Relaxin is essential for renal vasodilation during pregnancy in conscious rats

Receptor-induced dilatation in the systemic and intrarenal adaptation to pregnancy in rats

Normal pregnancy is associated with systemic and intrarenal vasodilatation resulting in an increased glomerular filtration rate. This adaptive response occurs in spite of elevated circulating levels of angiotensin II (Ang II). In the present study, we evaluated the potential mechanisms responsible for this adaptation. The reactivity of the mesangial cells (MCs) cultured from 14-day-pregnant rats to Ang II was measured through changes in the intracellular calcium concentration ([Cai]). The expression levels of inducible nitric oxide synthase (iNOS), the Ang II-induced vasodilatation receptor AT2, and the relaxin (LGR7) receptor were evaluated in cultured MCs and in the aorta, renal artery and kidney cortex by real time-PCR. The intrarenal distribution of LGR7 was further analyzed by immunohistochemistry. The MCs displayed a relative insensitivity to Ang II, which was paralleled by an impressive increase in the expression level of iNOS, AT2 and LGR7. These results suggest that the MCs also adapt to the pregnancy, thereby contributing to the maintenance of the glomerular surface area even in the presence of high levels of Ang II. The mRNA expression levels of AT2 and LGR7 also increased in the aorta, renal artery and kidney of the pregnant animals, whereas the expression of the AT1 did not significantly change. This further suggests a role of these vasodilatation-induced receptors in the systemic and intrarenal adaptation during pregnancy. LGR7 was localized in the glomeruli and on the apical membrane of the tubular cells, with stronger labeling in the kidneys of pregnant rats. These results suggest a role of iNOS, AT2, and LGR7 in the systemic vasodilatation and intrarenal adaptation to pregnancy and also suggest a pivotal role for relaxin in the tubular function during gestation.

Relaxin as a natural agent for vascular health

Hypertension, atherothrombosis, myocardial infarction, stroke, peripheral vascular disease, and renal failure are the main manifestations of cardiovascular disease (CVD), the leading cause of death and disability in developed countries. Continuing insight into the pathophysiology of CVD can allow identification of effective therapeutic strategies to reduce the occurrence of death and/or severe disabilities. In this context, a healthy endothelium is deemed crucial to proper functioning and maintenance of anatomical integrity of the vascular system in many organs. Of note, epidemiologic studies indicate that the incidence of CVD in women is very low until menopause and increases sharply thereafter. The loss of protection against CVD in post-menopausal women has been chiefly attributed to ovarian steroid deficiency. However, besides steroids, the ovary also produces the peptide hormone relaxin (RLX), which provides potent vasoactive effects which render it the most likely candidate as the elusive physiological shield against CVD in fertile women. In particular, RLX has a specific relaxant effect on peripheral and coronary vasculature, exerted by the stimulation of endogenous nitric oxide (NO) generation by cells of the vascular wall, and can induce angiogenesis. Moreover, RLX inhibits the activation of inflammatory leukocytes and platelets, which play a key role in CVD. Experimental studies performed in vascular and blood cell in vitro and in animal models of vascular dysfunction, as well as pioneer clinical observations, have provided evidence that RLX can prevent and/or improve CVD, thus offering background to clinical trials aimed at exploring the broad therapeutic potential of human recombinant RLX as a new cardiovascular drug.

Hormones restore biomechanical properties of the vagina and supportive tissues after surgical menopause in young rats

OBJECTIVE

The objective of the study was to determine the impact of hormones on the biomechanical properties of the vagina and its supportive tissues following surgical menopause in young vs middle-aged rats.

STUDY DESIGN

Long-Evans rats (4-month virgin [n = 34], 4-month parous [n = 36], and 9-month parous [n = 34]), underwent ovariectomy (OVX) or sham surgery. OVX animals received hormones (estrogen [E2] or estrogen plus progesterone [E2 plus P4]), placebo, or a matrix metalloproteinase inhibitor (chemically modified tetracycline-8 [CMT-8]). Animals were euthanized after 8 weeks and the biomechanical properties of the vagina and supportive tissues determined. Data were analyzed using a 1-way analysis of variance and posthoc tests.

RESULTS

OVX induced a rapid decline in the biomechanical properties of pelvic tissues in young but not middle-aged rats. Supplementation with E2, E2 plus P4, or CMT-8 restored tissues of young rats to control levels with no effect on middle-aged tissues. Parity did not have an impact on tissue behavior.

CONCLUSION

OVX has a differential effect on the tissues of young vs middle-aged rats.

Identification of the N-linked glycosylation sites of the human relaxin receptor and effect of glycosylation on receptor function

The relaxin receptor, RXFP1, is a member of the leucine-rich repeat-containing G-protein-coupled receptor (LGR) family. These receptors are characterized by a large extracellular ectodomain containing leucine-rich repeats which contain the primary ligand binding site. RXFP1 contains six putative Asn-linked glycosylation sites in the ectodomain at positions Asn-14, Asn-105, Asn-242, Asn-250, Asn-303, and Asn-346, which are highly conserved across species. N-Linked glycosylation is the most common post-translational modification of G-protein-coupled receptors, although its role in modulating receptor function differs. We herein investigate the actual N-linked glycosylation status of RXFP1 and the functional ramifications of these post-translational modifications. Site-directed mutagenesis was utilized to generate single- or multiple-glycosylation site mutants of FLAG-tagged human RXFP1 which were then transiently expressed in HEK-293T cells. Glycosylation status was analyzed by immunoprecipitation and Western blot and receptor function analyzed with an anti-FLAG ELISA, (33)P-H2 relaxin competition binding, and cAMP activity measurement. All of the potential N-glycosylation sites of RXFP1 were utilized in HEK-293T cells, and importantly, disruption of glycosylation at individual or combinations of double and triple sites had little effect on relaxin binding. However, combinations of glycosylation sites were required for cell surface expression and cAMP signaling. In particular, N-glycosylation at Asn-303 of RXFP1 was required for optimal intracellular cAMP signaling. Hence, as is the case for other LGR family members, N-glycosylation is essential for the transport of the receptor to the cell surface. Additionally, it is likely that glycosylation is also essential for the conformational changes required for G-protein coupling and subsequent cAMP signaling.

Anatomic and Functional Changes of the Upper Urinary Tract During Pregnancy

An appreciation of the maternal physiologic adaptations that occur in the renal system during pregnancy is fundamental to the understanding and proper clinical management of normal pregnancy, renal disorders in the gravid patient, and pregnancy-specific conditions such as pre-eclampsia. In this article, the authors first address the anatomic changes that occur in the upper urinary tract in normal pregnancy, followed by the dramatic alterations in maternal renal hemodynamics and glomerular filtration. They also briefly discuss renal handling of various substrates in pregnancy, including protein, uric acid, and glucose. An understanding of these changes is important and relevant to the clinician caring for pregnant women.

Effects of uteroplacental restriction on the relaxin-family receptors, Lgr7 and Lgr8, in the uterus of late pregnant rats

The peptide hormone relaxin stimulates uterine growth and endometrial angiogenesis and inhibits myometrial contractions in a variety of species. The receptor for relaxin is a leucine-rich repeat containing G-protein-coupled receptor Lgr7 (RXFP1) that is highly expressed in the myometrium of late pregnant mice, with a significant decrease in receptor density observed at term. The present study first compared the expression of Lgr7 with another relaxin-family receptor Lgr8 (RXFP2) in the uterus and placenta of late pregnant rats. The uterus was separated into endometrial and myometrial components, and the myometrium into fetal and non-fetal sites, for further analysis. We then assessed the response of these receptors to uteroplacental restriction (UPR). Expression of the Lgr7 gene was significantly higher in the uterus compared with the placenta. Within the uterus, on Day 20 of gestation, there was equivalent expression of Lgr7 in fetal and non-fetal sites of the myometrium, as well as in the endometrium v. myometrium. The second receptor investigated, Lgr8, was also expressed in the endometrium and myometrium, but at significantly lower levels than Lgr7. Bilateral ligation of the maternal uterine blood vessels on Day 18 of gestation resulted in uteroplacental restriction, a decrease in fetal weight and litter size, and a significant upregulation in uterine, but not placental, Lgr7 and Lgr8 gene expression in UPR animals compared with controls. These data suggest that both relaxin family receptors are upregulated in response to a reduction in uteroplacental blood flow in rats.

Evidence for local relaxin ligand-receptor expression and function in arteries

Relaxin is a 6 kDa protein hormone produced by the corpus luteum and secreted into the blood during pregnancy in rodents and humans. Growing evidence indicates that circulating relaxin causes vasodilatation and increases in arterial compliance, which may be among its most important actions during pregnancy. Here we investigated whether there is local expression and function of relaxin and relaxin receptor in arteries of nonpregnant females and males. Relaxin-1 and its major receptor, Lgr7, mRNA are expressed in thoracic aortas, small renal and mesenteric arteries from mice and rats of both sexes, as well as in small renal arteries from female tammar wallabies (an Australian marsupial). Using available antibodies for rat and mouse Lgr7 receptor and rat relaxin, we also identified protein expression in arteries. Small renal arteries isolated from relaxin-1 gene-deficient mice demonstrate enhanced myogenic reactivity and decreased passive compliance relative to wild-type (WT) and heterozygous mice. Taken together, these findings reveal an arterial-derived, relaxin ligand-receptor system that acts locally to regulate arterial function.

Relaxin is essential for systemic vasodilation and increased global arterial compliance during early pregnancy in conscious rats

During early pregnancy, there are marked increases in cardiac output (CO) and global arterial compliance (AC), as well as decreases in systemic vascular resistance (SVR). We recently reported that administration of recombinant human relaxin to nonpregnant female rats elicits changes in systemic hemodynamics and arterial mechanical properties similar to those observed during normal pregnancy. In the present study, we directly tested whether endogenous relaxin mediates the cardiovascular adaptations of pregnancy by neutralizing circulating relaxin with monoclonal antibodies during early gestation. Relaxin neutralizing antibodies were administered daily, beginning on d 8 of rat gestation, to block the functional effects of circulating relaxin. Systemic hemodynamics and arterial properties were assessed between gestational d 11 and 15 using techniques we have previously reported. Pregnant rats administered the neutralizing antibodies failed to exhibit the gestational increases in stroke volume, CO, and global AC or decreases in SVR that were observed in control pregnant rats administered an irrelevant antibody against fluorescein or PBS. In fact, in the pregnant rats administered the relaxin neutralizing antibodies, cardiovascular parameters were not statistically different from those in virgin rats. Interestingly, small renal and first-order mesenteric arteries isolated from midterm pregnant rats administered either relaxin-neutralizing or control antibodies did not exhibit any changes in passive mechanical properties compared with virgin rats. These findings indicate that circulating relaxin mediates the transition of the systemic circulation from the virgin to the pregnant state in the gravid rat model, suggesting a potential role for aberrant relaxin regulation in abnormal pregnancies wherein these cardiovascular adaptations are inadequate or excessive.

Relaxin—a pleiotropic hormone and its emerging role for experimental and clinical therapeutics

The insulin-related peptide hormone relaxin (Rlx) is known as pregnancy hormone for decades. In the 1980s, researchers began to recognize the highly intriguing fact that Rlx plays a role in a multitude of physiological processes far beyond pregnancy and reproduction. So, Rlx's contribution to the regulation of vasotonus, plasma osmolality, angiogenesis, collagen turnover, and renal function has been established. In addition, the peptide has been demonstrated to represent a mediator of cardiovascular pathology. The ongoing efforts to identify Rlx receptors eventually precipitated the discovery of the G protein-coupled receptors (GPCR) LGR7 and LGR8 as membrane receptors for human Rlx-2 in 2002. This review will summarize the current state of insight into this rapidly evolving field, which has further been expanded by the discovery of GPCR135 and GPCR142 as receptors for Rlx-3. In addition, Rlx has also been shown to activate the human glucocorticoid receptor (GR). There is evidence from Rlx and Rlx receptor knockouts suggesting that LGR7 is the only relevant receptor for mouse Rlx-1 (corresponding to human Rlx-2) in vivo and that insulin-like peptide (INSL)-3 represents the physiological ligand for LGR8. Regarding Rlx signal transduction, the cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) pathways will be characterized as major cascades. Investigation of downstream signaling remains an important field for future research. Finally, the current state of therapeutical strategies using Rlx in animal models as well as in humans is summarized.

Relaxin in cardiovascular and renal disease

Fibrosis (organ scarring) is a hallmark of many forms of cardiovascular and renal disease, and causes organ dysfunction and structural changes when normal tissue is replaced with scar tissue; the accumulation of scar tissue being a leading cause of death around the world. Despite deep organ scarring potentially existing in many forms (including myocardial and vascular sclerosis, renal interstitial fibrosis, and glomerulosclerosis), current therapies have only had limited success in delaying end-stage disease. The peptide hormone relaxin is emerging as a potent antifibrotic therapy with rapid-occurring efficacy. Recent studies have demonstrated the antifibrotic actions of relaxin in experimental models of cardiac and renal disease in vivo, and the various levels at which relaxin acts to inhibit fibroblast-induced collagen overproduction leading to fibrosis, in vitro. Separate studies using relaxin gene-knockout mice have demonstrated the significance of endogenous relaxin as a naturally occurring and protective moderator of collagen turnover, while the therapeutic potential of relaxin has been enhanced by its ability to promote vasodilation and renal hyperfiltration. This review will summarize these coherent findings as a means of highlighting the clinical potential of relaxin in cardiovascular and renal disease.

Upregulation of histidine decarboxylase expression in superficial cortical nephrons during pregnancy in mice and women

Mechanisms regulating pregnancy-induced changes in renal function are incompletely understood. Few candidate genes have been identified and data suggest that alternate mechanisms remain to be elucidated. Our objective was to screen thousands of genes expressed in kidneys from mice throughout gestation to identify possible key regulators of renal function during pregnancy. Mouse complementary DNA microarrays were used to screen for differences in expression during pregnancy in C57BL/6 mice. Interesting candidate genes whose expression varied with pregnancy were further analyzed by reverse transcription-PCR and Northern blot. Expression was localized by in situ hybridization and immunohistochemistry. Follow-up immunohistochemical analyses in archival human kidney sections from the fetus, non-pregnant, and pregnant women were also performed. Histidine decarboxylase (HDC), the enzyme that synthesizes histamine, was markedly upregulated in the mouse kidney during pregnancy. HDC expression localized to proximal tubule cells of fetal and adult mice. Females showed strong expression in the juxtamedullary zone before pregnancy and upregulation in the superficial cortical zone (SCZ) by mid-gestation. Histamine colocalized with HDC. Male mice showed only low HDC expression. Similar expression patterns were observed in human kidneys. Our results show that HDC expression and histamine production are increased in the SCZ during pregnancy. If histamine acts as a vasodilator, we speculate that increasing production in the SCZ may increase renal blood flow to this zone and recruit superficial cortical nephrons during pregnancy.

Matrix metalloproteinase-2 activity, protein, mRNA, and tissue inhibitors in small arteries from pregnant and relaxin-treated nonpregnant rats

Vascular gelatinase activity is essential for pregnancy- and relaxin (Rlx)-induced renal vasodilation and hyperfiltration in rats. The objective of this study was to further elucidate the mechanisms for the increase in vascular matrix metalloproteinase (MMP)-2 activity caused by pregnancy and Rlx. We first corroborated our earlier work by showing that pro- and active forms of MMP-2 were increased in small renal arteries from pregnant compared with virgin rats and Rlx-treated compared with vehicle-treated nonpregnant rats. We next investigated other artery types and showed that MMP-2 activity was upregulated in mesenteric arteries from pregnant rats (pro-MMP-2 by 50% and active MMP-2 by 40%, both P<0.05) and from Rlx-treated nonpregnant rats (pro-MMP-2 by 50% and active MMP-2 by 90%, both P<0.005) compared with their respective controls. To corroborate these results obtained by gelatin zymography, pro-MMP-2 protein was determined by Western analysis in the same small arteries. Pro-MMP-2 protein was increased in small renal arteries from pregnant compared with virgin rats and from Rlx- compared with vehicle-treated nonpregnant rats: pro-MMP-2-to-beta-actin ratio=0.29 vs. 0.21 (P<0.01) and 0.43 vs. 0.32 (P<0.005). Findings were similar for mesenteric arteries. MMP-2 mRNA as measured by real-time PCR was increased in small renal arteries from pregnant and Rlx-treated nonpregnant rats compared with their respective controls. There were no significant differences in tissue inhibitor of metalloproteinase (TIMP-1 or TIMP-2) activity by reverse zymography in small renal arteries. Thus increases in MMP-2 mRNA and protein expression are major factors contributing to increased MMP-2 activity in small arteries from pregnant and Rlx-treated nonpregnant rats.

Renal Hemodynamic Effects of Relaxin in Humans

Rat studies have convincingly demonstrated the essential role of the ovarian hormone relaxin in mediating gestational renal hemodynamic and osmoregulatory changes in that species. We describe a model in nonpregnant volunteers using exogenous hCG to stimulate the production and release of ovarian relaxin in order to assess renal hemodynamic responses. Women (n = 10) were serially studied +/- hCG stimulation during menstrual cycles with measurement of inulin, PAH, and neutral dextran clearances (to determine glomerular filtration rate [GFR], renal plasma flow [RPF], and glomerular porosity, respectively). Controls were women without ovarian function (n = 6) and men (n = 10). GFR and RPF were increased in the luteal phase compared to the follicular phase (15.3% increase in GFR, P < 0.005; 17.8% increase in RPF, P < 0.05). In controls, GFR and RPF were not significantly different between study occasions. Although exogenous hCG did not stimulate relaxin secretion in women without ovarian function or in men, it did so in normal women, but not into the pregnancy range. In no group were renal hemodynamics augmented by administered hCG. In naturally occurring cycles, increased serum relaxin is associated with augmented renal hemodynamics. As luteal stimulation with hCG failed to yield pregnancy relaxin levels, the use of exogenous relaxin for human administration is needed to further elucidate the renal vasodilatory properties of relaxin.

Recombinant Human Relaxin (rhRLX) Modifies Systemic Arterial Properties in Conscious Rats Irrespective of Gender, but in a Biphasic Fashion

Chronic administration of recombinant human relaxin (rhRLX) to conscious female nonpregnant rats that reaches serum concentrations of 10-30 ng/mL increases cardiac output and reduces systemic arterial load comparable to levels observed in midterm pregnancy. Chronic administration of the hormone to male rats increases cardiac output and reduces systemic arterial load to a similar extent. Short-term or chronic administration of rhRLX to conscious female rats that reaches serum concentrations of approximately 80 ng/mL results in minimal and insignificant changes. We conclude that: (1) rhRLX increases cardiac output and reduces arterial load irrespective of gender, and (2)the rhRLX dose response is biphasic.

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.

Acute and chronic effects of relaxin on vasoactivity, myogenic reactivity and compliance of the rat mesenteric arterial and venous vasculature

We investigated the effect of relaxin on vasodilation, myogenic reactivity, and compliance of small mesenteric arteries and veins. In acute experiments, small (second order) mesenteric arteries and veins from female rats were mounted in a pressure myograph, perfused intraluminally with relaxin, and exposed to incremental increases in intraluminal pressure (20-120 mm Hg for arteries, 2-12 mm Hg for veins). We expressed myogenic reactivity as the ratio of active to passive diameter at each pressure step. In chronic experiments, relaxin was administered to rats (4 microg/h) for 3 days prior to isolating the vessels and measuring myogenic reactivity. Arteries were more sensitive than were veins to the acute vasodilatory activity of relaxin (EC50: arteries=1.32+/-0.18x10(-8) M; veins=3.19+/-0.88x10(-8) M, P<0.05). Acute relaxin reduced myogenic reactivity of mesenteric arteries, but not veins. Chronic pretreatment with relaxin did alter the pressure/diameter relationship in Ca(2+)-containing medium, but this was due to increased passive compliance (control: 2.96+/-0.14 microm mm Hg(-1), n=5; relaxin: 3.72+/-0.16 microm mm Hg(-1), n=5) rather than to reduced myogenic reactivity. Chronic relaxin did not alter myogenic reactivity or compliance (control: 43.8+/-1.4 microm mm Hg(-1), n=5; relaxin: 46.1+/-2.3 microm mm Hg(-1), n=5) of veins. Thus, although relaxin reduces total peripheral resistance, it does not affect splanchnic venous capacitance or tone. In the face of elevated plasma relaxin levels, such as during pregnancy, cardiac preload may thus be maintained, concurrent with a reduction in cardiac afterload and blood pressure. We caution that, if an experimental treatment alters compliance, myogenic reactivity must be expressed as the ratio of active:passive diameter.

Relaxin Increases Cardiac Output and Reduces Systemic Arterial Load in Hypertensive Rats

Chronic administration of recombinant human relaxin (rhRLX) to conscious, normotensive rats (male and female) increases cardiac output (CO) and global arterial compliance (ACg) and reduces systemic vascular resistance (SVR) with no change in mean arterial pressure (MAP). Effects (magnitude and temporal pattern) of relaxin on systemic hemodynamics and arterial properties in hypertensive animal models are not known. Accordingly, the major goal of the present study was to determine the cardiovascular effects of rhRLX in hypertensive rats using 2 models: Long-Evans rats chronically administered angiotensin II (AII) and spontaneously hypertensive rats (SHR). CO and systemic arterial load, as quantified by SVR and ACg, were obtained using methods reported previously by us. In rats with AII-induced hypertension, acute rhRLX administration (up to 6 hours) significantly increased CO and ACg (24.9+/-3.9 and 34.3+/-12.6% above baseline, respectively) and significantly decreased SVR (17.2+/-3.5%) without changing MAP. In contrast, acute rhRLX administration to SHR and normotensive rats for up to 6 hours failed to produce any significant changes in CO, ACg, SVR, or MAP. However, chronic rhRLX administration (1 to 7 days) to SHR yielded significant changes (24.0+/-8.1 and 22.3+/-6.6% increases in CO and ACg, respectively, and a 13.3+/-5.3% decrease in SVR, with no change in MAP). In conclusion, rhRLX increases CO and reduces arterial load in hypertensive rats without reducing MAP. However, the time course of response to rhRLX treatment is dependent on the model of hypertension such that rats characterized by AII-mediated hypertension responded more rapidly to rhRLX administration than SHR.

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.

Evidence against the hypothesis that endothelial endothelin B receptor expression is regulated by relaxin and pregnancy

The endothelial endothelin B (ET(B)) receptor subtype is critical for renal vasodilation induced by relaxin in nonpregnant rats and during pregnancy (the latter via endogenous circulating relaxin). Here we tested whether expression of vascular ET(B) receptor protein is regulated by relaxin. Small renal arteries were harvested from virgin and midterm pregnant rats as well as nonpregnant rats that were administered recombinant human relaxin (rhRLX) at 4 mug/h or vehicle for 5 d or 4-6 h. Small renal arteries dissected from additional virgin rats were incubated in vitro with rhRLX or vehicle for 3 h at 37 C. ET(B) expression was also evaluated in cultured human endothelial cells: aortic, coronary, umbilical vein, and dermal microvascular endothelial cells. Cells were incubated for 4, 8, or 24 h with rhRLX (5, 1, or 0.1 ng/ml) or vehicle. ET(B) protein expression in arteries and cells was evaluated by Western analysis. No regulation of ET(B) expression was observed in small renal arteries in any of the experimental protocols, nor was there an increase in the vasorelaxation response to ET-3 in small renal arteries incubated in vitro with rhRLX. rhRLX only sporadically altered ET(B) expression in human coronary artery endothelial cells and human umbilical vein endothelial cells at certain time points or doses, and no regulation was observed in human aortic endothelial cells or human dermal microvascular endothelial cells. These results suggest that regulation of ET(B) receptor protein has little or no role in relaxin stimulation of the endothelial ET(B)/nitric oxide vasodilatory pathway.

Nitric oxide synthase isoforms in the rat kidney during pregnancy

OBJECTIVE

Nitric oxide mediates renal vasodilation and hyperfiltration during pregnancy in conscious rats through the endothelin B (ETB) receptor subtype. We tested the hypothesis that immunoreactive levels of endothelial nitric oxide synthase (eNOS) would be greater in the kidneys of midterm pregnant rats compared with virgin rats.

METHODS

We studied midterm pregnancy because renal plasma flow and glomerular filtration rate are maximal at this gestational stage. Western analysis was used to determine the level of eNOS in the three major zones of the kidney-inner medulla, outer medulla, and cortex-and in isolated small renal arteries, and in purified renal microvessels from the cortex.

RESULTS

There were no significant differences in eNOS expression between virgin and midterm pregnant rats in any of those renal tissues, regardless of whether immunoreactivity was expressed as arbitrary densitometry units, as "microg placental equivalents" interpolated from the linear portion of a dose-response curve of placental villous protein (2.5-30 microg, positive control) run concurrently on each gel, or normalized for beta-actin. We also investigated other NOS isoforms. In particular, immunoreactive neuronal NOS (nNOS) was detectable in the inner and outer medulla, but it was not significantly different between groups. nNOS immunoreactivity was below the level of detection in the cortex, but mRNA expression was not significantly different between pregnant and virgin rats by reverse transcriptase polymerase chain reaction.

CONCLUSION

Our results suggest that an increase in eNOS isoform mass does not contribute to the endothelin and NO-dependent renal vasodilation in rat gestation.

Effects of relaxin on systemic arterial hemodynamics and mechanical properties in conscious rats: sex dependency and dose response

We previously showed that chronic administration of recombinant human relaxin (rhRLX; 4 μg/h) to conscious female, nonpregnant rats to reach serum levels corresponding to early to midgestation (∼20 ng/ml) increases cardiac output (CO) and global arterial compliance (AC) and decreases systemic vascular resistance (SVR), comparable to changes observed in midterm pregnancy. The goals of this study were to test whether chronic administration of rhRLX (4 μg/h) to conscious male rats will yield similar changes in CO and systemic arterial load and to determine whether higher infusion rates of rhRLX (50 μg/h) administered to nonpregnant female rats yielding serum concentrations corresponding to late pregnancy (∼80 ng/ml) will further modify CO and SVR and global AC comparable to late gestation. CO and systemic arterial load, as quantified by SVR and AC, were obtained by using the same methods as in our previous studies. With respect to baseline, chronic rhRLX administration to male rats over 10 days at 4 μg/h increased both CO (20.5 ± 4.2%) and AC (19.4 ± 6.9%) and reduced SVR (12.7 ± 3.9%). These results were comparable to those elicited by the hormone in nonpregnant female rats. In contrast, neither acute (over 4 h) nor chronic (over 6 days) infusion of the higher dose of rhRLX administered to conscious female rats resulted in significant changes in CO, AC, or SVR from baseline. We conclude that 1) rhRLX increases CO and AC and reduces SVR irrespective of sex, and 2) the rhRLX dose response is biphasic insofar as significant alterations in CO and systemic arterial load fail to occur at high serum concentrations.

The Epidemiology of Chronic Venous Insufficiency and Varicose Veins

Chronic venous disease is a common condition presenting to physicians in Western Europe and the United States. This article provides a comprehensive review of the published literature in the English language, from 1942 to the present, and focuses on the prevalence of chronic venous insufficiency and varicose veins, as well as the involved risk factors. Prevalence estimates vary widely by geographic location, with the highest reported rates in Western countries. Reports of prevalence of chronic venous insufficiency vary from < 1% to 40% in females and from < 1% to 17% in males. Prevalence estimates for varicose veins are higher, <1% to 73% in females and 2% to 56% in males. The reported ranges in prevalence estimations presumably reflect differences in the population distribution of risk factors, accuracy in application of diagnostic criteria, and the quality and availability of medical diagnostic and treatment resources. Established risk factors include older age, female gender, pregnancy, family history of venous disease, obesity, and occupations associated with orthostasis. Yet, there are several factors that are not well documented, such as diet, physical activity and exogenous hormone use, which may be important in the development of chronic venous disease and its clinical manifestations.

Agonistic autoantibodies to the AT1 receptor in a transgenic rat model of preeclampsia

We used rats transgenic for the human angiotensinogen (hAogen) gene and the human renin (hRen) gene and crossed the strains to produce a model of preeclampsia in the dams. The female (n=9) hAogen x male hRen cross had severe (telemetry-measured) hypertension and albuminuria, which developed during the last trimester of pregnancy and subsided after delivery. The converse cross (n=9) and control (n=9) SD rats did not. We demonstrated that the female hAogen x male hRen cross had agonistic antibodies capable of activating the angiotensin (Ang) II AT1 receptor (AT1R-AA) and defined the epitope on the receptor's second extracellular loop. The phenomenon also occurs in humans with preeclampsia. The rats displayed renal histology reminiscent of preeclampsia, including fibrin deposition confined to the glomeruli. The complement system was activated in glomeruli and IgG deposits were present that may represent AT1R-AA. Finally, we observed an atherosis-like lesion in the spiral arteries of the placental bed, which we called placental-bed arteriolosclerosis. Our model may be relevant to preeclampsia in humans.

Relaxin-induced changes in renal sodium excretion in the anesthetized male rat

Pregnancy is associated with profound changes in renal hemodynamics and electrolyte handling. Relaxin, a hormone secreted by the corpus luteum, has been shown to induce pregnancy-like increases in renal blood flow and glomerular filtration rate (GFR) and alter osmoregulation in nonpregnant female and male rats. However, its effects on renal electrolyte handling are unknown. Accordingly, the influence of short (2 h)- and long-term (7 day) infusion of relaxin on renal function was determined in the male rat. Short term infusion of recombinant human relaxin (rhRLX) at 4 μg·h−1·100 g body wt−1 induced a significant increase in effective renal blood flow (ERBF) within 45 min, which peaked at 2 h of infusion (vehicle, n = 6, 2.1 ± 0.4 vs. rhRLX, n = 7, 8.1 ± 1.1 ml·min−1·100 g body wt−1, P < 0.01). GFR and urinary excretion of electrolytes were unaffected. After a 7-day infusion of rhRLX at 4 μg/h, ERBF (1.4 ± 0.2 vs. 2.5 ± 0.4 ml·min−1·100 g body wt−1, P < 0.05), urine flow rate (3.1 ± 0.3 vs. 4.3 ± 0.4 μl·min−1·100 g body wt−1, P < 0.05) and urinary sodium excretion (0.8 ± 0.1 vs. 1.2 ± 0.1 μmol·min−1·100 g body wt−1, P < 0.05) were significantly higher; plasma osmolality and sodium concentrations were lower in rhRLX-treated rats. These data show that long-term relaxin infusion induces a natriuresis and diuresis in the male rat. The mechanisms involved are unclear, but they do not involve changes in plasma aldosterone or atrial natriuretic peptide concentrations.

Animal models of preeclampsia

There have been many attempts to produce animal models that mimic the hypertensive disorders of pregnancy, especially preeclampsia, but most are incomplete when compared to the full spectrum of the human disease. This review assesses a number of these models, organized according to the investigators attempt to focus on a specific pathogenic mechanism believed to play a role in the human disease. These mechanisms include uterine ischemia, impairments in the nitric oxide system, insulin resistance, overactivity of the autonomic nervous and/or renin-angiotensin systems, activation of a systemic inflammatory response, and most recently, activation of circulating proteins that interfere with angiogenesis. In addition a model of renal disease that mimics superimposed preeclampsia is discussed. Defining these animal models should help in our quest to understand the cause, as well as to test preventative and therapeutic strategies in the management of these hypertensive disorders of pregnancy.

Emerging role of relaxin in renal and cardiovascular function

Although traditionally associated with reproductive processes, relaxin is emerging as an important player in renal and cardiovascular function. Much of our recently acquired understanding of relaxin in this new context has arisen from studies of maternal renal and cardiovascular adaptations to pregnancy in rats where the hormone is turning out to be an important mediator. First, we highlight the influence of relaxin on renal hemodynamics and glomerular filtration rate, as well as on other peripheral circulations. Second, we discuss the effect of relaxin on both the steady and pulsatile systemic arterial load, as well as on the heart, in particular, coronary blood flow. Third, we consider the impact of the hormone on cultured endothelial and vascular smooth muscle cells. Fourth, we address the interaction of relaxin with renal and cardiac disease, as well as its role in angiogenesis. Finally, in Perspectives, we point out several key research questions in need of investigation that relate to a potential autocrine/paracrine role of relaxin in renal and cardiovascular tissues. Furthermore, on the basis of its potent vasodilatory and matrix-degrading attributes, we speculate about the therapeutic potential of relaxin in renal and cardiovascular diseases.

Relaxin modifies systemic arterial resistance and compliance in conscious, nonpregnant rats

Relaxin emanates from the corpus luteum of the ovary and circulates during pregnancy. Because the hormone is a potent renal vasodilator and mediates the renal vasodilation and hyperfiltration of pregnancy in conscious rats, we reasoned that it might also contribute to the broader cardiovascular changes of pregnancy. We began investigating this concept by testing whether relaxin can modify systemic arterial hemodynamics and load when chronically administered to nonpregnant rats. The major objectives of the present work were to determine whether relaxin administration to nonpregnant rats 1) modifies cardiac output (CO), systemic vascular resistance, and global arterial compliance (AC), and 2) regulates the passive mechanics of isolated arteries. To accomplish the first objective, we developed a conscious rat model for assessment of global AC. Passive mechanics of small renal arteries were assessed using a pressure arteriograph. Chronic administration of recombinant human relaxin by sc osmotic minipump to conscious, female, nonpregnant rats reduced the steady arterial load by decreasing systemic vascular resistance, increased CO, and reduced the pulsatile arterial load by increasing global AC as quantified by two indices-AC estimated from the diastolic decay of aortic pressure and CO and AC estimated by the ratio of stroke volume-to-pulse pressure. In another group of rats, relaxin administration also regulated the passive mechanics of small renal arteries, indicating that, in addition to reduction in vascular smooth muscle tone, modification of the vascular structure (e.g. extracellular matrix) contributes to the increase in global AC. These findings suggest a role for relaxin in the systemic hemodynamic changes of pregnancy, as well as novel therapeutic potential for relaxin in modifying arterial stiffness and cardiac afterload.

Relaxin-1–deficient mice develop an age-related progression of renal fibrosis

BACKGROUND

Relaxin (RLX) is a peptide hormone that stimulates the breakdown of collagen in preparation for parturition and when administered to various models of induced fibrosis. However, its significance in the aging kidney is yet to be established. In this study, we compared structural and functional changes in the kidney of aging relaxin-1 (RLX-/-) deficient mice and normal (RLX+/+) mice.

METHODS

The kidney cortex and medulla of male and female RLX+/+ and RLX-/- mice at various ages were analyzed for collagen content, concentration, and types. Histologic analysis, reverse transcription-polymerase chain reaction (RT-PCR) of relaxin and relaxin receptor mRNA expression, receptor autoradiography, glomerular isolation/analysis, and serum/urine analysis were also employed. Relaxin treatment of RLX-/- mice was used to confirm the antifibrotic effects of the peptide.

RESULTS

We demonstrate an age-related progression of renal fibrosis in male, but not female, RLX-/- mice with significantly (P < 0.05) increased tissue dry weight, collagen (type I) content and concentration. The increased collagen expression in the kidney was associated with increased glomerular matrix and to a lesser extent, interstitial fibrosis in RLX-/- mice, which also had significantly increased serum creatinine (P < 0.05) and urinary protein (P < 0.05). Treatment of RLX-/- mice with relaxin in established stages of renal fibrosis resulted in the reversal of collagen deposition.

CONCLUSION

This study supports the concept that relaxin may provide a means to regulate excessive collagen deposition during kidney development and in diseased states characterized by renal fibrosis.

Relaxin's physiological roles and other diverse actions

Relaxin has vital physiological roles in pregnant rats, mice, and pigs. Relaxin promotes growth and softening of the cervix, thus facilitating rapid delivery of live young. Relaxin also promotes development of the mammary apparatus, thus enabling normal lactational performance. The actions of relaxin on the mammary apparatus vary among species. Whereas relaxin is required for development of the mammary nipples in rats and mice, it is essential for prepartum development of glandular parenchyma in pregnant pigs. During pregnancy relaxin also inhibits uterine contractility and promotes the osmoregulatory changes of pregnancy in rats. Recent studies with male and nonpregnant female rodents revealed diverse therapeutic actions of relaxin on nonreproductive tissues that have clinical implications. Relaxin has been reported to reduce fibrosis in the kidney, heart, lung, and liver and to promote wound healing. Also, probably through its vasodilatory actions, relaxin protects the heart from ischemia-induced injury. Finally, relaxin counteracts allergic reactions. Knowledge of the diverse physiological and therapeutic actions of relaxin, coupled with the recent identification of relaxin receptors, opens numerous avenues of investigation that will likely sustain a high level of research interest in relaxin for the foreseeable future.

Impaired nipple development and parturition in LGR7 knockout mice

LGR7 is a G-protein coupled receptor with structural homology to the gonadotrophin and thyrotrophin receptors. Recently, LGR7 was deorphanized, and it was shown that relaxin is the ligand for LGR7. To further study the function of this receptor, mice deficient for LGR7 were generated by replacing part of the transmembrane-encoding region with a LacZ reporter cassette. Here we show that LGR7 is expressed in various tissues, including the uterus, heart, brain, and testis. Fertility studies using female LGR7-/- mice showed normal fertility and litter size. However, some females were incapable of delivering their pups, and several pups were found dead. Moreover, all offspring died within 24 to 48 h after delivery because female LGR7-/- mice were unable to feed their offspring due to impaired nipple development. In some male LGR7-/- mice, spermatogenesis was impaired, leading to azoospermia and a reduction in fertility. Interestingly, these phenomena were absent in mutant mice at older ages or in later generations. Taken together, results from LGR7 knockout mice indicate an essential role for the LGR7 receptor in nipple development during pregnancy. Moreover, a defect in parturition was observed, suggesting a role for LGR7 in the process of cervical ripening.

Cloning, characterization, and expression of the rat relaxin gene

Relaxin, a hormone in the insulin superfamily, is synthesized by the corpus luteum of the rat ovary. Expression of relaxin precursor mRNA in rats is sharply induced after day 10 of pregnancy and plateaus on days 15 to 20 (parturition occurs on day 23). In an effort to understand this induction, we cloned the gene and carried out promoter analyses by transient transfection and chromatin immunoprecipitation methods. The single gene is 2.9 kilobases and is composed of two exons and one intron. There are alternative splice acceptor sites, 3 base pairs apart, which account for the inclusion of an extra codon in about 10% of the transcripts. The induction of transcription by day 15 was observed by the binding of polymerase II and histone H3 acetylation at the promoter region. There is a functional STAT binding site, about 3.8 kb upstream from the transcriptional start site, that is occupied by STAT3 on day 6 of pregnancy, when relaxin expression is minimal; on day 15, when expression is maximal, STAT3 is replaced by STAT5a. These data are consistent with STAT5 playing a role in the induction of relaxin expression.

Fetal growth restriction and maternal oxygen transport during high altitude pregnancy

High altitude reduces birth weights, averaging a 100-g fall per 1000 m elevation gain, as the result of restriction of third trimester fetal growth. Intrauterine growth restriction (IUGR) raises neonatal or infant mortality at low as well as at high altitude, but existing studies are unclear as to whether IUGR-specific mortality at high altitude is similar to, less than, or greater than at low altitude. Pregnancy increases maternal ventilation and raises arterial O(2) saturation at high altitude, which helps to protect against altitude-associated IUGR. Chronic hypoxia interferes with the maternal circulatory adjustments to pregnancy such that blood volume is lower and the rise in cardiac output diminished compared with sea level. The growth and remodeling of the uterine artery and other uteroplacental vessels is incomplete at high compared with low altitude, with the result that there is less redistribution of common iliac flow from the external iliac to the uterine arteries and lower uterine artery blood flow near term. Adaptations in multigenerational high altitude populations (e.g., Andeans and Tibetans) permit higher uterine artery blood flows and protect against altitude-associated IUGR.

Essential role for vascular gelatinase activity in relaxin-induced renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small arteries

During pregnancy, relaxin stimulates nitric oxide (NO)-dependent renal vasodilation, hyperfiltration and reduced myogenic reactivity of small renal arteries via the endothelial ETB receptor subtype. Our objective in this study was to elucidate the mechanisms by which relaxin stimulates the endothelial ETB receptor/NO vasodilatory pathway. Using chronically instrumented conscious rats, we demonstrated that a specific peptide inhibitor of the gelatinases MMP-2 and -9, cyclic CTTHWGFTLC (cyclic CTT), but not the control peptide, STTHWGFTLS (STT), completely reversed renal vasodilation and hyperfiltration in relaxin-treated rats. Comparable findings were observed with a structurally different and well-established, general antagonist of MMPs, GM6001. In contrast, phosphoramidon, an inhibitor of endothelin-converting enzyme, did not significantly change the renal vasodilatory response to relaxin administration. When small renal arteries were incubated with either of the general MMP inhibitors, GM6001 or TIMP-2 (tissue inhibitor of MMP), or with the specific gelatinase inhibitor, cyclic CTT, the reduced myogenic reactivity of these blood vessels from relaxin-treated nonpregnant and midterm pregnant rats was totally abolished. Moreover, a neutralizing antibody specific for MMP-2 completely abrogated the reduced myogenic reactivity of small renal arteries from relaxin-treated nonpregnant and midterm pregnant rats. In contrast, phosphoramidon did not significantly affect the reduction in myogenic reactivity. Using gelatin zymography, we showed increased pro and active MMP-2 activity in small renal arteries from relaxin-treated nonpregnant and midterm pregnant rats relative to the control animals. Thus, inhibitors of MMPs in general and of gelatinases in particular reverse the renal vascular changes induced by pregnancy or relaxin administration to nonpregnant rats. Finally, the typical reduction in myogenic reactivity of small renal arteries from relaxin-treated nonpregnant rats was absent in ETB receptor-deficient rats, despite an increase in vascular MMP-2 activity. These results indicate an essential role for vascular gelatinase, which is in series with, and upstream of, the endothelial ETB receptor/NO signaling pathway in the renal vasodilatory response to relaxin and pregnancy.

Time course and dose response of relaxin-mediated renal vasodilation, hyperfiltration, and changes in plasma osmolality in conscious rats

The pregnancy hormone relaxin elicits renal vasodilation, hyperfiltration, and osmoregulatory changes when chronically administered to conscious, nonpregnant rats. The objective in this study was to determine the dose response and time course of hormone action, as well as the time required for recovery on stopping its administration. The threshold dose of recombinant human relaxin (rhRLX) for renal vasodilation and reduction in plasma osmolality was 0.15 microg/h when given by subcutaneous osmotic minipump for 2 days (an infusion rate that achieved circulating levels of approximately 6 ng/ml). The peak response was observed during the 0.4 microg/h infusion rate (serum rhRLX of approximately 11 ng/ml), which was comparable to our previous work using a 4.0 microg/h (serum rhRLX of approximately 20 ng/ml). In contrast, a dose of 40 microg/h was ineffective (serum rhRLX of approximately 80 ng/ml). When 4.0 microg/h rhRLX was administered by osmotic minipump for shorter periods (</=24 h), renal circulatory and osmoregulatory changes were observed by </=6 h. After removal of the osmotic minipump, these changes persisted for at least 12 h, but they were fully restored by 24 h. Even briefer administration of 4.0 microg/h rhRLX by intravenous infusion showed an onset of action in the kidney by 1-2 h. In contrast, the 40 microg/h dose of rhRLX elicited minimal effects, and comparable to our earlier report, 4.0 microg/h purified porcine relaxin was also relatively ineffective during short-term intravenous administration. In conclusion, the effect of relaxin on the renal circulation and osmoregulation is biphasic, insofar as high doses are relatively inactive, and the onset of action is more rapid than previously believed. These findings may be important to consider when evaluating relaxin in the treatment of renal disease.

Relaxin signaling in reproductive tissues

The insulin/relaxin peptide family includes insulin, IGFs, relaxin1-3, INSL3/RLF, INSL4, INSL5/RIF2 and INSL6/RIF1, many without functional characterization. Based on analysis of transgenic phenotypes and phylogenetic profiling, we have discovered that two orphan leucine-rich repeat-containing G protein-coupled receptors, LGR7 and LGR8, are cognate receptors for relaxin whereas INSL3 is a specific ligand for LGR8. With the identification of the relaxin receptors, it is now possible to investigate specific cells and tissues that are responsive to relaxin in diverse physiological and pathological conditions as well as to develop agonists and antagonists for LGR7 and LGR8 as therapeutics to treat different labor disorders. Furthermore, future functional characterization of the specificity of these pluripoentent receptors with peptide ligands could lead to the understanding of related orphan ligands and receptors.

Relaxin, a pregnancy hormone, is a functional endothelin-1 antagonist: attenuation of endothelin-1-mediated vasoconstriction by stimulation of endothelin type-B receptor expression via ERK-1/2 and nuclear factor-kappaB

We have recently demonstrated that relaxin (RLX) acts as compensatory mediator in human heart failure. RLX inhibits the stimulation of endothelin-1, the most potent vasoconstrictor in heart failure. Upregulation of the endothelin type-B receptor (ET(B)), which mediates endothelin-1 clearance and endothelial release of NO, represents a pivotal mode of RLX action. However, signal transduction and abundance of this phenomenon are unknown. Therefore, we investigated RLX-induced regulation of ET(B) in human umbilical vein endothelial, epithelial (HeLa), and vascular smooth muscle cells. In human umbilical vein endothelial cells and HeLa cells, but not in human vascular smooth muscle cells, RLX upregulated ET(B) expression and activated extracellular signal-regulated kinase-1/2 (ERK-1/2) and nuclear factor-kappaB (NF-kappaB), a transcription factor. PD-98059, a selective inhibitor of the mitogen-activated protein kinase kinase-1 (MEK-1)-ERK-1/2 pathway, abolished ERK-1/2 and NF-kappaB activation and ET(B) upregulation. NF-kappaB inhibition also prevented RLX-mediated ET(B) stimulation. In NF-kappaB-luciferase reporter assays, we demonstrated complete inhibition of RLX-induced NF-kappaB activation in cells transfected with dominant-negative Raf-1, MEK-1, or ERK-1/2 constructs, whereas dominant-negative Ras had no effect. In rat aorta and mesenteric artery, RLX pretreatment, in an ET(B)-dependent fashion, mitigated the maximum contractile response to endothelin-1, by 38+/-4% and 43+/-6%, and the endothelin-1 sensitivity (-log[EC(50)]: aorta, 8.2+/-0.2 for vehicle versus 7.2+/-0.2 for RLX; mesenteric artery, 8.0+/-0.2 for vehicle versus 7.1+/-0.1 for RLX). RLX pretreatment augmented the dilator effect of the ET(B) agonist endothelin-3 by 100+/-8% and 133+/-13%. In conclusion, RLX stimulates endothelial and epithelial ET(B) via a Ras-independent Raf-1-MEK-1-ERK-1/2 pathway that activates NF-kappaB. On vascular smooth muscle cells, ET(B), a contributor to endothelin-mediated vasoconstriction, remains unaffected. This renders RLX a functional endothelin-1 antagonist.

Relaxin peptides are new global players

Relaxin (RLX) has come of age. From being one of the earliest hormones described with a very specific function in parturition, recent research has now shown that it is involved in a variety of roles, from endometrial differentiation during embryo implantation, to being a response factor in infarct and wound situations. It ameliorates fibrosis, and might also be involved in tumour growth and progression. And it is not alone: two other closely related peptide hormones have recently been identified, one specific for the brain, the other with roles in testicular descent and ovarian apoptosis. Finally, the recent cloning of the RLX receptors now provides the basis for a new molecular pharmacology for these peptide hormones, and preliminary studies suggest that their signal transduction is both interesting and unusual.

Myogenic reactivity is reduced in small renal arteries isolated from relaxin-treated rats

Administration of the ovarian hormone relaxin to nonpregnant rats vasodilates the renal circulation comparable to pregnancy. This vasodilation is mediated by endothelin (ET), the ET(B) receptor, and nitric oxide. Furthermore, endogenous relaxin mediates the renal vasodilation and hyperfiltration that occur during gestation. The goal of this study was to investigate whether myogenic reactivity of small renal and mesenteric arteries is reduced in relaxin-treated rats comparable to the pregnant condition. Relaxin or vehicle was administered to virgin female Long-Evans rats for 5 days at 4 microg/h, thereby producing midgestational blood levels of the hormone. The myogenic responses of small renal arteries (200-300 microm in diameter) isolated from these animals were evaluated in an isobaric arteriograph system. Myogenic reactivity was significantly reduced in the small renal arteries from relaxin-treated compared with vehicle-treated rats. The reduced myogenic responses were mediated by the ET(B) receptor and nitric oxide since the selective ET(B) receptor antagonist RES-701-1 and the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester restored myogenic reactivity to virgin levels. The influence of relaxin was not limited to the renal circulation because myogenic reactivity was also reduced in small mesenteric arteries isolated from relaxin-treated rats. Thus relaxin administration to nonpregnant rats mimics pregnancy, insofar as myogenic reactivity of small renal and mesenteric arteries is reduced in both conditions.

Nitric oxide synthase upregulation and the predelivery blood pressure decrease in spontaneously hypertensive rats

OBJECTIVE

The pregnant spontaneously hypertensive rat (SHR) exhibits a decrease in arterial blood pressure shortly before delivery; however, the mechanisms are unknown. Nitric oxide may be involved.

DESIGN

Blood pressure in stroke-prone SHR (SHRSP) and Wistar-Kyoto control rats (WKY) was telemetrically measured. Four groups were studied: pregnant and non-pregnant WKY and SHRSP rats, respectively. Mean blood pressure in pregnant SHRSP rats decreased from 148 +/- 2 mmHg at conception to 120 +/- 4 mmHg at day 15, compared to 112 +/- 1 mmHg in pregnant WKY rats. At delivery, we determined the vasodilatory responses of isolated preconstricted aortic strips.

RESULTS

Vasodilatory responses from late-term SHRSP rats were significantly greater following acetylcholine than either those from non-pregnant SHRSP or pregnant and non-pregnant WKY rats (acetylcholine IC50: 5, 22.8, 398, 1000 nmol/l, respectively), while contractile responses to increasing doses of norepinephrine were not different. Similar results were obtained with substance P. Indomethacin had no effect on the relaxation responses. Relaxation in response to sodium nitroprusside was not different in the groups. Western blot analysis showed that endothelial nitric oxide synthase (eNOS) levels were significantly increased in the pregnant SHRSP vessels compared to non-pregnant SHRSP, pregnant WKY, and non-pregnant WKY vessels.

CONCLUSION

Increased NOS may explain the blood pressure decrease during late pregnancy in genetically hypertensive rats.

Activation of orphan receptors by the hormone relaxin

Relaxin is a hormone important for the growth and remodeling of reproductive and other tissues during pregnancy. Although binding sites for relaxin are widely distributed, the nature of its receptor has been elusive. Here, we demonstrate that two orphan heterotrimeric guanine nucleotide binding protein (G protein)-coupled receptors, LGR7 and LGR8, are capable of mediating the action of relaxin through an adenosine 3',5'-monophosphate (cAMP)-dependent pathway distinct from that of the structurally related insulin and insulin-like growth factor family ligand. Treatment of antepartum mice with the soluble ligand-binding region of LGR7 caused parturition delay. The wide and divergent distribution of the two relaxin receptors implicates their roles in reproductive, brain, renal, cardiovascular, and other functions.