Pial Collateral Reactivity During Hypertension and Aging: Understanding the Function of Collaterals for Stroke Therapy

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Background and Purpose—

We investigated vasoactive properties of leptomeningeal arterioles (LMAs) under normotensive conditions and during hypertension and aging that are known to have poor collateral flow and little salvageable tissue.

Methods—

LMAs, identified as distal anastomotic arterioles connecting middle and anterior cerebral arteries, were studied isolated and pressurized from young (18 weeks) or aged (48 weeks) normotensive Wistar Kyoto (WKY18, n=14; WKY48, n=6) rats and spontaneously hypertensive rats (SHR18, n=16; SHR48, n=6). Myogenic tone and vasoactive responses to pressure as well as endothelial function and ion channel activity were measured.

Results—

LMAs from WKY18 had little myogenic tone at 40 mm Hg (8±3%) that increased in aged WKY48 (30±6%). However, LMAs from both WKY groups dilated to increased pressure and demonstrated little myogenic reactivity, a response that would be conducive to collateral flow. In contrast, LMAs from both SHR18 and SHR48 displayed considerable myogenic tone (56±8% and 43±7%; P<0.01 versus WKY) and constricted to increased pressure. LMAs from both WKY and SHR groups had similar basal endothelial nitric oxide and IK channel activity that opposed tone. However, dilation to sodium nitroprusside, diltiazem and 15 mmol/L KCl was impaired in LMAs from SHR18.

Conclusions—

This study shows for the first time that LMAs from young and aged SHR are vasoconstricted and have impaired vasodilatory responses that may contribute to greater perfusion deficit and little penumbral tissue. These results also suggest that therapeutic opening of pial collaterals is possible during middle cerebral artery occlusion to create penumbral tissue and prevent infarct expansion.

The Importance of Considering Sex Differences in Translational Stroke Research

Stroke is the second leading cause of death worldwide, and differences between men and women have been documented in incidence, prevalence, and outcome. Here, we reviewed the literature on sex differences in stroke severity, mortality, functional outcome, and response to therapies after ischemic stroke. Many of the sex differences in stroke severity and mortality are explained by differences in baseline demographics such as older age in women. However, women account for more stroke deaths, consistently suffer from worse stroke outcomes, and are more often institutionalized and permanently disabled than men. These sex differences in functional outcome are equalized after treatment with tissue plasminogen activator (tPA) and women may benefit more from treatment than men. However, this may depend on race, as African-American women have less of a response to tPA than other groups. Regarding endovascular treatments, the few existing studies that have investigated sex differences in stroke outcome point to equal benefit in both sexes; however, many clinical trials are relatively underpowered to detect sex differences. Further, we considered sex-specific effects in animal models of stroke and present recommendations for the performance of stroke studies in female animals. The male-biased use of research animals is distinguished from the clinical situation where there is a disproportionate and growing female stroke population. Stroke in women is greatly understudied, and including both sexes is especially important in both preclinical and clinical studies that evaluate potential stroke therapies.

Effects of Acute Stroke Serum on Non-Ischemic Cerebral and Mesenteric Vascular Function

We investigated the effects of circulating factors in serum obtained from patients in the acute phase of different subtypes of ischemic stroke on non-ischemic cerebral and mesenteric arteries, as a potential mechanism involved in influencing regional perfusion and thus clinical evolution. Posterior cerebral arteries (PCAs) and mesentery arteries (MAs) isolated from Wistar Kyoto rats were perfused with serum from acute stroke patients with large vessel disease without (LVD) or with hypertension (LVD + HTN), cardioembolism with hypertension (CE + HTN), or physiologic saline as controls. Myogenic activity and nitric oxide-dependent vasorelaxation were assessed after 2 h of intraluminal exposure to serum. Vascular function was differentially affected by sera. Exposure to LVD serum increased myogenic tone and produced endothelial dysfunction in both PCAs and MAs. However, CE + HTN serum increased tone and decreased smooth muscle sensitivity to NO in vessels from both vascular beds. LVD + HTN serum was associated with reduced smooth muscle sensitivity to NO in vessels from both vascular beds but increased tone only in PCAs. Inflammation and oxidative stress, determined by measurement of high sensitivity C-reactive protein, uric acid, and free 8-isoprostane, were enhanced in all the serum groups. These results demonstrate vasoactive properties of acute stroke serum related to stroke subtypes that could potentially contribute to the pathogenesis of early hemodynamic-based clinical events.

Cerebrovascular Dysfunction in Preeclamptic Pregnancies

Preeclampsia is a hypertensive, multisystem disorder of pregnancy that affects several organ systems, including the maternal brain. Cerebrovascular dysfunction during preeclampsia can lead to cerebral edema, seizures, stroke, and potentially maternal mortality. This review will discuss the effects of preeclampsia on the cerebrovasculature that may adversely affect the maternal brain, including cerebral blood flow (CBF) autoregulation and blood-brain barrier disruption and the resultant clinical outcomes including posterior reversible encephalopathy syndrome (PRES) and maternal stroke. Potential long-term cognitive outcomes of preeclampsia and the role of the cerebrovasculature are also reviewed.

Ondansetron-related hemorrhagic posterior reversible encephalopathy syndrome (PRES) following gastric bypass

Background

Posterior reversible encephalopathy syndrome (PRES) is a clinical-radiographic syndrome formally recognized in 1996, which describes specific changes noted on neuroimaging thought to be related to impaired cerebral blood flow autoregulation and endothelial dysfunction. We report a case of PRES in the setting of increased ingestion of ondansetron; complicated by hemorrhagic transformation and refractory intracranial hypertension. We hypothesize an association of 5-HT₃ antagonism and PRES.

Findings

This is a case study report; with review of previously published literature through PubMed search. We describe the case of a 25 year old man following bariatric surgery who increased his ingestion of ondansetron, taking up to 40 tablets/day due to excessive nausea and vomiting. The patient was hospitalized for progressively more severe headache of 1 week’s duration. Computed tomography (CT) revealed bilateral cerebral edema in the parietal and occipital lobes in the setting of elevated blood pressure (BP). Three days into his admission, following improvement in his BP with oral anti-hypertensive but continued use of the ondansetron, the patient developed near complete blindness. CT head imaging revealed progression of the posterior cerebral edema and intraparenchymal hemorrhage. He was admitted to our ICU and despite supportive treatment, his neurological examination worsened while CT head imaging findings remained stable. Invasive multimodality monitoring revealed elevated intracranial pressure. The patient was aggressively treated and after a prolonged hospitalization and rehabilitation course, made a significant recovery.

Conclusion

This case highlights a very rare potential neurological complication of ondansetron, a commonly used medication. We hypothesize an underlying association between PRES and 5-HT₃ antagonism, via the latter’s potential role in endothelial dysfunction. Prompt recognition and treatment of PRES is essential, in order to prevent secondary cerebral injury and the associated potentially grave consequences.

The cerebral circulation during pregnancy: adapting to preserve normalcy

The adaptation of the brain and cerebral circulation to pregnancy are unique compared with other organs and circulatory systems, ultimately functioning to maintain brain homeostasis. In this review, the effect of pregnancy on critical functions of the cerebral circulation is discussed, including changes occurring at the endothelium and blood-brain barrier, and changes in the structure and function of cerebral arteries and arterioles, hemodynamics, and cerebral blood flow autoregulation.

Myogenic tone as a therapeutic target for ischemic stroke

Ischemic stroke causes vascular paralysis and impaired autoregulation in the brain, the degree of which is dependent on the depth and duration of ischemia and reperfusion (I/R). Ischemic stroke also impairs the myogenic response of middle cerebral arteries (MCA) that may be an underlying mechanism by which autoregulation is impaired. Myogenic responses are affected by I/R through several mechanisms, including production of peroxynitrite, depolymerization of F-actin in vascular smooth muscle, and circulating vasoactive factors. The vascular endothelium is also significantly affected during focal ischemia that has a particularly large influence on vascular tone in the cerebral circulation. Endothelial nitric oxide (NO) and endothelin-1 (ET-1) are important endothelium-dependent vasoactive substances that can influence the level of myogenic tone in cerebral arteries and arterioles that are significantly affected during ischemic stroke. Unlike MCA, brain penetrating arterioles have considerable myogenic tone that appears less affected by focal ischemia. The persistent tone of brain parenchymal arterioles during focal ischemia could contribute to perfusion deficit and infarct expansion. These arterioles within the cerebral cortex are also unique from MCA in that they constrict to small- and intermediate- conductance calcium-activated potassium channel (SKCa and IKCa, respectively) inhibition, suggesting basal endothelium-derived hyperpolarizing factor (EDHF) is preserved during focal ischemia. This review will highlight our current understanding of the effects of I/R on myogenic response in both MCA and parenchymal arterioles and discuss underlying mechanisms by which focal ischemia affects myogenic tone in these different vascular segments.

Effect of hypertension and carotid occlusion on brain parenchymal arteriole structure and reactivity

We studied the effect of hypertension and chronic hypoperfusion on brain parenchymal arteriole (PA) structure and function. PAs were studied isolated and pressurized from 18-wk-old Wistar-Kyoto (WKY18; n = 8) and spontaneously hypertensive stroke prone (SHRSP18; n = 8) and 5-wk-old prehypertensive (SHRSP5; n = 8) rats. In separate groups, unilateral common carotid artery occlusion (UCCAo) was performed for 4 wk to cause chronic hypoperfusion in 18-wk-old WKY (WKY18-CH; n = 8) and SHRSP (SHRSP18-CH; n = 8). UCCAo caused PAs to have significantly diminished myogenic tone (31 ± 3 vs. 14 ± 6% at 60 mmHg; P < 0.05) and reactivity to pressure from WKY18-CH vs. WKY18 animals. The effect of UCCAo was limited to normotensive animals, as there was little effect of chronic hypoperfusion on vascular reactivity or percent tone in PAs from SHRSP18 vs. SHRSP18-CH animals (53 ± 4 vs. 41 ± 3%; P > 0.05). However, PAs from SHRSP18 and SHRSP5 animals had significantly greater tone compared with WKY18, suggesting an effect of strain and not hypertension per se on PA vasoconstriction. Structurally, PAs from SHRSP18 and SHRSP5 animals had similar sized lumen diameters, but increased wall thickness and distensibility compared with WKY18. Interestingly, chronic hypoperfusion did not affect the structure of PAs from either WKY18-CH or SHRSP18-CH animals. Thus PAs responded to UCCAo with active vasodilation, but not structural remodeling, an effect that was absent in SHRSP. The increased tone of PAs from SHRSP animals, combined with lack of response to chronic hypoperfusion, may contribute to the propensity for ischemic lesions and increased perfusion deficit during hypertension.

The Contribution of Normal Pregnancy to Eclampsia

Eclampsia, clinically defined as unexplained seizure in a woman with preeclampsia, is a life threatening complication unique to the pregnant state. However, a subpopulation of women with seemingly uncomplicated pregnancies experience de novo seizure without preeclamptic signs or symptoms, suggesting pregnancy alone may predispose the brain to seizure. Here, we hypothesized that normal pregnancy lowers seizure threshold and investigated mechanisms by which pregnancy may affect seizure susceptibility, including neuroinflammation and plasticity of gamma-aminobutyric acid type A receptor (GABA_AR) subunit expression. Seizure threshold was determined by quantifying the amount of pentylenetetrazole (PTZ) required to elicit electrical seizure in Sprague Dawley rats that were either nonpregnant (Nonpreg, n = 7) or pregnant (Preg; d20, n = 6). Seizure-induced vasogenic edema was also measured. Further, activation of microglia, a measure of neuroinflammation (n = 6-8/group), and GABA_AR δ- and γ2-subunit protein expression in the cerebral cortex and hippocampus (n = 6/group) was determined. Seizure threshold was lower in Preg compared to Nonpreg rats (36.7±9.6 vs. 65.0±14.5 mg/kg PTZ; p<0.01) that was associated with greater vasogenic edema formation (78.55±0.11 vs. 78.04±0.19% water; p<0.05). The % of active microglia was similar between groups; however, pregnancy was associated with downregulation of cortical GABA_AR-δ and hippocampal GABA_AR-γ2 expression. Overall, pregnancy appears to be a state of increased seizure susceptibility that is not due to neuroinflammation, but rather is associated with reduced expression of GABA_AR subunits and greater edema. Understanding neurophysiological changes occurring in normal pregnancy could allow for better prevention and management of de novo seizure, including pathologic states such as eclampsia.

Formerly eclamptic women have lower nonpregnant blood pressure compared with formerly pre-eclamptic women: a retrospective cohort study

OBJECTIVE

To compare nonpregnant blood pressure and circulating metabolic factors between formerly pre-eclamptic women who did and did not deteriorate to eclampsia.

DESIGN

Retrospective observational cohort study.

SETTING

Tertiary referral centre.

POPULATION

Formerly pre-eclamptic women with (n = 88) and without (n = 698) superimposed eclampsia.

METHODS

Women who experienced pre-eclampsia with or without superimposed eclampsia during their pregnancy or puerperium were tested for possible underlying cardiovascular risk factors at least 6 months postpartum. We measured blood pressure and determined cardiovascular and metabolic risk markers in a fasting blood sample. Groups were compared using Mann-Whitney U test, Spearman's Rho test or Fisher's Exact test (odds ratios).

MAIN OUTCOME MEASURES

Differences in postpartum blood pressures and features of the metabolic syndrome between formerly pre-eclamptic and formerly eclamptic women.

RESULTS

Formerly pre-eclamptic women who developed eclampsia differed from their counterparts without eclampsia by a lower blood pressure (P < 0.01) with blood pressure correlating inversely with the likelihood of having experienced eclampsia (P < 0.001). In addition, formerly eclamptic women had higher circulating C-reactive protein levels than formerly pre-eclamptic women (P < 0.05). All other circulating metabolic factors were comparable. Finally, 40% of all eclamptic cases occurred in the puerperium.

CONCLUSIONS

Formerly pre-eclamptic women with superimposed eclampsia have lower nonpregnant blood pressure compared with their counterparts without neurological sequelae with blood pressure negatively correlated to the occurrence of eclampsia. As about 40% of all eclamptic cases occur postpartum, routine blood pressure monitoring postpartum should be intensified.

Plasma from patients with HELLP syndrome increases blood-brain barrier permeability

Circulating inflammatory factors and endothelial dysfunction have been proposed to contribute to the pathophysiology of hemolysis, elevated liver enzymes, and low platelet count (HELLP) syndrome. To date, the occurrence of neurological complications in these women has been reported, but few studies have examined whether impairment in blood-brain barrier (BBB) permeability or cerebrovascular reactivity is present in women having HELLP syndrome. We hypothesized that plasma from women with HELLP syndrome causes increased BBB permeability and cerebrovascular dysfunction. Posterior cerebral arteries from female nonpregnant rats were perfused with 20% serum from women with normal pregnancies (n = 5) or women with HELLP syndrome (n = 5), and BBB permeability and vascular reactivity were compared. Plasma from women with HELLP syndrome increased BBB permeability while not changing myogenic tone and reactivity to pressure. Addition of the nitric oxide (NO) synthase inhibitor N(ω)-nitro-L-arginine methyl ester caused constriction of arteries that was not different with the different plasmas nor was dilation to the NO donor sodium nitroprusside different between the 2 groups. However, dilation to the small- and intermediate-conductance, calcium-activated potassium channel activator NS309 was decreased in vessels exposed to HELLP plasma. Thus, increased BBB permeability in response to HELLP plasma was associated with selective endothelial dysfunction.

Postischemic reperfusion causes smooth muscle calcium sensitization and vasoconstriction of parenchymal arterioles

BACKGROUND AND PURPOSE

Parenchymal arterioles (PAs) are high-resistance vessels in the brain that connect pial vessels to the microcirculation. We previously showed that PAs have increased vasoconstriction after ischemia and reperfusion that could increase perfusion deficit. Here, we investigated underlying mechanisms by which early postischemic reperfusion causes increased vasoconstriction of PAs.

METHODS

Isolated and pressurized PAs from within the middle cerebral artery territory were studied in male Wistar rats that were either nonischemic control (n=34) or after exposure to transient middle cerebral artery occlusion (MCAO) by filament occlusion for 2 hours with 30 minutes of reperfusion (MCAO; n=38). The relationships among pressure-induced tone, smooth muscle calcium (using Fura 2), and membrane potential were determined. Sensitivity of the contractile apparatus to calcium was measured in permeabilized arterioles using Staphylococcus aureus α-toxin. Reactivity to inhibition of transient receptor potential melastanin receptor type 4 (9-phenanthrol), Rho kinase (Y27632), and protein kinase C (Gö6976) was also measured.

RESULTS

After MCAO, PAs had increased myogenic tone compared with controls (47±2% versus 35±2% at 40 mm Hg; P<0.01), without an increase in smooth muscle calcium (177±21 versus 201±16 nmol/L; P>0.05) or membrane depolarization (-38±4 versus -36±1 mV; P>0.05). In α-toxin-permeabilized vessels, MCAO caused increased sensitivity of the contractile apparatus to calcium. MCAO did not affect dilation to transient receptor potential melastanin receptor type 4 or protein kinase C inhibition but diminished dilation to Rho kinase inhibition.

CONCLUSIONS

The increased vasoconstriction of PAs during early postischemic reperfusion seems to be due to calcium sensitization of smooth muscle and could contribute to infarct expansion and limit neuroprotective agents from reaching their target tissue.

Increased pressure-induced tone in rat parenchymal arterioles vs. middle cerebral arteries: role of ion channels and calcium sensitivity

Brain parenchymal arterioles (PAs) are high-resistance vessels that branch off pial arteries and perfuse the brain parenchyma. PAs are the target of cerebral small vessel disease and have been shown to have greater pressure-induced tone at lower pressures than pial arteries. We investigated mechanisms by which brain PAs have increased myogenic tone compared with middle cerebral arteries (MCAs), focusing on differences in vascular smooth muscle (VSM) calcium and ion channel function. The amount of myogenic tone and VSM calcium was measured using Fura 2 in isolated and pressurized PAs and MCAs. Increases in intraluminal pressure caused larger increases in tone and cytosolic calcium in PAs compared with MCAs. At 50 mmHg, myogenic tone was 37 ± 5% for PAs vs. 6.5 ± 4% for MCAs (P < 0.01), and VSM calcium was 200 ± 20 nmol/l in PAs vs. 104 ± 15 nmol/l in MCAs (P < 0.01). In vessels permeabilized with Staphylococcus aureus α-toxin, PAs were not more sensitive to calcium, suggesting calcium sensitization was not at the level of the contractile apparatus. PAs were 30-fold more sensitive to the voltage-dependent calcium channel (VDCC) inhibitor nifedipine than MCAs (EC50 for PAs was 3.5 ± 0.4 vs. 82.1 ± 2.1 nmol/l for MCAs;P < 0.01); however, electrophysiological properties of the VDCC were not different in VSM. PAs had little to no response to the calcium-activated potassium channel inhibitor iberiotoxin, whereas MCAs constricted ∼15%. Thus increased myogenic tone in PAs appears related to differences in ion channel activity that promotes VSM membrane depolarization but not to a direct sensitization of the contractile apparatus to calcium.

Abstract 133: Inhibition of TRPV4 is Protective of the Brain and Cerebral Circulation During Ischemic Stroke

Background: Transient receptor potential vanilloid 4 (TRPV4) channels are highly calcium permeable channels expressed in brain and cerebral endothelium and are regulated by a diverse array of stimuli including shear stress, cell swelling and second messengers. We hypothesized that inhibition of TRPV4 during postischemic reperfusion would be protective of the brain and cerebral endothelium.

Methods: Transient MCAO was used for all experiments. Infarction (by TTC) was measured in TRPV4 knockout mice (n=3) or C57B mice treated upon reperfusion with the TRPV4 inhibitor HC067047 (1mg/kg; n=7) or vehicle (n=8) after 90 minutes of ischemia with 24 hours of reperfusion. To determine the effect of TRPV4 inhibition on BBB permeability, hyperglycemic male Wistar rats (3 days by STZ) were treated with HC067047 (1mg/kg; n=6) or vehicle (n=6) upon reperfusion after 2 hours of ischemia and edema measured at 2 hours by wet:dry weights. Lastly, basal tone of MCAs isolated from male Wistar rats after 2 hours of ischemia with 2 hours of reperfusion (MCAO, n=7) or sham controls (Sham, n=6) was measured before and after addition of HC067047 (1μM) at 75 mmHg.

Results: Infarct volume was significantly reduced in TRPV4 knockout mice and with HC067047 treatment vs. vehicle (17.8 ± 2.8% for KO, 22.9 ± 2.7% for HC067047 vs. 39.8 ± 3.3% for vehicle; p<0.05). Hyperglycemic stroke caused considerable edema formation the ipsilateral vs. contralateral brain in vehicle-treated rats (81.38 ± 0.21% vs. 78.64 ± 0.12%; p<0.01) that was prevented by treatment with HC067047 (78.58 ± 0.22% vs. 78.24 ± 0.11%;p>0.05). MCAO diminished basal tone of MCAs vs. sham (16.3 ± 3.3 vs. 26.4 ± 2.8%; p<0.05) that was restored by HC067047 (25.9 ± 3.0%; p<0.05). The constriction to HC067047 did not occur in MCA that were denuded of endothelium or from sham animals.

Conclusion: These results suggest that postischemic reperfusion activates TRPV4 in cerebral endothelium to cause dilation and BBB permeability. The decreased infarct size after TRPV4 inhibition suggests that activation of TRPV4 during ischemia contributes to neuronal cell death. Thus, inhibition of TRPV4 may be an effective treatment for acute ischemic stroke.

Effect of hyperglycemia on brain penetrating arterioles and cerebral blood flow before and after ischemia/reperfusion

The effect of preexisiting hyperglycemia on cerebral blood flow (CBF) and brain penetrating arterioles before and after 2 h of ischemia and 30 min of reperfusion was determined. Male Wistar rats that were either hyperglycemic (50 mg/kg streptozotocin; n=24) or normoglycemic (n=24) were subjected to transient ischemia by filament occlusion or nonischemic. CBF was measured prior to ischemia using microspheres and during transient ischemia using laser Doppler. Edema was compared by wet/dry weights. Constriction to apamin, TRAM-34, and L-NNA, inhibitors of small- and intermediate-conductance calcium-activated potassium channels (SK and IK) and nitric oxide, were compared in penetrating arterioles from the ischemic hemisphere to investigate changes in basal tone and endothelium-dependent vasodilator responses. Preexisiting hyperglycemia did not affect CBF in non-ischemic animals or after transient ischemia; however, edema was significantly greater. Ischemia and reperfusion caused decreased basal tone in penetrating arterioles similarly in normoglycemic and hyperglycemic animals that was restored by apamin, and further increased by TRAM-34 and L-NNA. The restoration of tone in penetrating arterioles by apamin and TRAM-34 suggests that transient ischemia activates SK and IK channels in penetrating arterioles. This effect of ischemia was not different between normoglycemic and hyperglycemic animals, suggesting that it was related to ischemia and reperfusion rather than hyperglycemia.

Mechanisms of enhanced basal tone of brain parenchymal arterioles during early postischemic reperfusion: role of ET-1-induced peroxynitrite generation

The contributions of vasoconstrictors (endothelin-1 (ET-1), peroxynitrite) and endothelium-dependent vasodilatory mechanisms to basal tone were investigated in parenchymal arterioles (PAs) after early postischemic reperfusion. Transient middle cerebral artery occlusion (tMCAO) was induced for 2 hours with 30 minutes reperfusion in male Wistar rats and compared with ischemia alone (permanent MCAO (pMCAO); 2.5 hours) or sham controls. Changes in lumen diameter of isolated and pressurized PAs were compared. Quantitative PCR was used to measure endothelin type B (ETB) receptors. Constriction to intravascular pressure ('basal tone') was not affected by tMCAO or pMCAO. However, constriction to inhibitors of endothelial cell, small- (SK) and intermediate- (IK) conductance, Ca(2+)-sensitive K(+) channels (apamin and TRAM-34, respectively) were significantly enhanced in PAs from tMCAO compared with pMCAO or sham. Addition of the ETB agonist sarafotoxin caused constriction in PAs from tMCAO but not from sham animals (21 ± 4% versus 3 ± 3% at 1 nmol/L; P<0.01) that was inhibited by the peroxynitrite scavenger FeTMPyP (5,10,15,20-tetrakis (N-methyl-4'-pyridyl) porphinato iron (III) chloride) (100 μmol/L). Expression of ETB receptors was not found on PA smooth muscle, suggesting that constriction to sarafotoxin after tMCAO was due to peroxynitrite and not ETB receptor expression. The maintenance of basal tone in PAs after tMCAO may restrict flow to the ischemic region and contribute to infarct expansion.

Inhibition of PPARγ during rat pregnancy causes intrauterine growth restriction and attenuation of uterine vasodilation

Decreased peroxisome proliferator-activated receptor gamma (PPARγ) activity is thought to have a major role in preeclampsia through abnormal placental development. However, the role of PPARγ in adaptation of the uteroplacental vasculature that may lead to placental hypoperfusion and fetal growth restriction during pregnancy is not known. Here, pregnant Sprague-Dawley rats (n = 11/group) were treated during the second half of pregnancy with the PPARγ inhibitor GW9662 (10 mg/kg/day in food) or vehicle. Pregnancy outcome and PPARγ mRNA, vasodilation and structural remodeling were determined in maternal uterine and mesenteric arteries. PPARγ was expressed in uterine vascular tissue of both non-pregnant and pregnant rats with ~2-fold greater expression in radial vs. main uterine arteries. PPARγ mRNA levels were significantly higher in uterine compared to mesenteric arteries. GW9662 treatment during pregnancy did not affect maternal physiology (body weight, glucose, blood pressure), mesenteric artery vasodilation or structural remodeling of uterine and mesenteric vessels. Inhibition of PPARγ for the last 10 days of gestation caused decreased fetal weights on both day 20 and 21 of gestation that was associated with impaired vasodilation of radial uterine arteries in response to acetylcholine and sodium nitroprusside. These results define an essential role of PPARγ in the control of uteroplacental vasodilatory function during pregnancy, an important determinant of blood flow to the placenta and fetus. Strategies that target PPARγ activation in the uterine circulation could have important therapeutic potential in treatment of pregnancies complicated by hypertension, diabetes or preeclampsia.

Treatment for cerebral small vessel disease: effect of relaxin on the function and structure of cerebral parenchymal arterioles during hypertension

We investigated the effect of hypertension on the function and structure of cerebral parenchymal arterioles (PAs), a major target of cerebral small vessel disease (SVD), and determined whether relaxin is a treatment for SVD during hypertension. PAs were isolated from 18-wk-old female normotensive Wistar-Kyoto (WKY) rats, spontaneous hypertensive rats (SHRs), and SHRs treated with human relaxin 2 for 14 d (4 μg/h; n=8/group) and studied using a pressurized arteriograph system. Hypertension reduced PA inner diameter (58±3 vs. 49±3 μm at 60 mmHg in WKY rats, P<0.05), suggesting inward remodeling that was reversed by relaxin (56±4 μm, P<0.05). Relaxin also increased PA distensibility in SHRs (34±2 vs. 10±2% in SHRs, P<0.05). Relaxin was detected in cerebrospinal fluid (110±30 pg/ml) after systemic administration, suggesting that it crosses the blood-brain barrier (BBB). Relaxin receptors (RXFP1/2) were not detected in cerebral vasculature, but relaxin increased vascular endothelial growth factor (VEGF) and matrix metalloproteinase 2 (MMP-2) expression in brain cortex. Inhibition of VEGF receptor tyrosine kinase (axitinib, 4 mg/kg/d, 14 d) had no effect on increased distensibility with relaxin, but caused outward hypertrophic remodeling of PAs from SHRs. These results suggest that relaxin crosses the BBB and activates MMP-2 in brain cortex, which may interact with PAs to increase distensibility. VEGF appears to be involved in remodeling of PAs, but not relaxin-induced increased distensibility.

Pregnancy causes diminished myogenic tone and outward hypotrophic remodeling of the cerebral vein of Galen

Pregnancy increases the risk of several complications associated with the cerebral veins, including thrombosis and hemorrhage. In contrast to the cerebral arteries and arterioles, few studies have focused on the effect of pregnancy on the cerebral venous side. Here, we investigated for the first time the effect of pregnancy on the function and structure of the cerebral vein of Galen in rats. Our major finding was that cerebral veins from late-pregnant (LP, n=11) rats had larger lumen diameters and thinner walls than veins from nonpregnant (NP, n=13) rats, indicating that pregnancy caused outward hypotrophic remodeling of the vein of Galen. Moreover, veins from NP animals had a small amount of myogenic tone at 10 mm Hg (3.9±1.0%) that was diminished in veins during pregnancy (0.8±0.3%; P<0.01). However, endothelium-dependent and -independent vasodilation of the veins was unchanged during pregnancy. Using immunohistochemistry, we show that the vein of Galen receives perivascular innervation, and that serotonergic innervation of cerebral veins is significantly higher in veins from LP animals. Outward hypotrophic remodeling and diminished tone of cerebral veins during pregnancy may contribute to the development of venous pathology through elevated wall tension and wall stress, and possibly by promoting venous blood stasis.

The adaptation of the cerebral circulation to pregnancy: mechanisms and consequences

The adaptation of the cerebral circulation to pregnancy is unique from other vascular beds. Most notably, the growth and vasodilatory response to high levels of circulating growth factors and cytokines that promote substantial hemodynamic changes in other vascular beds is limited in the cerebral circulation. This is accomplished through several mechanisms, including downregulation of key receptors and transcription factors, and production of circulating factors that counteract the vasodilatory effects of vascular endothelial growth factor (VEGF) and placental growth factor. Pregnancy both prevents and reverses hypertensive inward remodeling of cerebral arteries, possibly through downregulation of the angiotensin type 1 receptor. The blood-brain barrier (BBB) importantly adapts to pregnancy by preventing the passage of seizure provoking serum into the brain and limiting the permeability effects of VEGF that is more highly expressed in cerebral vasculature during pregnancy. While the adaptation of the cerebral circulation to pregnancy provides for relatively normal cerebral blood flow and BBB properties in the face of substantial cardiovascular changes and high levels of circulating factors, under pathologic conditions, these adaptations appear to promote greater brain injury, including edema formation during acute hypertension, and greater sensitivity to bacterial endotoxin.

Increased oxidized low-density lipoprotein causes blood-brain barrier disruption in early-onset preeclampsia through LOX-1

Early-onset preeclampsia (EPE) is a severe form of preeclampsia that involves life-threatening neurological complications. However, the underlying mechanism by which EPE affects the maternal brain is not known. We hypothesized that plasma from women with EPE increases blood-brain barrier (BBB) permeability vs. plasma from women with late-onset preeclampsia (LPE) or normal pregnancy (NP) and investigated its underlying mechanism by perfusing cerebral veins from nonpregnant rats (n=6-7/group) with human plasma from women with EPE, LPE, or NP and measuring permeability. We show that plasma from women with EPE significantly increased BBB permeability vs. plasma from women with LPE or NP (P<0.001). BBB disruption in response to EPE plasma was due to a 260% increase of circulating oxidized LDL (oxLDL) binding to its receptor, LOX-1, and subsequent generation of peroxynitrite (P<0.001). A rat model with pathologically high lipid levels in pregnancy showed symptoms of preeclampsia, including elevated blood pressure, growth-restricted fetuses, and LOX-1-dependent BBB disruption, similar to EPE (P<0.05). Thus, we have identified LOX-1 activation by oxLDL and subsequent peroxynitrite generation as a novel mechanism by which disruption of the BBB occurs in EPE. As increased BBB permeability is a primary means by which seizure and other neurological symptoms ensue, our findings highlight oxLDL, LOX-1, and peroxynitrite as important therapeutic targets in EPE.

Effect of pregnancy and nitric oxide on the myogenic vasodilation of posterior cerebral arteries and the lower limit of cerebral blood flow autoregulation

Hemorrhage during parturition can lower blood pressure beyond the lower limit of cerebral blood flow (CBF) autoregulation that can cause ischemic brain injury. However, the impact of pregnancy on the lower limit of CBF autoregulation is unknown. We measured myogenic vasodilation, a major contributor of CBF autoregulation, in isolated posterior cerebral arteries (PCAs) from nonpregnant and late-pregnant rats (n = 10/group) while the effect of pregnancy on the lower limit of CBF autoregulation was studied in the posterior cerebral cortex during controlled hemorrhage (n = 8). Pregnancy enhanced myogenic vasodilation in PCA and shifted the lower limit of CBF autoregulation to lower pressures. Inhibition of nitric oxide synthase (NOS) prevented the enhanced myogenic vasodilation during pregnancy but did not affect the lower limit of CBF autoregulation. The shift in the autoregulatory curve to lower pressures during pregnancy is likely protective of ischemic injury during hemorrhage and appears to be independent of NOS.

Abstract WP440: Relaxin Selectively Affects Brain Parenchymal Arteriolar Structure during Hypertension

Background: Cerebral small vessel disease (SVD) is a common hypertension-related vasculopathy of the penetrating arterioles (PA) that causes lacunar stroke, white matter lesions and vascular dementia. We investigated the effect of chronic hypertension on PA structure and if treatment with relaxin, a hormone with cardioprotective effects previously shown to promote selective outward remodeling of PA in normotensive animals, could reverse these effects.

Methods: Female SHR rats (n=7-8/group) were treated in vivo with relaxin (4μg/hr) or vehicle for 2 weeks and studied at 18 weeks of age. Aged-matched WKY rats were used as controls. Middle cerebral arteries (MCA) and PA from the same animals were isolated and studied under pressurized conditions in zero calcium buffer to obtain structural measurements. Blood pressures were measured using tail cuff after 3 days of training.

Results: PA inner and outer diameters were smaller in SHR vs. WKY: at 100 mmHg PA inner and outer diameters were 59 ± 3 and 72 ± 3 μm for WKY vs. 50 ± 3 and 63 ± 3 μm for SHR (p<0.01). Relaxin treatment of SHR reversed this rarefaction such that inner and outer diameters were similar to WKY: 58 ± 4 and 69 ± 4 μm (p<0.01 vs. SHR; Figure). The increase in PA size with relaxin treatment was related to a large increase in distensibility (Figure). Percent distensibility of PA for WKY, SHR and SHR+relaxin was: 18 ± 3%, 13 ± 2% and 40 ± 4% (p<0.01). Relaxin treatment had no effect on MCA in SHR that were smaller than WKY: MCA inner and outer diameters were 258 ± 4 and 285 ± 5 μm in WKY, 228 ± 5 and 260 ± 5 μm in SHR and 225 ± 4 and 256 ± 4 μm in SHR+relaxin (p>0.05). Relaxin also did not lower blood pressures that were higher in SHR: MAP for WKY, SHR and SHR+relaxin was: 94 ± 3, 138 ± 2 and 144 ± 5 mmHg (p<0.01 vs. WKY).

Conclusion: These results suggest that relaxin may be an effective treatment for hypertension-induced rarefaction and increased stiffness of PA that underlies SVD in the brain.

Pregnancy enhances the effects of hypercholesterolemia on posterior cerebral arteries

In preeclampsia, hyperlipidemia is enhanced compared to normal pregnancy that could adversely affect vascular function. In the cerebral vasculature, this could lead to dysregulation of cerebral blood flow and neurological complications. Here, we examined the effect of excessive hyperlipidemia, as seen in preeclampsia, on cerebral artery function and expression of inflammatory markers in pregnancy. Pregnant and nonpregnant rats received a 14-day high-cholesterol diet or normal chow and posterior cerebral artery function was compared. High cholesterol significantly increased sensitivity of posterior cerebral arteries to the nitric oxide donor sodium nitroprusside that was accompanied by a ~12-fold increased messenger RNA (mRNA) expression of inducible nitric oxide synthase in late-pregnant rats only. Further, high cholesterol significantly increased peroxynitrite-induced dilation and decreased myogenic tone in cerebral arteries from late pregnant compared to nonpregnant animals. These results suggest that pathologically high levels of cholesterol in pregnancy enhance inflammatory responses and peroxynitrite generation in cerebral arteries.