Structure and mechanical properties of resistance arteries in hypertension: role of adhesion molecules and extracellular matrix determinants

The plastic nature of the vascular wall: a continuum of remodeling events contributing to control of arteriolar diameter and structure

The diameter of resistance arteries has a profound effect on the distribution of microvascular blood flow and the control of systemic blood pressure. Here, we review mechanisms that contribute to the regulation of resistance artery diameter, both acutely and chronically, their temporal characteristics, and their interdependence. Furthermore, we hypothesize the existence of a remodeling continuum that allows for the vascular wall to rapidly modify its structural characteristics, specifically through the re-positioning of vascular smooth muscle cells. Importantly, the concepts presented more closely link acute vasoregulatory responses with adaptive changes in vessel wall structure. These rapid structural adaptations provide resistance vessels the ability to maintain a desired diameter under presumed optimal energetic and mechanical conditions.

Cellular mediators of renal vascular dysfunction in hypertension

The renal vasculature plays a major role in the regulation of renal blood flow and the ability of the kidney to control the plasma volume and blood pressure. Renal vascular dysfunction is associated with renal vasoconstriction, decreased renal blood flow, and consequent increase in plasma volume and has been demonstrated in several forms of hypertension (HTN), including genetic and salt-sensitive HTN. Several predisposing factors and cellular mediators have been implicated, but the relationship between their actions on the renal vasculature and the consequent effects on renal tubular function in the setting of HTN is not clearly defined. Gene mutations/defects in an ion channel, a membrane ion transporter, and/or a regulatory enzyme in the nephron and renal vasculature may be a primary cause of renal vascular dysfunction. Environmental risk factors, such as high dietary salt intake, vascular inflammation, and oxidative stress further promote renal vascular dysfunction. Renal endothelial cell dysfunction is manifested as a decrease in the release of vasodilatory mediators, such as nitric oxide, prostacyclin, and hyperpolarizing factors, and/or an increase in vasoconstrictive mediators, such as endothelin, angiotensin II, and thromboxane A(2). Also, an increase in the amount/activity of intracellular Ca(2+) concentration, protein kinase C, Rho kinase, and mitogen-activated protein kinase in vascular smooth muscle promotes renal vasoconstriction. Matrix metalloproteinases and their inhibitors could also modify the composition of the extracellular matrix and lead to renal vascular remodeling. Synergistic interactions between the genetic and environmental risk factors on the cellular mediators of renal vascular dysfunction cause persistent renal vasoconstriction, increased renal vascular resistance, and decreased renal blood flow, and, consequently, lead to a disturbance in the renal control mechanisms of water and electrolyte balance, increased plasma volume, and HTN. Targeting the underlying genetic defects, environmental risk factors, and the aberrant renal vascular mediators involved should provide complementary strategies in the management of HTN.

Vascular remodeling: implications for small artery function and target organ damage

At the level of the small artery, essential hypertension is associated with eutrophic inward remodeling. This involves reduction in lumen diameter by an increase in wall thickness. Previously thought to involve either hypertrophy or hyperplasia of the vascular smooth muscle cells in the media, it is now felt to be mediated by a functional property of the wall: myogenic tone. This is the ability of an artery to contract in response to an increase in intraluminal pressure. This autoregulatory function is also vital to ensure stabilisation of distal capillary pressures and so prevent, or limit, organ damage. Indeed in any animal model studied, when myogenic autoregulation is affected, target organ damage ensues. We have also observed, in two studies, that when myogenic autoregulation is damaged in the context of hypertension, eutrophic remodeling is replaced by an outward growth of the arterial wall with preservation of lumen diameter. This is called hypertrophic remodeling and, independently, has been observed by a number of groups in small arteries from patients with type 2 diabetes. We believe that this is a key reason for the unique propensity to hypertensive injury seen in patients with diabetes. We also discuss the significance of integrins, transmembrane proteins with wide ranging functions; from initiation of cell migration to intracellular signalling. Two particular integrins, alpha5beta1 and alphanubeta3, have been found to be necessary for both normal myogenic autoregulation and eutrophic remodeling and the possibility that damage to these may occur in diabetes is examined.

Flow-mediated vascular remodeling in hypertension: relation to hemodyamics

BACKGROUND AND PURPOSE

Changes in shear and medial wall stress induced by blood flow contribute to vascular remodeling, but details of these relations remain undefined. We hypothesized that remodeling has a strong genetic component and that phenotypic responses to hemodynamic stress will differ among rat strains. Here, we characterized phenotypic traits related to carotid remodeling in the 2 rat strains that we previously showed the greatest difference in shear stress regulation: Genetically Hypertensive (GH) and Brown Norway (BN) rat strains.

METHODS

Left internal and external carotid arteries were ligated and blood flow was reduced in the left common (LCA) by 90% and increased in the right common carotid artery (RCA) by 60%. Rats were studied for up to 28 days after flow modification and carotid outer diameters were measured in vivo, and wall and luminal components by histomorphometry, to obtain indices of remodeling. Blood flow and pressure measurements were made at corresponding time points.

RESULTS

By day 28, remodeling in the GH was greater in response to high flow than in BN, and shear stress was normalized. In contrast, remodeling in the BN was greater in the low flow LCA than in GH. Media stress was greater in GH than BN for any value of carotid shear stress and remained relatively unchanged in low flow, but markedly increased in high flow remodeling. Importantly, pressure was not a major determinant of flow remodeling in these conditions.

CONCLUSIONS

There are key differences in the ability of carotids in GH and BN rats to adhere to hemodynamic laws during vascular remodeling. GH rats exhibit intact regulatory mechanisms for increased, but not reduced, shear stress. Moreover, the ability to maintain physiological shear and media stresses during vascular remodeling in response to modified flow appears to be intrinsically "genetically" determined.

Role of elevated EGFR phosphorylation in the induction of structural remodelling and altered mechanical properties of resistance artery from type 2 diabetic mice

BACKGROUND

Type 2 diabetes is associated with microvascular complications. We hypothesized that the sustained elevated EGFR phosphorylation produces structural wall remodelling and altered mechanical properties of mesenteric resistance artery (MRA) in type 2 diabetes.

METHODS

Freshly isolated MRA (80-100 microm diameter) from type 2 diabetic (db(-)/db(-), diabetic) and non-diabetic (db(-)/db(+), control) mice were subjected to pressure-passive diameter and wall thickness relationships; western blot analysis and immunohistology.

RESULTS

Data indicated that MRA from diabetic mice have a smaller passive diameter than MRA from control mice under intra-luminal pressure range from 25 to 125 mmHg. Measurements of wall thickness : lumen diameter ratios (21 +/- 1.8 vs 14 +/- 1.2 at 75 mmHg diabetic vs control, respectively), wall thickness and remodelling index (38 +/- 5% vs control) revealed eutrophic structural remodelling of MRA from diabetic mice, which was strengthened with histology. Mechanical properties revealed a great strain-stress relationship in MRA from control versus diabetic mice indicating increased stiffness in MRA from diabetic mice. Western blot analysis showed increased collagen type 1 content in a freshly isolated MRA from the type 2 diabetic mice when compared to control mice. Diabetic mice treated with EGFR inhibitor (AG1478, 10 mg/kg/day) for 2 weeks showed reduced EGFR phosphorylation, wall thickness, collagen type 1 content, and improved the altered mechanical properties of MRA.

CONCLUSION

These data provide evidence regarding the role of EGFR in morphological wall remodelling and altered mechanical properties of MRA from type 2 diabetic mice. This may identify new therapeutic targets for the control of vascular structure and therefore have important implications in type 2 diabetes.

Analysis of retinal arteriolar structure in never-treated patients with essential hypertension

OBJECTIVE

Increased wall-to-lumen ratio of small arteries is a predictor of adverse cardiovascular prognosis. We aimed to analyze retinal arteriolar structure in never-treated patients with essential hypertension and to test whether elevated blood pressure is associated with an increased wall-to-lumen ratio of retinal arterioles.

METHODS

The study cohort comprised 21 untreated male patients with essential hypertension (mean age 39.1 +/- 5.4 years) and 29 untreated normotensive men (mean age 36.7 +/- 5.9 years). Wall-to-lumen ratio of retinal arterioles was assessed in vivo using scanning laser Doppler flowmetry.

RESULTS

Patients with essential hypertension had a higher wall-to-lumen ratio of retinal arterioles than normotensive individuals (0.36 +/- 0.1 vs. 0.28 +/- 0.1, P = 0.028). Wall cross-sectional area of retinal arterioles did not differ between the study groups. The growth index, indicating the percentage of difference in average wall cross-sectional area of retinal arterioles between both groups, was 18%. Both systolic (r = 0.360, P = 0.010) and diastolic (r = 0.536, P < 0.001) blood pressures were related to wall-to-lumen ratio of retinal arterioles. Multiple regression analysis including a variety of known cardiovascular risk factors revealed that blood pressure is independently associated with an increased wall-to-lumen ratio of retinal arterioles (systolic blood pressure: beta = 0.417, P = 0.012; diastolic blood pressure: beta = 0.548, P = 0.001).

CONCLUSION

The changes in arteriolar structure of retinal vessels in our study cohort revealed a similar pattern to that observed previously by other investigators in subcutaneous small arteries in essential hypertension. Blood pressure emerged as an important and independent determinant of wall-to-lumen ratio of retinal arterioles.

Changes in the structure-function relationship of elastin and its impact on the proximal pulmonary arterial mechanics of hypertensive calves

Extracellular matrix remodeling has been proposed as one mechanism by which proximal pulmonary arteries stiffen during pulmonary arterial hypertension (PAH). Although some attention has been paid to the role of collagen and metallomatrix proteins in affecting vascular stiffness, much less work has been performed on changes in elastin structure-function relationships in PAH. Such work is warranted, given the importance of elastin as the structural protein primarily responsible for the passive elastic behavior of these conduit arteries. Here, we study structure-function relationships of fresh arterial tissue and purified arterial elastin from the main, left, and right pulmonary artery branches of normotensive and hypoxia-induced pulmonary hypertensive neonatal calves. PAH resulted in an average 81 and 72% increase in stiffness of fresh and digested tissue, respectively. Increase in stiffness appears most attributable to elevated elastic modulus, which increased 46 and 65%, respectively, for fresh and digested tissue. Comparison between fresh and digested tissues shows that, at 35% strain, a minimum of 48% of the arterial load is carried by elastin, and a minimum of 43% of the change in stiffness of arterial tissue is due to the change in elastin stiffness. Analysis of the stress-strain behavior revealed that PAH causes an increase in the strains associated with the physiological pressure range but had no effect on the strain of transition from elastin-dominant to collagen-dominant behavior. These results indicate that mechanobiological adaptations of the continuum and geometric properties of elastin, in response to PAH, significantly elevate the circumferential stiffness of proximal pulmonary arterial tissue.

Induction of immature dendritic cell apoptosis by foot and mouth disease virus is an integrin receptor mediated event before viral infection

Foot and mouth disease virus (FMDV) has been demonstrated to infect dendritic cells (DC) and reduced its ability to stimulate host immune responses. This study aimed to determine whether non‐replicating FMDV could induce apoptosis of the host immune cells. In this study, we have demonstrated that bone morrow derived dendritic cells (BMDCs) were induced to undergo apoptosis in a dose‐dependent manner, which was determined by the annexin‐V staining, DNA fragmentation, and TUNEL staining methods, after they were treated with the chemically inactivated FMDV in vitro. The initiation of apoptosis was apparently via an interaction of the integrin receptor on BMDCs and the RGD motif within the VP1 capsid protein of FMDV. The initiation activated a cascade of apoptotic pathway including reduced expression of Bcl‐2, activation of caspases, and release of cytochrome c from mitochondria. Pretreatment with BMDCs with LPS prevented the inactivated FMDV induced apoptosis, suggesting immature BMDCs are susceptible to such apoptosis. Taken together, the data demonstrate that the inactivated FMDV induces the apoptosis in BMDCs via the integrin receptor and subsequently triggers the apoptosis signal, suggesting that such induction of apoptosis is likely to impair immune responses against FMDV infection. J. Cell. Biochem. 102: 980–991, 2007. © 2007 Wiley‐Liss, Inc.

Potentiation of large conductance, Ca2+-activated K+ (BK) channels by alpha5beta1 integrin activation in arteriolar smooth muscle

Injury/degradation of the extracellular matrix (ECM) is associated with vascular wall remodelling and impaired reactivity, a process in which altered ECM-integrin interactions play key roles. Previously, we found that peptides containing the RGD integrin-binding sequence produce sustained vasodilatation of rat skeletal muscle arterioles. Here, we tested the hypothesis that RGD ligands work through alpha5beta1 integrin to modulate the activity of large conductance, Ca(2+)-activated K(+) (BK) channels in arteriolar smooth muscle. K(+) currents were recorded in single arteriolar myocytes using whole-cell and single-channel patch clamp methods. Activation of alpha5beta1 integrin by an appropriate, insoluble alpha5beta1 antibody resulted in a 30-50% increase in the amplitude of iberiotoxin (IBTX)-sensitive, whole-cell K(+) current. Current potentiation occurred 1-8 min after bead-antibody application to the cell surface. Similarly, the endogenous alpha5beta1 integrin ligand fibronectin (FN) potentiated IBTX-sensitive K(+) current by 26%. Current potentiation was blocked by the c-Src inhibitor PP2 but not by PP3 (0.1-1 mum). In cell-attached patches, number of open channels x open probability (NP(o)) of a 230-250 pS K(+) channel was significantly increased after FN application locally to the external surface of cell-attached patches through the recording pipette. In excised, inside-out patches, the same method of FN application led to large, significant increases in NP(o) and caused a leftward shift in the NP(o)-voltage relationship at constant [Ca(2+)]. PP2 (but not PP3) nearly abolished the effect of FN on channel activity, suggesting that signalling between the integrin and channel involved an increase in Ca(2+)sensitivity of the channel via a membrane-delimited pathway. The effects of alpha5beta1 integrin activation on both whole-cell and single-channel BK currents could be reproduced in HEK 293 cells expressing the BK channel alpha-subunit. This is the first demonstration at the single-channel level that integrin signalling can regulate an ion channel. Our results show that alpha5beta1 integrin activation potentiates BK channel activity in vascular smooth muscle through both Ca(2+)- and c-Src-dependent mechanisms. This mechanism is likely to play a role in the arteriolar dilatation and impaired vascular reactivity associated with ECM degradation.

Hypertension induces compensatory arterial remodeling following arteriotomy

BACKGROUND

Hypertension has been traditionally considered a risk factor for restenosis following carotid arteriotomy. Genetic and morphological response to carotid arteriotomy in normotensive Wystar-Kyoto (WKY), spontaneously hypertensive (SHR), and Milan hypertensive (MHS) rats were analyzed.

MATERIAL AND METHODS

C-myc, angiotensin II receptor-1 (AT1), angiotensin II receptor-2 (AT2), endothelin-1 receptor A (ET(A)), endothelin-1 receptor B (ET(B)), Bcl-2 family-members (Bcl-2/Bax, Bcl-X(L/S)) were analyzed in surgically injured as well as uninjured carotids of WKY and hypertensive strains (HS). Thirty-day histology and morphometry were accomplished on injured and uninjured carotids.

RESULTS

C-myc mRNA is activated earlier and/or to a greater extent in hypertensive strains than in WKY. AT1 mRNA increases in WKY after injury, while it decreases in SHR and MHS. AT2 shows the opposite, decreasing in WKY and increasing in hypertensive strains. ET(A) mRNA decreases in all strains although with different timing and levels, associated with a replacement by ET(B) mRNA. Bcl-2/Bax ratio gradually decreases in WKY, while it shows only a transient decrease in SHR and MHS 4 h after the injury. Negative remodeling is observed in all injured carotids, although neointima was detected in WKY only. Thirty days following arteriotomy, morphometry demonstrated a significant decrease of luminal area, with consistent gain in the medial area in WKY, whereas hypertensive strains showed significant increase of the luminal area, consistent with a contemporary decrease of the medial area.

CONCLUSIONS

Vaso-relaxant AT2 and ET(B) induced limited vasoconstriction in HS. Less apoptosis in hypertensive rats reduced cell proliferation, contrasting c-myc. These responses favorably modulated media/lumen area ratio following arteriotomy in HS.

Role of alpha1beta1-integrin in arterial stiffness and angiotensin-induced arterial wall hypertrophy in mice

We examined the arterial phenotype of mice lacking alpha(1)-integrin (alpha(1)(-/-)) at baseline and after 4 wk of ANG II or norepinephrine (NE) administration. Arterial mechanical properties were determined in the carotid artery (CA). Integrin expression, MAPK kinases, and focal adhesion kinase (FAK) were assessed in the aorta. No change in arterial pressure was observed in alpha(1)(-/-) mice. Elastic modulus-wall stress curves were similar in alpha(1)(-/-) and alpha(1)(+/+) animals, indicating no change in arterial stiffness. The rupture pressure was lower in alpha(1)(-/-) mice, demonstrating decreased mechanical strength. Lack of alpha(1)-integrin was accompanied by an increase in beta(1)-, alpha(v)-, and alpha(5)-integrins but no change in alpha(2)-integrin. ANG II increased medial cross-sectional area of the CA in alpha(1)(+/+), but not alpha(1)(-/-), mice, whereas equivalent pressor doses of NE did not produce a significant increase in either group. In alpha(1)(+/+) mice, ANG II induced alpha(1)-integrin expression and smooth muscle cell (SMC) hypertrophy in the CA in association with increased aortic expression of alpha-smooth muscle actin and smooth muscle myosin heavy chain and phosphorylation of ERK1/2, p38 MAPK, and FAK. ANG II did not induce SMC hypertrophy or phosphorylation of p38 MAPK and FAK in alpha(1)(-/-) mice. A functional anti-alpha(1)-integrin antibody inhibited in vitro the ANG II-induced phosphorylation of FAK and p38 MAPK. In conclusion, alpha(1)(-/-) mice exhibit a reduced mechanical strength at baseline and a lack of ANG II-induced SMC hypertrophy. These results emphasize the importance of alpha(1)beta(1)-integrin in p38 MAPK and FAK phosphorylation during vascular hypertrophy in response to ANG II.

Transient middle cerebral artery occlusion causes different structural, mechanical, and myogenic alterations in normotensive and hypertensive rats

Transient focal cerebral ischemia in the rat alters vessel properties, and spontaneously hypertensive rats (SHR) show a poorer outcome after ischemia. In the present study we examined the role of hypertension on vessel properties after ischemia-reperfusion. The right middle cerebral artery (MCA) was occluded (90 min) and reperfused (24 h) in SHR (n = 12) and Wistar-Kyoto rats (WKY; n = 11). Sham-operated rats (SHR, n = 10; WKY, n = 10) were used as controls. The structural, mechanical, and myogenic properties of the MCA were assessed by pressure myography. Nuclei distribution and elastin content and organization were analyzed by confocal microscopy. Infarct volume was larger in SHR than in WKY rats. Ischemia-reperfusion induced adventitial hypertrophy associated with an increase in the total number of adventitial cells. In addition, fenestrae area and arterial distensibility increased and myogenic tone decreased in the MCA of WKY rats after ischemia-reperfusion. Hypertension per se induced hypertrophic inward remodeling. Ischemia-reperfusion decreased the cross-sectional area of the MCA in SHR, without significant changes in distensibility, despite an increase in fenestrae area. In addition, MCA myogenic properties were not altered after ischemia-reperfusion in SHR. Our results indicate that in normotensive rats, MCA develops a compensatory mechanism (i.e., enhanced distensibility and decreased myogenic tone) that counteracts the effect of ischemia-reperfusion and ensures correct cerebral irrigation. These compensatory mechanisms are lost in hypertension, thereby explaining, at least in part, the greater infarct volume observed in SHR.

Effect of dietary chromium on resistance artery function and nitric oxide signaling in the sucrose-fed spontaneously hypertensive rat

BACKGROUND

Consumption of high-glycemic index foods contributes to the development of hypertension in some patients. Likewise, in spontaneously hypertensive rats (SHR), high sucrose promotes a secondary rise in systolic blood pressure (SBP). Chromium (III) (Cr(3+)) prevents sucrose-induced hypertension, but leaves the basal hypertension that characterizes SHR intact.

METHODS

Since hypertension entails increased peripheral resistance, we compared effects of Cr(3+) on resistance arteries from SHR fed low-glycemic (starch) versus high-glycemic (sucrose) index diets. Subgroups of SHR also received Cr(3+). Structure, stiffness, and vasodilation of mesenteric resistance arteries were studied using pressurized myography.

RESULTS

Sucrose increased SBP in SHR and, exclusively in sucrose-fed SHR, Cr(3+) reduced SBP and augmented acetylcholine or nitroprusside-dependent vasodilation. Neither sucrose nor Cr(3+) affected artery structure or stiffness. Since Cr(3+) enhanced vasodilation, we assessed endothelial NO synthase (eNOS), guanylate cyclase, cGMP-dependent protein kinase (PKG-1alpha and 1beta), and PKG activity by immunoblotting. Sucrose reduced eNOS, PKG-1beta, and PKG activity. Cr(3+) prevented the effects of sucrose on NO signaling.

CONCLUSION

In hypertension exacerbated by high-glycemic index diet, Cr(3+) reduces SBP. The BP-lowering effect of Cr(3+), selectively on sucrose-induced but not basal hypertension in SHR, involves at least in part, improving vasodilatory function vis-à-vis restoration of NO signaling in resistance arteries.

Resistance artery remodeling in deoxycorticosterone acetate-salt hypertension is dependent on vascular inflammation: evidence from m-CSF-deficient mice

Deoxycorticosterone acetate (DOCA)-salt hypertension has an important endothelin-1 (ET-1)-dependent component. ET-1-induced vascular damage may be mediated in part by oxidative stress and vascular inflammation. Homozygous osteopetrotic (Op/Op) mice, deficient in macrophage colony-stimulating factor (m-CSF), exhibit reduced inflammation. We investigated in osteopetrotic (Op/Op) mice the effects of DOCA-salt hypertension on vascular structure, function, and oxidative stress, the latter as manifested by reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase activity. Mice were implanted with DOCA (200 mg/mouse, under 5% isofluorane anesthesia) and given saline for 14 days. Systolic blood pressure (mmHg) was significantly increased (146 +/- 2 and 138 +/- 1; P < 0.001 vs. basal 115 +/- 3 and 115 +/- 3, respectively) by DOCA-salt in wild-type (+/+) and heterozygous (Op/+) mice, but not in Op/Op mice (130 +/- 1 vs. basal 125 +/- 3). Norepinephrine contractile response was significantly enhanced, while acetylcholine endothelium-dependent vasodilation was significantly impaired in DOCA-salt-treated +/+ and Op/+ mice compared with control mice. No changes in norepinephrine-induced contraction and acetylcholine-induced relaxation were observed in DOCA-salt Op/Op mice. DOCA-salt +/+ and Op/+ mice had significantly increased mesenteric resistance artery media-to-lumen ratio and media cross-sectional area, neither of which were altered in Op/Op mice. Basal vascular superoxide production and NAD(P)H oxidase activity, vascular cell adhesion molecule-1 expression, and macrophage infiltration were significantly increased only in DOCA-salt +/+ mice. Thus m-CSF-deficient mice developed less endothelial dysfunction, vascular remodeling, and oxidative stress induced by DOCA-salt than +/+ and Op/+ mice, suggesting that inflammation may play a role in DOCA-salt hypertension, a model that results in part from effects of ET-1, which has proinflammatory actions.

Elevated endothelin-1 levels are associated with decreased arterial elasticity in hypertensive patients

Endothelin-1 stimulates collagen synthesis and is increased in hypertension, but its effect on collagen degradation remains unknown. The current study tested the hypothesis that elevated endothelin-1 levels are associated with decreased collagenase activity, markers of collagen degradation, and arterial compliance in hypertensive patients. Normotensive (n = 10) and hypertensive (n = 13) patients who were not on any antihypertensive medication were recruited, and small and large artery elasticity index, systemic vascular resistance, pulse pressure, and blood pressure were determined using blood pressure waveform analysis. Large artery elasticity index and collagen degradation products were decreased whereas endothelin-1, systemic vascular resistance, and pulse pressure were elevated in hypertensive patients. Plasma endothelin-1 was negatively correlated with a cross-linked C-terminal telopeptide of collagen type I, a collagen degradation marker (r = -0.43; p = 0.04), collagenase matrix metalloproteinase-1 (r = -0.48; p = 0.02), and large artery elasticity (r = -0.45; p = 0.03) and positively correlated with pulse pressure (r = 0.68; p = 0.0005). These results suggest that endothelin-1 contributes to decreased arterial compliance in hypertension via inhibition of collagen degradation.

Microgravity-induced changes in aortic stiffness and their role in orthostatic intolerance

Microgravity (microG)-induced orthostatic intolerance (OI) in astronauts is characterized by a marked decrease in cardiac output (CO) in response to an orthostatic stress. Since CO is highly dependent on venous return, alterations in the resistance to venous return (RVR) may be important in contributing to OI. The RVR is directly dependent on arterial compliance (C(a)), where aortic compliance (C(ao)) contributes up to 60% of C(a). We tested the hypothesis that microG-induced changes in C(a) may represent a protective mechanism against OI. A retrospective analysis on hemodynamic data collected from astronauts after 5- to 18-day spaceflight missions revealed that orthostatically tolerant (OT) astronauts showed a significant decrease in C(a) after spaceflight, while OI astronauts showed a slight increase in C(a). A ground-based animal model simulating microG, hindlimb-unweighted rats, was used to explore this phenomenon. Two independent assessments of C(ao), in vivo pulse wave velocity (PWV) of the thoracic aorta and in vitro pressure-diameter squared relationship (PDSR) measurements of the excised thoracic aorta, were determined. PWV showed a significant increase in aortic stiffness compared with control, despite unchanged blood pressures. This increase in aortic stiffness was confirmed by the PDSR analysis. Thus both actual microG in humans and simulated microG in rats induces changes in C(ao). The difference in C(a) in OT and OI astronaut suggests that the microG-induced decrease in C(a) is a protective adaptation to spaceflight that reduces the RVR and allows for the maintenance of adequate CO in response to an orthostatic stress.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.

Losartan improves resistance artery lesions and prevents CTGF and TGF-beta production in mild hypertensive patients

Although structural and functional changes of resistance arteries have been proposed to participate in arterial hypertension (HTA) outcome, not all therapies may correct these alterations, even if they normalize the blood pressure (BP). The aim of this study was to investigate the mechanisms of the protection afforded by the angiotensin receptor antagonist losartan in resistance arteries from patients with essential HTA. In all, 22 untreated hypertensive patients were randomized to receive losartan or amlodipine for 1 year and the morphological characteristics of resistance vessels from subcutaneous biopsies were evaluated. Protein expression of connective tissue growth factor (CTGF), transforming growth factor beta (TGF-beta), and collagens III and IV was detected by immunohistochemistry. In comparison with normotensive subjects, resistance arteries from hypertensive patients showed a significant media:lumen (M/L) ratio increment and a higher protein expression of CTGF, TGF-beta, and collagens. After 1 year of treatment, both losartan and amlodipine similarly controlled BP. However, M/L only decreased in patients under losartan treatment, whereas in the amlodipine-treated group this ratio continued to increase significantly. The administration of losartan prevented significant increments in CTGF, TGF-beta, and collagens in resistance arteries. By contrast, amlodipine-treated patients showed a higher vascular CTGF, TGF-beta, and collagen IV staining than before treatment. Our results show that the administration of losartan, but not amlodipine, to hypertensive patients improves structural abnormalities and prevents the production of CTGF and TGF-beta in small arteries, despite similar BP lowering. These data may explain the molecular mechanisms of the better vascular protection afforded by drugs interfering with the renin-angiotensin system.

Arteriolar remodeling in essential hypertension: are connective tissue growth factor and transforming growth factor involved?

Gómez-Garre et al. report increased expression of connective tissue growth factor and transforming growth factor-beta and worsening of remodeling in subcutaneous arterioles of patients with essential hypertension during amlodipine treatment. Losartan improves remodeling without changes in cytokine expression. The discordant effects of two well-accepted treatments for hypertension underline the need for further studies on therapy-induced changes in arteriolar remodeling.

A novel strategy for increasing wall thickness of coronary venules prior to retroperfusion

The sudden exposure of veins to arterial pressures during coronary venous retroperfusion may cause rupture of small venules. Our rationale is to first occlude the coronary vein, which will cause an increase in pressure intermediate to arterial and venous values, and hence lead to remodeling and increased wall thickness of the veins prior to retroperfusion. To accomplish this objective, five pigs were subjected to left anterior descending (LAD) vein ligation while six pigs served as sham. Myocardial tissue samples were obtained from the area adjacent to the LAD vein at four transmural locations of the left ventricular free wall: epicardial surface, subepicardium, midmyocardium, and endocardium. Arterioles and venules from the experimental and sham control groups were photographed, and the following measurements were made: inner and outer circumferences, inner and outer areas, major and minor diameters, and intima-media thickness. Each vessel was categorized in four different orders according to lumen diameter. Our results show that intima-media thickness was larger in the experimental group in all four regions of the heart and in all four orders of the vessels, although venules from the epicardial region showed the largest increase in thickness. The intima-media thickness-to-radius ratio was also larger in the experimental group and decreased from epicardial to endocardial region of the heart and from order 1 to order 4 of the vessels. The present study provides a rationale for the development of coronary retroperfusion strategy that avoids vessel rupture and hemorrhage in the postcapillary venules.

Changes in extracellular matrix in subcutaneous small resistance arteries of patients with primary aldosteronism

CONTEXT AND OBJECTIVE

It has been previously demonstrated that aldosterone may possess a strong profibrotic action in vitro and in animal models of genetic or experimental hypertension. Our aim was to evaluate whether such a profibrotic action is present also in the human microcirculation.

DESIGN AND PATIENTS

We investigated 13 patients with primary aldosteronism, seven patients with essential hypertension, and 10 normotensive controls. All subjects were submitted to a biopsy of gluteal sc fat tissue. Small resistance arteries were dissected and mounted on an isometric myograph, and the tunica media to internal lumen ratio was measured.

MAIN OUTCOME MEASURES

The total collagen content within the tunica media was detected (Sirius red staining and image analysis), and collagen subtypes were evaluated using polarized light microscopy; under this condition thicker type I collagen fibers appear orange or red, whereas thinner type III collagen fibers are yellow or green.

RESULTS

Tunica media to internal lumen ratio was significantly increased in primary aldosteronism and in essential hypertension compared with normotensive controls. Clinic blood pressure values were similar in primary aldosteronism and in essential hypertension, and greater than in normotensive controls. Normotensive controls had less total and type III collagen (3.23 +/- 0.58 and 1.60 +/- 0.22%, respectively) in respect to the two hypertensive groups (P < 0.001). Total collagen and type III vascular collagen were significantly greater in primary aldosteronism (total collagen, 8.17 +/- 1.38%; type III collagen, 6.06 +/- 0.74%; P < 0.05) than in essential hypertension (total collagen, 6.84 +/- 1.15%; type III collagen, 5.25 +/- 0.80%).

CONCLUSIONS

Our results indicate that, in small resistance arteries of patients with primary aldosteronism, a pronounced fibrosis may be detected, even more evident than in blood-pressure-matched patients with essential hypertension.

Role of the renin–angiotensin–aldosterone system in vascular remodeling and inflammation: a clinical review

The concept of hypertension as primarily a consequence of altered hemodynamics has changed. Many factors are now implicated in the development of hypertensive vascular disease, and the renin-angiotensin-aldosterone system (RAAS) appears to be one of the most significant. Angiotensin II, the principal effector peptide of the RAAS, has far-reaching effects on vascular structure, growth and fibrosis, and is a key regulator of vascular remodeling and inflammation. Reactive oxygen species and a network of signaling pathways mediate angiotensin II and cellular mechanisms that promote remodeling and inflammation. The involvement of aldosterone in vessel-wall and myocardial remodeling has also come under intensive research scrutiny. Treatments that block the pathologic effects of the RAAS at several points have been shown to limit target-organ damage in hypertension and to decrease cardiovascular morbidity and mortality. Understanding the molecular and cellular mechanisms that participate in the early development of hypertensive vascular disease may lead to more targeted treatment and improved outcomes.

Integrin Receptor Activation Triggers Converging Regulation of Cav1.2 Calcium Channels by c-Src and Protein Kinase A Pathways

L-type, voltage-gated Ca2+ channels (CaL) play critical roles in brain and muscle cell excitability. Here we show that currents through heterologously expressed neuronal and smooth muscle CaL channel isoforms are acutely potentiated following alpha5beta1 integrin activation. Only the alpha1C pore-forming channel subunit is critical for this process. Truncation and site-directed mutagenesis strategies reveal that regulation of Cav1.2 by alpha5beta1 integrin requires phosphorylation of alpha1C C-terminal residues Ser1901 and Tyr2122. These sites are known to be phosphorylated by protein kinase A (PKA) and c-Src, respectively, and are conserved between rat neuronal (Cav1.2c) and smooth muscle (Cav1.2b) isoforms. Kinase assays are consistent with phosphorylation of these two residues by PKA and c-Src. Following alpha5beta1 integrin activation, native CaL channels in rat arteriolar smooth muscle exhibit potentiation that is completely blocked by combined PKA and Src inhibition. Our results demonstrate that integrin-ECM interactions are a common mechanism for the acute regulation of CaL channels in brain and muscle. These findings are consistent with the growing recognition of the importance of integrin-channel interactions in cellular responses to injury and the acute control of synaptic and blood vessel function.

Estrogen metabolites and systolic blood pressure in a population-based sample of postmenopausal women

CONTEXT

Lower systolic blood pressure (SBP) and lower rates of coronary heart disease among premenopausal women compared with similarly aged men and postmenopausal women suggest that female sex hormones may confer cardiovascular protection. 2-Hydroxyestradiol, a product of 17beta-estradiol oxidative metabolism, inhibits the proliferation of vascular smooth muscle cells in vitro. The other major product of 17beta-estradiol oxidative metabolism, 16alpha-hydroxyestradiol, does not demonstrate similar inhibitory effects. Concentrations of 2-hydroxyestrone (2-OHE) and 16alpha-hydroxyestrone (16-OHE) in urine reflect the relative activity of the 2- and 16alpha-hydroxylation pathways of 17beta-estradiol.

OBJECTIVE

The objective of this study was to determine the relationship between SBP and the ratio of 2-OHE to 16-OHE in urine.

DESIGN AND PARTICIPANTS

This was a cross-sectional study of 80 postmenopausal women living in Cook County, Illinois.

SETTING

This study was performed in an academic clinical laboratory.

MAIN OUTCOME MEASURE

The main outcome measure was SBP.

RESULTS

Women taking hormone replacement therapy had higher levels of urinary 2-OHE and 16-OHE, but their mean 2:16-OHE ratio and SBP did not differ from that of women not taking hormone replacement therapy. In a multivariate regression model that controlled for age, body mass index, race/ethnicity, and antihypertensive medication use, a sd increase in the 2:16-OHE ratio was associated with a 6.7-mm Hg decrease (P < 0.05) in SBP.

CONCLUSIONS

The ratio of urinary 2-OHE to 16-OHE is a significant predictor of SBP among postmenopausal women and may reflect the effects of 2-hydroxyestradiol, a potent inhibitor of vascular smooth muscle cell proliferation.

Elastic fibres and vascular structure in hypertension

Blood vessels are dynamic structures composed of cells and extracellular matrix (ECM), which are in continuous cross-talk with each other. Thus, cellular changes in phenotype or in proliferation/death rate affect ECM synthesis. In turn, ECM elements not only provide the structural framework for vascular cells, but they also modulate cellular function through specific receptors. These ECM-cell interactions, together with neurotransmitters, hormones and the mechanical forces imposed by the heart, modulate the structural organization of the vascular wall. It is not surprising that pathological states related to alterations in the nervous, humoral or haemodynamic environment-such as hypertension-are associated with vascular wall remodeling, which, in the end, is deleterious for cardiovascular function. However, the question remains whether these structural alterations are simply a consequence of the disease or if there are early cellular or ECM alterations-determined either genetically or by environmental factors-that can predispose to vascular remodeling independent of hypertension. Elastic fibres might be key elements in the pathophysiology of hypertensive vascular remodeling. In addition to the well known effects of hypertension on elastic fibre fatigue and accelerated degradation, leading to loss of arterial wall resilience, recent investigations have highlighted new roles for individual components of elastic fibres and their degradation products. These elements can act as signal transducers and regulate cellular proliferation, migration, phenotype, and ECM degradation. In this paper, we review current knowledge regarding components of elastic fibres and discuss their possible pathomechanistic associations with vascular structural abnormalities and with hypertension development or progression.

Alterations in structure and mechanics of resistance arteries from ouabain-induced hypertensive rats

We have previously described that chronic administration of ouabain induces hypertension and functional alterations in mesenteric resistance arteries. The aim of this study was to analyze whether ouabain treatment also alters the structural and mechanical properties of mesenteric resistance arteries. Wistar rats were treated for 5 wk with ouabain (8.0 microg/day sc). The vascular structure and mechanics of the third-order branches of the mesenteric artery were assessed with pressure myography and confocal microscopy. Total collagen content was determined by picrosirius red staining, collagen I/III was analyzed by Western blot, and elastin was studied by confocal microscopy. Vascular reactivity was analyzed by wire myography. Internal and external diameters and cross-sectional area were diminished, whereas the wall-to-lumen ratio was increased in arteries from ouabain-treated rats compared with controls. In addition, arteries from ouabain-treated rats were stiffer. Ouabain treatment decreased smooth muscle cell number and increased total and I/III collagens in the vascular wall. However, this treatment did not modify adventitia and media thickness, nuclei morphology, elastin structure, and vascular reactivity to norepinephrine and acetylcholine. The present work shows hypotrophic inward remodeling of mesenteric resistance arteries from ouabain-treated rats that seems to be the consequence of a combination of decreased cell number and impaired distension of the artery, possibly due to a higher stiffness associated with collagen deposition. The narrowing of resistance arteries could play a role in the pathogenesis of hypertension in this model.

Atomic force-multi-optical imaging integrated microscope for monitoring molecular dynamics in live cells

A novel hybrid imaging system is constructed integrating atomic force microscopy (AFM) with a combination of optical imaging techniques that offer high spatial resolution. The main application of this instrument (the NanoFluor microscope) is the study of mechanotransduction with an emphasis on extracellular matrix-integrin-cytoskeletal interactions and their role in the cellular responses to changes in external chemical and mechanical factors. The AFM allows the quantitative assessment of cytoskeletal changes, binding probability, adhesion forces, and micromechanical properties of the cells, while the optical imaging applications allow thin sectioning of the cell body at the coverslip-cell interface, permitting the study of focal adhesions using total internal reflection fluorescence (TIRF) and internal reflection microscopy (IRM). Combined AFM-optical imaging experiments show that mechanical stimulation at the apical surface of cells induces a force-generating cytoskeletal response, resulting in focal contact reorganization on the basal surface that can be monitored in real time. The NanoFluor system is also equipped with a novel mechanically aligned dual camera acquisition system for synthesized Forster resonance energy transfer (FRET). The integrated NanoFluor microscope system is described, including its characteristics, applications, and limitations.

Hemodynamic conditions alter axial and circumferential remodeling of arteries engineered ex vivo

We previously demonstrated that growth and remodeling was stimulated in arteries elongated ex vivo using step increases in axial strain. Viability and vasoactivity were similar to fresh arteries, however there was a substantial decrease in the ultimate circumferential stress. To test the hypothesis that the subphysiological perfusion conditions (i.e., low pressure and flow) previously used caused the reduction, arteries were subjected to the identical elongation protocol (50% increase over 9 days) while being perfused with physiological levels of flow, viscosity and pulsatile pressure. A significant increase in unloaded length was achieved by elongation under both perfusion conditions, although the increase was less under physiological (7 +/- 1%) than under subphysiological conditions (19 +/- 2%, p < 0.005). When length at physiological stress was estimated using mechanical testing data the values were similar. The ultimate circumferential stress of arteries elongated under physiological conditions was increased (33%), whereas the ultimate axial stress was decreased (50%) as compared with arteries elongated under subphysiological conditions. Elongated arteries under both perfusion conditions showed significant increases in proliferation and collagen mass, and similar viability and appearance to fresh arteries. These data suggest that there is substantial cross-talk between perfusion conditions and axial strain that modulates arterial remodeling and length.

The effect of morphine in rat small mesenteric arteries

We investigated the effect of morphine in phenylephrine (PE)- or KCl-precontracted rat small mesenteric arteries. Morphine (10(-6)-10(-4) M) administration caused concentration-dependent relaxation responses in small mesenteric arteries precontracted by PE or KCl. Removal of endothelium did not significantly alter the relaxation responses to morphine. The relaxant responses to morphine were partially inhibited by pre-treatment of tissues with naloxone (NAL, 10(-5) M) for 20 min. The inhibitory effect of NAL on relaxant responses to morphine in PE- or KCl-precontracted arteries did not differ significantly between endothelium-intact and endothelium-denuded preparations. Incubation of endothelium-intact or endothelium-denuded arterial segments with NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 10(-4) M) or cyclooxygenase (COX) inhibitor indomethacin (10(-5) M) or histamine H(1)-receptor blocker diphenhydramine (10(-6) M), for 20 min did not inhibit the relaxation responses to morphine. Small mesenteric arterial segment contractions induced by stepwise addition of calcium to high KCl solution with no calcium were almost completely inhibited by morphine. These findings suggested that morphine-induced relaxation responses in isolated rat small mesenteric arteries were neither dependent on endothelium nor blocked by NOS or COX inhibition but they rather seem to depend on an interaction of morphine with calcium influx pathways.

Mechanical properties of native and ex vivo remodeled porcine saphenous veins

When grafted into an arterial environment in vivo, veins remodel in response to the new mechanical environment, thereby changing their mechanical properties and potentially impacting their patency as bypass grafts. Porcine saphenous veins were subjected for one week to four different ex vivo hemodynamic environments in which pressure and shear stress were varied independently, as well as an environment that mimicked that of an arterial bypass graft. After one week of ex vivo culture, the mechanical properties of intact saphenous veins were evaluated to relate specific aspects of the mechanical environment to vein remodeling and corresponding changes in mechanics. The compliance of all cultured veins tended to be less than that of fresh veins; however, this trend was more due to changes in medial and luminal areas than changes in the intrinsic properties of the vein wall. A combination of medial hypertrophy and eutrophic remodeling leads to significantly smaller (p<0.05) wall stresses measured in all cultured veins except those subjected to bypass graft conditions relative to stresses measured in fresh veins at corresponding pressures. Our results suggest that the mechanical environment effects changes in vessel size, as well as the nature of the remodeling, which contribute to altering vein mechanical properties.

Type 2 diabetes causes remodeling of cerebrovasculature via differential regulation of matrix metalloproteinases and collagen synthesis: role of endothelin-1

The risk of cerebrovascular disease is four- to sixfold higher in patients with diabetes. Vascular remodeling, characterized by extracellular matrix deposition and an increased media-to-lumen ratio, occurs in diabetes and contributes to the development of complications. However, diabetes-induced changes in the cerebrovascular structure remain unknown. Endothelin-1 (ET-1), a potent vasoconstrictor with profibrotic properties, is chronically elevated in diabetes. To determine diabetes-mediated changes in the cerebrovasculature and the role of ET-1 in this process, type 2 diabetic Goto-Kakizaki (GK) rats were administered an ET(A) receptor antagonist for 4 weeks. Middle cerebral arteries were harvested and studies were performed to determine vascular structure. Tissue and plasma ET-1 levels were increased in GK rats compared with controls. Significant medial hypertrophy and collagen deposition resulted in an increased wall-to-lumen ratio in diabetic rats that was reduced by ET(A) receptor antagonism. Vascular matrix metalloproteinase (MMP)-2 activity was higher, but MMP-1 levels were significantly reduced in GK rats, and MMP levels were restored to control levels by ET(A) receptor antagonism. We conclude that ET-1 promotes cerebrovascular remodeling in type 2 diabetes through differential regulation of MMPs. Augmented cerebrovascular remodeling may contribute to an increased risk of stroke in diabetes, and ET(A) receptor antagonism may offer a novel therapeutic target.

Shear stress and pressure modulate saphenous vein remodeling ex vivo

Vein graft failure remains an important clinical challenge, but factors contributing to vein graft failure have not clearly been defined. We investigated the role of the mechanical environment in vein remodeling in an ex vivo perfusion system. Porcine saphenous veins were subjected to five different ex vivo hemodynamic environments, including one mimicking an arterial bypass graft, for one week in order to independently assess the effects of shear stress and pressure on vein remodeling. The extent of intimal hyperplasia decreased with culture under increasing shear stress, with veins cultured under the lowest levels of shear stress exhibiting the greatest ratio of intimal/medial area, 0.15+/-0.03, which was greater than that of fresh veins (0.06+/-0.01, p<0.05). All perfused veins displayed characteristics of both medial hypertrophy and eutrophic remodeling, with those veins cultured under elevated pressures showing greater increases in mass and area than those cultured under venous pressures. Medial area correlated with the average pressure under which veins were cultured (R2=0.95, p<0.001), with veins cultured under bypass graft conditions, which were exposed to the greatest pressure during the one week culture, exhibiting the largest medial area (1.69+/-0.15 mm2), which was significantly greater than that of fresh veins (1.08+/-0.05 mm2, p<0.05). However, pulsatility was not a necessary stimulus for medial growth, as increases in medial area were observed in culture conditions in which steady flow and pressure were present. Our results suggest that pressure and shear stress act independently to regulate vein remodeling, influencing changes in vessel size as well as the nature of the remodeling.

Comparative effects of the angiotensin II receptor blocker, telmisartan, and the angiotensin-converting enzyme inhibitor, ramipril, on cerebrovascular structure in spontaneously hypertensive rats

OBJECTIVE

Antihypertensive treatment with angiotensin-converting enzyme inhibitors (ACEIs) reverses cerebral arteriolar remodeling, thus restoring dilatation and hence the lower limit of cerebral blood flow (CBF) autoregulation (LLCBF). The objective of this study was to determine whether angiotensin II receptor AT1 blockers (ARBs) produce the same effect.

DESIGN

We examined the effects of treatment with an ARB [telmisartan (TEL), 1.93 +/- 0.04 mg/kg per day] or an ACEI [ramipril (RAM), 1.00 +/- 0.02 mg/kg per day] on the cerebral circulation in spontaneously hypertensive rats (SHR).

METHODS

Arteriolar pressure and diameter (cranial window) and CBF (laser Doppler) were measured during stepwise hypotensive hemorrhage, before and after deactivation (ethylenediamine tetraacetic acid), in untreated Wistar-Kyoto (WKY) rats and SHR untreated or treated for 3 months with TEL or RAM in the drinking water.

RESULTS

Treatment normalized arteriolar internal diameter (SHR, 38 +/- 3 microm; TEL, 52 +/- 2 microm; RAM, 50 +/- 2 microm; WKY, 58 +/- 4 microm), essentially by reversing eutrophic inward remodeling, and the LLCBF (SHR, 80 +/- 11 mmHg; TEL, 60 +/- 4 mmHg; RAM, 71 +/- 6 mmHg; WKY, 57 +/- 5 mmHg).

CONCLUSION

The fact that the ARB (TEL) is as effective as an ACEI (RAM) in reversing cerebral arteriolar remodeling suggests that the cerebrovascular AT1 receptor is an underlying mechanism that promotes hypertensive eutrophic inward remodeling.

Microvascular rarefaction and decreased angiogenesis in rats with fetal programming of hypertension associated with exposure to a low-protein diet in utero

In hypertension, increased peripheral vascular resistance results from vascular dysfunction with or without structural changes (vessel wall remodeling and/or microvascular rarefaction). Humans with lower birth weight exhibit evidence of vascular dysfunction. The current studies were undertaken to investigate whether in utero programming of hypertension is associated with in vivo altered response and/or abnormal vascular structure. Offspring of Wistar dams fed a normal (CTRL) or low (LP)-protein diet during gestation were studied. Mean arterial blood pressure response to ANG II was significantly increased, and depressor response to sodium nitroprusside (SNP) infusions significantly decreased in male LP adult offspring relative to CTRL. No arterial remodeling was observed in male LP compared with CTRL offspring. Capillary and arteriolar density was significantly decreased in striated muscles from LP offspring at 7 and 28 days of life but was not different in late fetal life [day 21 of gestation (E21)]. Angiogenic potential of aortic rings from LP newborn (day of birth, P0) was significantly decreased. Striated muscle expressions (Western blots) of ANG II AT(1) receptor subtype, endothelial nitric oxide synthase, angiopoietin 1 and 2, Tie 2 receptor, vascular endothelial growth factor and receptor, and platelet-derived growth factor C at E21 and P7 were unaltered by antenatal diet exposure. In conclusion, blood pressure responses to ANG II and SNP are altered, and microvascular structural changes prevail in this model of fetal programming of hypertension. The capillary rarefaction is absent in the fetus and appears in the neonatal period, in association with decreased angiogenic potential. The study suggests that intrauterine protein restriction increases susceptibility to postnatal factors resulting in microvascular rarefaction, which could represent a primary event in the genesis of hypertension.

Systolic hypertension in the elderly: arterial wall mechanical properties and the renin-angiotensin-aldosterone system

BACKGROUND

Systolic hypertension in the elderly involves an increase of arterial stiffness and early wave reflections, both of them causing a predominant or selective increase of systolic blood pressure. The mechanisms for such alterations remain largely unknown.

DESCRIPTION AND RESULTS

The development of systolic hypertension includes constantly an age-related increase of sodium sensitivity and of endothelial dysfunction, both responsible for phenotypic changes of aortic smooth muscle cells with collagen accumulation and increased stiffness. In the presence of a high sodium diet and under the influence of angiotensin II and aldosterone, a higher number of attachments between vascular smooth muscle cells and collagen fibers develop, causing a supplementary increase in stiffness independent of the mean blood pressure together with the occurrence of early wave reflections. Gene polymorphisms related to the renin-angiotensin system may participate in this evolution.

CONCLUSION

This process contributes to accelerating the increase in pulse pressure and arterial stiffness with age, and therefore to the development of cardiovascular risk.

Vascular endothelin in hypertension

Endothelins are powerful vasoconstrictor peptides that also play numerous other functions in many different organs. Endothelin-1 (ET-1) is the most abundant and important of this family of peptides in blood vessels. Production of ET-1 is increased in the endothelium and the kidney in salt-dependent models of hypertension (e.g.: DOCA-salt rats and Dahl salt-sensitive rats, in salt-loaded SHR-SP, in angiotensin II-infused and in diabetic rats). ET-1 elicits an inflammatory response by increasing oxidant stress in the vascular wall, which induces vascular remodeling and endothelial dysfunction found in the hypertensive models that exhibit an endothelin-mediated component. Endothelin receptor antagonism reduces blood pressure and vascular hypertrophic remodeling present in these hypertensive models. Patients with stage 2 hypertension have enhanced vascular expression of ET-1. Endothelin receptor antagonists lower blood pressure in hypertensive patients. They could become therapeutic agents for prevention of target organ damage in hypertension and in type 2 diabetes, chronic renal failure and congestive heart failure. Side effects of endothelin receptor blockers have prevented up to the present their development for these indications. New endothelin antagonists devoid of these side effects, or alternatively inhibitors of the endothelin converting enzymes that generate ET-1 may in the future become available to block the endothelin system. However, to date endothelin antagonists have been approved only for the treatment of primary pulmonary hypertension, a rapidly fatal condition in which the endothelin system plays an important role and endothelin antagonists exert favorable effects.

Vascular gap junctions and implications for hypertension

Four connexin (Cx) molecules, namely Cx37, Cx40, Cx43 and Cx45, are expressed in the gap junctions that exist within and between the cellular layers of arteries. Endothelial cells are well coupled by large gap junctions expressing Cx37, Cx40 and, to a lesser extent, Cx43, whose expression may be more subject to regulation by physical factors. Smooth muscle cells are more heterogeneously coupled by gap junctions that are small and rare. The identity of the Cx expressed in the media may vary among different arteries. Myoendothelial gap junctions are small and more common in resistance arteries with fewer layers of smooth muscle cells. Given the small size of these gap junctions and the rapid turnover rate of Cxs, homocellular coupling in the media and heterocellular coupling between the cell layers may be subject to more dynamic control than coupling in the endothelium. Vascular gap junctions have been implicated in a number of vasomotor responses that may regulate vascular tone and blood pressure. These include the mechanism of action of the vasodilator, endothelium-derived hyperpolarizing factor (EDHF), the myogenic constriction to intramural pressure increase, the spontaneous or agonist-induced vasomotion of arteries and arterioles and the spreading vasodilation and constriction observed in microcirculatory networks. Few data are available on Cx expression in the media of resistance arteries during hypertension. Changes in the expression of Cx43 described in the media of the aorta of hypertensive rats vary with the hypertensive model studied and are likely to represent adaptations to structural changes in the vascular wall. In contrast, in the endothelium of the caudal and mesenteric arteries of spontaneously hypertensive rats, expression of Cxs is significantly decreased compared with arteries from normotensive rats and this decrease is reversed by inhibitors of the renin-angiotensin system. During hypertension, the activity of EDHF is decreased in the mesenteric artery, but this occurs much later than the initial increase in blood pressure and the decrease in endothelial Cxs, suggesting that changes in EDHF may not be causally related to hypertension or to the changes in endothelial Cxs. Upregulation of the myogenic response and the incidence of vasomotion has been reported in hypertension. Little is currently known of the effects of hypertension on spreading vasomotor responses. Deletion of specific Cxs in genetically modified mice is complicated by neonatal lethality or coordinate regulation and compensatory changes in the remaining Cxs. Nevertheless, mice in which Cx40 has been deleted are hypertensive and spreading vasodilatory responses are significantly impaired. Determination of a role for specific Cxs in the control of blood pressure must await the development of animals in which Cx expression can be modulated in a more complex temporal and tissue-specific manner.