Evidence against the hypothesis that prostaglandins are the vasodepressor agents of pregnancy. Serial studies in chronically instrumented, conscious rats

Endogenous nitric oxide suppresses rat myometrial connexin 43 gap junction protein expression during pregnancy

Nitric oxide (NO) synthase (NOS) is active in the gravid uterus, and its activity decreases prior to the onset of parturition. We tested the hypothesis that NO helps maintain uterine quiescence by suppressing the expression of genes necessary for parturition. Pregnant rats (18 days gestation) were treated with inducible NOS (iNOS) inhibitor N-iminoethyl-L-lysine (NIL) or endothelial NOS inhibitor nitro-L-arginine methyl ester (L-NAME); 24 h later, uteri were analyzed for myometrial connexin 43 (Cx43) protein by immunoblotting and mRNA by Northern analysis. Myometrial oxytocin receptors (OTR) were measured by radioligand binding, and decidual prostaglandin H synthase (PGHS) protein by immunoblotting. Uterine NOS blockade was verified by NOS activity assay. We found that NIL, but not L-NAME, significantly increased myometrial Cx43 protein to parturitional levels with treatment at 19 but not 17 days gestation. Steady state mRNA concentrations were not changed at 24 h. NOS inhibition did not increase the concentrations of OTR, or PGHS protein, nor did it decrease maternal serum progesterone. We conclude that endogenous uterine NO from iNOS suppresses myometrial Cx43 gap junction protein expression during rat pregnancy. Although the exact mechanism is unknown, an increase of uterine wall stretch due to inhibition of relaxation could account for increased Cx43 gene transcription.

Endothelin mediates renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Profound vasodilation of the kidneys and other nonreproductive organs transpires during early pregnancy. Because nitric oxide (NO) was found to mediate renal vasodilation and hyperfiltration in conscious pregnant rats, and endogenous endothelin (ET) was suggested to be vasodilatory in the renal circulation of nonpregnant rats, we tested whether endothelin mediates the NO-dependent changes in the renal circulation during pregnancy. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were measured in conscious pregnant and virgin rats before and during infusion of 30 micrograms/min RES-701-1 (a selective ETB receptor subtype antagonist). Baseline GFR and ERPF were significantly increased by 35% in gravid rats relative to virgin controls. During infusion of RES-701-1, the pregnant rats responded more robustly, showing a greater decline in both GFR and ERPF such that renal function converged in the two groups of rats. ERPF also converged in pregnant and virgin rats during infusion of SB-209760, a nonselective ETA/B receptor subtype antagonist. Combined infusion of Nomega-nitro-L-arginine methyl ester [L-NAME, an NO synthase (NOS) inhibitor] and RES-701-1 reduced GFR and ERPF to levels comparable to those reached with either agent given alone, suggesting inhibition of a common vasodilatory pathway. RES-701-1 and SB-209670 significantly lowered the cGMP content of small renal arteries from gravid and virgin rats in vitro, strengthening the link between the renal endothelial ETB receptor subtype and NO. Importantly, we showed that RES-701-1 is not a direct inhibitor of NOS. We conclude that endothelin mediates the NO-dependent changes in the renal circulation of conscious rats during pregnancy.

Relaxin is a potent renal vasodilator in conscious rats

The kidneys and other nonreproductive organs vasodilate during early gestation; however, the "pregnancy hormones" responsible for the profound vasodilation of the renal circulation during pregnancy are unknown. We hypothesized that the ovarian hormone relaxin (RLX) contributes. Therefore, we tested whether the administration of RLX elicits renal vasodilation and hyperfiltration in conscious adult, intact female rats. After several days of treatment with either purified porcine RLX or recombinant human RLX 2 (rhRLX), effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) increased by 20%-40%. Comparable renal vasodilation and hyperfiltration was also observed in ovariectomized rats, suggesting that estrogen and progesterone are unnecessary for the renal response to rhRLX. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester completely abrogated the increase in ERPF and GFR elicited by chronic administration of purified porcine RLX. In contrast, the renal vasoconstrictory response to angiotensin II was attenuated by the RLX treatment. Short-term infusion of purified porcine RLX to conscious rats over several hours failed to increase ERPF and GFR. Plasma osmolality was consistently reduced by the chronic administration of both RLX preparations. In conclusion, the renal and osmoregulatory effects of chronic RLX administration to conscious rats resemble the physiological changes of pregnancy in several respects: (a) marked increases in ERPF and GFR with a mediatory role for nitric oxide; (b) attenuation of the renal circulatory response to angiotensin II; and (c) reduction in plasma osmolality.

The immunology of preeclampsia

The immune maladaptation hypothesis of preeclampsia is concordant with cytokine-mediated oxidative stress, chronology of endothelial activation, lipid changes, adverse effect of changing partners, and the protective effect of sperm exposure. Genetic factors may involve underlying hereditary thrombophilic disorders and hyperhomocysteinemia, essential hypertension and/or obesity, or control of the Th1/Th2 balance and thus affect the maternal response against fetal antigens. Placental ischemia and increased syncytiotrophoblast deportation are probably end-stage disease phenomena.

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.

Contribution of vasomotion to vascular resistance: a comparison of arteries from virgin and pregnant rats

Intrinsic oscillatory activity, or vasomotion, within the microcirculation has many potential functions, including modulation of vascular resistance. Alterations in oscillatory activity during pregnancy may contribute to the marked reduction in vascular resistance. The purpose of this study was 1) to mathematically model the oscillatory changes in vessel diameter and determine the effect on vascular resistance and 2) to characterize the vasomotion in resistance arteries of pregnant and nonpregnant (virgin) rats. Mesenteric arteries were isolated from Sprague-Dawley rats and studied in a pressurized arteriograph. Mathematical modeling demonstrated that the resistance in a vessel with vasomotion was greater than that in a static vessel with the same mean radius. During constriction with the alpha1-adrenergic agonist phenylephrine, the amplitude of oscillation was less in the arteries from pregnant rats. We conclude that vasomotor activity may provide a mechanism to regulate vascular resistance and blood flow independent of static changes in arterial diameter. During pregnancy the decrease in vasomotor activity in resistance arteries may contribute to the reduction in peripheral vascular resistance.

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.

The acute pressure natriuresis response blunted and the blood pressure response reset in the normal pregnant rat

OBJECTIVES

Our purpose was to test the hypothesis that the acute pressure natriuresis curve was reset in pregnancy to facilitate the volume expansion.

STUDY DESIGN

Studies were done with 14- to 16-day pregnant (n = 8) and age-matched virgin female (n = 6) Sprague-Dawley rats that were under general anesthesia. The left kidney was denervated, and mechanical clamps were placed on the aorta above and below the renal arteries for manipulation of renal perfusion pressure. Rats received intravenous 0.9% sodium chloride (1.5% body weight/h) and a cocktail of vasoactive factors to suppress variation in endogenous hormones. Renal perfusion pressure was varied acutely from 125 to 95 mm Hg, and glomerular filtration rate, renal plasma flow, sodium excretion, and urine flow were measured in both kidneys at each renal perfusion pressure. Data were analyzed by unpaired t test and by homogeneity by slopes.

RESULTS

The acute pressure natriuresis curve was blunted in pregnant rats versus virgins, and the renal nerves were not responsible. The blunted natriuretic response in pregnancy was due to loss of tubular epithelial responsiveness to increased blood pressure.

CONCLUSION

The pressure natriuretic response is markedly blunted in pregnancy, permitting the cumulative plasma volume expansion to occur. Contrary to nongravid states, blunting of the acute pressure natriuresis curve in pregnancy is not associated with increased blood pressure because of the profound peripheral vasodilation. This suggests an alteration in the mechanism(s) normally linking blood pressure control to the acute pressure natriuresis relationship.

Possible involvement of endothelium-derived hyperpolarizing factor in vascular responses of abdominal aorta from pregnant rats

Increased relaxant response to acetylcholine during pregnancy is proposed to be due to an estrogen-mediated increase in nitric oxide release. We studied acetylcholine-induced pathways of relaxation in the thoracic and abdominal aortic rings from pregnant and nonpregnant Wistar-Kyoto rats and measured basal and stimulated release of nitrites in these vessels. Endothelium-dependent relaxation was significantly greater in pregnant than in nonpregnant rats. Acetylcholine provoked a concentration-dependent relaxation on thoracic and abdominal aortic rings from nonpregnant and pregnant rats. After N118-nitro-L-arginine methyl ester pretreatment, the relaxation was significantly inhibited in the two preparations of nonpregnant and pregnant rodents. The relaxation was not inhibited by indomethacin in any of the aortic segments from pregnant and nonpregnant rats. After cytochrome P450 arachidonic acid metabolism inhibitor clotrimazole, a nonsignificant decrease in the Emax to acetylcholine-induced relaxation was observed in the thoracic segments of pregnant and nonpregnant rats. On the other hand, in abdominal aorta, clotrimazole decreased maximal relaxation in rings from pregnant rats (P<.05) but did not change the acetylcholine-induced relaxation from nonpregnant rats. Our results show an increase in the acetylcholine-stimulated release of nitrites in thoracic aortic rings from pregnant rats compared with rings from nonpregnant rats, which cannot be evidenced in abdominal aortic rings. These results suggest that acetylcholine-induced vasodilation in the abdominal segment from pregnant rats is mediated only in part by nitric oxide, the remainder apparently due to an endothelium-derived vasodilator, cytochrome P450-dependent, which may be endothelium-derived hyperpolarizing factor/epoxyeicosatrienoic acid.

Demonstration of the existence of nitric oxide-independent as well as nitric oxide-dependent vasodilator mechanisms in the in situ renal circulation in near term pregnant rats

1. We have investigated the role of endogenous nitric oxide on renal vascular reactivity in late pregnancy in in situ blood perfused kidneys of alpha-chloralose anaesthetized Wistar-Kyoto rats. Nitric oxide synthesis inhibition was achieved by intravenous administration of NG-nitro-L-arginine or NG-nitro-L-arginine methyl ester. 2. Intra-arterial mean blood pressure was lower in pregnancy compared with nonpregnant controls. Following nitric oxide synthesis inhibition mean blood pressure increased in both pregnant and nonpregnant groups, but remained lower in pregnant animals. 3. Basal renal perfusion pressure was similar in pregnant and nonpregnant groups. Intravenous administration of Ng-nitro-L-arginine resulted in dose-dependent increases in renal perfusion pressure but responses were substantially depressed in pregnancy. 4. Renal vasoconstrictor responses to regional angiotensin II (AII) were decreased in pregnancy, whereas those to noradrenaline (NA) did not differ from nonpregnant controls. NG-nitro-L-arginine (5 mg kg-1) potentiated renal responses to regional AII and NA in both groups, but AII responses remained lower in pregnancy. Blunted renal AII responses in pregnancy were still evident following large doses of NG-nitro-L-arginine methyl ester (100 mg kg-1). 5. The results demonstrate that nitric oxide synthesis inhibition increases renal perfusion pressure to a lesser extent in pregnant compared with nonpregnant rats, and that reduced renal pressor responses to AII are still evident in pregnancy after nitric oxide synthesis inhibition. 6. These results suggest that although endogenous nitric oxide synthesis modulates renal vasoconstrictor responses in both pregnant and nonpregnant animals, this mechanism does not fully account for the blunted renal vasconstrictor responses to regional AII or nitric oxide inhibitors in near term pregnant rats. The nature of this important physiological vasodilator mechanism in pregnancy remains to be elucidated.

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

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

Effects of 17 beta-oestradiol on vascular responses in the in situ blood-perfused mesentery of Wistar-Kyoto rats

1. To determine whether endogenous oestrogen plays a role in pregnancy induced decreased vascular reactivity we have examined the effects of 17 beta-oestradiol on vasoconstrictor responses to various stimuli using an in situ blood-perfused mesenteric vascular preparation in Wistar-Kyoto (WKY) rats. 2. Daily administration of 17 beta-oestradiol (500 micrograms/kg, s.c.) for 15 days significantly enhanced mesenteric vasoconstrictor responses to noradrenaline (NA), without affecting responses to the electrical stimulation of sympathetic nerves (ES) and angiotensin II (AngII). 3. Nitric oxide (NO) synthesis inhibition by nitro-L-arginine methyl ester (L-NAME; 100 mg/kg, i.v.) significantly potentiated mesenteric vasoconstrictor responses to all stimuli in both 17 beta-oestradiol-treated and control animals. The difference in NA responses between groups was diminished following NO synthesis inhibition. 4. These findings do not support the hypothesis that increased endogenous oestrogen plays a role in decreased mesenteric vascular reactivity in pregnancy. However, responses to oestrogen may be dose-dependent and enhancement of vasoconstrictor responses to NA may be relevant to oral contraceptive-induced hypertension.

The effect of chronic nitric oxide synthesis inhibition on blood pressure and angiotensin II responsiveness in the pregnant rat

OBJECTIVES

Our purpose was to determine whether blockade of inducible or endothelial nitric oxide synthesis prevents maternal vasodilation and blunting of angiotensin II responsiveness in the pregnant rat.

STUDY DESIGN

Pregnant and nonpregnant rats were given (1) drinking water alone (untreated), (2) drinking water containing the inducible nitric oxide synthase inhibitor aminoguanidine (0.5 gm/L), or (3) drinking water containing the nonselective nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (0.5 gm/L) from postmating days 5 to 21. On days 7, 14, and 20, 24-hour urinary nitrate-nitrite excretion, urine protein concentration, hematocrit, mean arterial blood pressure, and pressor responses to angiotensin II (12.5 to 200 ng/kg) were measured. On day 21 litter size, fetal weight, and fetal mortality were determined.

RESULTS

Urinary nitrate-nitrite excretion was increased, and hematocrit and blood pressure were decreased by day 20 of pregnancy. Angiotensin II pressor responses were decreased on days 14 and 20 of pregnancy. Aminoguanidine slightly decreased nitrate-nitrite excretion in pregnant, but not nonpregnant rats, and abolished the late pregnancy increase. Aminoguanidine did not affect hematocrit, blood pressure, or angiotensin II responsiveness in either pregnant or nonpregnant rats. N omega-nitro-L-arginine methyl ester greatly reduced nitrate-nitrite excretion and induced hypertension in both nonpregnant and pregnant rats, but on day 20 blood pressure of the pregnant rats was significantly lower than that of the nonpregnant rats. N omega-nitro-L-arginine methyl ester increased angiotensin II responsiveness on days 14 and 20 only in the pregnant rats. N omega-nitro-L-arginine methyl ester, but not aminoguanidine, increased fetal mortality and decreased fetal weight.

CONCLUSIONS

Inducible nitric oxide synthesis accounts for increased nitrate-nitrite excretion during pregnancy. Endothelium-derived nitric oxide may attenuate angiotensin II responsiveness but does not cause vasodilation and the fall in blood pressure during the last week of gestation.

Prostaglandins maintain renal vasodilation and hyperfiltration during chronic nitric oxide synthase blockade in conscious pregnant rats

Rats demonstrate renal vasodilation and hyperfiltration in pregnancy. Because both NO and cGMP biosynthesis are increased in gravid rats and because acute administration of NO synthase inhibitors abrogates renal vasodilation and hyperfiltration, NO most likely mediates the renal circulatory changes of gestation. In the present study, we tested the effect of chronic inhibition of NO synthase on effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) in chronically instrumented, conscious, gravid rats. Because gestation is a relatively long-term condition, we postulated that chronic withdrawal of NO would result in sustained inhibition of renal vasodilation and hyperfiltration. Contrary to our hypothesis, the renal circulatory changes of pregnancy were maintained during chronic blockade of NO synthase. That is, subcutaneous administration of 10 micrograms/min N omega-nitro-L-arginine methyl ester (NAME) for 48 hours did not significantly reduce GFR in either virgin or pregnant rats; thus, hyperfiltration persisted in the latter despite chronic NO synthase blockade. In contrast, ERPF was reduced and effective renal vascular resistance (ERVR) increased in both groups of rats during NAME administration but in a parallel fashion, such that renal vasodilation persisted in the gravid animals despite chronic inhibition of NO synthase. However, with superimposition of acute prostaglandin synthesis inhibition (meclofenamate, 10 mg/kg IV), renal vasodilation and hyperfiltration were abolished; ie, the combined treatments of chronic NO synthase blockade and acute prostaglandin synthesis inhibition led to the equalization of GFR, ERPF, and ERVR in conscious virgin and pregnant rats. Inhibition of prostaglandin synthesis alone had little affect on the renal circulation, as previously reported. In summary, prostaglandins are recruited to maintain renal vasodilation and hyperfiltration during chronic NO synthase blockade in conscious pregnant rats.

Bradykinin-mediated relaxation of isolated maternal resistance arteries in normal pregnancy and preeclampsia

OBJECTIVE

The aim of the study was to investigate bradykinin-mediated vasodilator function in small arteries from normotensive pregnant and nonpregnant women and from women with preeclampsia.

STUDY DESIGN

Small subcutaneous arteries (approximately 250 microns luminal diameter) were dissected from biopsy specimens obtained at cesarean section from 24 normotensive pregnant women and 6 women with preeclampsia and during abdominal surgery in 15 nonpregnant women. Vascular function was assessed after arteries were mounted on a small vessel myograph.

RESULTS

Preconstricted arteries from normotensive pregnant women demonstrated enhanced relaxation to bradykinin compared with those from nonpregnant women (p < 0.05), whereas arteries from women with preeclampsia showed blunted responses compared with those from normotensive pregnant women (P < 0.01). Relaxation in all groups was attenuated in the presence of the nitric oxide synthase inhibitor N omega-nitro-L-arginine so that it became similar in the three groups. Indomethacin had a small but significant inhibitory effect on bradykinin-induced relaxation, but this component of relaxation was no different among groups. Sensitivity of arteries to norepinephrine and sodium nitroprusside showed no significant differences in the three groups of women.

CONCLUSION

This study provides evidence for an increase in bradykinin-mediated nitric oxide synthesis from the vascular endothelium of small arteries from the peripheral circulation of normotensive pregnant women and a relative reduction in women with preeclampsia. In turn, these changes may contribute to vasodilation in normal pregnancy and elevation of the blood pressure in preeclampsia.

L-arginine reverses the adverse pregnancy changes induced by nitric oxide synthase inhibition in the rat

OBJECTIVE

Inhibition of nitric oxide synthase with N omega-nitro-L-arginine methyl ester (L-NAME) induces a preeclampsia-like syndrome of hypertension, proteinuria, intrauterine growth restriction, and renal glomerular capillary endothelial lesions in pregnant rats. We attempted to reverse these changes with late-pregnancy administration of L-arginine.

STUDY DESIGN

Sprague Dawley rats with timed pregnancies received infusions of either saline solution (n = 12) (group SC) or L-NAME (n = 12) (group LC) (160 mg/kg per day) on gestational day 10 through term. On gestational day 16 half of the saline solution group (group SA) and half of the L-NAME group (group LA) received L-arginine (21 mg/kg per day) through delivery. Systolic blood pressures were determined via tail cuff on days 10, 16, and 21. Pup weights were assessed at delivery, serum and urine were collected and analyzed for nitrites and nitrates, and renal tissue was processed for histologic examination. Data were analyzed with the one-way analysis of variance and the Newman-Keuls test for multiple comparisons.

RESULTS

In the L-NAME-treated animals L-arginine significantly lowered systolic blood pressure at late pregnancy (125 +/- 2.42 vs 153 +/- 3.0 mm Hg) (p < 0.01), increased mean pup weight (5.6 +/- 0.11 gm in group LA vs. 5.0 +/- 0.02 gm in group LC) (p < 0.001), decreased the degree of proteinuria (2+ vs trace), and decreased the proportion of injured glomeruli (7% vs 64%) (p < 0.001).

CONCLUSIONS

Lesions induced by chronic inhibition of endothelium-derived nitric oxide synthesis (hypertension, intrauterine growth restriction, proteinuria, renal glomerulus injury) are reversed by treatment with L-arginine. These findings lend support to the potential for use of nitric oxide donors in the treatment and prevention of preeclampsia.

Effects of pregnancy on femoral microvascular responses in the rat

OBJECTIVE

Our purpose was to test the hypothesis that augmented basal and vasodilator agonist-stimulated synthesis of endothelium-derived nitric oxide would blunt vasoconstrictor-induced contraction and enhance relaxation in isolated femoral resistance vessels from pregnant rats.

STUDY DESIGN

Potassium chloride-, phenylephrine-, and angiotensin II-induced contraction and acetylcholine-induced relaxation were assessed in small (approximately 240 microns diameter) femoral resistance arteries from gravid and virgin animals studied under isometric conditions; the effects of potassium chloride, phenylephrine, and acetylcholine were also studied in the presence of nitroarginine.

RESULTS

Maximal vasoconstriction was similar in vessels from virgin and pregnant rats, whereas phenylephrine potency was slightly enhanced in vessels from gravid animals. Nitroarginine augmented contraction in both groups but with greater effect on phenylephrine potency and on submaximal potassium chloride contractions in virgins. Acetylcholine-induced relaxation did not differ between groups and was similarly, but only partially, inhibited by nitroarginine.

CONCLUSIONS

Pregnancy does not result in vasoconstrictor resistance, increased basal nitric oxide synthesis, or augmented agonist-stimulated endothelium-dependent relaxation in this particular isolated rat microvascular preparation. Further, endothelium-dependent relaxation of these vessels depends only partly on nitric oxide.

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 long-term nitric oxide synthesis inhibition on plasma volume expansion and fetal growth in the pregnant rat

We conducted the present study to investigate whether the vasodilator nitric oxide plays a role in plasma volume homeostasis during pregnancy. Pregnant Sprague-Dawley rats were randomly assigned to a control group (n = 18) or to groups receiving 0.69 mmol/L (n = 11) or 1.7 mmol/L (n = 14) N omega-nitro-L-arginine, a competitive inhibitor of nitric oxide synthetase, from gestational days 7 through 21. On day 20 systolic pressure was measured. On day 21 blood samples were taken for plasma volume, hematocrit, and hormonal measurements. Fetal and placental weights also were determined. Systolic pressure was significantly higher in experimental rats (101 +/- 6 and 115 +/- 6 mm Hg in the 0.69 and 1.7 mmol/L groups, respectively) than in controls (79.7 +/- 7.5 mm Hg), and plasma volume was lower (18.4 +/- 1.1 and 17.1 +/- 0.5 mL) than in controls (21.5 +/- 0.8 mL). Both experimental groups had increased hematocrit levels. Plasma renin activity was significantly lower in the experimental groups (11.5 +/- 3 and 7.2 +/- 1.5 ng angiotensin I/mL per hour) than in controls (21.9 +/- 2.7 ng angiotensin I/mL per hour); however, no changes were observed in aldosterone levels. Experimental groups had lower fetal weight (4.6 +/- 0.1 and 5.1 +/- 0.1 g) than controls (5.5 +/- 0.1 g). In addition, fetal hindlimb hypoplasia was observed in the experimental groups. In conclusion, the present data indicate that long-term N omega-nitro-L-arginine administration to pregnant rats leads to increased blood pressure, reduced plasma volume expansion, lower plasma renin activity, and fetal growth retardation.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelial modulation of vascular tone and 5-hydroxytryptamine-induced responses in human chorionic arteries and veins

1. In vitro studies were undertaken to evaluate the potential role of endothelium through release of vasodilators compounds on basal tension and on vasoconstrictor response induced by 5-hydroxytryptamine (5-HT) in chorionic arteries and veins rings from normotensive gestants. 2. N omega-nitro-L-arginine (100 microM) and methylene blue (10 microM) increased significantly the basal tension in chorionic arteries but not in veins. However, indomethacin (10 microM) inhibited the basal tension in both kinds of vessels. 3. The sensitivity to 5-HT was significantly lower in placental arteries than in veins. Removal of endothelium increased the sensitivity of placental arteries to 5-HT in 2.2-fold; however, in veins denudation had no significant effect. 4. N omega-nitro-L-arginine potentiated the 5-HT-induced tone in both types of vessels, but the increase of contraction was greater in chorionic arteries than in veins. However, indomethacin decreased the 5-HT-induced contractions in arteries and veins. 5. These results indicate that NO is more important than vasodilators prostanoids in the control of vascular tone and in 5-HT-induced placental contractile response.

Acute blockade of nitric oxide synthase inhibits renal vasodilation and hyperfiltration during pregnancy in chronically instrumented conscious rats

Because the kidneys are vasodilated and the endogenous production of nitric oxide is increased in gravid rats, we tested whether nitric oxide mediates the renal vasodilatory response to pregnancy. Chronically instrumented, conscious rats of gestational days 12-14 were studied concurrently with age-matched virgin control animals. GFR and effective renal plasma flow (ERPF) were determined by the renal clearances of inulin and para-aminohippurate before and during acute infusion of N omega-nitro-L-arginine methyl ester (NAME; 2, 20, and 50 micrograms/min) or NG-monomethyl-L-arginine (100 micrograms/min). Baseline GFR and ERPF were significantly increased, and effective renal vascular resistance was decreased by 30-40% in gravid rats compared with virgin controls. During infusion of all three dosages of NAME and NG-monomethyl-L-arginine, effective renal vascular resistance, GFR, and ERPF were equalized in the pregnant and virgin rats (the only exception being GFR during the 20 micrograms/min NAME infusion). When compared with virgin rats, the gravid animals were more responsive to nitric oxide synthase inhibition, showing a significantly greater decline in GFR and ERPF and rise in effective renal vascular resistance at each timepoint during the infusion of inhibitor. To exclude the possibility that nonspecific renal vasoconstriction per se led to equalization of renal function in the two groups of rats, we investigated angiotensin II. In contrast to the results observed with nitric oxide synthase inhibitors, pregnant rats were less responsive to the renal vasoconstrictory effects of angiotensin II, such that the baseline differences in renal parameters measured before infusion of the hormone were increased during the infusion. To determine whether nitric oxide synthase was inhibited to a similar extent in gravid and virgin rats, aortic and renal cortical cGMP content was assayed ex vivo at the end of inhibitor infusion. The lower 2-micrograms/min dose of NAME consistently reduced cGMP content of these tissues to comparable levels in the two groups of rats. In conclusion, we suggest that nitric oxide mediates reduced renal vascular resistance and hyperfiltration during pregnancy in conscious rats.

Calcium supplementation prevents pregnancy-induced hypertension by increasing the production of vascular nitric oxide

Pregnancy-induced hypertension (PIH) remains a common cause of maternal and fetal morbidity and mortality. During the past 7 years, some progress has been made in the prevention of PIH. Specifically, clinical studies have shown that supplementation with calcium can significantly reduce the frequency of PIH, specially in populations with a low calcium intake. We have suggested that, in such a population, calcium supplementation is a safe and effective measure for reducing the frequency of PIH. Thus, the purpose of this article is to advance a hypothesis about the mechanism by which calcium supplementation reduces the risk of PIH. We propose that dietary calcium supplementation reduces the frequency of PIH by maintaining the serum ionized calcium level which is crucial for the production of endothelial nitric oxide, the increased generation of which maintains the vasodilatation that is characteristic of normal pregnancy.

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)

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.

Nitric oxide biosynthesis during pregnancy: implications for circulatory changes

1. The biosynthesis of NO and its second messenger, cGMP, increases from pre-pregnant levels during rat gestation. An increase in plasma level and urinary excretion of cGMP is also evident during human pregnancy. However, the relative contribution of the maternal vasculature and other tissues to increased NO and cGMP biosynthesis during gestation is uncertain. Consensus is lacking about the contribution of NO to reduced maternal vascular tone and reactivity during gestation in various organ beds; clearly, further investigation is still needed. That NO may also regulate vascular smooth muscle behaviour during pregnancy by altering membrane potential is another intriguing possibility. 2. The syncytiotrophoblast of the human placenta expresses significant NO synthase activity, and along with the fetoplacental endothelium undoubtedly contributes to NO production during pregnancy. 3. Finally, it should be emphasized that vascular studies in gravid animal models need to be extended to pregnant women.

Endothelium-dependent relaxation of human resistance arteries in pregnancy

OBJECTIVE

Reduced peripheral vascular resistance is an integral component of vascular adaptation in pregnancy. The precise mediators of the reduced resistance are unknown, however, and are often assessed by indirect means. In this study we investigated endothelium-dependent relaxation directly by examining acetylcholine-mediated relaxation in small arteries dissected from the subcutaneous fat layer undergoing biopsy at the time of gynecologic surgery and cesarean section.

STUDY DESIGN

By means of a small vessel myograph we measured tension in resistance arteries of normal pregnant (n = 22) and nonpregnant (n = 10) women and assessed the contributions of vasodilatory prostanoids and endothelium-derived relaxing factor on endothelium-dependent relaxation, as elicited by acetylcholine (1 nmol/L to 10 mumol/L) after precontraction with 3 mumol/L norepinephrine.

RESULTS

Endothelium-dependent relaxation was similar in arteries of pregnant and nonpregnant women. Sodium nitroprusside-induced relaxation was similar in arteries of pregnant and nonpregnant women.

CONCLUSION

This study fails to demonstrate any increase in acetylcholine-induced endothelium-dependent relaxation in small subcutaneous arteries in pregnancy.

Effects of prostaglandin inhibition on the renal function curve during ovine pregnancy

OBJECTIVE

This study was designed to assess the influence of prostaglandins on the steady-state relationship of mean arterial pressure and urinary sodium excretion.

STUDY DESIGN

Ten nonpregnant and nine pregnant ewes were chronically prepared with indwelling catheters and maintained in metabolism cages. All the ewes received a continuous intravenous infusion of indomethacin (5 mg/kg/day) for 24 days. Step increases in sodium intake (20, 100, 400, and 1200 mmol per day), each maintained for 6 days, were administered as a fixed dietary (20 mmol) component with the balance infused intravenously. Mean arterial pressure was continuously monitored. The renal function curves were constructed from the steady-state mean arterial pressure-urinary sodium excretion relationships after sodium balance was attained.

RESULTS

Indomethacin inhibited urinary excretion of prostaglandin E2 by 62% +/- 4% in nonpregnant and 47% +/- 6% in pregnant ewes, and 6 keto-prostaglandin F1 alpha, the stable metabolite of prostaglandin I2, by 55% +/- 4% and 65% +/- 6% in nonpregnant and pregnant ewes, respectively. Blood volume was not affected by indomethacin or salt in either group. Plasma angiotensin II was suppressed less in pregnant ewes (60% vs 80%) by increases in salt intake. Chronic treatment with indomethacin did not alter the renal function curves of either nonpregnant or pregnant ewes.

CONCLUSIONS

These results suggest that prostaglandins do not influence the steady-state relationship between mean arterial pressure and urinary sodium excretion in either nonpregnant or pregnant sheep.

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.

Induction of calcium-dependent nitric oxide synthases by sex hormones

We have examined the effects of pregnancy and sex hormones on calcium-dependent and calcium-independent nitric oxide synthases (NOSs) in the guinea pig. Pregnancy (near term) caused a > 4-fold increase in the activity of calcium-dependent NOS in the uterine artery and at least a doubling in the heart, kidney, skeletal muscle, esophagus, and cerebellum. The increase in NOS activity in the cerebellum during pregnancy was inhibited by the estrogen-receptor antagonist tamoxifen. Treatment with estradiol (but not progesterone) also increased calcium-dependent NOS activity in the tissues examined from both females and males. Testosterone increased calcium-dependent NOS only in the cerebellum. No significant change in calcium-independent NOS activity was observed either during pregnancy or after the administration of any sex hormone. Both pregnancy and estradiol treatment increased the amount of mRNAs for NOS isozymes eNOS and nNOS in skeletal muscle, suggesting that the increases in NOS activity result from enzyme induction. Thus both eNOS and nNOS are subject to regulation by estrogen, an action that could explain some of the changes that occur during pregnancy and some gender differences in physiology and pathophysiology.

Estrogen withdrawal selectively increases serotonin reactivity in rabbit basilar artery

Clinical observations and laboratory investigations suggest that gender and menstrual status modulate cerebrovascular reactivity. We prepared 7 groups of rabbits (I) males (II) oophorectomized untreated females, (III) testosterone treated oophorectomized females, (IV) superovulated females, (V) superovulated estrogen withdrawn females, (VI) estrogen treated oophorectomized females, and (VII) estrogen withdrawn females to mimic phases of the estrous cycle and compare cerebral basilar artery reactivity to serotonin (5-HT) and norepinephrine (NE) in vitro. Basilar artery sensitivity to 5-HT vasoconstriction was increased in oophorectomized, acutely estrogen withdrawn females (Group VII) when compared to estrogen maintained and the other groups (p < 0.0001). There was a significant reduction in 5-HT sensitivity in superovulated females (Group IV) (p < 0.001). The change in 5-HT sensitivity is selective and was not observed for NE. Nitroarginine treatment and mechanical denudement resulted in higher Tmax and lower ED50 for both NE and 5-HT regardless of hormonal manipulation. We conclude that estrogen withdrawal increases 5-HT vasoreactivity by an endothelium independent mechanism.

Indomethacin does not modify the role of nitric oxide on blood pressure regulation of SHR

1. The endothelium-dependent relaxation is impaired in spontaneously hypertensive rats (SHR) by the release of a vasoconstrictor prostanoid. We evaluated whether such a vasoconstrictor prostanoid is masking the vasodilatation induced by nitric oxide (NO). 2. For this we observed, in SHR, whether indomethacin (INDO) modified both the pressor response to the inhibition of NO biosynthesis with L-nitro-arginine methyl ester (L-NAME) and the acute hypotensive response to acetylcholine. 3. INDO did not modify basal mean arterial pressure (MAP), either the pressor response to L-NAME, or the depressor response to acetylcholine. 4. It shows that, in awake SHR, a vasoconstrictor prostanoid, did not seem to affect the acute regulatory function of NO on MAP.

Efficacy and safety of indomethacin therapy for polyhydramnios

The maternal and perinatal outcome of seven gravidas receiving 2.2-2.5 mg/kg per day of indomethacin for polyhydramnios are reported. Such therapy was started between 26 and 33 weeks of gestational age (mean, 30.4 weeks) and lasted for 20.1 days (range, 2-37 days). Median of amniotic fluid index ranged from 47 at the start of therapy (range, 32-53) to 15 (range, 2-50) when indomethacin was ended. Interval between the end of the therapy and the delivery ranged from 0 to 45 days (mean, 15 days). On average, pregnancies were prolonged by 5.1 weeks (range, 2-8 weeks). The newborn weight was 2678 g on average (range, 620-3700 g). Oligohydramnios was seen in two instances; one patient developed constriction of the fetal ductus arteriosus, which returned to normality after indomethacin suppression; one newborn in which other causes of neonatal bleeding could be excluded, developed a disseminated intravascular coagulation and died 15 h after birth. Finally, one mother presented an acute renal failure immediately after indomethacin administration; this patient completely recovered after indomethacin withdrawal. Thus, the benefit of pregnancy prolongation should be balanced against the increased risks for the newborn, mainly fetal ductus arteriosus constriction and possible bleeding disorders. A causal relationship of indomethacin administration to the latter complication warrants further investigation.

The role of endothelium in phenylephrine- and potassium-induced contractions of the rat aorta during pregnancy

The involvement of endothelium in the contractile responses of rat aortic rings to phenylephrine and potassium chloride in pregnancy was examined. Contractions in response to both agents were significantly greater in rings from non-pregnant rats than in rings from pregnant rats, and they were unaltered by treatment of the rings with indomethacin. De-endothelialization potentiated the contractions of rings from pregnant rats in response to phenylephrine, but had no significant effect on similar rings contracted with potassium chloride. Whereas de-endothelialization had no significant effect on the contractions to phenylephrine in rings from non-pregnant rats, it decreased those of rings from the same type of rats, contracted with potassium chloride. Pregnancy significantly inhibited contractions in response to calcium chloride of rings treated with phenylephrine or potassium chloride. The effect of endothelium removal on contractions to calcium chloride in rings from pregnant and non-pregnant rats treated with phenylephrine or potassium chloride was similar to that observed for phenylephrine-induced and potassium chloride-induced contractions, respectively. Contractions of intact aortic rings from pregnant and non-pregnant rats to phenylephrine in calcium-free medium were similar. Results of the study suggest that the effect of pregnancy on the contractions of the rat aorta in response to phenylephrine and potassium chloride is at least partly mediated by the endothelium and is independent of prostaglandin synthesis. The endothelial factor involved in this effect appears to modulate contractions by interfering with calcium influx through the receptor-operated calcium channels and the voltage-operated calcium channels.

Inhibition of nitric oxide synthesis in rats during pregnancy produces signs similar to those of preeclampsia

OBJECTIVES

Preeclampsia is associated with hypertension, fetal growth retardation, and proteinuria. We hypothesized that impaired vascular nitric oxide synthesis during pregnancy may be an important causal factor in preeclampsia.

STUDY DESIGN

An inhibitor of nitric oxide synthase, L-nitro-arginine methyl ester, or a nitric oxide donor, nitroglycerin, was infused subcutaneously to rats at a constant rate from day 17 of gestation. Systolic blood pressure, day of spontaneous delivery, weight, and mortality rate of pups were recorded.

RESULTS

Systolic blood pressures in rats infused with L-nitro-arginine methyl ester at daily doses of both 25 and 50 mg were significantly elevated compared with controls. This treatment also caused a substantial decrease in the weight of pups, with an increase in mortality rate, without affecting the gestational length. These effects were dose dependent. Nitroglycerin infusion, on the other hand, affected neither the weight and mortality rate of the pups nor the length of gestation.

CONCLUSIONS

Infusion of an inhibitor of nitric oxide synthesis during pregnancy causes hypertension and fetal growth retardation, without affecting gestational length. These signs are similar to those of preeclampsia and indicate that an alteration in nitric oxide synthesis may be one of the factors responsible for this disorder. Treatment with nitric oxide inhibitors may be used in an animal model for preeclampsia, to test various therapeutic strategies.

Pregnancy and lipid peroxide-induced alterations of eicosanoid-metabolizing enzymes in the aorta of the rat

OBJECTIVES

We examined whether pregnancy modulates the expression of prostaglandin endoperoxide synthase, prostacyclin synthase, and thromboxane A2 synthase in the systemic vasculature. Further, we examined whether elevated lipid peroxidation during pregnancy (induced by vitamin E deprivation) affects the normal adaptive process to pregnancy.

STUDY DESIGN

Western immunoblotting was performed on aortas from normal and vitamin E-deprived late pregnant (18 to 19 days) and age-matched virgin control rats.

RESULTS

Normal pregnancy resulted in an increased expression of prostaglandin endoperoxide synthase (2.91 vs 1.06 fmol/ng deoxyribonucleic acid, p < 0.05). Surprisingly, the expression for both prostacyclin and thromboxane A2 synthases were significantly decreased by pregnancy: prostacyclin synthase 2.60 versus 13.82 units/ng deoxyribonucleic acid and thromboxane A2 synthase 1.32 versus 9.85 units/ng of deoxyribonucleic acid. Elevation of endogenous lipid peroxidation partially reversed this normal pregnancy trend in enzyme expression: prostaglandin endoperoxide synthase 1.85 fmol/ng deoxyribonucleic acid, prostacyclin synthase 9.38 units/ng deoxyribonucleic acid, thromboxane A2 synthase 4.36 units/ng deoxyribonucleic acid.

CONCLUSION

Changes in prostanoid activity in the systemic vasculature during pregnancy may be mediated by concerted induction and down-regulation of specific enzymes. Increased lipid peroxidation interferes with this normal pregnant pattern. Further studies on the cell-specific expression of these genes will help to define the cardiovascular role of prostaglandins in pregnancy and in preeclampsia.

Mechanisms of vasodilatation in pregnancy: studies of the role of prostaglandins and nitric-oxide in changes of vascular reactivity in the in situ blood perfused mesentery of pregnant rats

1. To examine the possible mechanisms of the vasodilatation and blunted pressor responses in late pregnancy, we have studied vascular reactivity of the in situ blood perfused mesenteric resistance vessels of 18-20 day pregnant Wistar-Kyoto rats (WKY). 2. Intra-arterial mean blood pressure (MBP) was lower in pregnant rats than in nonpregnant controls. There was no significant difference in basal mesenteric perfusion pressure (BPP) between groups. 3. Vascular reactivity to electrical stimulation (ES) or intra-arterial noradrenaline (NA), angiotensin II (AII) and arginine vasopressin (AVP) was decreased in the preparations from pregnant rats compared to that from nonpregnant controls. Noradrenaline spillover into mesenteric venous blood following ES was similar in pregnant and nonpregnant animals. 4. Indomethacin (5 mg kg-1, i.v.), an inhibitor of cyclo-oxygenase, induced significant increases in reactivity to ES in both pregnant and nonpregnant groups while potentiating the responses to NA and AII in nonpregnant animals only and having no effect on AVP-induced contractions in the preparations from either pregnant or nonpregnant animals. 5. NG-nitro-L-arginine (L-NOARG) (5 mg kg-1, i.v.), an inhibitor of nitric-oxide synthase, increased MBP and BPP in both pregnant and nonpregnant animals, but the difference in MBP between groups was still evident. 6. L-NOARG enhanced mesenteric vascular responses to ES, NA and AII in both pregnant and nonpregnant groups. Only the difference in NA responses between groups was abolished after pretreatment with L-NOARG. 7. These data show that vasoconstrictor responses to a variety of agonists are decreased in the in situ blood-perfused mesenteric resistance vessels of pregnant rats. Increase in endothelial-dependent nitric oxide generation could contribute to the vasodilatation seen in pregnancy but other mechanisms might also be involved. Cyclo-oxygenase products are not responsible for any decreased contractile responses in this preparation.

Endogenous modulation of the blunted adrenergic response in resistance-sized mesenteric arteries from the pregnant rat

OBJECTIVE

We tested the hypothesis that during pregnancy the endothelium mediates the blunted response to adrenergic vasoconstriction.

STUDY DESIGN

Mesenteric resistance arteries from late pregnant (n = 6) and age-matched virgin control (n = 6) Sprague-Dawley rats were studied in a myograph.

RESULTS

Arteries from pregnant rats were 35% less sensitive to phenylephrine vasoconstriction than were those from nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.48 mumol/L, pregnant vs nonpregnant, p < 0.01). Meclofenamate had no effect on the vasoconstrictor response in arteries from either group. Inhibition of endothelium-derived relaxing factor with N(o)-nitro-L-arginine methyl ester or endothelial cell removal had a similar twofold increase in phenylephrine sensitivity in arteries from both the pregnant and nonpregnant rats (mean effective concentration that produced a 50% response 2.26 vs 1.11 mumol/L for pregnant rats and 1.48 vs 0.72 mumol/L for nonpregnant rats, p < 0.01). However, methacholine relaxation response was potentiated in pregnant versus nonpregnant rats (mean effective concentration that produced a 50% response 0.030 vs 0.049 mumol/L, p < 0.01).

CONCLUSION

Although the potential for endothelium-dependent relaxation is augmented in mesenteric arteries of the pregnant rat, the decreased sensitivity to phenylephrine during pregnancy is not modulated acutely by endothelium-derived relaxing factor or by prostaglandin products of the cyclooxygenase pathway.

Endothelium-derived relaxing factor inhibition augments vascular angiotensin II reactivity in the pregnant rat hind limb

OBJECTIVE

The purpose of our study was to determine whether endothelium-derived relaxing factor plays a role in the blunting of maternal vascular reactivity in pregnancy.

STUDY DESIGN

We measured the concentration-pressor responses to norepinephrine (10(-8) to 10(-4) mol/L and angiotensin II (10(-10) to 10(-6) mol/L) in isolated, perfused hind limbs of nonpregnant and pregnant (postmating day 20 to 21) normotensive Wistar-Kyoto and spontaneously hypertensive rats. The hind limbs were perfused at 4 ml/min with Krebs-Ringer solution containing indomethacin (10(-5) mol/L to inhibit prostaglandin production and were either infused with N omega-monomethyl-L-arginine (10(-4) mol/L), a specific inhibitor of endothelium-derived relaxing factor synthesis, or 0.9% saline solution (untreated).

RESULTS

Baseline perfusion pressure was similar in the nonpregnant and pregnant hind limbs of both strains, and N omega-monomethyl-L-arginine had no effect on perfusion pressure. Norepinephrine induced similar pressor responses in the nonpregnant and pregnant hind limbs of both strains, and N omega-monomethyl-L-arginine did not alter these responses. Angiotensin II pressor responses were significantly attenuated in the pregnant rat hind limbs compared with the nonpregnant rat hind limbs. N omega-monomethyl-L-arginine enhanced the angiotensin II responses in the pregnant, but not in the nonpregnant, rat hind limbs.

CONCLUSION

The results suggest that rat pregnancy is not associated with generalized refractoriness to all vasoconstrictors and that endothelium-derived relaxing factor plays a role in attenuating vascular reactivity to angiotensin II.

N omega-nitro-L-arginine, an inhibitor of nitric oxide synthesis, increases blood pressure in rats and reverses the pregnancy-induced refractoriness to vasopressor agents

OBJECTIVE

With N omega-nitro-L-arginine, a potent inhibitor of nitric oxide synthesis, we tested the hypothesis that nitric oxide plays a functional role in the blunted pressor responsiveness seen during pregnancy.

STUDY DESIGN

A group of six pregnant rats were instrumented on the fourteenth day of gestation and studied on days 19 and 20, as well as 7 days post partum. Another group of six virgin rats were similarly prepared and used 5 days after surgery. Blood pressure and heart rate were monitored in conscious freely moving animals before and during the administration of drugs or placebo. Results were analyzed, by one-way repeated-measures analysis of variance, with Dunnett's t test, or by paired t test where applicable.

RESULTS

Basal mean arterial pressure and heart rate were 90.8 +/- 3.0 mm Hg and 330 +/- 6 beats/min in pregnant animals and 107.1 +/- 3.2 mm Hg and 315 +/- 7 beats/min in nonpregnant animals. Pressor responses to angiotensin II, vasopressin, and norepinephrine were attenuated in gravid animals. Infusion of N omega-nitro-L-arginine significantly and in a dose-dependent manner increased mean arterial pressure and reduced heart rate. These effects could be completely reversed by L-arginine administration. Changes in mean arterial pressure were higher during pregnancy as compared with postpartum values. N omega-nitro-L-arginine infusion potentiated pressor responses to all three vasopressors, resulting in dose-response curves that were significantly shifted to the left, making them virtually identical in pregnant and postpartum rats.

CONCLUSION

Our data support the emerging view that nitric oxide plays a key role in the regulation of blood pressure during pregnancy.

The effect of pregnancy on renal function: physiology and pathophysiology

Marked changes in renal function occur with pregnancy. We present a summary of these changes in this review and give insight into possible mechanisms if they are known. Controversies exist regarding the therapy of pregnancy-induced hypertension and asymptomatic and recurrent bacteriuria. The current views on these topics are given. Specific renal diseases are summarized, including transplantation, and optimum management strategies and maternal and fetal prognosis during pregnancy are given.

Prostaglandin- or endothelium-mediated vasodilation is not involved in the blunted responses of blood vessels to vasoconstrictors in pregnant rats

Pregnancy is associated with decreases of blood pressure and vascular sensitivity to vasopressor agents. We have hypothesized that the increased liberation of endogenous vasodilator(s) by the vascular endothelium or other structures could mediate these blunted responses. Thoracic aorta rings of nonpregnant, 21 days pregnant, and first day post partum rats respond similarly to acetylcholine, an endothelium-dependent vasorelaxant. In contrast, the potency of the response to sodium nitroprusside, an endothelium-independent vasorelaxant, is unchanged in tissues of pregnant rats and increased (p less than 0.05) in those of post partum animals. In the presence of indomethacin (10 mumol/L) the three groups of tissues show a decreased potency. The effects of phenylephrine on aortic rings of both nonpregnant and pregnant rats are markedly increased in the presence of Ng-monomethyl-L-arginine. Indeed, the concentration producing 50% of the maximum response of phenylephrine decreases (p less than 0.001) from 50.7 to 8.02, from 93.8 to 37.6, and from 60.4 to 5.97 nmol/L with the use of Ng-monomethyl-L-arginine (0.1 mmol/L) in rings from nonpregnant, pregnant, and postpartum rats, respectively. Simultaneously, the maximum response to phenylephrine increases markedly in the three groups of tissues. In the presence of Ng-monomethyl-L-arginine, indomethacin does not influence the response to phenylephrine. Our results do not support the possible involvement of an endogenous vasodilator (prostaglandin-like or endothelium-derived) in the blunted responses to vasoconstrictors during pregnancy.

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

Pregnancy is normally associated with vasodilation that, in hypertensive animals such as the spontaneously hypertensive rat, causes a profound decrease in blood pressure. To test the possibility that enhanced basal endothelium-derived relaxing factor activity has a role in the vasodilation of pregnancy, we measured the changes in mean arterial pressure and heart rate induced by NG-monomethyl-L-arginine, a specific inhibitor of endothelium-derived relaxing factor synthesis, in conscious nonpregnant and pregnant (postmating day 20 to 21) normotensive Wistar-Kyoto and spontaneously hypertensive rats. NG-monomethyl-L-arginine caused similar dose-dependent increases in mean arterial pressure in nonpregnant and pregnant Wistar-Kyoto rats, but the accompanying decrease in heart rate was significantly greater in nonpregnant rats than in pregnant ones. In the spontaneously hypertensive rats, NG-monomethyl-L-arginine caused significantly greater dose-dependent increases in mean arterial pressure in pregnant compared with nonpregnant rats; there were no differences in the decreases in heart rate. These pressor responses were partially reversed by excess L-arginine but not D-arginine. Indomethacin had no effect on the pressor response to NG-monomethyl-L-arginine or the depressor response to L-arginine after NG-monomethyl-L-arginine. Therefore basal endothelium-derived relaxing factor plays a role in vascular tone and blood pressure regulation in vivo, and pregnancy may be associated with enhanced basal endothelium-derived relaxing factor activity in the hypertensive spontaneously hypertensive rats.

Evidence for attenuation of myo-inositol uptake, phosphoinositide turnover and inositol phosphate production in aortic vasculature of rats during pregnancy

We postulated that vascular phosphoinositide metabolism is attenuated during pregnancy, and thereby could contribute to maternal vasodilation and reduced vascular reactivity. The basal rate of incorporation of [3H]myo-inositol and [3H]glycerol into phosphoinositides of aortae from pregnant rats in vitro was significantly reduced, when compared with vessels from virgin animals. After injection of [3H]myo-inositol intravenously into chronically instrumented conscious pregnant and virgin rats, the incorporation of the label by phosphatidylinositol was 66 +/- 4% less in aortae of gravid versus virgin animals (P less than 0.001), despite comparable plasma concentrations of radioactivity. Fold stimulation of total [3H]inositol phosphates by arginine vasopressin, norepinephrine, and angiotensin II over a 15-min period was not different between aortic segments from virgin and gravid rats, although both absolute basal and stimulated levels were significantly less in vessels from pregnant animals. After 45 s of incubation with 10(-7) M arginine vasopressin, however, the fold-stimulation of [3H]inositol trisplus tetrakisphosphate was reduced in aortae from gravid rats, when compared with vessels from virgin animals (P less than 0.005). By HPLC, greater than 90% of the radioactivity in the [3H]inositol trisplus tetrakisphosphate column fraction after 30 and 60 s of agonist stimulation was [3H]inositol-1,4,5-trisphosphate. We further observed that the rate of uptake of [3H]myo-inositol by aortic vasculature obtained from gravid rats was significantly (24%) less than uptake by vessels from virgin animals. Plasma myo-inositol concentrations were not significantly different, but presumably as a consequence of reduced uptake, aortic segments freshly isolated from pregnant rats contained 22 +/- 6% less myo-inositol than vessels from virgin controls as measured by gas chromatography-mass spectrometry (P less than 0.03). We conclude that myo-inositol uptake and content, phosphoinositide turnover, and inositol phosphate production are reduced in aortic vasculature of gravid rats.

Prostaglandin E2 and gestational hypotension in rabbits

Arterial levels of 13,14-dihydro-15-keto-PGE2 (PGE2-M), a stable metabolite of prostaglandin E2 (PGE2) were compared between unanesthetized pregnant (n = 12) and nonpregnant (n = 8) rabbits with the aim of elucidating the role PGE2 in the development of physiological hypotension associated with pregnancy. On the 20th and 22nd days of the 30 day gestation period the mean arterial concentrations of PGE2-M were about 10-times higher (p less than 0.05) and largely variable as compared to that of nonpregnant rabbits. Mean arterial pressure was not lower on either the 20th (69 +/- 4 mmHg, mean +/- SD) or the 22nd (70 +/- 3 mmHg) days of gestation (dg) than in nonpregnant rabbits (69 +/- 4 and 73 +/- 6 mmHg, respectively). On the 23rd dg hypotension was invariably present (61 +/- 5 mmHg vs 72 +/- 4 in nonpregnants, p less than 0.001), but arterial levels of PGE2-M (31.0 +/- 31.6 ng/ml) did not overcome those measured on earlier, normotensive days of gestation. Hypotension was also evident in a subgroup of pregnant rabbits (n = 4) with low PGE2-M concentrations in the nonpregnant range (3.2 +/- 1.5 ng/ml vs 1.9 +/- 1.2 in nonpregnant rabbits, ns). Since the arterial level of PGE2-M proved to correlate (p less than 0.001) with both the uteroplacental venous and renal venous PGE2 concentrations, we suggest that a key role of uteroplacental and renal PGE2 played in the development of gestational hypotension is not probable in rabbits.

Role of the endothelium in adrenergic responses of mesenteric artery rings of pregnant rats

Responses to adrenergic agents were measured on rings of mesenteric, artery of nonpregnant and pregnant rats at 21 days' gestation. In tissues from pregnant rats, norepinephrine, and phenylephrine were less sensitive than in rings from nonpregnant animals; maximum responses were not affected. The effect of isoproterenol was not modified in tissues from pregnant rats. The presence of endothelium in tissue rings was evaluated as fractional relaxation to acetylcholine. Sensitivity to phenylephrine and norepinephrine was inversely related to the effect of acetylcholine. This relationship was similar in slope in both groups but different in intercept. The sensitivity to isoproterenol was directly related to the presence of endothelium (curves similar in both slope and intercept). This demonstrates that the responses of mesenteric artery rings depends on the presence of endothelium. Furthermore, vascular endothelium is not involved in the blunted responses to adrenergic vasoconstrictor during pregnancy.

Pressor responsiveness to endothelin is not attenuated in gravid rats

The aim of this study was to evaluate the effect of synthetic human/porcine endothelin (ET-1) on mean arterial blood pressure (MAP) in pregnant and non-pregnant rats and to compare this to the effects of two well characterized agonists, angiotensin II (AII), and vasopressin (VP). On day 14 of gestation (parturition day 22) polyethylene catheters were chronically implanted in the abdominal aorta for monitoring of MAP and in the vena cava for administration of drugs. Pressor responsiveness was measured in conscious freely moving animals on day 20 and again on the 7th day post-partum. All three agonists increased MAP in a dose related manner. However, whereas the sensitivity of pregnant rats (P) to AII and VP was significantly blunted compared to postpartal (PP) measurements, the MAP responses to ET-1 were the same in both groups. Moreover, the combined administration of ET-1 at a subpressor dose (0.05 pmol/100 g bw) and AII or VP at effective doses significantly potentiated (particularly in P) the pressor effects of AII and VP. These results demonstrate that ET-1 and possibly other vasoactive substances of endothelial origin, override the compensatory mechanism of normal pregnancy with respect to the blunted responsiveness to AII and VP. Such a mechanism may be of particular relevance in the evolution of pregnancy-induced hypertension.