Decreased in vitro responses to vasoconstrictors during gestation in normotensive and spontaneously hypertensive rats

The Involvement of MicroRNAs in SARS-CoV-2 Infection Comorbid with HIV-Associated Preeclampsia

Purpose of Review

This review investigated the potential role of microRNAs (miRNAs) in the synergy of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, preeclampsia (PE), and human immunodeficiency virus (HIV) infection. Maternal health is a great concern when treating pregnant women fighting this triad of diseases, which is highly prevalent in South Africa. MicroRNAs are involved in fine-tuning of physiological processes. Disruptions to the balance of this minute protein can lead to various physiological changes that are sometimes pathological.

Recent Findings

MicroRNAs have recently been implicated in PE and have been linked to the anti-angiogenic imbalance evident in PE. Recent in silico studies have identified potential host miRNAs with anti-viral properties against SARS-CoV-2 infection. Studies have demonstrated dysregulated expression of several miRNAs in HIV-1 infection along with the ability of HIV-1 to downregulate anti-viral host microRNAs.

Summary

This review has highlighted the significant gap in literature on the potential of miRNAs in women with HIV-associated PE in synergy with the novel SARS-CoV-2 infection. In addition, this review has provided evidence of the critical role that the epigenetic regulatory mechanism of miRNA plays in viral infections and PE, thereby providing a foundation for further research investigating the potential of therapeutic miRNA development with fewer side-effects for pregnant women.

Kinetics of Postpartum Mesenteric Artery Structure and Function Relative to Pregnancy and Lactation in Mice

Epidemiological evidence suggests that normal pregnancy in women is associated with decreased cardiovascular risk in later life. Clinical studies have provided evidence that alterations in vascular function and structure are detectable long after delivery. To understand these findings, we examined mesenteric artery reactivity at both early (3 days and 2–4 weeks) and late (12 weeks) postpartum (PP) time points in relation to late pregnancy (LP) and lactation. Vessels from virgin controls, LP, PP, and nursing and non-nursing mothers were tested for responses to phenylephrine (PE), high potassium solutions (high K⁺), and acetylcholine (ACh). Passive arterial distensibility, vessel dimensions, and collagen and elastin content were evaluated for the studied groups. We observed that (1) there was a significant inhibition of vascular reactivity to PE in LP, 3 days and 2 weeks PP vessels that returned to pre-pregnancy levels at 4 and 12 weeks PP; (2) inhibition of NO production in PP vessels restored PE-induced constriction to pre-pregnancy levels; (3) vasodilator responses to ACh were similar at all PP periods; (4) LP and early PP was associated with a persistent increase in arterial distensibility that correlates with a PP-induced reduction in wall collagen, and regressed to pre-conception levels at 12 weeks PP; (5) vessels from non-nursing PP mice demonstrated an increased PE reactivity, diminished responses to ACh, and reduced distensibility compared to breastfeeding mice. These studies provide a timeframe for mesenteric artery adaptations that occur during pregnancy and extend to the PP period, but which may be modified by PP events.

Loss of Anticontractile Effect of Perivascular Adipose Tissue on Pregnant Rats: A Potential Role of Tumor Necrosis Factor-α

The present investigation examined the effect of pregnancy on the anticontractile effect of perivascular adipose tissue (PVAT) on the rat. Ring segments of the aorta, with and without PVAT, were set up in organ baths for isometric tension recording. In both groups, concentration-response curves to 5-hydroxytryptamine (5-HT) were displaced to the right with a reduction of the maximum response in aorta segments with PVAT. The anticontractile effect of PVAT was attenuated on segments from pregnant rats. 4-Aminopyridine (4-AP), an inhibitor of voltage-gated potassium (Kv) channels, enhanced 5-HT-induced contractions of aorta segments from pregnant and nonpregnant rats only when PVAT was attached. There was no difference in the effect of 4-aminopyridine on 5-HT-induced contractions of aorta segments with PVAT from pregnant and nonpregnant rats. There was also no significant difference in the expression of Kv7.4 channels in aorta segments (with PVAT) between pregnant and nonpregnant rats. Tumor necrosis factor-α (TNF-α) was detected in PVAT from pregnant and nonpregnant rats. The level of TNF-α was significantly greater in PVAT from pregnant rats. Treatment of pregnant rats with pentoxyphyline significantly reduced the level of TNF-α in the PVAT and restored the anticontractile effect of PVAT on aorta segments from pregnant rats. Finally, TNF-α (10 ng/mL) potentiated 5-HT-induced contractions of PVAT-containing pregnant rat aorta. These results would suggest that the loss of anticontractile effect of PVAT in pregnant rat aorta could be due to enhanced production of TNF-α in the PVAT in these rats.

Chronic Nicotine Exposure Abolishes Maternal Systemic and Renal Adaptations to Pregnancy in Rats

Pregnancy is characterized by maternal systemic and intrarenal vasodilation, leading to increases in the renal plasma flow (RPF) and glomerular filtration rate (GFR). These responses are mainly mediated by nitric oxide (NO) and relaxin. The impact of cigarette smoking on the maternal adaptations to pregnancy is unclear. Here we evaluated the effects of chronic exposure to nicotine on systemic and intrarenal parameters in virgin (V) and 14-day pregnant (P) Wistar rats. V and P groups received saline or nicotine (6 mg·kg^-1·day^-1) respectively, via osmotic minipumps for 28 days, starting 14 days before pregnancy induction. Nicotine induced a 10% increase in blood pressure in the V group and minimized the characteristic pregnancy-induced hypotension. Renal sympathetic nerve activity (rSNA) and baroreflex sensitivity were impaired by nicotine mainly in the P group, indicating that the effect of nicotine on blood pressure was not mediated by nervous system stimulation. Nicotine had no effect on GFR in the V rats but reduced GFR of the P group by 30%. Renal expression of sodium and water transporters was downregulated by nicotine, resulting in increased fractional sodium excretion mainly in the P group, suggesting that nicotine compromised the sodium and water retention required for normal gestation. There was a reduction in the expression of inducible NO synthase (iNOS) in both the kidney tissue and renal artery, as well as in the expression of the relaxin receptor (LGR7). These results clearly show that nicotine induced deleterious effects in both virgin and pregnant animals, and abolished the maternal capacity to adapt to pregnancy.

The PI3K-Akt-eNOS pathway is involved in aortic hyporeactivity to Phenylephrine associated with late pregnancy in spontaneously hypertensive rats

AIM

This study aimed to evaluate the effects of Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt), on aortic hyporeactivity to Phenylephrine (Phe) and nitric oxide bioavailability associated with pregnancy in hypertensive rats.

MAIN METHODS

The intact aortic rings of pregnant and non-pregnant Wistar or spontaneously hypertensive rats (SHRs) were stimulated with Phe (1nmol/L to 10mmol/L) before and after incubation with Wortmannin (10nmol/L, 30min). Western blot experiments analyzed the expression of phosphorylated PI3K [p85-PI3K], Akt [p-Akt (Ser 473)] and eNOS [p-eNOS (Ser 1177)] in aorta homogenates of pregnant and non-pregnant Wistar rats or SHRs. The effect of Wortmannin (10nmol/L) on the cytosolic concentrations of nitric oxide (NO; measured using 4,5-diaminofluorescein diacetate [DAF-2DA], 10mmol/L), Ca(2+) (using Fluo 3-AM, 5μmol/L) and reactive oxygen species (ROS; using dihydroethidium [DHE], 2.5mmol/L) were measured fluorimetrically in freshly isolated endothelial cells.

KEY FINDINGS

Wortmannin increases the reactivity of the aorta to Phe and decreases NO concentrations in the aortic endothelial cells of pregnant Wistar rats and SHR.

SIGNIFICANCE

The PI3/AKT/endothelial nitric oxide synthase (eNOS) pathway contributes to aortic hyporeactivity to Phenylephrine associated with pregnancy in normo- and hypertensive rats.

Adaptive changes of mesenteric arteries in pregnancy: a meta-analysis

The vascular response to pregnancy has been frequently studied in mesenteric artery models by investigating endothelial cell (EC)- and smooth muscle cell (SMC)-dependent responses to mechanical (flow-mediated vasodilation, myogenic reactivity, and vascular compliance) and pharmacological stimuli (G protein-coupled receptor responses: Gq(EC), Gs(SMC), Gq(SMC)). It is unclear to what extent these pathways contribute to normal pregnancy-induced vasodilation across species, strains, and/or gestational age and at which receptor level pregnancy affects the pathways. We performed a meta-analysis on responses to mechanical and pharmacological stimuli associated with pregnancy-induced vasodilation of mesenteric arteries and included 55 (188 responses) out of 398 studies. Most included studies (84%) were performed in Wistar and Sprague-Dawley rats (SDRs) and compared late gestation versus nonpregnant controls (80%). Pregnancy promotes flow-mediated vasodilation in all investigated species. Only in SDRs, pregnancy additionally stimulates both vasodilator Gq(EC) sensitivity (EC(50) reduced by -0.76 [-0.92, -0.60] log[M]) and Gs(SMC) sensitivity (EC(50) reduced by -0.51 [-0.82, -0.20] log[M]), depresses vasopressor Gq(SMC) sensitivity (EC(50) increase in SDRs by 0.23 [0.16, 0.31] log[M]), and enhances arterial compliance. We conclude that 1) pregnancy facilitates flow-mediated vasodilation at term among all investigated species, and the contribution of additional vascular responses is species and strain specific, and 2) late pregnancy mediates vasodilation through changes at the receptor level for the substances tested. The initial steps of vasodilation in early pregnancy remain to be elucidated.

Increased alpha-1 adrenoceptor expression in pregnant rats with subrenal aortic coarctation

The progression of pregnancy is associated with attenuation in vasopressor response to adrenergic agonists. In pregnancy-induced hypertension this attenuation is reverted. It is not known if this reversion involves alpha-1 adrenoceptor expression.

OBJECTIVE

In this work we propose that in pregnant rats with subrenal aortic coarctation there are changes in the expression of alpha-1 adrenergic receptors in the thoracic and abdominal aorta during pregnancy.

METHODS

We used non-pregnant, normal pregnant and pregnant with subrenal aortic coarctation female Wistar rats. Pregnancy-induced hypertension indicators, systolic blood pressure, 24 hours proteinuria, pup weight and maternal weight were measured. Dose response curves to phenylephrine were carried out to determine vascular reactivity along pregnancy. Alpha 1-adrenoceptors were detected from thoracic and abdominal aorta using immunoblot.

RESULTS

Results show significant increases in arterial pressure and proteinuria in pregnant rats with SRAC at the end of the third week. Pregnancy reduces alpha-(1-A, -B) and (-D) adrenoceptor expression and this event is reverted by subrenal aortic coarctation. This phenomenon is more apparent in the abdominal segment of the aorta.

CONCLUSIONS

These findings suggest that subrenal aortic coarctation is a good animal model of pregnancy-induced hypertension and that alpha1-adrenoceptors participate in its physiopathology increasing their expression in a segment-dependent manner.

ACE2 expression and activity are enhanced during pregnancy

In the current study, we investigated the expression and activity of ACE2 during pregnancy in normotensive and hypertensive rats, focusing on the relative contribution of the uterus and the placentas, the kidney serving as a reference. We used the Sabra rat model of salt-sensitive hypertension. We confirmed a systemic vasodilatory state during the third trimester of pregnancy, as evidenced by a reduction in blood pressure, both in normotensive and hypertensive rats. At the time that blood pressure was reduced, ACE2 was expressed abundantly in the reproductive organs. The relative levels of ACE2 mRNA in the pregnant animal were placenta > kidneys > or = uterus and of ACE2 activity kidney > placenta > uterus. In the uterus and the placenta, ACE2 expression was unaffected by strain, salt-loading, or the level of blood pressure. ACE2 activity in the uterus of the nonpregnant rat was not affected by any of these variables either, but during pregnancy increased in salt-loaded animals. When estimating the total contribution of the uterus to ACE2 mRNA and activity during pregnancy, we found that the amount of ACE2 mRNA increased in both strains irrespective of diet, but that ACE2 activity increased only in salt-loaded animals. We further estimated the relative total contribution of the uterus, placentas, and kidneys to ACE2 expression and activity during pregnancy by adjusting for mass and number of organs and found that the placentas were the major contributors, followed by the kidney and the uterus. We conclude that during pregnancy, the placentas, in particular, but also the uterus, constitute important sources of ACE2, in addition to its normal production in the kidney, leading to an estimated twofold increase in total ACE2 activity. These data are consistent the hypothesis that transient ACE2 overexpression and increased activity during pregnancy may be important in modulating systemic, as well as local hemodynamics in the uteroplacental unit.

Vasorelaxant action of 17 -estradiol in rat uterine arteries: role of nitric oxide synthases and estrogen receptors

The uterine vasculature plays an important role during pregnancy by providing adequate perfusion of the maternal-fetal interface. To this end, substantial remodeling of the uterine vasculature occurs with consequent changes in responsiveness to contractile agents. The purpose of our study was to characterize the vasorelaxant effects of estrogens on vascular smooth muscles of the rat uterine artery during pregnancy and to evaluate the involvement of estrogen receptors (ESR) and nitric oxide synthases (NOS). To do so, we measured NOS expression in the whole uterine and mesenteric circulatory bed by Western blotting. Vasorelaxant effects of 17beta-estradiol (17beta-E(2)) were assessed on endothelium-denuded uterine arteries with wire myographs in the absence and presence of pharmacological modulators [nitro-L-arginine methyl ester (L-NAME), ICI-182780, tamoxifen]. All experiments were performed on arteries from nonpregnant (NP) and late pregnant (P) rats. In the uterine vasculature of the latter group, NOS3 (endothelial NOS) expression was increased, while NOS1 (neuronal NOS) was reduced compared with NP rats. Expression of the NOS2 (inducible NOS) isoform was undetectable in the two groups. Both 17beta-E(2) and 17alpha-E(2) induced uterine artery relaxation, but the latter evoked lower responses. Endothelium-denuded arteries from NP rats showed larger relaxation with 17beta-E(2) than P rats. This larger relaxation disappeared in the presence of L-NAME. The ESR antagonist ICI-182780 did not affect acute relaxation with 17beta-E(2) and 17alpha-E(2). Moreover, membrane-nonpermeant 17beta-E(2):BSA (estradiol conjugated to bovine serum albumin) did not induce any vasorelaxation. Our results indicate that estrogens exert direct acute vasorelaxant effects in smooth muscles of the rat uterine artery that are mediated by mechanisms independent of ESR activation, but with some stereospecificity. Part of this effect, in NP rats only, is due to nitric oxide produced from muscle NOS1.

Vasorelaxant effect of levcromakalim on isolated umbilical arteries of preeclamptic women

OBJECTIVE

Potassium channel openers are revealed to be a new type of antihypertensive drug. We aimed to clarify the effects of levcromakalim, an ATP-sensitive potassium channel opener, on human isolated umbilical artery (UA) and to compare them with those of nifedipine and magnesium sulphate, which are currently used in the treatment of preeclampsia (PE).

STUDY DESIGN

A total of 52 umbilical arteries, isolated immediately after delivery from 27 healthy and 25 preeclamptic pregnant women, were placed into 10-ml organ baths filled with Kreb's solution at physiological pH and temperature. The concentration-dependent relaxations in response to levcromakalim, nifedipine and magnesium sulphate were compared in vessels precontracted with serotonin (1 micromol/l).

RESULTS

The maximal relative relaxation responses (E(max), expressed as percentage of serotonin-induced precontraction) to magnesium sulphate, nifedipine and levcromakalim in umbilical arteries were identical in the healthy (85.06+/-3.31, 84.80+/-3.01 and 80.37+/-5.32%, respectively) and preeclamptic (77.20+/-5.30, 83.36+/-2.37 and 79.13+/-4.30%, respectively) groups.

CONCLUSION

Levcromakalim has a vasodilatory effect on the umbilical artery like magnesium sulphate and nifedipine, and serves as an antihypertensive potential that might be used in the treatment of preeclampsia. However, further experimental and clinical studies are needed to propose that ATP-sensitive potassium channel openers are beneficial drugs in cases of clinical preeclampsia.

Pregnancy-induced modulation of calcium mobilization and down-regulation of Rho-kinase expression contribute to attenuated vasopressin-induced contraction of the rat aorta

Normal pregnancy is characterized by attenuated vascular reactivity to a variety of contractile agonists and this, in part, has been attributed to increased circulating vasodilators and/or impaired Ca(2+)-influx through L-type Ca(2+)-channels. Our hypothesis in this study was that reduced Ca(2+)-dependent (influx) and Ca(2+)-independent (involving the RhoA/Rho-kinase pathway) mechanisms contributed to attenuated vasopressin-induced contraction of the pregnant rat aorta. AVP (10(-10) -3 x 10(-7) M) induced concentration-dependent contraction of aortic ring segments from nonpregnant and pregnant rats with no significant change in pD(2) values (8.53+/-0.11 and 8.33+/-0.18 in nonpregnant and pregnant rats, respectively). The maximum response was however significantly reduced in aorta segments from pregnant rats. Nifedipine (10(-6) M) significantly inhibited AVP-induced contraction in artery segments from nonpregnant but not pregnant rats indicating a reduced role for Ca(2+)-influx through L-type Ca(2+)-channels in AVP-induced contractions of the pregnant rat aorta. Western blot analysis revealed the expression of ROCK-1 and ROCK-II isoforms in aorta segments from both groups. There was a significant reduction in the expression of ROCK-1 and ROCK-II isoforms in aortic tissues from pregnant rats. This is consistent with the reduced potency of Y-27632 in inhibiting AVP (10(-7) M) induced contraction in aorta segments from pregnant rats. It was concluded that pregnancy-induced attenuated vascular response to AVP was due to decreased Ca(2+)-influx through L-type Ca(2+)-channels and decreased sensitization of the contractile myofilaments to Ca(2+).

Increased expression and activity of heme oxygenase-2 in pregnant rat aorta is not involved in attenuated vasopressin-induced contraction

Pregnancy is associated with attenuated vascular reactivity to a variety of contractile agonists. Heme oxygenases are expressed in the placenta, and it has been suggested that the heme oxygenase/carbon monoxide (HO/CO) pathway plays a significant role in regulating blood flow through the feto-placental unit. In this study we investigated the possible involvement of heme oxygenases in the reduced vascular reactivity associated with pregnancy. Arginine vasopressin (AVP) (10(-10)-3x10(-7) M) induced concentration-dependent contraction of aortic ring segments from non-pregnant and pregnant (16-19 days) rats. Pregnancy did not alter the sensitivity to AVP (pD2=8.5+/-0.1 and pD2=8.4+/-0.2 in non-pregnant and pregnant rats, respectively) but significantly reduced the maximum response (107.9+/-12.7% and 38.6+/-7.4%, respectively, relative to noradrenaline-induced contraction). Western blot analysis revealed the expression of HO-2 but not HO-1 isoform in both groups. There was a significant increase in the expression and activity of HO-2 protein in aortic tissues from pregnant rats compared with those from age-matched non-pregnant rats. In the presence of L-NAME to inhibit nitric oxide (NO) synthesis, tin protoporphyrin IX (SnPP-IX, 10(-5) M), an inhibitor of heme oxygenase, did not significantly affect AVP-induced contraction in aorta segments from pregnant and non-pregnant rats. It was concluded that, though pregnancy increased the expression and activity of HO-2 in the aorta, HO-2 was not involved in the attenuated response to AVP.

Modulation of body fluids and angiotensin II receptors in a rat model of intra-uterine growth restriction

We previously reported that sodium restriction during pregnancy reduces plasma volume expansion and promotes intra-uterine growth restriction (IUGR) in rats while it activates the renin-angiotensin-aldosterone system (RAAS). In the present study, we proceeded to determine whether expression of the two angiotensin II (ANGII) receptor subtypes (AT(1) and AT(2)) change in relation to maternal water-electrolyte homeostasis and fetal growth. To this end, pregnant (gestation day 15) and non-pregnant Sprague-Dawley rats were randomly assigned to two groups fed either normal, or Na(+)-restricted diets for 7 days. At the end of the treatment period, plasma aldosterone and renin activity as well as plasma and urine electrolytes were measured. Determinations for AT(1) and AT(2) mRNA and protein were made by RNase protection assay and photoaffinity labelling, respectively, using a number of tissues implicated in volume regulation and fetal growth. In non-pregnant rats, Na(+) restriction decreases Na(+) excretion without altering plasma volume, plasma Na(+) concentration or the expression of AT(1) and AT(2) mRNA or protein in the tissues examined. In normally fed pregnant rats when compared to non-pregnant controls, AT(1) mRNA increases in the hypothalamus as well as pituitary and declines in uterine arteries, while AT(1) protein decreases in the kidney and AT(2) mRNA declines in the adrenal cortex. In pregnant rats, Na(+) restriction induces a decrease in plasma Na(+), an increase in plasma urea, as well as a decline in renal urea and creatinine clearance rates. Protein levels for both AT(1) and AT(2) in the pituitary and AT(2) mRNA in the adrenal cortex are lower in the Na(+)-restricted pregnant group when compared to normally fed pregnant animals. Na(+) restriction also induces a decrease in AT(1) protein in the placenta. In conclusion, these results suggest that pregnancy may increase sensitivity to Na(+) depletion by the tissue-specific modulation of ANGII receptors. Finally, these receptors may be implicated in the IUGR response to low Na(+).

Increased Na+ intake during gestation in rats is associated with enhanced vascular reactivity and alterations of K+ and Ca2+ function

Gestation is associated with decreased blood pressure and resistance to the effects of vasoconstrictor agents. A recent study showed that pregnant rats, on increased sodium intake, present physiological changes that resemble those observed in preeclampsia. We investigated the effects of sodium supplementation on reactivity and on potassium and Ca2+ channel activity in blood vessels during gestation. Sodium supplements, 0.9% or 1.8% NaCl as drinking water, were given to nonpregnant and pregnant rats for 7 days (last week of gestation). Reactivity to phenylephrine (PE), KCl, arginine vasopressin (AVP), and tetraethylammonium (TEA) was measured in aortic rings under modulation of potassium and calcium channels. TEA, a nonselective K+ channel inhibitor, induced concentration-dependent responses in aortic rings from nonpregnant but not in those from pregnant rats. The response to TEA was restored in rings from pregnant rats after preincubation with 10 mmol/l KCl. Sodium supplementation did not affect the response to TEA in the aortas of pregnant animals. After sodium supplementation, maximum responses to PE and AVP were decreased and increased in aortic rings from nonpregnant and pregnant rats, respectively. Cromakalim (an ATP-sensitive K+ channel activator)-induced inhibition of the responses to the three vasoconstrictors was more striking in aorta from nonpregnant than pregnant rats on regular diet, whereas it produced similar inhibition in tissues from both groups of animals on 0.9% and 1.8% NaCl. NS-1619 (a Ca2+-sensitive K+ activator) elicited heightened effects in the aortas of pregnant animals receiving 0.9% NaCl supplementation. Nifedipine (a Ca2+ channel blocker) caused greater inhibition of the contractile responses in tissues from nonpregnant rats on regular diet, and its action was increased in pregnant rats on sodium-supplemented diets. These data demonstrate that augmented sodium intake during gestation in the rat is linked with the reversal of gestational-associated resistance to vasopressors and indicate that this is an experimental model showing some features of gestational hypertension.

Pregnancy-associated increase in rat systemic arteries endothelial nitric oxide production diminishes vasoconstrictor but does not enhance vasodilator responses

Late pregnancy in rats is characterized by a decrease in arterial pressure and in isolated arterial vessels response to vasoconstrictors. In uterine arteries the pregnancy-associated attenuation of the response to vasoconstrictors has been attributed to an increase in basal and agonist-induced endothelial NO production. However, the role of NO in pregnancy-associated changes of systemic arteries reactivity to vasoactive agents remains to be fully elucidated. We examined whether pregnancy influences the reactivity of systemic arteries to vasodilator or vasoconstrictor agents through NO-dependent mechanisms. Thoracic aortic rings and mesenteric arterial bed of late pregnant rats showed refractoriness to phenylephrine-induced vasoconstriction that was abolished by NO synthase inhibition. The potency of L-NNA to enhance tension of aortic rings preconstricted with phenylephrine (10-20% of their maximal response) was significantly lower in preparations from pregnant animals. In phenylephrine-contracted aortas and mesenteric bed, the effects of the endothelium-dependent vasodilators acetylcholine, A23187 and bradykinin, were not influenced by pregnancy. Similarly, pregnancy did not affect the vasodilator responses of adenosine, isoproterenol, capsaicin, nitroprusside, forskolin, and Hoe234 in the mesenteric bed. NO synthase activity measured by determining the conversion of L-[(3)H]-arginine to L-[(3)H]-citrulline in aorta and mesenteric arteries homogenates was not altered by pregnancy. These findings show that endothelial-dependent and -independent vasodilators action as well as NO synthase activity in systemic arteries is uninfluenced by pregnancy, whereas pregnancy-associated hyporeactivity of systemic arteries to vasoconstrictors is related to an enhanced endothelial NO production either spontaneous or elicited directly or indirectly by vasoconstrictor agents. This interpretation implies that the enhanced NO production observed in systemic arteries during late pregnancy involves cellular pathways other than the ones involved in the response to endothelium-dependent vasodilators such as acetylcholine.

Oxytocin does not directly affect vascular tone in vessels from nonpregnant and pregnant rats

Recent evidence suggests oxytocin (OT) may regulate vascular tone. OT and its receptor (OTR) have been identified in the rat heart and great vessels. Expression of OT and OTR is increased in some tissues during pregnancy. We hypothesized that the OT/OTR system may be a physiological regulator of vascular tone and mediate the decreased vascular resistance noted during pregnancy. Using a wire myograph system, we measured changes in vascular tone in response to OT in small mesenteric arteries, uterine arcuate arteries, and thoracic aorta from nonpregnant and pregnant rats. Additionally, we used reverse transcriptase-polymerase chain reaction (RT-PCR) to measure mRNA for OTR in these vascular tissues. Although OTR mRNA was identified by RT-PCR, OT did not elicit a vasodilatory effect in any of the vessels studied. High concentrations of OT (>10(-8) M) caused vasoconstriction that was eliminated by a specific vasopressin V(1a) receptor antagonist. Although it may have an indirect effect in regulation of peripheral resistance, we conclude that OT is unlikely to play a direct role in the physiological regulation of vascular tone.

Relaxin and drinking in pregnant rats

Work reported in this chapter describes the potential role of relaxin in resetting cardiovascular thresholds in pregnant rats. Relaxin, a polypeptide produced primarily by the ovary in pregnant animals in many species, is also produced in the brain. Exogenous administration of relaxin into the brain causes a profound drinking response which is negated by pretreatment with a specific monoclonal antibody to rat relaxin when the antibody is injected into the brain. Neutralizing the action of endogenous brain relaxin in pregnant rats also blocks the normal increase in drinking that is observed in rats at night during the second half of pregnancy. Relaxin acts through the forebrain angiotensin system at the level of the subfornical organ (an important interface between the blood, the brain and the cerebrospinal fluid) as blockade of the angiotensin II receptor action negates several central actions of relaxin. Expression of angiotensin II AT1 receptors in the subfornical organ increases in parallel with the increase in circulating relaxin seen in the second half of pregnancy. Neutralizing the effects of endogenous brain relaxin, using central injections of the monoclonal antibody, blocks this increase in the expression of angiotensin II AT1 receptors in subfornical organ. These data imply that relaxin in the brain may act to affect central cardiovascular thresholds in rats and this may be important for the normal physiology of pregnancy.

Chronic and acute effects of oestrogens on vascular contractility

In addition to their role as sex hormones, it has been known for many years that oestrogens have protective effects on the vasculature. These have been implicated in the reduced incidence of cardiovascular disorders in premenopausal women and in post-menopausal women receiving oestrogen replacement therapy. This protection has been found to be due, in part at least, to direct effects of oestrogens on blood vessels. This review will summarize the available literature regarding oestrogenic effects on vascular contractility. Two major influences of oestrogens will be discussed; first the genomic effects induced by chronic administration of steroid hormones, and second, the rapid effects on vascular smooth muscle by non-genomic, and as yet not fully identified, mechanisms. In so doing, the diversity of oestrogenic actions on vascular contractility will be highlighted and the protective role of these agents against adverse cardiovascular events discussed.

Endothelium and aortic contraction to endothelin-1 in the pregnant rat

Endothelium-derived factors modulate tone and may be involved in hyporeactivity to vasoconstrictors, such as norepinephrine or angiotensin II, as has been previously described during gestation. The endothelium produces endothelin-1, a major vasoconstrictor peptide, therefore aortic contractions to endothelin-1 (10-10 to 3 ×10-7 M) were used to assess the role of the endothelium in pregnant Wistar rats (at 20 days of gestation). Late pregnancy is characterized by a significantly diminished systolic blood pressure in conscious rats (-17 mmHg, P < 0.001, n = 14). In pregnant and in age-matched nonpregnant female rats, endothelin-1 induced aortic contraction was greater when endothelium was present (at least P < 0.01). Indomethacin significantly reduced this contraction in aortic rings with intact endothelium in all groups. In aortic rings that had endothelium physically removed, contraction to endothelin-1 was greater in pregnant rats than in nonpregnant ones. Indomethacin decreased contraction of aortic rings in pregnant rats only. These results suggest an enhanced synthesis of vasoconstrictors by cyclooxygenases in vascular smooth muscle during pregnancy. In vessels with intact endothelium, we did not find hyporeactivity to endothelin-1 during late pregnancy. Contraction to endothelin-1 involved ETA receptors because it was decreased by BQ-123, an ETA receptor antagonist, whereas there was no significant change when using BQ-788, an ETB receptor antagonist.

Key words: endothelin-1, endothelium, contraction, aorta, gestation.

Effects of potassium channel modulators on myotropic responses of aortic rings of pregnant rats

The contribution of potassium channels [ATP-sensitive potassium (K(ATP)) and high-conductance calcium-activated potassium (BK(Ca)) channels] in the resistance of aortic rings of term pregnant rats to phenylephrine (Phe), arginine vasopressin (AVP), and KCl was investigated. Concentration-response curves to tetraethylammonium (TEA), a nonselective K(+) channel inhibitor, were obtained in the absence or presence of KCl. TEA induced by itself concentration-dependent responses only in aortic rings of nonpregnant rats. These responses to TEA could be modulated in both groups of rings by preincubation with different concentrations of KCl. Concentration-response curves to Phe, AVP, and KCl were obtained in the absence or presence of cromakalim or NS-1619 (K(ATP) and BK(Ca) openers, respectively) and glibenclamide or iberiotoxin (K(ATP) and BK(Ca) inhibitors, respectively). Cromakalim significantly inhibited the responses to the three agonists in a concentration-dependent manner in both groups of rats. Alternatively, in the pregnant group of rats, glibenclamide increased the sensitivity to all three agonists. NS-1619 also inhibited the response to all agonists. With AVP and KCl, its effect was greater in aortic rings of pregnant than nonpregnant rats. Finally, iberiotoxin increased the sensitivity to all three agents. This effect was more important in aortic rings of nonpregnant rats and was accompanied by an increase of the maximal response to Phe and AVP. These results suggest that potassium channels are implicated in the control of basal membrane potential and in the blunted responses to these agents during pregnancy.

Increased acetylcholine-induced vasodilation in pregnant rats: A role for gap junctional communication

We have tested the hypothesis that increased gap junctional communication contributes to the augmented endothelium-dependent vasodilation in pregnancy. Contractile force and connexin43 expression were measured in aortic rings from nonpregnant and pregnant rats. Norepinephrine-constricted aortas from pregnant rats were more sensitive to acetylcholine, but not to sodium nitroprusside, compared with those from nonpregnant rats. Vessels from pregnant rats, constricted either with 45 mmol/L KCl or with norepinephrine + 10(-4) mol/L N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthase, also exhibited greater relaxation to acetylcholine. Heptanol, an uncoupler of gap junctional communication, inhibited acetylcholine responses in norepinephrine-constricted aortas from nonpregnant rats but greatly impaired acetylcholine relaxation in aortas from pregnant rats. Heptanol also inhibited in both groups acetylcholine responses in vessels constricted with KCl, only minimally affected acetylcholine relaxation in arteries constricted with norepinephrine + L-NMMA, and did not change sodium nitroprusside-induced relaxation. Tetraethylammonium chloride induced greater contractions in control aortas compared with aortas from pregnant rats. Increased connexin43 mRNA levels were found in the uterus and in the mesenteric, uterine, and thoracic aortic arteries, but not in the heart and brain, from pregnant rats. These results suggest that increased gap junctional communication, possibly due to increased gap junction protein expression, may facilitate the effects of endothelium-derived relaxing factors, contributing to the augmented endothelium-dependent relaxation in arteries from pregnant rats.

Endothelin-1 and relaxation of the rat aorta during pregnancy in nitroarginine-induced hypertension

In pregnant rats during hypertension induced by NO synthase inhibition, endothelin (ET) plasma levels are increased as in some preeclamptic women. Previously, the enhanced vasodepressor effect of endothelin-1 (ET-1) has been observed in this model, thus we decided to study the relaxation induced by ET-1 on the aorta. Non-pregnant or pregnant Wistar rats (n = 7 by group) were fed for 7 days (day 13-day 20) on a nitroarginine-enriched diet (L-NNA, 0.063% i.e. 30 mg/kg/day) or a control diet. Systolic blood pressure, measured by the tail cuff method on conscious rats at day 20 of gestation, was raised by the chronic L-NNA treatment (mean +/- s.e.m., mmHg, p < 0.001: pregnant L-NNA treated, 145 +/- 1.84 vs. pregnant control, 101 +/- 2.00 and non-pregnant L-NNA treated, 148 +/- 3.11 vs. non-pregnant control, 119 +/- 1.80). On day 20 ex vivo aortic ring relaxation was produced by ET-1 in vessels previously precontracted with norepinephrine only when endothelium was present. In control rats, ET-1 (10(-8) to 5 x 10(-8) M) produced a short but significant relaxation (mean value between 4 to 19%) followed by a long-lasting contracting phase, and a higher ET-1 concentration (10(-7) M) only produced contraction. Chronic L-NNA treatment decreased the level of relaxation (at least p < 0.05, in non-pregnant and pregnant rats) and with a 30 min L-NAME (10(-4) M) preincubation, relaxation was completely inhibited in non-pregnant and pregnant rats. BQ-123, an ETA receptor antagonist, did not produce any effect on ET-1 induced relaxation. BQ-788, an ETB receptor antagonist, significantly decreased it. In conclusion, in female rats, as in male rats, ET-1 induces a transient relaxation in the preconstricted aorta which involves endothelial ETB receptors. Despite a decrease in the systemic vascular reactivity during late gestation, the vasodilating and vasoconstricting properties of ET-1 on the aorta are not changed.

Modulation of calcium mobilization in aortic rings of pregnant rats: Contribution of extracellular calcium and of voltage-operated calcium channels

Pregnancy is associated with decreased vascular responsiveness to vasopressor stimuli. We have tested the involvement of Ca2+ mobilization in myotropic responses of aortic rings obtained from pregnant and virgin rats. Contractions of the rings to phenylephrine, in the absence of calcium in the bathing medium, were lower in tissues from virgin than from pregnant rats. Concentration-response curves to CaCl2 that were measured after stimulation by phenylephrine in the absence of Ca2+ were shifted to higher levels of contraction. This was not observed when KCl was used to prestimulate the aorta. D-600, a phenylalkylamine calcium channel blocker, similarly inhibited these responses to CaCl2 in tissues from both pregnant and virgin animals. D-600 exerted a concentration-dependent inhibition of responses to phenylephrine and KCl. However, the calcium antagonist was less effective in aortic rings of pregnant than of virgin rats. Basal 45Ca2+ uptake was lower in aortic rings from pregnant than from virgin rats, and Bay K 8644 was unable to reverse this difference. The time course of basal and stimulated (KCl) 45Ca2+ influx was lower in aorta of pregnant rats at all times studied. Moreover, when the intracellular calcium pools were emptied with phenylephrine, the refilling of these pools was delayed in aortic rings of pregnant rats. These results indicate an altered extracellular calcium mobilization of aortic rings from pregnant rats. These changes may be due to a functional alteration of the voltage-operated calcium channels during pregnancy.

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.

The dipsogenic effects of rat relaxin: The effect of photoperiod and the potential role of relaxin on drinking in pregnancy

Experiments were done to examine whether rat relaxin is dipsogenic and whether such dipsogenic effects of rat relaxin are related to time of injection during the light-dark cycle. Female rats were fitted with a chronic intra-cerebro-ventricular (i.c.v.) cannula. Rat relaxin (2.5, 5, 10, 25, 50, or 100 ng/2 microl in 0.9% saline) was injected into the right lateral ventricle at either morning (0800-1000 h), afternoon (1400-1600 h), or night (2200-2400 h), and water consumption was measured. Relaxin caused a dose-dependent dipsogenesis at doses > or = 5 ng, but the sensitivity and magnitude of the response varied with the photoperiod. Water consumption was smallest (3.5 +/- 0.7 ml at 50 ng) and least sensitive (minimal effective dose at 25 ng) in the afternoon and maximal (17.7 +/- 2.3 ml at 50 ng) and most sensitive (minimal effective dose 5 ng) at night. The latency from injection to drinking was 55.8 +/- 10.4 sec (mean +/- SEM) and did not vary significantly with either the dose or time of day. A second set of experiments was done to examine the effects of neutralizing the central actions of relaxin on drinking behavior in pregnancy. Pregnant rats were injected daily, through a chronically implanted i.c.v. cannula, with either a specific monoclonal antibody raised against rat relaxin from day 12 to day 22 of gestation or with saline as a control. Drinking and eating behavior and weight gain were monitored every 12 h during pregnancy. There was a significant decrease in water consumed at night, but no effect on drinking during the day in relaxin-neutralized rats. These animals also showed a decrease in weight gain during pregnancy compared with controls and gave birth to lighter-weight litters. These data provide evidence that the dipsogenic response to exogenous rat relaxin in female rats varies with time of injection during the light-dark cycle and suggest that relaxin in the brain may have a role in nighttime drinking behavior during the second half of pregnancy.

Nitric oxide blunts sympathetic response of pregnant normotensive and hypertensive rat arteries

Rat pregnancy is associated with a blunted response to vasocontrictors both in vivo and in vitro as well as a decrease in arterial pressure. We examined the influence of pregnancy on neurally induced vasoconstrictor and vasodilator responses of the isolated mesenteric arterial bed from normotensive Wistar and spontaneously hypertensive nonpregnant and 20-day pregnant rats and determined the possible role of nitric oxide (NO) in modulating these responses. MAP (mm Hg) in pregnant normotensive (98+/-1, n=13) and hypertensive (136+/-5, n=13) rats was lower (P<.05) than in nonpregnant controls (114+/-2, n=14, and 174+/-3, n=12, respectively). In isolated mesenteric arterial beds, electrical field stimulation (EFS; 34 V, 3 ms, 10-64 Hz) of perivascular nerves at basal tone induced a frequency-dependent increase in perfusion pressure that was significantly (P<.001) greater in preparations from hypertensive compared with normotensive rats. Pregnancy was associated with a significant decrease in the maximal vasoconstrictor response elicited by EFS in both normotensive and hypertensive groups compared with their nonpregnant controls. In phenylephrine-preconstricted mesenteric beds, EFS (60 V, 1 ms, 1-8 Hz) elicited a similar frequency-dependent decrease in perfusion pressure in normotensive and hypertensive groups, but pregnancy did not influence these responses. In the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine (200 micromol/L), the maximal vasoconstrictor response induced by EFS was significantly (P<.001) augmented in both normotensive and hypertensive groups, and the differences observed between pregnant and nonpregnant groups were abolished. Responses to sodium nitroprusside were not affected by pregnancy, although they were greater in preparations from hypertensive rats. These results indicate that NO contributes to pregnancy-associated diminished vasoconstrictor response to sympathetic stimulation in the mesenteric arterial bed of both normotensive and hypertensive rats.

Roles of NO-synthase and cyclooxygenase in sex- and pregnancy-dependent arterial and venous pressures in the rat

The roles of NO synthase (NOS) and cyclooxygenase on vascular pressures were studied as a function of sex and pregnancy. After anesthesia, mean arterial pressure (MAP) and mean circulatory filling pressure were lower in pregnant rats compared with male and virgin rats, but N(G)-nitro-L-arginine methyl ester (L-NAME; 30 mg/kg) induced similar increases in MAP. Pithing abolished these pressure differences, suggesting a diminished autonomic reflex in pregnancy, and led in pregnant rats to a lower arterial and venous NO modulation. In separately perfused mesenteries, the lower responses to KCI observed in venous beds of female compared with male rats do not involve any dysfunction of NOS activity in the mesenteries isolated from virgin and pregnant rats. The cyclooxygenase pathway is implicated in the KCl-induced responses of vessels taken from male rats and of venous mesentery from pregnant rats. But prostanoids do not share in the acetylcholine (ACh)-induced relaxations in the arterial and venous K+-contracted mesenteric vasculatures isolated from any of the groups of rats.

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.

Comparison of endothelium-derived relaxing factor activity between nonpregnant and pregnant rats

The tension of isolated ring preparation of aorta from nonpregnant and pregnant rats was measured isometrically to study the effect of pregnancy on endothelium-derived relaxing factor activity. Contraction in response to norepinephrine and potassium chloride was greater in aortae from nonpregnant rats than in those from pregnant rats. The endothelium-dependent relaxation that was caused by acetylcholine (10(-10)-3 x 10(-9) M) in aortae precontracted with norepinephrine was significantly enhanced in aortae from pregnant rats compared with the relaxation in those from nonpregnant rats. NG-nitro-L-arginine methyl ester (L-NAME) inhibited the endothelium-dependent relaxation in both aorta from pregnant and nonpregnant rats. L-Arginine reversed the inhibition of L-NAME. Those results suggest that the enhanced endothelium-derived relaxing factor activity in rats aortae is associated with pregnancy.

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.

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)

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.

Effect of autonomic blockade on pressor responses to vasopressin in pregnant rats

OBJECTIVE

We tested the hypothesis that gestational changes in reflex neural control of the heart and vasculature contribute to altered cardiovascular responses to vasopressin during pregnancy.

STUDY DESIGN

Changes in mean arterial pressure, cardiac output, total peripheral resistance, and heart rate were measured in response to constant infusion of arginine vasopressin (0.15 to 2.5 mU/kg/min) in conscious pregnant and virgin rats (n = 9) with total autonomic blockade plus restoration of baseline hemodynamics by norepinephrine infusion.

RESULTS

Resting cardiac output was 40% higher and total peripheral resistance 30% lower in pregnant animals (p < 0.01). Constant infusion of arginine vasopressin evoked equivalent changes in mean arterial pressure in both groups, but the respective contributions of cardiac output and total peripheral resistance to mean arterial pressure differed between groups. Cardiac output was unchanged and the increase in total peripheral resistance was significantly blunted in pregnant vs virgin rats during arginine vasopressin infusion. Control data in nonblocked revealed similar pressor responses to arginine vasopressin in gravid compared with virgin rats but no differences in the contributions of cardiac output and total peripheral resistance to the change in mean arterial pressure.

CONCLUSION

These findings suggest that neural modulation of arginine vasopressin-induced hypertension is altered during pregnancy and are consistent with a reduction in intrinsic vascular sensitivity to arginine vasopressin during gestation.

The role of extracellular calcium in pregnancy-induced attenuation of phenylephrine contraction in rat aorta with functional endothelium

The effect of pregnancy on the supply of calcium ions for the contractile responses of rat aortic rings to phenylephrine was investigated. The contractility of intact aortic rings from pregnant rats, compared with that of similar rings from non-pregnant rats, to phenylephrine and potassium chloride was significantly decreased. Contractions of rings from non-pregnant rats, pretreated with phenylephrine or potassium chloride, in response to calcium chloride were greater than those of similarly treated rings from pregnant rats. When the concentration of calcium chloride in the medium bathing the rings was reduced to 0.8 mmol.l-1, the contractile response to phenylephrine was significantly (P < 0.005) inhibited in rings from both pregnant and non-pregnant rats but to a greater extent in rings from non-pregnant rats. Contractions of aortic rings from pregnant rats in response to phenylephrine in calcium-free medium were similar to those of rings from non-pregnant rats, suggesting equal dependence on calcium from intracellular stores. The results suggest that pregnancy decreased the response to calcium influx into the aortic smooth muscle cells through both receptor- and voltage-operated calcium entry pathways. Since de-endothelialization reversed the pregnancy-induced diminished contraction to phenylephrine, it is likely that pregnancy interfered with contractions induced by activation of receptors with phenylephrine through enhanced production of endothelium-derived relaxing factor(s).

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.

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.

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.

Neurovascular function during pregnancy in the spontaneously hypertensive rat

1. Blood pressure (BP) declines dramatically in the final week of gestation in the pregnant spontaneously hypertensive rat (SHR). This study investigated the hypothesis that alterations of vascular neuroeffector function in the pregnant SHR and normotensive Wistar-Kyoto (WKY) rat are responsible for this decline. 2. Pregnancy in SHR and WKY rats was associated with a significant drop in BP in the last week of gestation. 3. Responses of the perfused mesenteric vasculature to bolus doses of noradrenaline (NA) and potassium chloride (KCl) were decreased in preparations from SHR rats 4 days before delivery. This decreased responsiveness was absent in preparations from SHR rats 1 day before delivery. Responses of the perfused mesenteric vasculature to sympathetic nerve stimulation were not influenced by pregnancy in the SHR. 4. It is concluded that while there are dynamic changes occurring in neurovascular function just prior to delivery, it is unlikely that they are wholly responsible for the dramatic decline in blood pressure in the SHR rat.

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.

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.

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.

Regulation of ANG II and AVP receptors in isolated hepatocytes of pregnant rats

Recent evidence suggests that angiotensin II (ANG II) and vasopressin (AVP) act on the liver via specific receptors. We have examined the binding properties of these receptors in isolated rat hepatocytes and studied the regulation of the biological responses to ANG II and AVP during pregnancy in the rat. In contrast to [3H]ANG II, 125I-labeled-[Sar1-Ile8]ANG II was markedly resistant to degradation by isolated liver cells. Displacement and saturation experiments with this iodinated antagonist revealed the presence of a single class of binding sites [2 x 10(5) sites/cell, dissociation constant (KD) = 1.0 nM]. The potency of ANG II analogues to displace 125I-[Sar1-Ile8]-ANG II agrees closely with data reported for vascular smooth muscle cells. Isolated hepatocytes have approximately 8 x 10(4) [3H]AVP binding sites/cell (KD = 1.0 nM) based on saturation experiments. AVP analogues selectively displaced [3H]AVP, suggesting the presence of V1-AVP receptor subtype. The maximum response of [Sar1]ANG II-induced glycogenolysis in the cells was decreased during gestation, whereas the effective concentration producing 50% of maximum response (EC50) was significantly increased (0.15-0.28 nM) when compared with cells from nonpregnant animals. In pregnancy, receptors for 125I-[Sar1-Ile8]ANG II were not changed in affinity (KD) or in density (Bmax). The maximum response and EC50 of AVP on liver glycogenolysis were not significantly decreased during pregnancy, whereas an increased number of AVP binding sites (from 5.0 +/- 0.5 x 10(4) to 11.0 +/- 1.7 x 10(4)) with similar KD was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro release of endothelium-derived relaxing factor by acetylcholine is increased during the guinea pig pregnancy

The relaxation of isolated blood vessels by acetylcholine is dependent on the presence of intact endothelium and its release of a smooth muscle relaxing nitroso-like compound. Pregnancy is associated with altered vascular responsiveness to a variety of agents. Because many of these agents stimulate the release of endothelium-derived relaxing factor, we investigated in vitro the effect of pregnancy on acetylcholine-mediated relaxation of guinea pig uterine and carotid artery rings. The presence of intact and functional endothelium was confirmed both by examination under the scanning electron microscope and by vessel relaxation after the addition of acetylcholine. The addition of acetylcholine to the vessel bath produced dose-dependent relaxation of both carotid and uterine artery segments obtained from pregnant and nonpregnant animals after they had been submaximally preconstricted with phenylephrine. There was a significant increase in both response and efficacy to acetylcholine during pregnancy for both uterine and carotid arteries (p less than 0.0001 for each). The concentrations of endothelial cells (cells per square micrometer) were similar in uterine arteries from pregnant and nonpregnant animals. We conclude that the most likely explanation for these findings is a pregnancy-mediated enhancement of endothelium-derived relaxing factor activity.

Lack of evidence of a vasodepressor role for relaxin in spontaneously hypertensive and normotensive pregnant rats

To test the possibility that the ovarian polypeptide hormone relaxin mediates the vasodepressor effect of pregnancy, we measured changes in blood pressure in three groups of spontaneously hypertensive and Wistar-Kyoto normotensive rats: (1) pregnant, bilaterally oophorectomized rats on postmating day 13, (2) pregnant, sham-oophorectomized rats on postmating day 13, and (3) nonpregnant, sham-oophorectomized rats after 13 days. The day before parturition we also measured the pressor responsiveness to angiotensin II and norepinephrine. Systolic blood pressure fell significantly during the last week of gestation in both strains, reaching normotensive levels in the hypertensive rats by term. The pressor responsiveness to angiotensin II, but not norepinephrine, of the pregnant rats was reduced compared with that of the nonpregnant rats in both strains. Oophorectomy did not prevent the fall in blood pressure or the decrease in vascular reactivity to angiotensin II. Therefore, although ovarian relaxin secretion increases during pregnancy, it apparently does not decrease vascular sensitivity to angiotensin II or cause vasodilation.