelines for assessing maternal cardiovascular physiology during pregnancy and postpartum

Maternal mortality rates are at an all-time high across the world and are set to increase in subsequent years. Cardiovascular disease is the leading cause of death during pregnancy and postpartum, especially in the United States. Therefore, understanding the physiological changes in the cardiovascular system during normal pregnancy is necessary to understand disease-related pathology. Significant systemic and cardiovascular physiological changes occur during pregnancy that are essential for supporting the maternal-fetal dyad. The physiological impact of pregnancy on the cardiovascular system has been examined in both experimental animal models and in humans. However, there is a continued need in this field of study to provide increased rigor and reproducibility. Therefore, these guidelines aim to provide information regarding best practices and recommendations to accurately and rigorously measure cardiovascular physiology during normal and cardiovascular disease-complicated pregnancies in human and animal models.

Growth Restriction in Preeclampsia: Lessons from Animal Models

Preeclampsia remains a major health concern for mother and child. Yet, treatment options remain limited to early delivery. Placental dysfunction in preeclampsia occurs in response to an increase in oxidative stress and inflammatory cytokines with vasoactive and anti-angiogenic factors contributing to impaired maternal and fetal health. Moreover, recent studies indicate a potential role for epigenetic mediators in the pathophysiology of placental ischemia. Numerous animal models are utilized to explore the pathogenesis of preeclampsia and fetal growth restriction. This review provides a brief overview of recent progress in preclinical studies regarding potential therapeutic targets for the treatment and prevention of preeclampsia with an emphasis on fetal growth restriction and the fetal programming of increased cardiovascular risk.

Glomerular hyperfiltration

Circulating blood is filtered across the glomerular barrier to form an ultrafiltrate of plasma in the Bowman's space. The volume of glomerular filtration adjusted by time is defined as the glomerular filtration rate (GFR), and the total GFR is the sum of all single-nephron GFRs. Thus, when the single-nephron GFR is increased in the context of a normal number of functioning nephrons, single glomerular hyperfiltration results in 'absolute' hyperfiltration in the kidney. 'Absolute' hyperfiltration can occur in healthy people after high protein intake, during pregnancy and in patients with diabetes, obesity or autosomal-dominant polycystic kidney disease. When the number of functioning nephrons is reduced, single-nephron glomerular hyperfiltration can result in a GFR that is within or below the normal range. This 'relative' hyperfiltration can occur in patients with a congenitally reduced nephron number or with an acquired reduction in nephron mass consequent to surgery or kidney disease. Improved understanding of the mechanisms that underlie 'absolute' and 'relative' glomerular hyperfiltration in different clinical settings, and of whether and how the single-nephron haemodynamic and related biomechanical forces that underlie glomerular hyperfiltration promote glomerular injury, will pave the way toward the development of novel therapeutic interventions that attenuate glomerular hyperfiltration and potentially prevent or limit consequent progressive kidney injury and loss of function.

Primary Aldosteronism and Pregnancy

OBJECTIVE

Primary aldosteronism (PA) may present at younger age and may thus complicate pregnancy. Our aim was to identify female patients in whom PA was diagnosed after pregnancy complicated with hypertension and to analyze possible hypertension-related complications during pregnancy.

METHODS

We performed retrospective analysis of female patients with PA diagnosed and treated at our Department who were pregnant before the diagnosis of PA.

RESULTS

We found 14 patients with PA (age at diagnosis 32.2 ± 4.2 years, hypertension duration 5.4 ± 3.6 years) suffering from hypertension 3 (IQR 0, 4) years before pregnancy (6 patients had hypertension diagnosed during pregnancy). Three subjects were pregnant twice, and 1 patient had been pregnant three times before the final diagnosis of PA was made. Ten subjects delivered by Caesarean section (in 3 cases due to early-onset preeclampsia and 2 subjects due to significantly increased blood pressure), and 9 cases spontaneously (1 subject complicated twice due to late-onset preeclampsia). Preterm delivery occurred in 5 cases - the earliest one in the sixth month of gestation. Subsequent diagnosis of PA (sometimes with a long delay up to a maximum of 12 years) was made on the basis of significantly low potassium values (2.7 ± 0.4 mmol/L; 2 subjects even suffered from muscle cramps) and hypertension (mostly moderate), elevated plasma/serum aldosterone (54.1 ± 20.2 ng/dL) and suppressed plasma renin activity (0.4 ± 0.2 ng/mL/h) or plasma renin (1.9 ± 1.6 ng/L). Thirteen subjects underwent laparoscopic adrenalectomy (in all but 2 cases, diagnosis of a large cortical adenoma [16 ± 5.9 mm] was made), and 1 subject was classified with bilateral hyperplasia according to adrenal venous sampling. Operation normalized BP in 10 subjects and improved BP control in the remaining 3 subjects. Two patients became pregnant after adrenalectomy, and their pregnancies were uneventful.

CONCLUSION

PA is associated with a high rate of pregnancy-related complications. The most frequent complication is preeclampsia, in some cases leading to preterm delivery. The optimal prevention of these complications is early diagnosis of PA, and in these particular hypertensive cases, the awareness of hypokalemia.

Animal models of preeclampsia: translational failings and why

Preeclampsia affects up to 8% of pregnancies worldwide and is a leading cause of both maternal and fetal morbidity and mortality. Our current understanding of the cause(s) of preeclampsia is far from complete, and the lack of a single reliable animal model that recapitulates all aspects of the disease further confounds our understanding. This is partially due to the heterogeneous nature of the disease, coupled with our evolving understanding of its etiology. Nevertheless, animal models are still highly relevant and useful tools that help us better understand the pathophysiology of specific aspects of preeclampsia. This review summarizes the various types and characteristics of animal models used to study preeclampsia, highlighting particular features of these models relevant to clinical translation. This review points out the strengths and limitations of these models to illustrate the importance of using the appropriate model depending on the research question.

Relaxin treatment reduces angiotensin II-induced vasoconstriction in pregnancy and protects against endothelial dysfunction†

The peptide relaxin has gained considerable attention as a new vasoactive drug, largely through its beneficial therapeutic effects in cardiovascular disease. In this study, we tested the hypothesis that relaxin treatment alleviates systemic vascular dysfunction characteristic of hypertensive diseases of pregnancy. We investigated vascular effects and mechanisms of relaxin action in (i) pregnant relaxin-deficient (Rln-/-) mice with enhanced responses to angiotensin II (AngII) and (ii) arteries pre-incubated ex vivo in trophoblast conditioned media (TCM) to induce endothelial dysfunction. Pregnant Rln-/- mice received 0.5 μg/h recombinant human H2 relaxin (rhRLX: n = 5) or placebo (20 nM sodium acetate; n = 7) subcutaneously via osmotic minipumps for 5 days prior to gestational day 17.5. This treatment protocol significantly reduced AngII-mediated contraction of mesenteric arteries and increased plasma 6-keto prostaglandin F1α. These vascular effects were endothelium independent and likely involve smooth muscle-derived vasodilator prostanoids. In the second study, mesenteric arteries were incubated ex vivo for 24 h at 37°C in TCM, which contained high levels of soluble Flt-1 (>20 ng/ml) and soluble Eng (>1 ng/ml). TCM incubation caused significant reduction in endothelium-dependent relaxation and increased sensitivity to AngII. Co-incubation of arteries with rhRLX for 24 h (n = 6-16/treatment) prevented endothelial dysfunction but had no effect on AngII-mediated contraction. In conclusion, relaxin treatment prevents and/or reverses vascular dysfunction in mesenteric arteries, but acts through different vascular pathways depending on duration of relaxin treatment and type of vascular dysfunction. Overall, our data suggest that relaxin is a potential therapeutic to alleviate maternal systemic vascular dysfunction associated with hypertensive diseases in pregnant women.

Preeclampsia and the brain: neural control of cardiovascular changes during pregnancy and neurological outcomes of preeclampsia

Preeclampsia (PE) is a form of gestational hypertension that complicates ∼5% of pregnancies worldwide. Over 70% of the fatal cases of PE are attributed to cerebral oedema, intracranial haemorrhage and eclampsia. The aetiology of PE originates from abnormal remodelling of the maternal spiral arteries, creating an ischaemic placenta that releases factors that drive the pathophysiology. An initial neurological outcome of PE is the absence of the autonomically regulated cardiovascular adaptations to pregnancy. PE patients exhibit sympathetic overactivation, in comparison with both normotensive pregnant and hypertensive non-pregnant females. Moreover, PE diminishes baroreceptor reflex sensitivity (BRS) beyond that observed in healthy pregnancy. The absence of the cardiovascular adaptations to pregnancy, combined with sympathovagal imbalance and a blunted BRS leads to life-threatening neurological outcomes. Behaviourally, the increased incidences of maternal depression, anxiety and post-traumatic stress disorder (PTSD) in PE are correlated to low fetal birth weight, intrauterine growth restriction (IUGR) and premature birth. This review addresses these neurological consequences of PE that present in the gravid female both during and after the index pregnancy.

Epoxyeicosatrienoic Acid Inhibition Alters Renal Hemodynamics During Pregnancy

In this study we examined the expression of cytochrome P450 (CYP) 2C and CYP2J Isoforms in renal proximal tubules and microvessels isolated from rats at different stages of pregnancy. We also selectively inhibited epoxyeicosatrienoic acid (EET) production by the administration of N-methanesulfonyl-6-(2-proparyloxyphenyl)hexanamide (MSPPOH 20 mg/kg/day iv) to rats during Days 14–17 of gestation and to age-matched virgin rats and determined the consequent effects on renal function. Western blot analysis showed that CYP2C11, CYP2C23, and CYP2J2 expression was significantly increased in the renal microvessels of pregnant rats on Day 12 of gestation. In the proximal tubules, CYP2C23 expression was significantly increased throughout pregnancy, while the expression of CYP2C11 was increased in early and late pregnancy and the expression of CYP2J2 was increased in middle and late pregnancy. MSPPOH treatment significantly Increased pregnant rats’ mean arterial pressure, renal vascular resistance, and sodium balance but significantly decreased renal blood flow, glomerular filtration rate, and urinary sodium excretion, as well as fetal pups’ body weight and length. In contrast, MSPPOH treatment had no effect on renal hemodynamics or urinary sodium excretion in age-matched virgin rats. In pregnant rats, MSPPOH treatment also caused selective inhibition of renal cortical EET production and significantly decreased the expression of CYP2C11, CYP2C23, and CYP2J2 in the renal cortex, renal microvessels, and proximal tubules. These results suggest that upregulation of renal vascular and tubular EETs contributes to the control of blood pressure and renal function during pregnancy.

The enigma of continual plasma volume expansion in pregnancy: critical role of the renin-angiotensin-aldosterone system

Pregnancy is characterized by avid renal sodium retention and plasma volume expansion in the presence of decreased blood pressure. Decreased maternal blood pressure is a consequence of reduced systemic vascular tone, which results from an increased production of vasodilators [nitric oxide (NO), prostaglandins, and relaxin] and decreased vascular responsiveness to the potent vasoconstrictor (angiotensin II). The kidneys participate in this vasodilatory response, resulting in marked increases in renal plasma flow and glomerular filtration rate (GFR) during pregnancy. In women, sodium retention drives plasma volume expansion (∼40%) and is necessary for perfusion of the growing uterus and fetus. For there to be avid sodium retention in the presence of the potent natriuretic influences of increased NO and elevated GFR, there must be modifications of the tubules to prevent salt wasting. The purpose of this review is to summarize these adaptations.

Agonistic Autoantibodies to the Angiotensin II Type 1 Receptor Enhance Angiotensin II-Induced Renal Vascular Sensitivity and Reduce Renal Function During Pregnancy

Preeclamptic women produce agonistic autoantibodies to the angiotensin II type 1 receptor (AT1-AA) and exhibit increased blood pressure (mean arterial pressure), vascular sensitivity to angiotensin II (ANG II), and display a decrease in renal function. The objective of this study was to examine the renal hemodynamic changes during pregnancy in the presence of AT1-AAs with or without a slow pressor dose of ANG II. In this study, mean arterial pressure was elevated in all pregnant rats treated with ANG II with or without AT1-AA. Glomerular filtration rate was reduced from 1.90±0.16 mL/min in normal pregnant (NP) to 1.20±0.08 in ANG II+AT1-AA rats. Renal blood flow was decreased in ANG II+AT1-AA versus NP rats to 7.4±1.09 versus 15.4±1.75 mL/min. Renal vascular resistance was drastically increased between ANG II+AT1-AA versus NP rats (18.4±2.91 versus 6.4±0.77 mm Hg/mL per minute). Isoprostane excretion was increased by 3.5-fold in ANG II+AT1-AA versus NP (1160±321 versus 323±52 pg/mL). In conclusion, ANG II and AT1-AA together significantly decrease glomerular filtration rate by 37% and renal blood flow by 50% and caused a 3-fold increase in renal vascular resistance and isoprostane levels versus NP rats. These data indicate the importance of AT1-AAs to enhance ANG II-induced renal vasoconstriction and reduce renal function as mechanisms to cause hypertension as observed during preeclampsia.

Vascular actions of relaxin: nitric oxide and beyond

The peptide hormone relaxin regulates the essential maternal haemodynamic adaptations in early pregnancy through direct actions on the renal and systemic vasculature. These vascular actions of relaxin occur mainly through endothelium-derived NO-mediated vasodilator pathways and improvements in arterial compliance in small resistance-size arteries. This work catalysed a plethora of studies which revealed quite heterogeneous responses across the different regions of the vasculature, and also uncovered NO-independent mechanisms of relaxin action. In this review, we first describe the role of endogenous relaxin in maintaining normal vascular function, largely referring to work in pregnant and male relaxin-deficient animals. We then discuss the diversity of mechanisms mediating relaxin action in different vascular beds, including the involvement of prostanoids, VEGF, endothelium-derived hyperpolarisation and antioxidant activity in addition to the classic NO-mediated vasodilatory pathway. We conclude the review with current perspectives on the vascular remodelling capabilities of relaxin.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

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.

Relaxin deficiency attenuates pregnancy-induced adaptation of the mesenteric artery to angiotensin II in mice

Pregnancy is associated with reduced peripheral vascular resistance, underpinned by changes in endothelial and smooth muscle function. Failure of the maternal vasculature to adapt correctly leads to serious pregnancy complications, such as preeclampsia. The peptide hormone relaxin regulates the maternal renal vasculature during pregnancy; however, little is known about its effects in other vascular beds. This study tested the hypothesis that functional adaptation of the mesenteric and uterine arteries during pregnancy will be compromised in relaxin-deficient ( Rln−/−) mice. Smooth muscle and endothelial reactivity were examined in small mesenteric and uterine arteries of nonpregnant (estrus) and late-pregnant ( day 17.5) wild-type ( Rln+/+) and Rln−/− mice using wire myography. Pregnancy per se was associated with significant reductions in contraction to phenylephrine, endothelin-1, and ANG II in small mesenteric arteries, while sensitivity to endothelin-1 was reduced in uterine arteries of Rln+/+ mice. The normal pregnancy-associated attenuation of ANG II-mediated vasoconstriction in mesenteric arteries did not occur in Rln−/− mice. This adaptive failure was endothelium-independent and did not result from altered expression of ANG II receptors or regulator of G protein signaling 5 ( Rgs5) or increases in reactive oxygen species generation. Inhibition of nitric oxide synthase with l-NAME enhanced ANG II-mediated contraction in mesenteric arteries of both genotypes, whereas blockade of prostanoid production with indomethacin only increased ANG II-induced contraction in arteries of pregnant Rln+/+ mice. In conclusion, relaxin deficiency prevents the normal pregnancy-induced attenuation of ANG II-mediated vasoconstriction in small mesenteric arteries. This is associated with reduced smooth muscle-derived vasodilator prostanoids.

Functional vascular changes of the kidney during pregnancy in animals: a systematic review and meta-analysis

Renal vascular responses to pregnancy have frequently been studied, by investigating renal vascular resistance (RVR), renal flow, glomerular filtration rate (GFR), and renal artery responses to stimuli. Nonetheless, several questions remain: 1. Which vasodilator pathways are activated and to what extent do they affect RVR, renal flow and GFR across species, strains and gestational ages, 2. Are these changes dependent on renal artery adaptation, 3. At which cellular level does pregnancy affect the involved pathways? In an attempt to answer the questions raised, we performed a systematic review and meta-analysis on animal data. We included 37 studies (116 responses). At mid-gestation, RVR and GFR change to a similar degree across species and strains, accompanied by variable change in renal flow. At least in rats, changes depend on NO activation. At late gestation, changes in RVR, renal flow and GFR vary between species and strains. In rats, these changes are effectuated by sympathetic stimulation. Overall, renal artery responsiveness to stimuli is unaffected by pregnancy, except for Sprague Dawley rats in which pregnancy enhances renal artery vascular compliance and reduces renal artery myogenic reactivity. Our meta-analysis shows that: 1. Pregnancy changes RVR, renal flow and GFR dependent on NO-activation and sympathetic de-activation, but adjustments are different among species, strains and gestational ages; 2. These changes do not depend on adaptation of renal artery responsiveness; 3. It remains unknown at which cellular level pregnancy affects the pathways. Our meta-analysis suggests that renal changes during pregnancy in animals are qualitatively similar, even in comparison to humans, but quantitatively different.

The renal circulation in normal pregnancy and preeclampsia: is there a place for relaxin?

During the first trimester of human pregnancy, the maternal systemic circulation undergoes remarkable vasodilation. The kidneys participate in this vasodilatory response resulting in marked increases in renal plasma flow (RPF) and glomerular filtration rate (GFR). Comparable circulatory adaptations are observed in conscious gravid rats. Administration of the corpus luteal hormone relaxin (RLN) to nonpregnant rats and humans elicits vasodilatory changes like those of pregnancy. Systemic and renal vasodilation are compromised in midterm pregnant rats by neutralization or elimination of circulating RLN and in women conceiving with donor eggs who lack a corpus luteum and circulating RLN. Although RLN exerts both rapid (minutes) and sustained (hours to days) vasodilatory actions through different molecular mechanisms, a final common pathway is endothelial nitric oxide. In preeclampsia (PE), maternal systemic and renal vasoconstriction leads to hypertension and modest reduction in GFR exceeding that of RPF. Elevated level of circulating soluble vascular endothelial growth factor receptor-1 arising from the placenta is implicated in the hypertension and disruption of glomerular fenestrae and barrier function, the former causing reduced Kf and the latter proteinuria. Additional pathogenic factors are discussed. Last, potential clinical ramifications include RLN replacement in women conceiving with donor eggs and its therapeutic use in PE. Another goal has been to apply knowledge gained from investigating circulatory adaptations in pregnancy toward identifying and developing novel therapeutic strategies for renal and cardiovascular disease in the nonpregnant population. So far, one candidate to emerge is RLN and its potential therapeutic use in heart failure.

Chronic vasodilation increases renal medullary PDE5A and α-ENaC through independent renin-angiotensin-aldosterone system pathways

We have previously observed that many of the renal and hemodynamic adaptations seen in normal pregnancy can be induced in virgin female rats by chronic systemic vasodilation. Fourteen-day vasodilation with sodium nitrite or nifedipine (NIF) produced plasma volume expansion (PVE), hemodilution, and increased renal medullary phosphodiesterase 5A (PDE5A) protein. The present study examined the role of the renin-angiotensin-aldosterone system (RAAS) in this mechanism. Virgin females were treated for 14 days with NIF (10 mg·kg(-1)·day(-1) via diet), NIF with spironolactone [SPR; mineralocorticoid receptor (MR) blocker, 200-300 mg·kg(-1)·day(-1) via diet], NIF with losartan [LOS; angiotensin type 1 (AT1) receptor blocker, 20 mg·kg(-1)·day(-1) via diet], enalapril (ENAL; angiotensin-converting enzyme inhibitor, 62.5 mg/l via water), or vehicle (CON). Mean arterial pressure (MAP) was reduced 7.4 ± 0.5% with NIF, 6.33 ± 0.5% with NIF + SPR, 13.3 ± 0.9% with NIF + LOS, and 12.0 ± 0.4% with ENAL vs. baseline MAP. Compared with CON (3.6 ± 0.3%), plasma volume factored for body weight was increased by NIF (5.2 ± 0.4%) treatment but not by NIF + SPR (4.3 ± 0.3%), NIF + LOS (3.6 ± 0.1%), or ENAL (4.0 ± 0.3%). NIF increased PDE5A protein abundance in the renal inner medulla, and SPR did not prevent this increase (188 ± 16 and 204 ± 22% of CON, respectively). NIF increased the α-subunit of the epithelial sodium channel (α-ENaC) protein in renal outer (365 ± 44%) and inner (526 ± 83%) medulla, and SPR prevented these changes. There was no change in either PDE5A or α-ENaC abundance vs. CON in rats treated with NIF + LOS or ENAL. These data indicate that the PVE and renal medullary adaptations in response to chronic vasodilation result from RAAS signaling, with increases in PDE5A mediated through AT1 receptor and α-ENaC through the MR.

Effects of relaxin on arterial dilation, remodeling, and mechanical properties

Administering relaxin to conscious rats and humans elicits systemic and renal vasodilation. The molecular mechanisms vary according to the duration of relaxin exposure-so-called "rapid" (within minutes) or "sustained" (hours to days) vasodilatory responses-both being endothelium-dependent. Rapid responses are mediated by G(αi/o) protein coupling to phosphoinositol-3 kinase/Akt (protein kinase B)-dependent phosphorylation and activation of nitric oxide synthase. Sustained responses are mediated by vascular endothelial and placental growth factors, as well as increases in arterial gelatinase activity. Thus, after hours or days of relaxin treatment, respectively, arterial MMP-9 or MMP-2 hydrolyze "big" endothelin (ET) at a gly-leu bond to form ET(1-32), which in turn activates the endothelial ET(B) receptor/nitric oxide vasodilatory pathway. Administration of relaxin to conscious rats also increases global systemic arterial compliance and passive compliance of select isolated blood vessels such as small renal arteries (SRA). The increase in SRA passive compliance is mediated by both geometric remodeling (outward) and compositional remodeling (decreased collagen). Relaxin-induced geometric remodeling has also been observed in brain parenchymal arteries, and this remodeling appears to be via the activation of peroxisome proliferator-activated receptor-γ. Given the vasodilatory and arterial remodeling properties of relaxin, the hormone may have therapeutic potential in the settings of abnormal pregnancies, heart failure, and pathologies associated with stiffening of arteries.

Impaired vascular responses to relaxin in diet-induced overweight female rats

Relaxin mediates renal and mesenteric vascular adaptations to pregnancy by increasing endothelium-dependent vasodilation and compliance and decreasing myogenic reactivity. Diet-induced overweight and obesity are associated with impaired endothelial dysfunction and vascular remodeling leading to a reduction in arterial diameter. In this study, we tested the hypothesis that local vascular responses to relaxin are impaired in diet-induced overweight female rats on a high-fat cafeteria-style diet for 9 wk. Rats were chronically infused with either relaxin or placebo for 5 days, and vascular responses were measured in isolated mesenteric arteries and the perfused kidney. Diet-induced overweight significantly increased sensitivity to phenylephrine (by 17%) and vessel wall thickness, and reduced renal perfusion flow (RPFF; by 16%), but did not affect flow-mediated vasodilation, myogenic reactivity, and vascular compliance. In the normal weight rats, relaxin treatment significantly enhanced flow-mediated vasodilation (2.67-fold), decreased myogenic reactivity, and reduced sensitivity to phenylephrine (by 28%), but had no effect on compliance or RPFF. NO blockade by l-NAME diminished most relaxin-mediated effects. In diet-induced overweight rats, the vasodilator effects of relaxin were markedly reduced for flow-mediated vasodilation, sensitivity to phenylephrine, and myogenic response compared with the normal diet rats, mostly persistent under l-NAME. Our data demonstrate that some of the vasodilator responses to in vivo relaxin administration are impaired in isolated mesenteric arteries and the perfused kidney in diet-induced overweight female rats. This does not result from a decrease in Rxfp1 (relaxin family peptide receptor) expression but is likely to result from downstream disruption to endothelial-dependent mechanisms in diet-induced overweight animals.

The kidney in normal pregnancy and preeclampsia

Complicating up to 8% of pregnancies, preeclampsia is, in fact, the most common glomerular disease worldwide. In this article, we review the effect of normal pregnancy on the kidney as well as the role of the kidney in preeclampsia. We discuss blood pressure in pregnancy and preeclampsia, followed by the physiology of hyperfiltration in normal pregnancy as well as the pathophysiology of hypofiltration and proteinuria in preeclampsia. Recent studies have suggested that the clinical syndrome of preeclampsia, which recovers rapidly after delivery of the placenta, is caused by impaired vascular endothelial growth factor signaling that disturbs the status of vascular dilatation as well as the symbiosis between the glomerular endothelium and the podocytes. Finally, we discuss the intriguing association between chronic kidney disease (CKD) and preeclampsia. We hypothesize that the imbalance between angiogenic and anti-angiogenic factors, which may be common to both preeclampsia and CKD, might explain why CKD predisposes pregnant women to develop preeclampsia.

Maternal vasodilation in pregnancy: the emerging role of relaxin

Pregnancy is a unique physiological condition of profound maternal renal and systemic vasodilation. Our goal has been to unveil the reproductive hormones mediating this remarkable vasodilatory state and the underlying molecular mechanisms. In addition to advancing our knowledge of pregnancy physiology, reaching this goal may translate into therapeutics for pregnancy pathologies such as preeclampsia and for diseases associated with vasoconstriction and arterial stiffness in nonpregnant women and men. An emerging player is the 6 kDa corpus luteal hormone relaxin, which circulates during pregnancy. Relaxin administration to rats and humans induces systemic and renal vasodilation regardless of sex, thus mimicking the pregnant condition. Immunoneutralization or elimination of the source of circulating relaxin prevents renal and systemic vasodilation in midterm pregnant rats. Infertile women who become pregnant by donor eggs (IVF with embryo transfer) lack a corpus luteum and circulating relaxin, and they show a markedly subdued gestational increase in glomerular filtration rate. These data implicate relaxin as one of the vasodilatory reproductive hormones of pregnancy. There are different molecular mechanisms underlying the so-called rapid and sustained vasodilatory actions of relaxin. The former is mediated by Gα(i/o) protein coupling to phosphatidylinositol-3 kinase/Akt (protein kinase B)-dependent phosphorylation and activation of endothelial nitric oxide synthase, the latter by vascular endothelial and placental growth factors, and increases in arterial gelatinase(s) activity. The gelatinases, in turn, hydrolyze big endothelin (ET) at a gly-leu bond to form ET(1-32), which activates the endothelial ET(B) receptor/nitric oxide vasodilatory pathway.

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.

Cholesterol induces renal vasoconstriction and anti-natriuresis by inhibiting nitric oxide production in anesthetized rats

Although hypercholesterolemia is implicated in the pathophysiology of many renal disorders as well as hypertension, its direct actions in the kidney are not yet clearly understood. In the present study, we evaluated renal responses to administration of cholesterol (8 microg x min(-1).100 g body wt(-1); bound by polyethylene glycol) into the renal artery of anesthetized male Sprague-Dawley rats. Total renal blood flow (RBF) was measured by a Transonic flow probe, and glomerular filtration rate (GFR) was determined by Inulin clearance. In control rats (n = 8), cholesterol induced reductions of 10 +/- 2% in RBF [baseline (b) 7.6 +/- 0.3 microg x min(-1).100 g(-1)], 17 +/- 3% in urine flow (b, 10.6 +/- 0.9 microg x min(-1).100 g(-1)), 29 +/- 3% in sodium excretion (b, 0.96 +/- 0.05 mumol.min(-1).100 g(-1)) and 24 +/- 2% in nitrite/nitrate excretion (b, 0.22 +/- 0.01 nmol.min(-1).100 g(-1)) without an appreciable change in GFR (b, 0.87 +/- 0.03 ml.min(-1).100 g(-1)). These renal vasoconstrictor and anti-natriuretic responses to cholesterol were absent in rats pretreated with nitric oxide (NO) synthase inhibitor, nitro-l-arginine methylester (0.5 microg x min(-1).100 g(-1); n = 6). In rats pretreated with superoxide (O(2)(-)) scavenger tempol (50 microg x min(-1).100 g(-1); n = 6), the cholesterol-induced renal responses remained mostly unchanged, although there was a slight attenuation in anti-natriuretic response. This anti-natriuretic response to cholesterol was abolished in furosemide-pretreated rats (0.3 microg x min(-1).100 g(-1); n = 6) but remained unchanged in amiloride-pretreated rats (0.2 microg x min(-1).100 g(-1); n = 5), indicating that Na(+)/K(+)/2Cl(-) cotransport is the dominant mediator of this effect. These data demonstrate that cholesterol-induced acute renal vasoconstrictor and antinatriuretic responses are mediated by a decrease in NO production. These data also indicate that tubular effect of cholesterol on sodium reabsorption is mediated by the furosemide sensitive Na(+)/K(+)/2Cl(-) cotransporter.

Canonical transient receptor potential 1 channel is involved in contractile function of glomerular mesangial cells

Contractility of mesangial cells (MC) is tightly controlled by [Ca(2+)](i). Ca(2+) influx across the plasma membrane constitutes a major component of mesangial responses to vasoconstrictors. Canonical transient receptor potential 1 (TRPC1) is a Ca(2+)-permeable cation channel in a variety of cell types. This study was performed to investigate whether TRPC1 takes part in vasoconstrictor-induced mesangial contraction by mediating Ca(2+) entry. It was found that angiotensin II (AngII) evoked remarkable contraction of the cultured MC. Downregulation of TRPC1 using RNA interference significantly attenuated the contractile response. Infusion of AngII or endothelin-1 in rats caused a decrease in GFR. The GFR decline was significantly reduced by infusion of TRPC1 antibody that targets an extracellular domain in the pore region of TRPC1 channel. However, the treatment of TRPC1 antibody did not affect the AngII-induced vasopressing effect. Electrophysiologic experiments revealed that functional or biologic inhibition of TRPC1 significantly depressed AngII-induced channel activation. Fura-2 fluorescence-indicated that Ca(2+) entry in response to AngII stimulation was also dramatically inhibited by TRPC1 antibody and TRPC1-specific RNA interference. These results suggest that TRPC1 plays an important role in controlling contractile function of MC. Mediation of Ca(2+) entry might be the underlying mechanism for the TRPC1-associated MC contraction.

Interaction of endothelial nitric oxide and angiotensin in the circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

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.

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.

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.

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.

AT(1) receptor blockade with losartan during gestation in Wistar rats leads to an increase in thirst and sodium appetite in their adult female offspring

We studied the effects of angiotensin II receptor blockade with losartan on thirst and sodium appetite in pregnant Wistar rats and on their adult female offspring. During maternal adaptation to pregnancy, average daily total water intake increased by 63% (P<0.01); NaCl intake by 214% (P<0.001). These changes were not blocked by daily s.c. injections of losartan (50 mg/kg bw i.p.) from gestation day (GD) 2 until GD 19 which implied that maternal AT(1) receptors were not involved in the up regulation of thirst and sodium appetite during pregnancy. Losartan blockade during gestation led to a significant and continued increase in thirst and sodium appetite in the adult female offspring. Daily water intakes were greater in the losartan (LO) group than in the vehicle-injected control group (CO), leading to a total water intake of 1114 +/- 80.6 ml/kg bw compared with 738 +/- 56.7 ml/kg bw (P<0.05) during the 8-day period of observation. Daily sodium intakes were usually 2-3 times greater in the LO group compared with the CO group, amounting to a final cumulative intake of 232 +/- 33 mmol/kg bw compared with 93.8 +/- 16.5 mmol/kg bw (P<0.05) in 8 days. These elevated sodium and water intakes were nearly counterbalanced by the increased renal excretion of water and sodium by fully functional kidneys that were not injured by the drug. Body weights were 10% lower in the LO group at the start but remained unchanged relative to the CO group during the entire 8-day period of observation. Plasma electrolytes, blood hematocrit and carotid MABP in the LO group did not differ from the CO group.

Physiological elevation in plasma angiotensinogen increases blood pressure

Hepatic angiotensinogen secretion is controlled by a complex pattern of physiological or pathophysiological mediators. Because plasma concentrations of angiotensinogen are close to the Michaelis-Menten constant, it was hypothesized that changes in circulating angiotensinogen affect the formation rate of ANG I and ANG II and, therefore, blood pressure. To further test this hypothesis, we injected purified rat angiotensinogen intravenously in Sprague-Dawley rats via the femoral vein and measured mean arterial blood pressure after arterial catheterization. In controls, mean arterial pressure was 131 +/- 2 mmHg before and after the injection of vehicle (sterile saline). The injection of 0.8, 1.2, and 2.9 mg/kg angiotensinogen caused a dose-dependent increase in mean arterial blood pressure of 8 +/- 0.4, 19.3 +/- 2.1, and 32 +/- 2.4 mmHg, respectively. In contrast, the injection of a purified rabbit anti-rat angiotensinogen antibody (1.4 mg/kg) resulted in a significant decrease in mean arterial pressure (-33 +/- 3.2 mmHg). Plasma angiotensinogen increased to 769 +/- 32, 953 +/- 42, and 1,289 +/- 79 pmol/ml, respectively, after substrate and decreased by 361 +/- 28 pmol/ml after antibody administration. Alterations in plasma angiotensinogen correlated well with changes in plasma renin activity. In summary, variations in circulating angiotensinogen can result in changes in blood pressure. In contrast to renin, which is known as a tonic regulator for the generation of ANG I, angiotensinogen may be a factor rather important for long-term control of the basal activity of the renin-angiotensin system.

Enhanced renal expression of preproendothelin mRNA during chronic angiotensin II hypertension

The purpose of this study was to determine the role of endothelin in mediating the renal hemodynamic and arterial pressure changes observed during chronic ANG II-induced hypertension. ANG II (50 ng x kg(-1) x min(-1)) was chronically infused into the jugular vein by miniosmotic pump for 2 wk in male Sprague-Dawley rats with and without endothelin type A (ET(A))-receptor antagonist ABT-627 (5 mg x kg(-1) x day(-1)) pretreatment. Arterial pressure increased in ANG II rats compared with control rats (149 +/- 5 vs. 121 +/- 6 mmHg, P < 0.05, respectively). Renal expression of preproendothelin mRNA was increased by approximately 50% in both the medulla and cortex of ANG II rats. The hypertensive effect of ANG II was completely abolished in rats pretreated with the ET(A)-receptor antagonist (114 +/- 5 mmHg, P < 0.05). Glomerular filtration rate was decreased by 33% in ANG II rats, and this response was attenuated in rats pretreated with ET(A)-receptor antagonist. These data indicate that activation of the renal endothelin system by ANG II may play an important role in mediating chronic renal and hypertensive actions of ANG II.

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.

Intrauterine endotoxin infusion in rat pregnancy induces preterm delivery and increases placental prostaglandin F2alpha metabolite levels

OBJECTIVE

This study was designed to examine the effects of intrauterine endotoxin (lipopolysaccharide) on rat pregnancy.

STUDY DESIGN

Pregnant Sprague-Dawley rats (N = 26) were implanted with uterine catheters on day 15 or 16 of a 22-day gestation. Animals were randomly assigned to receive either lipopolysaccharide (25 or 50 microg) or sodium chloride solution (1 mL) on day 17 and then were either sacrificed on day 19 or observed until delivery. Placentas were harvested at the time of death, homogenates were prepared, and prostaglandin F(2)(alpha) metabolite levels were determined by means of radioimmunoassay. Data were analyzed by analysis of variance, Student-Newman-Keuls, and Mann-Whitney tests.

RESULTS

Lipopolysaccharide-treated groups (25 and 50 microg) displayed a shorter interval to delivery (mean +/- SE, 82 +/- 13 and 63 +/- 8 hours, respectively) than control animals (117 +/- 3 hours). Pups of lipopolysaccharide-treated (25 and 50 microg) female animals had lower live birth weights (4.92 +/- 0.01 and 5.12 +/- 0. 24 g, respectively) compared with control animals (6.04 +/- 0.07 g). Placental homogenates from lipopolysaccharide-treated female animals contained higher levels of prostaglandin F(2)(alpha) metabolite (1567 +/- 64 and 1475 +/- 59 pg/mL) than those from sodium chloride solution-infused control animals (804 +/- 68 pg/mL).

CONCLUSION

Bacterial products induce the preterm delivery of low-birth-weight pups in rats, possibly by increasing local prostaglandin biosynthesis.

The effect of indomethacin and prostacyclin agonists on blood pressure in a rat model of preeclampsia

OBJECTIVE

This study was designed to determine the effects of cyclooxygenase inhibition and prostacyclin agonists on the hypertension induced by nitric oxide synthase blockade in a previously characterized rat model of preeclampsia.

STUDY DESIGN

A condition similar to preeclampsia was induced by infusing pregnant rats with the nitric oxide synthase inhibitor N G -nitro- L -arginine methyl ester through subcutaneously implanted osmotic minipumps. Blood pressure was measured with the tail cuff method. In the first experiment the rats received either vehicle alone (control group), N G -nitro- L -arginine methyl ester (50 mg/d), indomethacin (0.1 mg/d), or N G -nitro- L -arginine methyl ester plus indomethacin beginning on day 17 of pregnancy. In the second experiment the rats received vehicle alone (control group), N G -nitro- L -arginine methyl ester (50 mg/d), or N G -nitro- L -arginine methyl ester plus iloprost (31 microgram/d). In a third experiment cicaprost (15 microgram/d) was substituted for iloprost.

RESULTS

Except for an increase on the day after insertion of the pump indomethacin had no significant effect on the hypertension induced by N G -nitro- L -arginine methyl ester. Both prostacyclin agonists (iloprost and cicaprost), however, attenuated the rise in blood pressure usually seen after N G -nitro- L -arginine methyl ester administration.

CONCLUSIONS

Nonselective inhibition of the cyclooxygenase enzymatic system does not influence the hypertension seen in the rat preeclampsia model induced by chronic nitric oxide deficiency. The hypertension in this model can be partially reversed with prostacyclin analogs.