Enhanced endothelium-derived relaxing factor activity in pregnant, spontaneously hypertensive rats

Vaso-active factors in pregnancy

Pregnancy results in profound physiological changes in the cardiovascular system, yet these changes are completely reversible. It is apparent that vaso-active factors, some as yet probably unidentified, which act as humoral or local autocrine or paracrine regulators of vasular resistance, play a major role in these cardio-vascular changes. This role may be heightened in pregnancy when there has to be a large increase in blood flow to the uterus and placenta while maintaining adequate flow to other vascular beds. Our knowledge of the mechanisms of action of these vaso-active factors and their interactions with each other still remains incomplete. Alterations in synthesis and action of these vaso-active factors may occur in pregnancies associated with pregnancy-induced hypertension, pre-eclampsia or intra-uterine growth retardation. Investigation of such alterations may help to elucidate the roles of vaso-active factors in both normal and pathological situations. The gestational hormones oestrogen and progesterone, are obviously prime candidates as overall regulators of the cardiovascular changes of pregnancy and as agents which alter the synthesis or action of other vaso-active factors. Currently, much attention is being focused on the role of local autocrine or paracrine vaso-active factors which may be produced by the endothelium or by the underlying vascular smooth muscle cells and alterations in their production or action in the hyptertensive disorders of pregnancy. The endothelium forms the largest endocrine organ within the body and so its importance in the mediation of vascular events should not be under-estimated. The principal objective of this review is to examine the roles of these many autocrine and paracrine vaso-active factors during pregnancy and their relation with the overall regulation of the vascular system. Changes which may occur and be involved in the aetiology of pre-eclampsia and growth retardation will also be examined.

The role of nitric oxide in female reproduction

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

Booking blood pressures and plasma nitrite in Jamaican women with pre-eclampsia

Our objective was to assess the levels of nitric oxide (NO) in pre-eclampsia and to investigate its effect on blood pressure (BP) in the Jamaican population. A total of 103 participants (50 pre-eclampsia, 53 controls) were recruited from the University Hospital of the West Indies (UHWI). Blood samples were collected in the fasting state and trimester BP measurements were obtained from their records. A commercially available kit supplied by Oxford Biomedical Research Inc. (MI, USA) was used to measure plasma levels of NO. All measures of booking BP were significantly higher in women who later developed pre-eclampsia compared with those whose pregnancies remained normotensive (p </= 0.004). Mean plasma nitrite (NO) levels in the cases was 21.9 +/- 5.4 micromol/L (n = 48), significantly higher than that of the controls, 19.2 +/- 5.7 micromol/L (n = 52) p = 0.015. NO levels were not directly associated with the level of BP, p > 0.05. We concluded that in the Jamaican population, booking BP measurements may be predictors of pre-eclampsia and NO production increases in pre-eclampsia but is not related to the height of the BP.

Effects of L-arginine oral supplements in pregnant spontaneously hypertensive rats

PURPOSE: To evaluate the effects of L-arginine oral supplementation in spontaneously hypertensive pregnant rats (SHR). METHODS: Thirty SHR and ten Wistar-EPM-1 virgin female rats were used in the study. Before randomization, females were caged with males of the same strain (3:1). Pregnancy was confirmed by sperm-positive vaginal smear (Day 0). Wistar-EPM-1 rats served as counterpart control (C-1). SHR rats were randomized in 4 groups (n=10): Group Control 2, non-treated rats; Group L-Arginine treated with L-arginine 2%; Group Alpha-methyldopa treated with Alpha-methyldopa 33mg/Kg; Group L-Arginine+Alpha-methyldopa treated with L-arginine 2%+Alpha-methyldopa 33mg/Kg. L-arginine 2% solution was offered ad libitum in drinking water and Alpha-methyldopa was administered by gavage twice a day during the length of pregnancy (20 days). Blood pressure was measured by tailcuff plethysmography on days 0 and 20. Body weight was measured on days 0, 10 and 20. Results were expressed as mean ± SD (Standard Deviation). One-Way ANOVA/Tukey (or Kruskal-Wallis/Dunn, as appropriate) was used for group comparisons. Statistical significance was accepted as p<0.05. RESULTS: There was no significant weight gain in isolated L-arginine treated SHR. Mean blood pressure decreased in L-arginine-treated SLR compared with untreated-SHR rats. CONCLUSION: L-arginine oral supplementation reduces blood pressure in spontaneously hypertensive rats during pregnancy.

The vasodilator potency of the endothelium-derived relaxing factor, L-S-nitrosocysteine, is impaired in conscious spontaneously hypertensive rats

OBJECTIVE

This study compared the hemodynamic responses elicited by the endothelium-derived relaxing factor (EDRF), L-S-nitrosocysteine (L-SNC), the non-prostanoid EDRF released by acetylcholine (ACh) and nitric oxide (NO)-donors such as MAHMA NONOate, in conscious spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats.

METHODS

The depressor and/or vasodilator responses elicited by intravenous injections of ACh, L-SNC and MAHMA NONOate were determined in adult WKY and SH rats before and after intravenous injection of the NO synthesis inhibitor, N(G)-nitro-L-arginine methylester (L-NAME), or the cyclooxygenase inhibitor, indomethacin.

RESULTS

The responses elicited by ACh and L-SNC were smaller in SH than in WKY rats whereas the responses elicited by MAHMA NONOate were augmented in SH rats. The ACh-induced responses were not diminished after injection of L-NAME in WKY or SH rats. Indomethacin did not affect the responses to any of the vasodilator agents in WKY or SH rats. Addition of L-SNC to whole blood or thoracic aortae from SH rats yielded similar amounts of NO to those of WKY rats.

CONCLUSIONS

The vasodilator potencies of ACh and L-SNC were diminished whereas that of NO was augmented in SH rats. The loss of potency of L-SNC in SH rats was not obviously due to differences in decomposition to NO or the overactivity of cyclooxygenase factors. This study provides the first evidence that diminished endothelium-dependent vasodilation in SH rats may involve a loss of vasodilator potency of endogenous L-SNC.

Interleukin-6 impairs endothelium-dependent NO-cGMP-mediated relaxation and enhances contraction in systemic vessels of pregnant rats

IL-6 is elevated in plasma of preeclamptic women, and twofold elevation of plasma IL-6 increases vascular resistance and arterial pressure in pregnant rats, suggesting a role of the cytokine in hypertension of pregnancy. However, whether the hemodynamic effects of IL-6 reflect direct effects of the cytokine on the mechanisms of vascular contraction/relaxation is unclear. The purpose of this study was to test the hypothesis that IL-6 directly impairs endothelium-dependent relaxation and enhances vascular contraction in systemic vessels of pregnant rats. Active stress was measured in aortic strips isolated from virgin and late pregnant Sprague-Dawley rats and then nontreated or treated for 1 h with IL-6 (10 pg/ml to 10 ng/ml). In endothelium-intact vascular strips, phenylephrine (Phe, 10(-5) M) caused an increase in active stress that was smaller in pregnant (4.2 +/- 0.3) than virgin rats (5.1 +/- 0.3 x 10(4) N/m(2)). IL-6 (1,000 pg/ml) caused enhancement of Phe contraction that was greater in pregnant (10.6 +/- 0.7) than virgin rats (7.5 +/- 0.4 x 10(4) N/m(2)). ACh and bradykinin caused relaxation of Phe contraction and increases in vascular nitrite production that were greater in pregnant than virgin rats. IL-6 caused reductions in ACh- and bradykinin-induced vascular relaxation and nitrite production that were more prominent in pregnant than virgin rats. Incubation of endothelium-intact strips in the presence of N(omega)-nitro-L-arginine methyl ester (10(-4) M) to inhibit nitric oxide (NO) synthase, or 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 10(-5) M) to inhibit cGMP production in smooth muscle, inhibited ACh-induced relaxation and enhanced Phe-induced stress in nontreated but to a lesser extent in IL-6-treated vessels, particularly those of pregnant rats. Removal of the endothelium enhanced Phe-induced stress in nontreated but not IL-6-treated vessels, particularly those of pregnant rats. In endothelium-denuded strips, relaxation of Phe contraction with sodium nitroprusside, an exogenous NO donor, was not different between nontreated and IL-6-treated vessels of virgin or pregnant rats. Thus IL-6 inhibits endothelium-dependent NO-cGMP-mediated relaxation and enhances contraction in systemic vessels of virgin and pregnant rats. The greater IL-6-induced inhibition of vascular relaxation and enhancement of contraction in systemic vessels of pregnant rats supports a direct role for IL-6 as one possible mediator of the increased vascular resistance associated with hypertension of pregnancy.

Reduced endothelial NO-cGMP-mediated vascular relaxation and hypertension in IL-6-infused pregnant rats

Placental ischemia during pregnancy is associated with increased plasma cytokines such as interleukin-6 (IL-6), which may contribute to increased vascular resistance and hypertension of pregnancy. We tested the hypothesis that an increase in plasma IL-6 during pregnancy is associated with impaired endothelium-dependent relaxation, enhanced vascular contraction, and hypertension. Systolic blood pressure was measured in virgin and pregnant Sprague-Dawley rats non-treated or infused with IL-6 (200 ng/kg per day for 5 days). Isometric contraction was measured in isolated aortic strips, and endothelial nitric oxide (NO) synthase (eNOS) was measured in aortic homogenate using Western blots. Blood pressure was greater in IL-6-infused (146+/-3) than in control pregnant rats (117+/-2 mm Hg). In endothelium-intact vascular strips, phenylephrine (Phe) caused greater increase in active stress in IL-6-infused (maximum: 10.6+/-0.6) than in control pregnant rats (maximum: 4.1+/-0.3x10(4) N/m2). Acetylcholine (ACh)-induced relaxation of Phe contraction and vascular eNOS protein and nitrite/nitrate production were less in IL-6-infused than in control pregnant rats. N(omega)-nitro-L-arginine methyl ester (10(-4) mol/L), inhibitor of NOS, or 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (10(-5) mol/L), inhibitor of cGMP production in smooth muscle, inhibited ACh-induced relaxation and enhanced Phe-induced stress in control but not IL-6-infused pregnant rats. Endothelium removal enhanced Phe-induced stress in control but not in IL-6-infused pregnant rats. The blood pressure and vascular Phe-induced contraction, ACh relaxation, and eNOS protein were not different between control and IL-6-infused virgin rats. Thus, an endothelium-dependent NO-cGMP-mediated relaxation pathway is inhibited in systemic vessels of pregnant rats infused with IL-6. The results support a role for IL-6 as a possible mediator of the increased vascular resistance during hypertension of pregnancy.

Possible mechanisms underlying pregnancy-induced changes in uterine artery endothelial function

The last 10 years has seen a dramatic increase in our understanding of the mechanisms underlying the pregnancy-specific adaptation in cardiovascular function in general and the dramatic changes that occur in uterine artery endothelium in particular to support the growing fetus. The importance of these changes is clear from a number of studies linking restriction of uterine blood flow (UBF) and/or endothelial dysfunction and clinical conditions such as intrauterine growth retardation (IUGR) and/or preeclampsia in both humans and animal models; these topics are covered only briefly here. The recent developments that prompts this review are twofold. The first is advances in an understanding of the cell signaling processes that regulate endothelial nitric oxide synthase (eNOS) in particular (Govers R and Rabelink TJ. Am J Physiol Renal Physiol 280: F193-F206, 2001). The second is the emerging picture that uterine artery (UA) endothelial cell production of nitric oxide (NO) as well as prostacyclin (PGI2) may be as much a consequence of cellular reprogramming at the level of cell signaling as due to tonic stimuli inducing changes in the level of expression of eNOS or the enzymes of the PGI2 biosynthetic pathway (cPLA2, COX-1, PGIS). In reviewing just how we came to this conclusion and outlining the implications of such a finding, we draw mostly on data from ovine or human studies, with reference to other species only where directly relevant.

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

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

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

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

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.

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

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

In vivo regulation of vasomotricity by nitric oxide and prostanoids during gestation

Pharmacological studies using the Doppler technique revealed that pregnancy decreases the systemic blood pressure and enhances uterine blood velocity in rats. The reactivity of the uterine artery to alpha-adrenoceptor and muscarinic receptor agonists was higher than that of systemic arteries. Sodium nitroprusside increased uterine arterial blood velocity slightly during gestation and markedly in non-pregnant rats. N(G)-L-Arginine methyl ester (L-NAME) decreased the uterine blood velocity mainly in gravid animals. The effect of diclofenac on uterine blood velocity was also more pronounced during pregnancy. The actions of sodium nitroprusside, L-NAME and diclofenac on systemic blood pressure were similar in pregnant and virgin rats. Altogether, these results indicate that pregnancy enhances nitric oxide (NO) and vasodilatory prostanoid production in the uterine vascular muscle which becomes less sensitive to exogenous NO. The uterine vasodilated status appears to be determined by conjugated actions of endothelial NO and vasodilator prostanoids of which the synthesis and the effects are weakly modified in systemic arteries during gestation.

High-salt diet enhances vascular reactivity in pregnant rats with normal and reduced uterine perfusion pressure

High-salt (HS) diet is often associated with increased vascular resistance and arterial pressure; however, the effects of HS intake on the vascular control mechanisms of arterial pressure during pregnancy are unclear. We investigated whether a HS diet during pregnancy is associated with increases in vascular reactivity. Active stress was measured in aortic strips of virgin and normal pregnant Sprague-Dawley rats and a hypertensive pregnant rat model produced by reduction in uterine perfusion pressure (RUPP), fed either normal-sodium (NS, 1%) or HS diet (8%) for 7 days. In endothelium-intact strips, phenylephrine (Phe) caused a concentration-dependent contraction that was greater in RUPP rats than in normal pregnant or virgin rats and was significantly enhanced in pregnant/HS and RUPP/HS rats compared with pregnant/NS and RUPP/NS rats, respectively. Removal of the endothelium enhanced the Phe-induced stress slightly in virgin rats and significantly in pregnant/NS but not in pregnant/HS, RUPP/NS, or RUPP/HS. In endothelium-intact strips, acetylcholine (ACh) caused a concentration-dependent relaxation that was reduced in RUPP/NS (max, 31%) compared with pregnant/NS rats (max, 75%). ACh relaxation was further reduced in pregnant/HS rats compared with pregnant/NS rats and in RUPP/HS rats compared with RUPP/NS rats. Pretreatment of endothelium-intact strips with N(G)-nitro-L-arginine methyl ester (L-NAME, 10(-4) mol/L), to inhibit NO synthase, or with 1H-[1,2,4]oxadiazolo[4,3]-quinoxalin-1-one (ODQ, 10(-6) mol/L), to inhibit cGMP production in smooth muscle, inhibited ACh-induced relaxation and enhanced Phe-induced contraction in pregnant/NS rats but not in pregnant/HS, RUPP/NS, or RUPP/HS rats. Basal and ACh-induced nitrite/nitrate production from aortic strips showed significant reduction in pregnant/HS rats compared with pregnant/NS rats but not in RUPP/HS rats compared with RUPP/NS rats. Sodium nitroprusside, an exogenous NO donor, caused relaxation of Phe contraction that was similar in virgin or pregnant rats on an NS or HS diet but was significantly reduced in RUPP/HS rats (ED(50) 6x10(-8) mol/L) compared with RUPP/NS rats (ED(50) 6x10(-9) mol/L). Thus, a HS diet in normal pregnant and RUPP rats is associated with increases in vascular reactivity. The enhanced vascular reactivity with the HS diet is possibly related to abnormalities in NO synthesis/release from the endothelium in normal pregnant rats and an additional decrease in the sensitivity of the smooth muscle to relaxation by NO in pregnant rats with reduced uterine perfusion pressure.

Effects of hypertension on maternal adaptations to pregnancy: experimental study on spontaneously hypertensive rats

CONTEXT

Animal models for essential hypertension have been used for understanding the human pathological conditions observed in pregnant hypertensive women.

OBJECTIVE

To study the possible effects of pregnancy on hypertension and of hypertension on pregnancy in spontaneously hypertensive rats (SHR), and in their normotensive Wistar-Kyoto (WKY) counterparts.

TYPE OF STUDY

Comparative study using laboratory animals.

SETTING

Animal Research Laboratory of Clinical Medicine at the Medical School of Botucatu, São Paulo State University, Brazil.

SAMPLE

Ten to twelve-week-old virgin female normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). The animals were separated into four groups: 15 pregnant spontaneously hypertensive rats (SHR-P), 10 non-pregnant spontaneously hypertensive rats (SHR-NP), 15 pregnant normotensive rats (WKY-P), and 10 non-pregnant normotensive rats (WKY-NP).

MAIN MEASUREMENTS

The blood pressure was evaluated by the tail cuff method, in rats either with or without prior training for the handling necessary for tail cuff measurements. The maternal volume expansion was indirectly evaluated by weight gain, and by systemic parameters as hematocrit, hemoglobin, total protein, albumin and sodium retention. The perinatal outcome of pregnancy was evaluated by analysis of resorptions, litter size, rate of low weight and number of stillbirths.

RESULTS

The late fall in blood pressure in the pregnant SHR strain and in the normotensive WKY strain can only be detected in rats previously trained to accept the handling necessary for the tail cuff measurement. During pregnancy the body weight gain was significantly higher in WKY than in SHR rats. Systemic parameters were significantly lower in pregnant WKY rats than in non-pregnant WKY rats, while no differences were observed between pregnant and non-pregnant SHR groups. In pregnant WKY rats the sodium retention was higher from the 13th day onwards, while in SHR rats this occurred only on the 21st day. The characteristics of reproductive function such as number and weight of fetus, perinatal mortality and the resorption rate were significantly affected in the SHR strain.

CONCLUSION

The SHR strain may be considered as a model for chronic hypovolemic maternal hypertension, with the fetal growth retardation being determined by this hypovolemic state.

Evidence of a nonendothelial source of nitric oxide in the isolated perfused hindlimb vasculature of the pregnant rat

OBJECTIVE

This study was undertaken to further elucidate the roles of the vascular endothelium and nitric oxide (NO) in the regulation of vascular tone and constrictor responsiveness in pregnancy.

METHODS

The perfusion pressure-flow relationship was measured in isolated, perfused, norepinephrine-constricted (1) endothelium-intact, (2) endothelium-denuded, and (3) N omega-nitro-L-arginine methyl ester (L-NAME)-treated hindlimbs from nonpregnant and term-pregnant rates.

RESULTS

Baseline perfusion pressure at a flow rate of 2 mL/min was similar (approximately 20 min Hg) in all hindlimbs. Norepinephrine (0.5 muM) increased perfusion pressure in both nonpregnant (+21.6 +/- 2.4 mm Hg) and pregnant (+13.6 +/- 0.9 mm Hg) endothelium-intact rat hindlimbs. In nonpregnant rat hindlimbs, endothelium removal and L-NAME increased norepinephrine vasoconstriction similarly (+44.3 +/- 4.0 mm Hg and +46.4 +/- 8.6 mm Hg, respectively). In pregnant-rat hindlimbs, L-NAME increased norepinephrine vasoconstriction by 43.5 +/- 10.8 mm Hg, similar to that in nonpregnant-rat hindlimbs, but endothelium removal only increased norepinephrine vasoconstriction by 28.0 +/- 2.2 mm Hg. Perfusion pressure increased linearly as the flow rate was increased from 2 to 4 mL/min, and the slope of the regression line of the endothelium-intact pregnant-rat hindlimbs (7.0 +/- 0.6) was slightly, but not significantly, lower than that of the nonpregnant-rat hindlimbs (9.6 +/- 0.9). Endothelium removal increased the slopes of the regression lines, but that of pregnant-rat hindlimbs (12.8 +/- 1.6) was significantly lower (p < or = 0.05) than that of the nonpregnant-rat hindlimbs (23.8 +/- 1.8). L-NAME caused a similar increase in the pressure-flow slopes of nonpregnant-rat (36.5 +/- 3.4) and pregnant-rat (32.1 +/- 5.3) hindlimbs.

CONCLUSIONS

These results suggest that nonendothelial nitric oxide production may be increased in the hindlimb resistance vasculature of the pregnant rat, which may play a role in the normal pregnancy blunting of constrictor responsiveness and reduction of vascular resistance.

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

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

The effect of chronic nitric oxide inhibition on vascular reactivity and blood pressure in pregnant rats

CONTEXT

The exact mechanism involved in changes in blood pressure and peripheral vascular resistance during pregnancy is unknown.

OBJECTIVE

To evaluate the importance of endothelium-derived relaxing factor (EDRF) and its main component, nitric oxide, in blood pressure and vascular reactivity in pregnant rats.

DESIGN

Clinical trial in experimentation animals.

SETTING

University laboratory of Pharmacology.

SAMPLE

Female Wistar rats with normal blood pressure, weight (152 to 227 grams) and age (90 to 116 days).

INTERVENTION

The rats were divided in to four groups: pregnant rats treated with L-NAME (13 rats); pregnant control rats (8 rats); virgin rats treated with L-NAME (10 rats); virgin control rats (12 rats). The vascular preparations and caudal blood pressure were obtained at the end of pregnancy, or after the administration of L-NAME in virgin rats.

MAIN MEASUREMENTS

The caudal blood pressure and the vascular response to acetylcholine in pre-contracted aortic rings, both with and without endothelium, and the effect of nitric oxide inhibition, Nw-L-nitro-arginine methyl-ester (L-NAME), in pregnant and virgin rats. The L-NAME was administered in the drinking water over a 10-day period.

RESULTS

The blood pressure decreased in pregnancy. Aortic rings of pregnant rats were more sensitive to acetylcholine than those of virgin rats. After L-NAME treatment, the blood pressure increased and relaxation was blocked in both groups. The fetal-placental unit weight of the L-NAME group was lower than that of the control group.

CONCLUSION

Acetylcholine-induced vasorelaxation sensitivity was greater in pregnant rats and that blood pressure increased after L-NAME administration while the acetylcholine-induced vasorelaxation response was blocked.

Pregnancy is not associated with altered morphology of the femoral artery

While pregnancy is associated with adjustments in cardiovascular function, the morphology of the vascular system during pregnancy has been generally viewed as being very stable. However, recently we have demonstrated that pregnancy remodels the aorta and the carotid artery. In the present study, we assessed the morphological characteristics of the guinea-pig femoral artery during different stages of pregnancy using light and electron microscopy. There were no significant differences between external and internal diameters, wall thickness, total cross-sectional area and cross-sectional areas of lumen, intima, media, and adventitia of femoral arteries from non-pregnant and early-, mid- and late-pregnant guinea-pigs (n = 8-10). In previous studies, we have shown that the morphology of vascular smooth muscle and endothelial cells in the aorta and the carotid artery may be altered by pregnancy. Therefore, to test this possibility we measured diameters as well as cross-sectional areas of femoral arterial muscle and endothelial cells using electron microscopy. These parameters, at the electron microscopy level, were also not significantly changed by pregnancy (n = 8-10). It is concluded that the morphology of the guinea-pig femoral artery is not altered during pregnancy. In this regard, this study demonstrated that pregnancy-induced vascular remodelling varies between blood vessels that undergo the same functional alterations. Therefore, this may suggest that pregnancy-induced changes in blood flow through different vascular beds are not the most important factor involved in vascular remodelling observed during pregnancy. Rather, it is possible that haemodynamic-independent factors regulate pregnancy-mediated structural changes of the vascular wall.

The effect of chronic nitric oxide synthase inhibition on blood pressure and heart rate in unrestrained pregnant rats as recorded by radiotelemetry

OBJECTIVES

The objective of this study was to determine the effect of chronic nitric oxide synthase inhibition on heart rate and intravascular blood pressure in unrestrained pregnant rats as recorded by radiotelemetry.

STUDY DESIGN

Heart rate and systolic and diastolic blood pressures were monitored beginning with day 6 of pregnancy and until 1 week post partum with a radiotelemetric device. On day 10 of pregnancy osmotic minipumps were implanted subcutaneously and loaded to continuously deliver N(G)-nitro-L -arginine methyl ester (50 mg/d per rat, n = 6 animals) or vehicle (control group, n = 6 animals).

RESULTS

Blood pressure in the animals treated with N(G)-nitro-L -arginine methyl ester significantly increased compared with that in the control group and heart rate significantly decreased immediately after nitric oxide synthase blockade. Blood pressure then trended downward as gestation progressed, until the difference between the control group and the group treated with N(G)-nitro-L -arginine methyl ester became nonsignificant after day 17. Refractoriness to nitric oxide synthase blockade was especially evident in the diastolic pressure. Systolic, diastolic, and mean blood pressures in the rats treated with N(G)-nitro-L -arginine methyl ester were again significantly higher than those in the control group immediately after delivery and remained so despite a lower heart rate until the experiment was ended on postpartum day 6.

CONCLUSIONS

Radiotelemetry can be used to monitor heart rate and intra-arterial blood pressure in unstressed, unrestrained animals. Chronic inhibition of nitric oxide does not cause sustained hypertension throughout pregnancy. Nitric oxide does not appear to be the only factor responsible for the vascular changes in pregnancy. The factors responsible for the refractoriness to nitric oxide synthase blockade are specific to pregnancy and disappear immediately after delivery.

Normal pregnancy is associated with enhanced endothelium-dependent flow-mediated vasodilation

Normal pregnancy is characterized by reduced systemic vascular resistance, which may be mediated by nitric oxide (NO). We compared endothelial vasomotor function in 71 normal pregnant women (13 in first, 29 in middle, and 29 in last trimester) to 37 healthy age-matched controls. With external ultrasound, brachial artery diameter was measured at rest, during reactive hyperemia [with increased flow causing endothelium-dependent dilation (FMD)], and after sublingual nitroglycerin (causing endothelium-independent dilation). Compared with controls, resting flow and brachial artery diameter were significantly higher during the middle and last trimesters. Reactive hyperemia was reduced in all pregnant groups. FMD increased from the first trimester (by 26%), reaching the highest value in the last trimester (to 47% above nonpregnant values). FMD was significantly correlated to pregnancy status (nonpregnant or pregnant) and to vessel size. Nitroglycerin-induced dilation was similar in pregnant and nonpregnant women. A longitudinal study of eight women evaluated in the first, middle, and last trimesters confirmed an increase in FMD throughout pregnancy. The study supports the idea that basal and stimulated NO activity is enhanced in normal pregnancy and may contribute to the decrease in peripheral resistance.

Endothelial nitric oxide release in isolated perfused ovine uterine arteries: effect of pregnancy

The present study was designed to determine the release of endothelial nitric oxide, measured as combined nitric oxide, nitrite and nitrate (NOx), in isolated perfused uterine arteries obtained from nonpregnant and pregnant sheep. Noradrenaline produced concentration-dependent increases in perfusion pressure in both nonpregnant and pregnant uterine arteries with pD2 values of 5.1+/-0.07 and 4.6+/-0.04, respectively. The maximum responses were 300.8+/-8.8 mmHg for nonpregnant arteries and 86.9+/-1.3 mmHg for pregnant ones. N(G)-nitro-L-arginine increased noradrenaline-mediated maximum response in the pregnant (86.9+/-1.3 to 144.6+/-5.1 mmHg), but not in the nonpregnant, uterine arteries. The basal level of NOx was significantly higher in pregnant than in nonpregnant uterine arteries (346.1+/-63.2 vs. 86.0+/-20.6 pmol/ml). The calcium ionophore A23187 and adenosine triphosphate produced concentration-dependent increases in NOx release in both nonpregnant and pregnant arteries. Compared to the nonpregnant tissue, the agonist-induced increase in NOx release was significantly enhanced in the pregnant uterine artery. In accordance, endothelial NO synthase protein expression in pregnant uterine artery was 197% of that in nonpregnant artery. These data indicate that in the uterine artery, pregnancy increases both basal and agonist-induced release of endothelial nitric oxide, which is likely to play a key role in attenuated vascular reactivity of the uterine artery to vasoconstrictors during the course of pregnancy.

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

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

The alterations of vascular smooth muscle reactivity in vitro by human chorionic gonadotrophin

The study tests the hypothesis that human chorionic gonadotrophin (hCG) alters vascular smooth muscle reactivity by examining the effect of hCG administration on the contractility and relaxation of isolated vascular smooth muscle. Aortic rings from rats pre-treated with intraperitoneal administration of 5,000 I.U of hCG and control animals were contracted to phenylephrine, angiotensin II, CaCl2 and KCl. The experiments with phenylephrine were repeated with rings that were either de-endothelialized, incubated with L-NMMA, or incubated with calcium ionophore A23187. Aortic rings precontracted with phenylephrine were relaxed to acetylcholine (endothelium-dependent), sodium nitroprusside, hydralazine (endothelium-independent) or in the presence of A23187. The contractile responses of aortic rings from hCG-treated animals of phenylephrine, angiotensin II, CaCl2 and KCl were significantly attenuated. This effect was not reversed by pre-treatment with L-NMMA or by de-endothelialisation. In aortic rings from hCG-treated animals, there was almost total inhibition of acetylcholine-induced relaxation, but unaltered relaxation responses to sodium nitroprusside and hydralazine. The inhibitory effects of hCG-treatment on both the contraction and relaxation responses were either fully or partially reversed in the presence of calcium ionophore A23187. These observations suggest that hCG attenuates both contractile and endothelium-dependent relaxation responses by a mechanism which involves inhibition of extracellular calcium ion influx and may indicate a new role for the hormone in the altered vascular responses of both normal and abnormal pregnancies.

Etiology and pathogenesis of preeclampsia: current concepts

The etiology of preeclampsia is unknown. At present, 4 hypotheses are the subject of extensive investigation, as follows: (1) Placental ischemia-Increased trophoblast deportation, as a consequence of ischemia, may inflict endothelial cell dysfunction. (2) Very low-density lipoprotein versus toxicity-preventing activity-In compensation for increased energy demand during pregnancy, nonesterified fatty acids are mobilized. In women with low albumin concentrations, transporting extra nonesterified fatty acids from adipose tissues to the liver is likely to reduce albumin's antitoxic activity to a point at which very-low density lipoprotein toxicity is expressed. (3) Immune maladaptation-Interaction between decidual leukocytes and invading cytotrophoblast cells is essential for normal trophoblast invasion and development. Immune maladaptation may cause shallow invasion of spiral arteries by endovascular cytotrophoblast cells and endothelial cell dysfunction mediated by an increased decidual release of cytokines, proteolytic enzymes, and free radical species. (4) Genetic imprinting-Development of preeclampsia-eclampsia may be based on a single recessive gene or a dominant gene with incomplete penetrance. Penetrance may be dependent on fetal genotype. The possibility of genetic imprinting should be considered in future genetic investigations of preeclampsia.

Extracellular superoxide dismutase localization and activity within the human placenta

Maintenance of low vascular tone within the placenta is mediated by nitric oxide (NO). The half-life of NO is very short, as superoxide anion (O2-) rapidly inactivates NO to form peroxynitrite. Superoxide dismutases compete with NO for O2-. By scavenging O2-, superoxide dismutase promotes the vasodilatory action of NO. Extracellular superoxide dismutase (EC-SOD) is present in high concentrations within the extracellular matrix of systemic arteries and has been proposed to mediate vascular smooth muscle tone by increasing NO bioavailability. The localization and activity of EC-SOD within the human placenta has not been determined. Placental EC-SOD may be involved in placental vascular tone, and abnormal activity may lead to pre-eclampsia secondary to increased O2--mediated inactivation of NO. To investigate this possibility, the activity and localization of human placental EC-SOD was determined in normal women, and then compared to pre-eclamptic women. Placental EC-SOD localized within the villous extracellular matrix around arterioles, and there were no differences in distribution between normal and pre-eclamptic women. There were no differences in placental EC-SOD activity between normal and pre-eclamptic subjects in either center (33.7+/-4.1 versus 33.1+/-2.5, P=0.6), or peripheral (34.3+/-5.6 versus 34.0+/-3.5, P=0.9) samples. EC-SOD localization around villous vessels suggests that EC-SOD serves potentially to protect the fetal vasculature from O2-, in both normal and pre-eclamptic pregnancies. Placental EC-SOD distribution and activity is not different between pre-eclamptic and normal women, suggesting that EC-SOD is not involved in the vascular changes seen in pre-eclampsia.

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

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

Regulation of cGMP-induced relaxation and cGMP-dependent protein kinase in rat myometrium during pregnancy

Increases in guanosine 3',5'-cyclic monophosphate (cGMP) induced by nitric oxide (NO), nitrovasodilators, and atrial peptides correlate with relaxation of vascular smooth muscle. Relaxation of myometrial smooth muscle by increases in cGMP, however, has required unusually high concentrations of the cyclic nucleotide. We tested the hypothesis that the sensitivity of myometrium to relaxation by cGMP is increased during pregnancy. Aortic smooth muscle was more sensitive to relaxation by cGMP than myometrial tissues, and, contrary to our hypothesis, myometrium from pregnant rats was least sensitive. Although levels of cGMP were elevated after treatment with the NO donor, S-nitroso-N-acetylpenicillamine, relaxation of myometrial tissues obtained from pregnant rats occurred only at extraordinarily high concentrations. The levels of cGMP-dependent protein kinase (PKG) were significantly decreased in myometrium from pregnant rats compared with myometrium from nonpregnant cycling animals or aortic smooth muscle. Administration of estradiol to ovariectomized rats increased myometrial PKG expression, and progesterone antagonized this response. We conclude that 1) myometrial tissues from pregnant rats are not sensitive to relaxation by cGMP and 2) this insensitivity to cGMP is accompanied by progesterone-mediated decreases in the level of PKG expression.

A regional variation of acetylcholine-induced relaxation in different segments of rat aorta

The tension of the isolated thoracic aorta of the rat was measured isometrically to determine if there is a regional variation of the relaxation to acetylcholine (ACh). Endothelium-derived nitric oxide (NO)-dependent relaxation to ACh in the thoracic aorta precontracted with norepinephrine was significantly greater in the middle and distal segments than in the proximal segments. The relaxation to ACh was inhibited by NG-nitro-L-arginine methyl ester, an inhibitor of NO synthase. The relaxation to the NO donor sodium nitroprusside did not differ between the same three segments of the thoracic aorta. These results suggest that there are regional variations in the ACh-induced release of endothelium-derived NO in the rat thoracic aorta.

Sex differences in the abundance of endothelial nitric oxide in a model of genetic hypertension

A deficiency of nitric oxide may be responsible for the increased vascular resistance associated with human essential hypertension and that seen in animal models of hypertension. Premenopausal females are relatively protected from hypertension and cardiovascular complications. Levels of superoxide can influence the availability of nitric oxide. We hypothesize that there are differences in nitric oxide availability between stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto rats (WKY) and that superoxide may be responsible for at least some of these differences. We studied vascular reactivity in endothelium-intact aortic rings from WKY and SHRSP. We measured nitric oxide synthase activity in endothelial cells removed from aortas and also measured circulating nitrite/nitrate levels. We found the response to N(G)-nitro-L-arginine methyl ester to be significantly greater in WKY compared with SHRSP (95% CI: 20 to 174; P=.015) and in females compared with males in WKY (95% CI: 143 to 333; P=.00004) and SHRSP (95% CI: 70 to 224; P=.0006). Endothelial nitric oxide synthase activity was significantly greater in SHRSP compared with WKY (95% CI: 2.3 to 17.6; P=.016). The EC50 for relaxation to carbachol was significantly greater in male rats compared with female rats (95% CI: -1.1 to -0.2; P=.003) within the SHRSP strain. The maximum relaxation to carbachol was significantly attenuated in stroke prone spontaneously hypertensive compared with Wistar-Kyoto rats (95% CI: 1.7 to 14.4; P=.015). Diethyldithiocarbamate had a significantly greater effect on the stroke prone spontaneously hypertensive rats' carbachol response than that of Wistar-Kyoto rats (95% CI: 14.3 to 47.0; P=.0008). We conclude that superoxide may be responsible for strain differences in vascular reactivity, whereas nitric oxide availability may be responsible for sex differences independently of endothelial nitric oxide synthase activity and superoxide.

Nitric oxide and pregnancy

We have hypothesized that an alteration in the production of endothelium-dependent factors by sex hormones is a potential unifying mechanism for both the decreased arterial contractility and the redistribution of cardiac output characteristic of normal pregnancy. Thus, the effect of pregnancy/ estradiol on any one vascular bed will reflect the number and distribution of estrogen receptors. In this article, we review what is known about the effects of pregnancy and estrogen on nitric oxide synthase. Pregnancy increases Ca(2+)-dependent NOS activity early in gestation. The timing of the increase parallels the increase in plasma estradiol concentration. The increase in maternal brain NOS during pregnancy is blocked by tamoxifen. cGMP content increases along a similar time course in most but not all tissues. The changes in cGMP more closely approximate the changes in blood flow during pregnancy. This suggests that multiple elements of the NO:cGMP pathway are altered by pregnancy. It also shows that cGMP content cannot always be used as a surrogate for NOS activity. Estradiol, but not progesterone or testosterone, increases CA(2+)-dependent NOS activity. NO accounts for some, but not all of the pregnancy-associated changes in maternal arterial contractile response. It is not involved in uterine quiescence. Nitric oxide synthase is developmentally regulated in the fetus and is likely important in regulating the distribution of fetal blood flow.

Estrogen maintains nitric oxide synthesis in arterioles of female hypertensive rats

We hypothesized that in female spontaneously hypertensive rats (SHR), estrogen moderates the dysfunction of arterioles by preserving nitric oxide synthesis. To this end, we conducted experiments on isolated gracilis muscle arterioles (approximately 55 microns in diameter) of 12-week-old (SHR divided into four groups: females (fSHR), ovariectomized females (fSHR-OV), ovariectomized females with estrogen replacement (fSHR-OV+ES, 50 micrograms/kg SC 17 beta-estradiol benzoate every 48 hours), and males (mSHR). Arteriolar diameter in the presence of perfusion pressures of 60, 80, 100, and 120 mm Hg were obtained, and diameter changes were measured (at 80 mm Hg) in response to various concentrations of substance P (10(-9) to 5 x 10(-8) mol/L), sodium nitroprusside (10(-8) to 10(-6) mol/L), and A23187 (5 x 10(-8) to 10(-6) mol/L). The pressure-induced diameter of mSHR and fSHR-OV arterioles was significantly less (by approximately 10%) than that of fSHR and fSHR-OV+ES arterioles. N omega-nitro-L-arginine (10(-4) mol/L), a nitric oxide synthase inhibitor, elicited a significant decrease in basal arteriolar diameter of fSHR (by approximately 19%) and fSHR-OV+ES (by approximately 17%), thereby eliminating the differences in tone among the various groups. Dilations of fSHR and fSHR-OV+ES arterioles to substance P were significantly greater (by 140% at a concentration of 5 x 10(-8) mol/L) than those of mSHR and fSHR-OV arterioles, whereas dilations to sodium nitroprusside were not different among the groups. A23187 (a nitric oxide releaser) elicited dilations in arterioles of fSHR (5.9 +/- 1.5%, 13.0 +/- 1.8%, and 19.2 +/- 2.1%) and fSHR-OV+ES (4.3 +/- 1.0%, 10.3 +/- 2.4%, and 15.0 +/- 4.0%) but constrictions in those of mSHR (-7.5 +/- 1.6%, -25.3 +/- 39%, and -36.9 +/- 4.1%) and fSHR-OV (-2.6 +/- 1.7%, 7.4 +/- 3.3%, and -11.5 +/- 6.1%). We conclude that estrogen in fSHR is responsible for the preservation of nitric oxide synthesis in skeletal muscle arterioles, resulting in a greater modulation of pressure-induced myogenic tone than in mSHR and maintenance of nitric oxide-mediated dilations.

Impact of nitric oxide deficiency on blood pressure and glomerular hemodynamic adaptations to pregnancy in the rat

Studies were conducted to investigate the impact of nitric oxide synthesis inhibition on blood pressure and glomerular hemodynamic adaptations to pregnancy in the rat. In normal pregnancy, urinary excretion of NO2 + NO3 (NOx), reflecting increased nitric oxide (NO) production, progressively increased. Blockade of NO production in virgin and late pregnant Sprague-Dawley rats caused systemic hypertension, increased renal vascular resistance (RVR), reductions in RPF but GFR remained unchanged. In cortical nephrons, preglomerular and efferent arteriolar resistance (RA and RE) were elevated and glomerular capillary blood pressure (PGC) increased markedly. Glomerular plasma flow (QA) and the glomerular capillary ultrafiltration coefficient, Kf, were reduced without change in single nephron glomerular filtration rate (SNGFR) because of the large elevation in PGC. The pressor and glomerular hemodynamic responses to NO blockade were similar in virgins and pregnancy. Urinary NOx excretion was markedly reduced in all groups with chronic NO blockade. Inhibition was incomplete in pregnancy, however, and a level of NO production that was adequate for normal BP and renal function in virgins, led to severe vasoconstriction in pregnancy. The present studies suggest that chronic NO deficiency leads to derangement of the hemodynamic adaptations of pregnancy.

The vasoconstrictor effects of L-NAME, a nitric oxide synthase inhibitor, in pregnant rabbits

1. We have used anaesthetized, acutely instrumented non-pregnant (NP) and late pregnant (P) New Zealand white rabbits to examine the possible role of nitric oxide (NO) in the pregnancy-induced fall of vascular tone and arterial pressure. Systemic, renal and pulmonary vascular resistance, as well as plasma concentrations of cyclic GMP (PcGMP) were compared before and after the inhibition of NO synthesis by N(G)-nitro-L-arginine methyl ester (L-NAME). 2. P rabbits had lower baseline total peripheral resistance (TPR), mean arterial pressure (MAP) and higher PcGMP than NP controls (all P < 0.05 or less). L-NAME (1, 10, 50 mg kg1, i.v.) resulted in dose-dependent elevation of TPR in both groups. However, the absolute, as well as percentage increases in TPR were greater (P < 0.05) in NP than in P rabbits. 3. Cardiac output (CO) was reduced more (P < 0.01) by NO inhibition in NP than P rabbits. Therefore, despite the smaller increase in TPR, the elevation of MAP was greater (P < 0.001) in P than NP rabbits. After L-NAME, NP rabbits developed more severe bradycardia and a greater increase of pulmonary vascular resistance which might have contributed to the more pronounced reduction of CO. 4. PcGMP increased in both groups following L-NAME, but more (P < 0.01) in NP than P rabbits. 5. Infusion of acetylcholine (ACh, 0.02 micromol l-1 kg-1) reduced MAP and TPR more (both P < 0.05) in NP than P rabbits and L-NAME reduced the ACh-induced depressor response only in NP rabbits. 6. These results suggest that the low vascular tone and arterial pressure in pregnant rabbits is not mediated by NO.

Mesenteric arterial function in the rat in pregnancy: role of sympathetic and sensory-motor perivascular nerves, endothelium, smooth muscle, nitric oxide and prostaglandins

1. The effects of pregnancy on mesenteric arterial function were examined in constantly perfused (5 ml min-1) mesenteric arterial beds isolated from 21-day pregnant rats. The function of sympathetic and sensory-motor perivascular nerves, endothelium and smooth muscle was examined. The role of nitric oxide and prostaglandins in vasoconstrictor function was tested by use of NG-nitro-L-arginine methyl ester (L-NAME; 100 microM) and indomethacin (10 microM), respectively. 2. Electrical field stimulation (EFS; 4-32 Hz, 1 ms, 90V, 30s) at basal tone elicited frequency-dependent vasoconstriction which was markedly reduced in preparations from pregnant rats at all frequencies. Vasoconstrictor responses to vasopressin and endothelin were also reduced in pregnancy and there was a trend towards a reduction in maximal responses to noradrenaline (NA). In contrast, there was no difference in vasoconstrictor responses to ATP, 5-hydroxytryptamine (5-HT) or angiotension II. 3. L-NAME (100 microM) augmented responses to EFS, NA, ATP and vasopressin in control mesenteric arterial preparations. In contrast, L-NAME augmented responses only to EFS in pregnancy, having no significant effect on responses to NA, ATP and vasopressin. 4. Indomethacin (10 microM) attenuated responses to NA and vasopressin, but not to EFS, in controls and in pregnancy. Responses to ATP were attenuated by indomethacin in controls but not in pregnancy. 5. Mesenteric preparations from pregnant rats were resistant to having tone raised by continuous perfusion with methoxamine. Despite an approximately 10 fold greater concentration of methoxamine, there was a significantly smaller increase in tone in preparations from pregnant, 34.27 +/- 4.8 mmHg (n = 11) compared to control, 65.92 +/- 5.4 mmHg (n = 11), rats. EFS (4-12 Hz, 60 V, 0.1 ms, 30s) in the presence of guanethidine (5 microM) to block sympathetic neurotransmission elicited frequency-dependent vasodilatation due to activation of sensory-motor nerves. Percentage relaxations were similar in preparations from pregnant and non-pregnant rats. 6. Dose-dependent endothelium-dependent vasodilatations to acetylcholine and ATP were similar in preparations from pregnant and non-pregnant rats. Endothelium-independent vasodilatation to sodium nitroprusside and to calcitonin gene-related peptide were also similar between the two groups. 7. There was no significant difference in the basal perfusion pressure of mesenteric arterial beds from control (21.3 +/- 1.0 mmHg, n = 24) and pregnant (20.2 +/- 1.2 mmHg, n = 23) rats. However, a step-wise increase in perfusate flow from 5 to 10, 15, 20 and 24ml min-1 produced smaller increases in perfusion pressure in pregnancy compared to the controls. L-NAME (100 microM) or indomethacin (10 microM) had no significant effect on the relationship between flow and perfusion pressure. 8. The present results show that prejunctional changes are involved in blunted sympathetic vasoconstriction of rat mesenteric arteries in pregnancy. Non-specific postjunctional changes are implicated in the reduced constrictor responses to applied methoxamine, vasopressin and endothelin, but not to ATP. In contrast, sensory-motor nerves and endothelium-dependent and -independent vasodilatation was unchanged. The decrease in receptor-mediated mesenteric arterial constrictor responsiveness in pregnancy does not appear to be due to acute modulation by NO or prostaglandins, but may involve changes in the distensibility of the bed and/or changes in wall thickness.

Endothelial dysfunction in preeclampsia. Part II: Reducing the adverse consequences of endothelial cell dysfunction in preeclampsia; therapeutic perspectives

Next to low-dose Aspirin there appear to be several new and promising pharmacologie approaches for reducing the adverse consequences of endothelial cell dysfunction in preeclampsia. Among these are selective thromboxane-A2 synthetase and/or thromboxane-A2 receptor antagonists, stable prostacyclin analogues, selective S(erotonin)2-receptor blockers, nitrovasodilators, glycoprotein IIb/IIIa antagonists, hirudin, and ticlopidine. Early-onset preeclampsia appears to be associated with certain disorders that are likely to provoke an arterial thrombotic process by impairing the normal endothelial cell-platelet interactions. Especially heterozygous hyperhomocysteinemia, protein S deficiency and anticardiolipin antibodies appear to be fairly common. The management of these 3 separate disease entities will be discussed.

Effects of nifedipine and Bay K 8644 on myotropic responses in aortic rings of pregnant rats

The hypothesis that Ca2+ channel function is altered during pregnancy was tested by comparing responses to potassium chloride (KCl) and phenylephrine in aortic rings of virgin and term-pregnant rats under the influence of nifedipine and Bay K 8644. Maximum response to KCl was progressively reduced by increasing nifedipine concentrations (1.0-100 nM) in both groups of tissues. Nifedipine produced a smaller inhibition of KCl-induced contraction in aortic rings of pregnant than of virgin rats. It exerted little inhibition on the concentration-response curve to phenylephrine. The Ca2+ channel antagonist (100 nM) reduced the maximum response to the alpha-adrenoceptor agonist in rings from virgin rats, but had no effect in pregnant rats. Bay K 8644, a Ca2+ channel activator, potentiated the responses to low concentrations of both phenylephrine and KCl in the tissues of both virgin and pregnant rats, but did not affect maximum responses. It also induced concentration-dependent contractions in rings of virgin but not of pregnant rats. The effects of Bay K 8644 were markedly potentiated by precontracting the aorta with 10mM KCl. Nevertheless tissues from pregnant rats were still less responsive to Bay K 8644. However, when the strips were precontracted to the same level by different concentrations of KCl, the concentration-response curves to Bay K 8644 were identical in both groups. [3H]Nitrendipine binding to membrane preparations of the thoracic aorta was similar in virgin and pregnant rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Gender difference in endothelial dysfunction in the aorta of spontaneously hypertensive rats

We investigated endothelium-dependent responses of thoracic aorta isolated from age-matched male and female spontaneously hypertensive rats (SHR) to explore gender differences in endothelial dysfunction that may contribute to the sexual dimorphism observed in the development of hypertension in this strain. Endothelium-dependent relaxation in response to acetylcholine (10(-9) to 10(-4) mol/L) was significantly greater in female rats than in male rats, although impaired responses were seen in both sexes compared with normotensive controls. Inhibition of cyclooxygenase by indomethacin (10(-5) mol/L) improved endothelium-dependent relaxation, but it did not abolish the gender difference. Relaxations in response to sodium nitroprusside were identical in denuded aortic rings from male and female SHR. Acetylcholine at higher concentrations (10(-6) to 10(-4) mol/L) induced endothelium-dependent contraction in intact, quiescent aortic rings from male SHR but not in those from female SHR. After incubation with NG-nitro-L-arginine (10(-4) mol/L), contraction in response to acetylcholine became apparent in rings from female SHR, but it was still significantly less pronounced than in similarly treated rings from male SHR. Endothelium-dependent contraction was prevented by indomethacin in both sexes, suggesting that a cyclooxygenase product such as endoperoxide may be mediating this effect. Because responses evoked by the thromboxane/endoperoxide receptor agonist U46619 (10(-10) to 10(-6) mol/L) were not greater in rings from male SHR than those from female SHR, increased smooth muscle responsiveness or higher thromboxane/endoperoxide receptor density in the males could not account for the differences in endothelium-dependent contraction. These results suggest that sex steroid hormones may control endothelium-dependent vascular reactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of nitric oxide in the altered vascular reactivity of pregnancy in the rat

1. Pregnancy is characterized by a decrease in systemic vascular resistance and a blunting of the angiotensin II (AII) pressor response. We studied the role of nitric oxide (NO) and prostanoids in these vascular changes of pregnancy in anaesthesized, ganglion blocked non-pregnant and pregnant rats. 2. Inhibition of NO synthesis with NG-nitro-L-arginine methyl ester (L-NAME) led to an increase in mean arterial pressure (MAP) which was of a significantly greater magnitude in pregnant rats in late gestation than in non-pregnant rats, or rats in mid-gestation. 3. The pressor response to varying doses of AII was attenuated during late pregnancy, and this attenuation was partially reversed by L-NAME. 4. The pressor response to varying doses of a vasoconstrictor, phenylephrine (PE), was also attenuated in late pregnancy. However, this attenuation was not reversed by L-NAME. 5. Inhibition of prostanoid biosynthesis with meclofenamate did not alter basal MAP, nor the pressor response to varying doses of AII or PE in pregnant and non-pregnant animals. 6. It is concluded that (a) increased NO synthesis occurs during late gestation and contributes both to the decrease in systemic vascular resistance, as well as the blunting of the pressor response to AII during pregnancy, and (b) prostaglandins are not important in the maintenance of basal vascular tone, or the blunting of the pressor response to AII during pregnancy.

Control of vascular resistance in the maternal and feto-placental arterial beds

This review aims to provide a comprehensive summary of the mechanisms involved in the physiological adaptation of the vasculature to pregnancy. Profound changes occur both systemically and in discrete circulations in the mother, but it is debatable which factors are responsible. Similarly, whilst the feto-placental circulation must be substantially controlled by humoral mechanisms, the exact role of each potential contributor is not known. In view of the hitherto unappreciated and very important role of the endothelium-derived vasodilator, nitric oxide, in the control of peripheral vascular resistance, considerable emphasis will be placed on the many recent investigations in this area.

The role of nitric oxide in the pathogenesis of preeclampsia

OBJECTIVE

Nitric oxide, a potent vasodilator released by endothelial cells, inhibits platelet aggregation and adhesion to vascular endothelial surfaces. Because endothelial cell damage is considered pivotal in the pathogenesis of preeclampsia, this study was initiated to determine whether nitric oxide production is decreased in patients with preeclampsia.

STUDY DESIGN

Twenty-six patients with preeclampsia (as defined by a blood pressure > or = 140 mm Hg systolic or 90 mm Hg diastolic plus proteinuria, > or = 300 mg per 24 hours or > or = 2+ by dipstick, both occurring on two occasions > or = 4 hours apart) and 26 normotensive women with singleton gestations in the third trimester were studied. Because nitric oxide is spontaneously oxidized to both nitrite and nitrate, two analytic assays were used serially. Serum nitrite levels were initially determined with the Greiss reagent and subsequently analyzed with Escherichia coli nitrate reductase.

RESULTS

With the Greiss reagent alone the mean +/- SEM of serum nitrite level in 26 patients with preeclampsia was significantly decreased compared with 26 normotensive patients (3.46 +/- 1.43 mumol/L vs 4.65 +/- 0.85 mumol/L, p = 0.02). With the addition of the nitrate reductase enzyme of Escherichia coli the mean +/- SEM of serum nitrite level in 26 preeclamptic patients was again significantly decreased compared with 26 normotensive patients (20.04 +/- 1.25 mumol/L vs 27.38 +/- 2.23 mumol/L, p = 0.02). One patient with the syndrome of hemolysis, elevated liver enzymes, and low platelets demonstrated a concurrent decrease in serum nitrite over a 2-week period, emphasizing the relationship of nitric oxide to the pathophysiologic features of the syndrome.

CONCLUSIONS

Circulating levels of nitrite are decreased in patients with preeclampsia. These data support the concept that diminished nitric oxide synthesis contributes to the pathophysiologic changes seen in preeclampsia.

Glomerular filtration and volume regulation in gravid animal models

The gestational increase in glomerular filtration rate that occurs in the normal rat is exclusively the result of an increase in renal plasma flow and there is no sustained increase in glomerular capillary blood pressure during a normal pregnancy. The factor or factors that initiate the gestational renal vasodilatation (and plasma volume expansion) are maternal, not fetoplacental in origin. The precise nature of the initiating factors has not yet been defined, although it is unlikely that the gestational plasma volume expansion can be the sole cause of the increased glomerular filtration rate seen in pregnancy. A number of vasoactive hormones are activated in pregnancy but as yet no clear candidate has emerged as 'the renal vasodilator'. Preliminary evidence suggests that nitric oxide may play an important role in gestational vasodilatation. The normal kidney in pregnancy exhibits substantial renal reserve to amino acid infusion and unimpaired autoregulatory ability despite being already vasodilated by the gestational stimulus. There are marked and sometimes contradictory changes in the various volume sensing and control systems in pregnancy. In general, the sensors perceiving and controlling intravascular volume are reset during a normal pregnancy to enable to mother to accommodate the increased plasma volume without provoking a natriuretic response. Whether the expanded plasma volume of pregnancy is perceived as normal or underfilled is not clear at this time and may vary according to the volume regulatory system. Repetitive pregnancies do not have any cumulative, long-term deleterious effects on renal function, when the underlying function is normal, when it has been compromised by removal of renal mass or during chronic systemic hypertension in the spontaneously hypertensive rat. In the short term, pregnancy does not worsen kidney function when underlying glomerular damage is due to immune stimuli, ablation of renal mass or gentamicin, or in the spontaneously hypertensive rat. Therefore, the chronic renal vasodilatation of pregnancy does not appear to be a damaging entity, unlike other states of low preglomerular arteriolar resistance, studied in the male rat. When pregnancy is superimposed on Adriamycin nephrosis or chronic blockade of nitric oxide, hypertension occurs and renal function declines. In both situations endothelial damage/dysfunction occurs, as is also seen in pre-eclampsia. Further study of the effects of pregnancy in animal models of endothelial dysfunction will prove rewarding.