Chronic N(G)-nitro-L-arginine methyl ester treatment does not prevent flow-induced remodeling in mesenteric feed arteries and arcading arterioles

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Dual Role of Thrombospondin-1 in Flow-Induced Remodeling

(1) Background: Chronic increases in blood flow, as in cardiovascular diseases, induce outward arterial remodeling. Thrombospondin-1 (TSP-1) is known to interact with matrix proteins and immune cell-surface receptors, but its contribution to flow-mediated remodeling in the microcirculation remains unknown. (2) Methods: Mesenteric arteries were ligated in vivo to generate high- (HF) and normal-flow (NF) arteries in wild-type (WT) and TSP-1-deleted mice (TSP-1-/-). After 7 days, arteries were isolated and studied ex vivo. (3) Results: Chronic increases in blood flow induced outward remodeling in WT mice (increasing diameter from 221 ± 10 to 280 ± 10 µm with 75 mmHg intraluminal pressure) without significant effect in TSP-1-/- (296 ± 18 to 303 ± 14 µm), neutropenic or adoptive bone marrow transfer mice. Four days after ligature, pro inflammatory gene expression levels (CD68, Cox2, Gp91phox, p47phox and p22phox) increased in WT HF arteries but not in TSP-1-/- mice. Perivascular neutrophil accumulation at day 4 was significantly lower in TSP-1-/- than in WT mice. (4) Conclusions: TSP-1 origin is important; indeed, circulating TSP-1 participates in vasodilation, whereas both circulating and tissue TSP-1 are involved in arterial wall thickness and diameter expansion.

Shear Stress Attenuates Inward Remodeling in Cultured Mouse Thoracodorsal Arteries in an eNOS-Dependent, but Not Hemodynamic Manner, and Increases Cx37 Expression

BACKGROUND

Arteries chronically constricted in culture remodel to smaller diameters. Conversely, elevated luminal shear stress (SS) promotes outward remodeling of arteries in vivo and prevents inward remodeling in culture in a nitric oxide synthase (NOS)-dependent manner.

OBJECTIVES

To determine whether SS-induced prevention of inward remodeling in cultured arteries is specifically eNOS-dependent and requires dilation, and whether SS alters the expression of eNOS and other genes potentially involved in remodeling.

METHODS

Female mouse thoracodorsal arteries were cannulated, pressurized to 80 mm Hg, and cultured for 2 days with low SS (<7 dyn/cm2), high SS (≥15 dyn/cm2), high SS + L-NAME (NOS inhibitor, 10-4 M), or high SS in arteries from eNOS-/- mice. In separate arteries cultured 1 day with low or high SS, eNOS and connexin (Cx) 37, Cx40, and Cx43 mRNA were assessed with real-time PCR.

RESULTS

High SS caused little change in passive diameters after culture (-4.7 ± 2.0%), which was less than low SS (-18.9 ± 1.4%; p < 0.0001), high SS eNOS-/- (-18.0 ± 1.5; p < 0.001), or high SS + L-NAME (-12.0 ± 0.6%; nonsignificant) despite similar constriction during culture. Cx37 mRNA expression was increased (p < 0.05) with high SS, but other gene levels were not different.

CONCLUSIONS

eNOS is involved in SS-induced prevention of inward remodeling in cultured small arteries. This effect does not require NO-mediated dilation. SS increased Cx37.

A novel method for comparison of arterial remodeling in hypertension: Quantification of arterial trees and recognition of remodeling patterns on histological sections

Remodeling of spatially heterogeneous arterial trees is routinely quantified on tissue sections by averaging linear dimensions, with lack of comparison between different organs and models. The impact of experimental models or hypertension treatment modalities on organ-specific vascular remodeling remains undefined. A wide variety of arterial remodeling types has been demonstrated for hypertensive models, which include differences across organs. The purpose of this study was to reassess methods for measurement of arterial remodeling and to establish a morphometric algorithm for standard and comparable quantification of vascular remodeling in hypertension in different vascular beds. We performed a novel and comprehensive morphometric analysis of terminal arteries in the brain, heart, lung, liver, kidney, spleen, stomach, intestine, skin, skeletal muscle, and adrenal glands of control and Goldblatt hypertensive rats on routinely processed tissue sections. Mean dimensions were highly variable but grouping them into sequential 5 μm intervals permitted creation of reliable linear regression equations and complex profiles. Averaged arterial dimensions demonstrated seven remodeling patterns that were distinct from conventional inward-outward and hypertrophic-eutrophic definitions. Numerical modeling predicted at least nineteen variants of arterial spatial conformations. Recognition of remodeling variants was not possible using averaged dimensions, their ratios, or the remodeling and growth indices. To distinguish remodeling patterns, a three-dimensional modeling was established and tested. The proposed algorithm permits quantitative analysis of arterial remodeling in different organs and may be applicable for comparative studies between animal hypertensive models and human hypertension. Arterial wall tapering is the most important factor to consider in arterial morphometry, while perfusion fixation with vessel relaxation is not necessary. Terminal arteries in organs undergo the same remodeling pattern in Goldblatt rats, except for organs with hemodynamics affected by the arterial clip. The existing remodeling nomenclature should be replaced by a numerical classification applicable to any type of arterial remodeling.

Thrombospondin-1 in early flow-related remodeling of mesenteric arteries from young normotensive and spontaneously hypertensive rats

We tested the hypotheses that TSP-1 participates in the initiation of remodeling of small muscular arteries in response to altered blood flow and that the N-terminal domain of TSP-1 (hepI) can reverse the pathological inward remodeling of resistance arteries from SHR.We measured (1) changes in gene/protein expression in MA of 6 week old WKY and SHR exposed to either increased (+ 100 %) or reduced blood flow (- 90 %) for 24-40 hours and (2) structural changes in MA of 12 week old SHR exposed for 3 days to hepI in organ culture.In both HF and LF of WKY, mRNA expression of eNOS, sGCα1 and PKG1β were significantly reduced (p < 0.05), whereas mRNA of TSP1 was markedly increased (p < 0.05). In MA of young SHR, similar results were obtained except that eNOS mRNA was not reduced in LF. Expression of TSP1 protein was significantly increased in LF of young WKY and SHR (p < 0.05). Exposure of MA of 12 week old SHR to hepI (1 µmol/L) resulted in a rapid lumen diameter increase (+ 12 ± 2% after 3 days) without alteration in vascular reactivity, distensibility, media surface area or cell number.These are the first observations of reduced gene expression of eNOS/sGC/PKG and increased expression of TSP1 at the initiation of arterial remodeling in young WKY and SHR, irrespective of its outward or inward outcome. Furthermore, a fragment of TSP-1 rapidly and directly reversed pathological inward arterial remodeling of SHR in vitro.

Antioxidants reverse age-related collateral growth impairment

Aging is a major risk factor for the development of cardiovascular diseases, including arterial occlusive disease. Oxidant stress increases with age, and may be a significant factor contributing to vascular dysfunction and disease. We have shown that aging and hypertension impair collateral growth, the natural compensatory response to arterial occlusive disease, and that antioxidants restore collateral growth in young hypertensive rats. The aim of this study was to test the hypothesis that oxidant stress mediates collateral growth impairment in nondiseased, aged rats. Ileal arteries were induced to become collaterals via ligation of adjacent arteries. Growth was assessed at 7 days by repeated in vivo measurements and comparison to same-animal control arteries. Collateral diameter enlargement did not occur in aged rats, but luminal expansion was stimulated by pretreatment with tempol. Co-administration of L-NAME with tempol prevented tempol-mediated collateral development. Expression of p22(phox) mRNA was increased in aged versus young rat arteries, suggesting NAD(P)H oxidase as a source of reactive oxygen species. Treatment with apocynin increased collateral growth capacity, whether administered prior to, or 7 days following, arterial ligation. The results suggest that antioxidant treatment may be useful in promoting collateral growth to compensate for age-related arterial occlusive disease.

Shear stress, reactive oxygen species, and arterial structure and function

Shear stress is well known to be a key factor in the regulation of small-artery tone and structure. Although nitric oxide is a major endothelium-derived factor involved in short- and long-term regulation of vascular caliber, it is clear that other mechanisms also can be involved. This review discusses the evidence for endothelium-derived reactive oxygen species (ROS) as mediators for shear-dependent arterial tone and remodeling. The work focuses on resistance vessels, because their caliber determines local perfusion. However, work on large vessels is included where needed. Attention is given to the shear-stress levels and profiles that exist in the arterial system and the differential effects of steady and oscillating shear on NO and ROS production. We furthermore address the relation between microvascular tone and remodeling and the effect of ROS and inflammation on the activity of remodeling enzymes such as matrix metalloproteinases and transglutaminases. We conclude that future work should address the role of H(2)O(2) as an endothelium-derived factor mediating tone and influencing structure of small arteries over the long term.

Maternal uterine vascular remodeling during pregnancy

Sufficient uteroplacental blood flow is essential for normal pregnancy outcome and is accomplished by the coordinated growth and remodeling of the entire uterine circulation, as well as the creation of a new fetal vascular organ: the placenta. The process of remodeling involves a number of cellular processes, including hyperplasia and hypertrophy, rearrangement of existing elements, and changes in extracellular matrix. In this review, we provide information on uterine blood flow increases during pregnancy, the influence of placentation type on the distribution of uterine vascular resistance, consideration of the patterns, nature, and extent of maternal uterine vascular remodeling during pregnancy, and what is known about the underlying cellular mechanisms.

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.

The estrogen effects on endothelial repair and mitogen-activated protein kinase activation are abolished in endothelial nitric-oxide (NO) synthase knockout mice, but not by NO synthase inhibition by N-nitro-L-arginine methyl ester

We have previously shown that estrogen exerts a vasoprotective effect by accelerating reendothelialization after perivascular artery injury through activation of the estrogen receptor alpha. Because 17beta-estradiol (E2) is known to increase the bioavailability of nitric oxide, in this study, we used the same perivascular model to characterize the role of the endothelial nitric oxide synthase (eNOS) pathway in reendothelialization. Surprisingly, we found that the stimulatory effect of E2 on reendothelialization was not altered following pharmacological inhibition of nitric-oxide synthase enzymatic activity by N-nitro-L-arginine methyl ester, whereas it was abolished in eNOS-deficient (eNOS-/-) mice. This discrepancy between eNOS gene inactivation and the pharmacological inhibition of eNOS was confirmed in a classical model of endovascular injury. When assessing the involvement of eNOS in short-term membrane-associated signaling events induced by E2, we found that E2 stimulated phosphorylation of extracellular signal-regulated kinase 1/2 in isolated perfused carotid arteries from wild-type mice in the absence or presence of N-nitro-l-arginine methyl ester, whereas this stimulation was abolished in carotid arteries from eNOS-/- mice. Similar results were obtained in primary cultures of mouse aortic endothelial cells. These data reveal an original and unexpected role of eNOS, in which its presence but not its enzymatic activity appears to be a determinant for estrogen signaling in the endothelium. The consequences of this novel function of eNOS with respect to vascular diseases should be explored.

Small Artery Remodeling and Erythrocyte Deformability in L-NAME-Induced Hypertension: Role of Transglutaminases

BACKGROUND

Hypertension is associated with inward remodeling of small arteries and decreased erythrocyte deformability, both impairing proper tissue perfusion. We hypothesized that these alterations depend on transglutaminases, cross-linking enzymes present in the vascular wall, monocytes/macrophages and erythrocytes.

METHODS AND RESULTS

Wild-type (WT) mice and tissue-type transglutaminase (tTG) knockout (KO) mice received the nitric oxide inhibitor Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME) to induce hypertension. After 1 week, mesenteric arteries from hypertensive WT mice showed a smaller lumen diameter (-6.9 +/- 2.0%, p = 0.024) and a larger wall-to-lumen ratio (11.8 +/- 3.5%, p = 0.012) than controls, whereas inward remodeling was absent in hypertensive tTG KO mice. After 3 weeks, the wall-to-lumen ratio was increased in WT (20.8 +/- 4.8%, p = 0.005) but less so in tTG KO mice (11.7 +/- 4.6%, p = 0.026), and wall stress was normalized in WT but not in tTG KO mice. L-NAME did not influence expression of tTG or an alternative transglutaminase, coagulation factor XIII (FXIII). Suppression of FXIII by macrophage depletion was associated with increased tTG in the presence of L-NAME. L-NAME treatment decreased erythrocyte deformability in the WT mice (-15.3% at 30 dynes/cm(2), p = 0.014) but not in the tTG KO mice.

CONCLUSION

Transglutaminases are involved in small artery inward remodeling and erythrocyte stiffening associated with nitric oxide inhibition-related hypertension.

Characterizing intramural stress and inflammation in hypertensive arterial bifurcations

A histology-based methodology was developed and used to determine whether intramural stress and combined monocyte/macrophage density positively correlate within hypertensive bifurcations. Hypertension was induced in Sprague-Dawley rats using Angiotensin II pumps. Analysis focused on mesenteric bifurcations harvested 7 days (n = 4) post implant, but also included normotensive (n = 2) and 21-day hypertensive (n = 1) samples. Mesentery was processed in a manner that preserves morphology, corrects for histology-related distortions and results in reconstructions suitable for finite element analysis. Peaks in intramural stress and monocyte/macrophage density occurred near bifurcations after the onset of hypertension. Cell density peaks occurred in regions where surface curvature is complex and tends to heighten intramural stress. Also, a strong positive correlation between mean stress and mean cell density suggests that they are related phenomena. A point-by-point comparison of stress and cell density throughout each bifurcation did not exhibit a consistent pattern. We offer reasons why this most stringent test did not corroborate our other findings that high intramural stress is correlated with increased inflammation near the center of the bifurcation.

Key role of the NO-pathway and matrix metalloprotease-9 in high blood flow-induced remodeling of rat resistance arteries

OBJECTIVE

Blood flow is altered in metabolic and ischemic diseases with dramatic consequences. Resistance arteries structure and function remodel in response to chronic blood flow changes through a mechanism remaining mainly unknown. We hypothesized that the NO pathway and matrix metalloproteases (MMPs) activation might play a role in flow (shear stress)-induced microvascular remodeling.

METHODS AND RESULTS

Mesenteric resistance arteries were ligated to alter blood flow in vivo for 4 or 14 days: arteries were submitted to high (HF), low (LF), or normal flow (NF). Rats were treated with L-NAME, the angiotensin converting enzyme inhibitor perindopril or the MMPs inhibitor doxycycline. After 14 days, outward hypertrophic remodeling occurred in HF arteries in association with eNOS overexpression. MMP9 activity increased in the early phase (day 4). HF-remodeling was prevented by L-NAME, eNOS gene knockout, and doxycycline. L-NAME prevented eNOS overexpression and MMPs activation whereas doxycycline only prevented MMPs activation. In LF arteries diameter reduction was associated with a decreased eNOS expression without change in MMPs expression and activation. LF-remodeling was reduced by perindopril.

CONCLUSIONS

In resistance arteries, high flow induced diameter enlargement and wall hypertrophy associated with the sequential activation of eNOS and MMP9.

High-Flow-Induced Arterial Remodeling in Rats with Different Susceptibilities to Cerebral Aneurysms

BACKGROUND

The higher incidence of cerebral aneurysms (CAs) induced by enhanced arterial blood flow in Long Evans (LE) compared to Brown Norway (BN) rats suggests that intrinsic differences in high-flow arterial remodeling may be involved in determining CA susceptibility. Some aspects of this remodeling were compared in LE and BN rats after creation of an abdominal aortocaval fistula (ACF).

METHODS AND RESULTS

At 4 days with ACF, aortic luminal cross-sectional area (LCSA) determined by morphometry was increased by 20% in LE but not in BN rats. mRNA levels, determined by RT-PCR, were higher in LE than in BN rats for collagen alpha1(I), collagen alpha1(III), MMP2 and its inhibitor TIMP1 at 19 days with ACF. Nitric oxide synthase (NOS) mRNA levels were higher in LE rats at 4 days for the inducible (NOS2) isoform and at 4 and 19 days for the neuronal (NOS1) isoform. Aortic LCSA and NOS1 mRNA levels were tightly correlated and NOS inhibition prevented ACF-induced aortic remodeling in the LE rat. MMP2 and MMP7 activity, evaluated by zymography at 4 days with ACF, did not greatly differ between BN and LE.

CONCLUSIONS

These data suggest that a higher intrinsic ability for high-flow-induced arterial enlargement associated with NOS gene overexpression may be a possible genetic determinant in CA susceptibility.

Continuous flow augments reactivity of rabbit carotid artery by reducing bioavailability of NO despite an increase in release of EDHF

Experiments were designed to investigate the influence of steady flow and pressure on endothelial function in the rabbit carotid artery. Increases and decreases in isometric force were compared in static rings and perfused (5 or 50 ml/min) segments of the same arteries in the presence and absence of endothelium. The alpha(1)-adrenoceptor agonist phenylephrine and the muscarinic agonist acetylcholine were applied as vasoconstrictor and vasodilator stimuli, respectively. Continuous flow (5 and 50 ml/min) reduced the cGMP content and shifted the concentration-response curve to phenylephrine to the left compared with nonperfused static rings. Removal of the endothelium abolished the differences in cGMP content and the sensitivity to phenylephrine between static rings and perfused segments. No difference in sensitivity to phenylephrine was observed in tissues treated with N(omega)-nitro-l-arginine methyl ester (l-NAME). Acetylcholine-evoked relaxations were increased in perfused segments. l-NAME nearly abolished the acetylcholine-evoked relaxation in static rings, whereas about one-half of the relaxation remained in segments exposed to flow. This remnant relaxation was blocked by inhibition of endothelial small- and intermediate-conductance calcium-activated potassium channels by apamin plus 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34). These experiments demonstrate that continuous flow increases the constriction evoked by alpha(1)-adrenergic activation in the rabbit carotid artery through a reduced influence of basally released endothelial NO and, furthermore, that luminal flow unmasks an ability of the endothelium to release a non-NO, noncyclooxygenase vasodilator, presumably endothelium-derived hyperpolarizing factor.

Remodeling characteristics of minimally diseased coronary arteries are consistent along the length of the artery

Using a method that creates anatomically correct, 3-dimensional arterial reconstructions, 55 minimally diseased coronary arteries from 40 patients were studied. Homogenous remodeling characteristics along the entire length of the artery were observed in 48 arteries (87%). In the aggregate, arteries exhibited compensatory expansive remodeling. Individually, the full spectrum of compensatory expansive remodeling (60%), excessive expansive remodeling (21%), and constrictive remodeling (19%) was observed across arteries. Each artery was consistent in its remodeling characteristics from proximal to distal portions of the artery, and the remodeling pattern of each artery was independent within the same patient.

Uterine Artery Remodeling and Reproductive Performance Are Impaired in Endothelial Nitric Oxide Synthase-Deficient Mice1

The progressive rise in uterine blood flow during pregnancy is accompanied by outward hypertrophic remodeling of the uterine artery (UA). This process involves changes of the arterial smooth muscle cells and extracellular matrix. Acute increases in blood flow stimulate endothelial production of nitric oxide (NO). It remains to be established whether endothelial NO synthase (eNOS) is involved in pregnancy-related arterial remodeling. We tested the hypothesis that absence of eNOS results in a reduced remodeling capacity of the UA during pregnancy leading to a decline in neonatal outcome. UA of nonpregnant and pregnant wild-type (Nos3+/+) and eNOS-deficient (Nos3-/-) mice were collected and processed for standard morphometrical analyses. In addition, cross sections of UA were processed for cytological (smoothelin, smooth muscle alpha-actin) and proliferation (Ki-67) immunostaining. We compared the pregnancy-related changes longitudinally and, together with the data on pregnancy outcome, transversally by analysis of variance with Bonferroni correction. During pregnancy, the increases in radius and medial cross sectional area of Nos3-/- UA was significantly less than those of Nos3+/+ UA. Smooth muscle cell dedifferentiation and proliferation were impaired in gravid Nos3-/- mice as deduced from the lack of change in the expression of smoothelin and smooth muscle alpha-actin, and the reduced Ki-67 expression. Until 17 days of gestation, litter size did not differ between both genotypes, but at birth the number of viable newborn pups and their weights were smaller in Nos3-/- than in Nos3+/+ mice. We conclude that absence of eNOS adversely affects UA remodeling in pregnancy, which may explain the impaired pregnancy outcome observed in these mice.

Toward functional genomics of flow-induced outward remodeling of resistance arteries

In resistance-sized arteries, a chronic increase in blood flow leads to increases in arterial structural luminal diameter and arterial wall mass. In this review, we summarize recent evidence that outward remodeling of resistance arteries 1) can help maintain and restore tissue perfusion, 2) is not intimately related to flow-induced vasodilatation, 3) involves transient dedifferentiation and turnover of arterial smooth muscle cells, and 4) is preceded by increased expression of matricellular proteins, which have been shown to promote disassembly of focal adhesion sites. Studies of experimental and physiological resistance artery remodeling involving differential gene expression analyses and the use of knockout and transgenic mouse models can help unravel the mechanisms of outward remodeling.

Impaired collateral artery development in spontaneously hypertensive rats

OBJECTIVE

To determine whether collateral artery development is impaired in spontaneously hypertensive (SHR) relative to normotensive (WKY) rats.

METHODS

Sequential mesenteric arteries were ligated to create a collateral pathway responsible for the perfusion of approximately 50 first-order arterioles. Collateral development was assessed by measurement of in vivo arterial diameter before and 1 week after ligation. Histological and morphometric measurements were made from cross-sectional preparations of these arteries to evaluate intimal and medial cell numbers and medial area. eNOS expression was evaluated with Western blotting.

RESULTS

One week after arterial ligation, collateral diameter was increased more in WKY than SHR both absolutely (137 +/- 9.1 versus 99 +/- 8.6 microm) and relative to same-animal controls (38 +/- 5.5% versus 13 +/- 7.1%). At the time of model creation, blood flow was elevated to comparable levels in both WKY and SHR, and wall shear rate in the SHR collateral was greater than both the SHR control and WKY collateral arteries. Endothelial cell number in arterial cross-section was increased in collaterals by 80% in WKY and only 22% in the SHR. eNOS expression was increased in the WKY (128%) but not in the SHR collateral.

CONCLUSIONS

For equivalent arterial occlusion, the data demonstrate that collateral development is suppressed in the SHR as indicated by luminal expansion. This impairment of luminal expansion is associated with a decreased endothelial proliferation and the lack of an increase in eNOS expression.

Chronic hydralazine improves flow (shear stress)-induced endothelium-dependent dilation in mouse mesenteric resistance arteries in vitro

Flow (shear stress)-mediated dilation (FMD) plays a key role in the local control of vascular diameter and blood flow supply. Although vasodilator treatments improve FMD in diverse models of hypertension, FMD may also change in situations where systemic blood pressure is not affected. In pathological situations such as ischemia, local blood flow and vascular density are increased by vasodilators not affecting systemic blood pressure. As the mechanisms involved remain obscure, we studied FMD in resistance arteries from mice treated chronically (1 month) with hydralazine (200 mg/L in drinking water). Blood flow in mesenteric arteries of mice treated with hydralazine was significantly increased (130 +/- 15 to 169 +/- 27 microl/min, n = 10/group), whereas mean arterial blood pressure was not affected (79 +/- 5 vs 82 +/- 3 mm Hg in controls). Mesenteric resistance arteries (90 microm internal diameter, 75 mm Hg) were isolated and mounted in vitro in an arteriograph. Pressure (myogenic tone)-, phenylephrine-, and KCl-induced contractions, as well as acetylcholine- and sodium nitroprusside-induced dilations, were unaffected by hydralazine. Flow-mediated dilation in arteries from hydralazine-treated mice was significantly increased, especially for low flow values (up to sevenfold). L-NAME-sensitive and indomethacin-sensitive FMD were both increased by hydralazine. Passive arterial diameter increased and arterial wall thickness decreased after chronic hydralazine. This is the first functional evidence that flow (shear stress)-mediated dilation in resistance arteries is improved by a chronic treatment with a nonselective vasodilator. This arteriolar adaptation to a chronic increase in blood flow might be of importance in the pathophysiology of ischemic diseases.

Effect of amlodipine compared to atenolol on small arteries of previously untreated essential hypertensive patients

In a previous retrospective study, long-term treatment of essential hypertensive patients with a slow-release calcium channel blocker resulted in normal resistance artery structure and endothelial function, which did not occur with a beta-blocker. In the present prospective study, 19 previously untreated essential hypertensive patients (aged 47 +/- 2 years, 75% male) were treated for 1 year in a double-blind randomized study with the long-acting calcium channel blocker amlodipine or the beta-blocker atenolol. Resistance arteries (lumen diameter, 150 to 350 microm) dissected from gluteal subcutaneous biopsies were studied on a pressurized myograph. Blood pressure (BP) control (129 +/- 2/85 +/- 2 mm Hg) was identical in both groups for the last 6 months of the study. After 1 year of treatment with amlodipine, the media-to-lumen ratio (M/L) of resistance arteries decreased from 7.89% +/- 0.40% to 6.81% +/- 0.41% (P < .05). Acetylcholine-induced endothelium-dependent relaxation tended to improve from 84.3% +/- 5.5% to 90.5% +/- 4.8% (P = .06), whereas sodium nitroprusside-induced relaxation was unchanged in the patients treated with amlodipine. In the beta-blocker-treated group there was no significant change in M/L or acetylcholine-induced relaxation. In conclusion, treatment with the calcium channel blocker amlodipine corrected altered resistance artery structure and tended to improve endothelial function in essential hypertensive patients, whereas similar good control of BP with the beta-blocker atenolol did not. Whether the vascular protective effect of amlodipine will result in improved outcomes in hypertension remains to be demonstrated.

Effect of crossing over hypertensive patients from a beta-blocker to an angiotensin receptor antagonist on resistance artery structure and on endothelial function

BACKGROUND

Treatment of essential hypertensive patients with an AT1 angiotensin receptor antagonist has previously resulted in correction of resistance artery structure and endothelial function, whereas in a parallel group treated with the beta-blocker atenolol there was no improvement of altered vascular structure and function. To test the hypothesis that patients previously treated with atenolol could present improvement of vascular structure and endothelial function if they were subjected to blockade of the renin-angiotensin system, we crossed over hypertensive patients that had been randomized to treatment with the beta-blocker atenolol to treatment with the AT1 antagonist irbesartan, and studied small artery structure and endothelial function before and after treatment.

METHODS

Eleven essential hypertensive patients (51 +/- 2 years, range 38-65; 75% male) that had previously been randomized to treatment with atenolol and treated for 1 year with good blood pressure control, were crossed over to treatment with the AT1 antagonist irbesartan for 1 year. Small resistance arteries were dissected from gluteal subcutaneous biopsies that were performed before and after 1 year of treatment. The structure and endothelial function of the resistance arteries were studied on a pressurized myograph.

RESULTS

Blood pressure control (129 +/- 3.3/85 +/- 1.8 mmHg) was identical to that achieved previously with atenolol (131 +/- 3.3/84 +/- 1.1 mmHg). Following 1 year of treatment, the arterial media width to lumen ratio (M/L) of resistance arteries (lumen diameter, 150-350 microm), which had remained unchanged under atenolol treatment, decreased from 8.44 +/- 0.45% when patients were on atenolol, to 6.46 +/- 0.30%, P < 0.01, when patients received irbesartan. Maximal acetylcholine-induced endothelium-dependent relaxation was 81.1 +/- 4.1% when patients were on atenolol, unchanged from before starting treatment with the beta-blocker, and was normalized by irbesartan (to 94.8 +/- 2.0%, P < 0.01).

CONCLUSION

Crossing over essential hypertensive patients with well-controlled blood pressure from the beta-blocker atenolol to the AT1 receptor antagonist irbesartan resulted in correction of previously persistently altered vascular structure and endothelial function, suggesting a structural and endothelial vascular protective effect of antihypertensive treatment with the AT1 receptor antagonist.

Shear level influences resistance artery remodeling: wall dimensions, cell density, and eNOS expression

The magnitude of shear stimulus has been shown to determine the level of growth factor expression in cell culture. However, little is known regarding what effect shear level has on specific arterial wall remodeling events in vivo. We have hypothesized that the rate of luminal diameter change and specific remodeling events within the arterial wall layers are dependent on shear level. Selective ligations were made to alter the number of microvascular perfusion units of mesenteric arteries within the same animal to approximately 50%, 200%, and 400% of control. Arterial blood flow and wall shear rate were correlated with the degree of alteration in perfusion units. Luminal diameters were decreased in 50% arteries by day 2 and increased approximately 17% and 33% respectively, in 200% and 400% arteries at day 7. The rate of diameter change was greatest in 50% and 400% arteries. Wall areas (medial +37%; intimal +18% at day 2) and cell densities (intimal +26%; adventitial +44% at day 2) were altered only in the 400% arteries. A positive correlation existed by day 2 between endothelial staining for endothelial nitric oxide synthase and shear level. The results demonstrate that shear level influences the rate of luminal expansion, specific remodeling events within each wall layer, and the degree of endothelial gene expression. A greater understanding of how shear level influences specific remodeling events within each wall layer should aid in the development of targeted therapies to manipulate the remodeling process in health and disease.