Effects of protection from pressure on resistance artery morphology and reactivity in spontaneously hypertensive and Wistar-Kyoto rats

Retinal arterial hypertrophy: the new LVH?

Prevention of target organ damage represents the El Dorado for clinicians who treat hypertension. Although many of the cardiovascular sequelae of chronic hypertension are due to large artery atherosclerosis, an equal number are due to small artery dysfunction. These microvascular complications include eye disease (retinopathy), kidney failure, diastolic dysfunction of the heart and small vessel brain disease leading to stroke syndromes, dementia and even depression. Examination of the retinal vasculature represents the only way to reliably derive information regarding small arteries responsible for these diverse pathologies. This review aims to summarise the rapidly accruing evidence indicating that easily observable abnormalities of retinal arteries reflect target organ damage elsewhere in the body of hypertensive patients. In tandem, we also present putative mechanisms by which hypertension and diabetes fundamentally change small artery structure and function and how these processes may lead to target organ damage.

Pulsatile hemodynamics of hypertension: systematic review of aortic input impedance

OBJECTIVE

Input impedance is the frequency-dependent afterload to pulsatile blood flow. Studies of input impedance have been performed as early as the 1960s and have been applied to hypertension (HTN). However, to date, these studies have not been systematically evaluated. This systematic review aims to summarize the literature, interpret existing data from the perspective of impedance theory, and to discuss their potential for generating physiological insights into HTN.

METHODS

We identified 11 studies wherein computed impedance moduli from both HTN and control (CNT) groups were reported. In addition, we performed bivariate analyses of raw data from three of these studies.

RESULTS

Major findings include HTN groups had consistently elevated impedance moduli at 0 Hz (Z(0)) and at heart rate frequency (Z(1)), an increased frequency wherein impedance phase first crosses 0 (f(0)), but no consistent pattern in characteristic impedance (Z(c)), when compared to CNT groups; SBP and DBP are highly correlated with Z(0) and Z(1), moderately correlated with f(0), less correlated with Z(c); the measurement and calculation methods for Z(c) are varied and inconsistent; and a not insignificant proportion of hypertensive individuals have 'normal' Z(0), Z(1) and Z(c) values. These findings are limited by the heterogeneous study populations and small sample sizes.

CONCLUSION

These findings suggest that Z(0), Z(1) and f(0) are significantly associated with HTN, whereas the role of Z(c) is less clear. Additional studies are needed to evaluate these input impedance variables in order to generate substantial implications in clinic settings.

Remodeling of retinal small arteries in hypertension

Vascular dysfunction due to elevated blood pressure constitutes an early step in the pathogenesis of atherosclerotic disease. A better understanding of the pathophysiology and of clinical correlates of vascular remodeling in retinal arteries and arterioles offers the opportunity for a better risk stratification and treatment. In vivo vascular changes can be best detected by direct imaging techniques. In this review, we summarize the main findings of several recent studies analyzing retinal-arteriolar parameters, such as outer diameter (OD) and lumen diameter (LD), retinal capillary flow (RCF), wall-to-lumen-ratio, and wall cross-sectional area by using scanning laser Doppler flowmetry (SLDF). Blood pressure emerged as an independent determinant of the wall-to-lumen ratio (WLR) of retinal arterioles. Retinal arterioles and small arteries of hypertensive subjects showed eutrophic inward remodeling as indicated by increased WLR, decreased LD and almost unchanged wall cross-sectional area compared to normotensive subjects. These findings are in accordance with those observed in small-resistance vessels analyzed ex vivo. In hypertensive patients, an increased retinal vascular resistance has been documented and basal nitric oxide activity emerged as an independent determinant of early arteriolar remodeling. Thus, SLDF emerged as a noninvasive research tool to assess early vascular changes in the retinal circulation.

Small artery structure and function in hypertension

It has been known for some considerable time that sustained hypertension changes the circulatory architecture both in the heart and blood vessels. The histopathological alterations are of considerable interest because once they have developed they appear to carry an adverse prognostic risk. In the heart it is apparent that there is hypertrophy. This extends also to the large- and medium-sized blood vessels but at the level of the smaller arteries that contribute to vascular resistance, this is not the case: it is clear that the physiological response to higher pressures is a change in the positional conformation of the pre-existing tissue constituents and as a result of this the lumen is narrowed. This brief review looks at our knowledge in this area and attempts to clarify our understanding of how hypertension brings these about and what happens when these homeostatic mechanisms break down. From a therapeutic perspective it appears imperative to control blood pressure in an attempt to reverse or prevent such alterations to cardiovascular structure. Our knowledge is fast expanding in this field and it is only to be anticipated that as detection methodology improves everyday practice will alter as we profile our patients in terms of structural alterations in the ventricle and blood vessels.

Low-dose ouabain constricts small arteries from ouabain-hypertensive rats: implications for sustained elevation of vascular resistance

Prolonged ouabain administration to normal rats causes sustained blood pressure (BP) elevation. This ouabain-induced hypertension (OH) has been attributed, in part, to the narrowing of third-order resistance arteries (approximately 320 microm internal diameter) as a result of collagen deposition in the artery media. Here we describe the structural and functional properties of fourth-order mesenteric small arteries from control and OH rats, including the effect of low-dose ouabain on myogenic tone in these arteries. Systolic BP in OH rats was 138 +/- 3 versus 124 +/- 4 mmHg in controls (P < 0.01). Pressurized (70 mmHg) control and OH arteries, with only a single layer of myocytes, both had approximately 165-microm internal diameters and approximately 20-microm wall thicknesses. Even after fixation, despite vasoconstriction, the diameters and wall thicknesses did not differ between control and OH fourth-order arteries, whereas in third-order arteries, both parameters were significantly smaller in OH than in controls. Myogenic reactivity was significantly augmented in OH fourth-order arteries. Nevertheless, phenylephrine- (1 microM) and high K(+)-induced vasoconstrictions and acetylcholine-induced vasodilation were comparable in control and OH arteries. Vasoconstrictions induced by 5 microM phenylephrine and by 10 mM caffeine in Ca(2+)-free media indicated that releasable sarcoplasmic reticulum Ca(2+) stores were normal in OH arteries. Importantly, 100 nM ouabain constricted both control and OH arteries by approximately 26 microm, indicating that this response was not downregulated in OH rats. This maximal ouabain-induced constriction corresponds to a approximately 90% increase in resistance to flow in these small arteries; thus ouabain at EC(50) of approximately 0.66 nM should raise resistance by approximately 35%. We conclude that dynamic constriction in response to circulating nanomolar ouabain in small arteries likely makes a major contribution to the increased vascular tone and BP in OH rats.

Urocortin inhibits mesenteric arterial remodeling in spontaneously hypertensive rats

Urocortin (UCN), a newly isolated corticotrophin-releasing factor (CRF) related peptide, has been found to have potent cardiovascular protective effects. This study aimed to investigate the long-term effects of UCN on arterial remodeling and related functional alterations. UCN (7 microg/kg/d) was administered to spontaneously hypertensive rats (SHR) for 8 weeks. Systolic blood pressure (SBP) was measured weekly. Functional studies were performed on isolated mesenteric arterial segments. Also, by light microscope and electron microscope, the morphology of mesenteric arteries was examined. Our results showed that mean SBP in UCN-treated SHRs was about 40 mm Hg lower than that of the control SHR group, and was similar to that of the enalapril-treated group. In the mesenteric arterial segments pre-contracted with norepinephrine (0.001-10 microM), the maximal relaxation rate induced by acetylcholine (10 microM) in UCN-treated group (about 93.3%) was higher than that in SHR control group (about 40.0%) (n=6, P<0.01). Furthermore examination under light microscope showed that UCN (3.5 microg/kg/d) treatment significantly reduced media thickness, media/lumen ratio, resulting in larger lumen diameter while analysis of transmission electron microscopic findings revealed that chromatin, internal elastic lamina and densely packed mitochondria displayed a close-to-normal distribution after UCN treatment. These results suggested that long-term UCN treatment not only had hypotensive effects but may also inhibited development of vascular remodeling in mesenteric arteries in SHR.

High intravascular pressure attenuates vascular dilation responses of small mesenteric arteries in the rat

BACKGROUND

Alterations in intravascular pressure can affect vascular function and the morphological properties of arteries. Although it has been shown that a transient elevation of blood pressure impairs endothelium-dependent dilation in small arteries, the vascular dilation responses during high intravascular pressure have not yet been investigated.

METHODS AND RESULTS

Using pressure myography, the endothelium-dependent and -independent dilation responses of small mesenteric arteries of rats were examined under 3 different intravascular pressure conditions (50, 80 and 120 mmHg). Endothelium-dependent dilation was evaluated by measuring vasodilator responses to increasing doses of acetylcholine (ACh, 10(-9) to 10(-4) mol/L) or increases in intraluminal flow (7-36 microl/min). Endothelium-independent vasodilator function was examined by using sodium nitroprusside (SNP, 10(-9) to 10(-4) mol/L). Flow-mediated dilation was significantly attenuated at 120 mmHg whereas ACh-induced dilation was progressively decreased with increases in intravascular pressure (at 80 and 120 mmHg). Significant attenuation in the vasodilator response to SNP was also observed at 80 and 120 mmHg.

CONCLUSION

Endothelium-dependent and-independent dilation responses are progressively attenuated with acute increases in intravascular pressure.

Hypertension and microvascular remodelling

In the present review, microvascular remodelling refers to alterations in the structure of resistance vessels contributing to elevated systemic vascular resistance in hypertension. We start with some historical aspects, underscoring the importance of Folkow's contribution made half a century ago. We then move to some basic concepts on the biomechanics of blood vessels, and explicit the definitions proposed by Mulvany for specific forms of remodelling, especially inward eutrophic and inward hypertrophic. The available evidence for the existence of remodelled resistance vessels in hypertension comes next, with relatively more weight given to human, in comparison with animal data. Mechanisms are discussed. The impact of antihypertensive drug treatment on remodelling is described, again with emphasis on human data. Some details are given on the three studies to date which point to remodelling of subcutaneous resistance arteries as an independent predictor of cardiovascular risk in hypertensive patients. We terminate by considering the potential role of remodelling in the pathogenesis of end-organ damage and in the perpetuation of hypertension.

Implications for repetitive application of acetylcholine in the determination of the mechanisms of endothelium-dependent relaxation

The influence of repetitive vasodilator concentration response curve determination was investigated in rat mesenteric and femoral small arteries. Arteries were precontracted with noradrenaline and relaxed with acetylcholine (ACh) or K+. Mesenteric arteries exhibited attenuation of ACh-stimulated relaxations during repetitive precontraction/relaxation cycles that was not prevented by SQ29548. Apamin, but not charybdotoxin, prevented the attenuation of this relaxation response. Borderline (p=0.064) statistical differences in the relaxations of mesenteric arteries in response to ACh remained in the presence of the nitric oxide synthase inhibitor L-NNA. In contrast, femoral arterial relaxations increased with repeated acetylcholine challenges. The enhanced responsiveness was prevented with L-NNA or Ba2+. In one experimental series, Ba2+ appeared to be without influence upon K+-stimulated relaxation of femoral arteries but a significant inhibitory effect was revealed when appropriate time control experiments were considered. These experiments reveal that impairment of SK(Ca) function and, to a lesser extent, an impaired NO signalling account for the attenuation of the relaxation responses of rat mesenteric arteries. In contrast, alterations of nitric oxide signalling and inward rectifier K channel activity contribute to enhanced relaxation responses in rat femoral arteries. These experiments highlight the importance of appropriate time control experiments for the proper interpretation of results derived from pharmacological experiments directed toward the elucidation of mechanisms of arterial vasorelaxation.

Marriage of resistance and conduit arteries breeds critical limb ischemia

Atherosclerosis in a major leg artery leads to impaired blood supply, which normally progresses to critical limb ischemia. Atherosclerosis produces substantial alterations of structure and endothelial function in the large conduit arteries. Pressure unloading and ischemia in the distal vasculature bring about alterations in microvascular function. Resistance arteries undergo significant wall thinning and changes in their contractile regulation. Optimization of large artery dimensions by the small arteries through flow-mediated vasodilation is impaired. Angiogenesis is stimulated, which can result in the formation of major collateral feeder vessels in addition to small nutritive blood vessels. However, angiogenesis can also contribute to instability of atherosclerotic plaques, which ultimately leads to further deterioration in blood supply. Surgical bypass grafting to restore blood supply to the distal leg generates a sudden increase of pressure in the weakened resistance vasculature, leading to uncontrolled changes in capillary hydrostatic pressure, extravasation of fluid, and tissue edema. This review aims to highlight the importance of the resistance vasculature in critical limb ischemia and the interdependence of pathophysiological changes in the large conduit and small resistance arteries. The major unresolved question is why the physiological mechanisms that regulate vascular structure and function ultimately break down, leading to circulatory failure within the distal limb.

Nitric oxide inhibition accelerates hypertension and induces perivascular inflammation in rats

1. Inflammatory changes in peripheral arteries have been reported in animal models of hypertension. Whether they occur in cerebral arteries (CA) with hypertension induced by deprivation of endogenous nitric oxide (NO) remains unknown. 2. In the present study, we compared the arteriolar injury score (AIS) and perivascular inflammation in CA between hypertensive and normotensive rats following NO deprivation with the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Five-week-old male spontaneously hypertensive rats (SHR) and Wistar -Kyoto (WKY) rats were fed with L-NAME (1 mg/mL) for 4 weeks. 3. Nitric oxide deprivation resulted in time-dependent elevations in tail-cuff pressure (representing systolic blood pressure (SBP)) in both SHR and WKY rats. The magnitude of increase in SBP was larger in SHR (+81.0 +/- 3.2 vs+25.0 +/- 2.2 mmHg; P < 0.01). Arteriolar hyalinosis and AIS in various segments of the CA were assessed with periodic acid-Schiff staining and inflammatory cells were immunostained with the antibody against macrophage/monocyte marker (ED1). The ED1+ cells appeared in the middle CA of L-NAME-treated SHR as early as 2 weeks after treatment. These cells were not observed in L-NAME-treated WKY rats and untreated SHR. More ED1+ cells were found in L-NAME-treated SHR than L-NAME-treated WKY rats after 4 weeks treatment. 4. The AIS and number of ED1+ cells around the perivascular area of the internal carotid artery were significantly higher in L-NAME-treated compared with untreated rats (AIS: 137 +/- 28 vs 46 +/- 10 for WKY rats, respectively; 169 +/- 18 vs 53 +/- 6 for SHR, respectively (P < 0.01); ED1+ cells: 7.9 +/- 0.6 vs 1.3 +/- 0.9 for WKY rats, respectively; 13.6 +/- 2.7 vs 2.1 +/- 0.9 for SHR, respectively (P < 0.01)), although SBP was higher in untreated SHR than in L-NAME-treated WKY rats (170 +/- 4 vs 137 +/- 4 mmHg, respectively; P < 0.05). 5. These findings suggest that ED1+ cells appeared in the middle CA of L-NAME-SHR as early as 2 weeks after treatment. Chronic inhibition of NO accelerates hypertension and induces perivascular inflammation.

Influence of experimental reduction of arterial media : lumen ratio on agonist-stimulated contractions in hypertension

OBJECTIVE

This study investigated the relationship between the arterial media thickness : lumen diameter (M : L) ratio and arterial contractile responses in the spontaneously hypertensive rat (SHR) and its Wistar-Kyoto control (WKY) under isobaric conditions.

METHODS

Femoral arteries with an experimentally reduced M : L ratio were produced in one hindlimb of both rat strains, by partial ligation of the ipsilateral iliac artery. Arterial structure and contractile responses were assessed in an arteriograph. Contractile responses of these vessels to the vasoconstrictor agonists noradrenaline and phenylephrine were determined after generation of spontaneous myogenic tone and compared with those from contralateral hindlimbs determined at both estimated in-vivo pressures and at 80 mmHg. Control SHR and WKY arterial responses were also compared.

RESULTS

Under both pressure conditions, relaxed M : L ratios were significantly greater in SHR than in WKY arteries and were also significantly reduced in arteries distal to the ligature in both strains. In no comparison was a greater M : L associated with a greater agonist-stimulated contractile response. However, increased M : L were associated with narrower lumens across the concentration response relationships under some, but not all, conditions.

CONCLUSION

An increased arterial M : L ratio does not effect an exaggerated contractile function in myogenically active resistance arteries in vitro. However, narrower lumen diameters associated with the increased M : L would provide a means for increased vascular resistance. These observations support the hypothesis that an increased M : L does not provide a means for a contractile response amplifier but may provide a means for a vascular resistance amplifier under some circumstances.

A model of physical factors in the structural adaptation of microvascular networks in normotension and hypertension

Adequate function of the microcirculation is vital to any tissue. To maintain an optimal function, microvascular networks must be able to adapt structurally to changes in the physical environment. Here we present a mathematical network model based on vessel wall mechanics. We assume based on experimental observations that longstanding change in transmural pressure elicits a change in the vascular wall-to-lumen ratio for maintaining circumferential wall stress at a certain level. In addition, experimental observations show that chronic change in fluid shear stress at the vascular wall elicits a persistent change in luminal diameter. On this basis we hypothesize that wall influencing substances released from the endothelium in response to shear stress have a certain optimal level in the vascular wall. Deviation from this level will cause vascular remodeling, i.e. a structural change in luminal diameter, until equilibrium is restored. The model explains several of the key features observed experimentally in the microcirculation in normotension and hypertension. Most importantly, it suggests a scenario where overall network structure and network hemodynamics depend on adaptation to local hemodynamic stimuli in the individual vessel. Simulated results show emanating microvascular networks with properties similar to those observed in vivo. The model points to an altered endothelial function as a key factor in the development of vascular changes characteristic of hypertension.

Role of endothelins in animal models of hypertension: focus on cardiovascular protection

Investigation of the regulation of vascular function by endothelium-derived factors has been a prominent topic of research in the field of hypertension during the last decade. Of the different endothelial factors, endothelins, which play an important role in vasodilatation-vasoconstriction balance, have been the subject of great interest and an impressive number of publications. This peptide, a very potent vasoconstrictor, triggers as well events involved in growth, proliferation, matrix production and local inflammation. In parallel, its role in hypertension has evolved from a simple vasoconstrictor to a central local regulator of vascular homeostasis contributing not only to the elevation of blood pressure, but also to the complications of hypertension. This review summarizes research on endothelins and its receptor antagonists in experimental hypertension, with special emphasis on vascular remodeling and target-organ protection.

Functional evidence that K+ is the non-nitric oxide, non-prostanoid endothelium-derived relaxing factor in rat femoral arteries

The mechanisms of K(+)-induced relaxation and of acetylcholine (ACh)-stimulated, endothelium-dependent relaxation were assessed in rat femoral arteries mounted in a myograph. ACh-stimulated (1 nM-1 microM) relaxation of arteries precontracted with 1 microM noradrenaline was mostly resistant to the combination of indomethacin (INDO; 10 microM) and N(omega)-nitro-L-arginine (L-NNA, 100 microM). The remaining relaxation was abolished by 30 mM K(+) or ouabain (1 mM) and significantly reduced by 30 microM Ba(2+) or charybdotoxin (ChTx; 100 nM) plus apamin (100 nM). K(+)-induced relaxation effected by raising [K(+)](o) by 0.5-4 mM was endothelium-independent and inhibited by ouabain and Ba(2+). These results indicate that ACh-stimulated relaxations are effected mainly by a non-prostanoid, non-nitric oxide mechanism, presumably an endothelium-derived hyperpolarising factor (EDHF). Relaxations stimulated by EDHF and K(+) are both mediated by Na(+)-K(+) ATPase and inward rectifier potassium channels (K(IR)). This study provides further functional evidence that EDHF is K(+) derived from endothelial cells that relaxes arterial smooth muscle subsequent to activation of Na(+)-K(+) ATPase and K(IR).

Myogenic response of rat femoral small arteries in relation to wall structure and [Ca(2+)](i)

The present study investigated the influence of media thickness on myogenic tone and intracellular calcium concentration ([Ca(2+)](i)) in rat skeletal muscle small arteries. A ligature was loosely tied around one external iliac artery of 5-wk-old spontaneously hypertensive rats. At 18 wk of age, femoral artery blood pressure was 102 +/- 11 mmHg (n = 15) on the ligated side and 164 +/- 6 mmHg (n = 15) on the contralateral side. Small arteries feeding the gracilis muscle had a reduced media cross-sectional area and a reduced media-to-lumen ratio on the ligated side, where also the range of myogenic constriction was shifted to lower pressures. However, when expressed as a function of wall stress, diameter responses were nearly identical. [Ca(2+)](i) was higher in vessels from the ligated hindlimb at pressures above 10 mmHg, but vasoconstriction was not accompanied by changes in [Ca(2+)](i). Thus the myogenic constriction here seems due primarily to changes in intracellular calcium sensitivity, which are determined mainly by the force per cross-sectional area of the wall and therefore altered by changes in vascular structure.

Small artery remodeling in hypertension

Hypertension is associated with altered structure of the resistance vessels, a process known as remodeling. This review summarizes current concepts concerning the structure of a subgroup of the resistance vessels, the small arteries, and the modes of remodeling, some of the determinants of remodeling, and some signaling pathways for remodeling. It is shown that the available evidence points to important roles for blood flow and growth factors, in addition to blood pressure, as causes of resistance artery remodeling. Finally, the relationship between vascular structure and blood pressure is discussed, in particular with regard to the effects of antihypertensive therapy. Here again, it appears that blood flow plays an important role in allowing the correction of the abnormal resistance vessel structure seen in hypertension.

Tyrosine phosphorylation following alterations in arteriolar intraluminal pressure and wall tension

Arterioles respond to increased transmural pressure with myogenic constriction. The present study investigated the role of tyrosine phosphorylation in myogenic activity. Cannulated segments of a rat cremaster arteriole were fixed under pressure, followed by incubation with fluorescein isothiocyanate (FITC)-conjugated anti-phosphotyrosine. Smooth muscle cell fluorescence intensity was measured with the use of confocal laser-scanning microscopy. Anti-phosphotyrosine fluorescence intensity in muscle cells of arterioles maintained at 100 mmHg was reduced by the tyrosine kinase inhibitor tyrphostin A47 (30 microM) and increased by the tyrosine phosphatase inhibitor pervanadate (100 microM). In time-course experiments, anti-phosphotyrosine fluorescence increased slowly (over 5 min) after an acute increase in intraluminal pressure, and was dissociated from myogenic contraction (within 1 min). In contrast, angiotensin II (0.1 microM) caused rapid constriction and increased tyrosine phosphorylation. Anti-phosphotyrosine fluorescence was also pressure dependent (10-100 mmHg). Abolition of myogenic activity, either through removal of extracellular Ca2+, or exposure to verapamil (5 microM) or forskolin (0.1 microM) caused a further increase in anti-phosphotyrosine fluorescence. We conclude that transmural pressure and/or wall tension in arterioles causes increased tyrosine phosphorylation; however, this is not involved in the acute phase of myogenic constriction but may be involved in later responses, such as sustained myogenic tone or mechanisms possibly related to growth.

The elastin-laminin receptor functions as a mechanotransducer in vascular smooth muscle

Laminin and elastin, two major constituents of the extracellular matrix, bind to cells via the elastin-laminin receptor (ELR), a receptor distinct from integrins. Despite the ubiquitous nature of elastin and laminin in the matrix, the consequences of activation of the ELR are unknown. Because integrins are capable of mechanosensitive transduction, we hypothesized that the ELR would exert a similar function. Accordingly, we examined the effects of cyclical stretch on canine coronary smooth muscle gene expression and proliferation that are mediated by the ELR. Northern blot analyses showed a 31% decrease in serum-induced expression of c-fos when cells were stretched for 30 min on elastin, but no change in expression was observed on collagen. Serum-induced proliferation of stretched cells was markedly attenuated on elastin when compared with collagen. Both the molecular (decreased c-fos expression) and biological (decreased proliferation) responses on elastin were restored after blockade of the ELR with the elastin fragment hexapeptide (valine-glycine-valine-alanine-proline-glycine, VGVAPG). The inhibition was specific for this peptide, as another hydrophobic hexapeptide (valine-serine-leucine-serine-proline-glycine, VSLSPG) did not inhibit the responses. These results demonstrate that cyclic stretch inhibits c-fos expression and proliferation of coronary vascular smooth muscle cells grown on elastin matrixes, a mechanosensitive response that is transduced by the ELR.

Contractility of resistance arteries of spontaneously hypertensive rats related to their media: lumen ratio

OBJECTIVE

The purpose of the study was to test the hypothesis that the increased media thickness: lumen diameter (M:L) ratio of spontaneously hypertensive rat (SHR) resistance arteries effects enhanced arterial contractile responses compared with those of Wistar-Kyoto (WKY) rat normotensive controls under pressurized conditions in vitro.

DESIGN

Contractile responses to the vasoconstrictor agonists noradrenaline and the thromboxane A2 analogue U46619 were assessed in femoral and mesenteric resistance arteries (internal diameters approximately 150 microm) after the development of spontaneous myogenic tone at 100 mmHg and at estimated in vivo pressures.

METHODS

Arterial contractile responses and structure were assessed in an arteriograph. Relaxed arterial structure was determined by light microscopy. Mean arterial pressure was determined subsequent to femoral artery cannulation.

RESULTS

Under relaxed conditions M:L ratios were significantly greater in SHR arteries at 100 mmHg (P < 0.01) and at in vivo pressures (P<0.01). Myogenic responses were not significantly different between SHR and WKY. Under both pressure conditions the contractile responses of SHR femoral arteries were not significantly different to those of WKY in response to either agonist SHR mesenteric arteries achieved smaller diameters in response to noradrenaline (P<0.05) and U46619 (P<0.05) at 100 mmHg. At in vivo pressures, concentration-response relationships of SHR mesenteric arteries for both agonists were not significantly different compared with those of WKY; however, the maximum percentage reduction of lumen diameter in SHR mesenteric arteries in the presence of noradrenaline was greater than in WKY (P<0.05).

CONCLUSION

The increased M:L ratio of SHR femoral resistance arteries does not impart an exaggerated contractile function in myogenically active resistance arteries in vitro. For mesenteric arteries the relationship is less clear because increased M:L ratio is associated with increased contractile responses under some, but not all, circumstances.

Renal vascular morphology and haemodynamics in Dahl salt-sensitive rats on high salt-low potassium diet: neural and genetic influences

OBJECTIVE

A dietary combination of high salt and low potassium (HS-LK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders Dahl salt-resistant (DR) rats hypertensive. In both strains, the hypertension is accompanied by remodelling of the renal resistance vasculature, and is attenuated by peripheral chemical sympathectomy. In the current study, we sought to determine whether the sympathetic nervous system is causally involved in mediating the renal vascular and haemodynamic alterations associated with HS-LK feeding in Dahl rats.

DESIGN

Two groups each of DS and DR rats were maintained on HS-LK diet (8% NaCl, 0.2% KCl) for 8 weeks. One group of DS (n = 9) and DR (n = 8) were treated with 6-hydroxydopamine (6-OHDA) in 0.001 N HCl vehicle to chemically ablate peripheral sympathetic nerve terminals. The two remaining groups (n = 8 each) received equivalent injections of vehicle.

METHODS

At the end of the dietary regimen, arterial blood pressure (ABP), glomerular filtration rate (GFR) and renal blood flow (RBF) were measured, and the structure of intra-renal resistance vessels was examined by planar morphometric analysis of coronal sections prepared from perfusion-fixed kidneys.

RESULTS

Both 6-OHDA-treated and untreated DS rats presented a greater degree of intra-renal vessel remodelling characterized by reduced lumen diameter in the absence (eutrophic) or presence (hypertrophic) of cross-sectional area expansion, higher renal vascular resistance (RVR) and lower GFR and RBF than DR rats. Chemical sympathectomy increased lumen diameters and reduced vascular wall expansion, resulting in a decrease in RVR and a concomitant increase in RBF and GFR in both strains; however, the effect was more prominent in the DS rats.

CONCLUSIONS

We conclude that HS-LK-induced changes in intra-renal vessel structure and renal haemodynamic function in Dahl rats are, at least in part, dependent on the activity of the sympathetic nervous system.

AT(1) receptor inhibition does not reduce arterial wall hypertrophy or PDGF-A expression in renal hypertension

To separate the role of ANG II from pressure in hypertrophy of the vascular wall in one-kidney, one-clip (1K1C) hypertension, experimental and sham-operated rats were given the AT(1)-receptor antagonist losartan (20 mg x kg(-1) x day(-1)) or tap water for 14 days. Mean arterial pressure was elevated in both experimental groups compared with controls. Rats were anesthetized with pentobarbital sodium, and the thoracic aorta and carotid, small mesenteric, and external spermatic arteries were harvested and embedded in paraffin. Tissue sections were used for morphological analysis, immunohistochemistry for 5-bromo-2'-deoxyuridine (BrdU) and platelet-derived growth factor (PDGF)-AA, stereological measurements, and in situ hybridization with a (35)S-labeled riboprobe for PDGF-A mRNA. Elevated cross-sectional areas of thoracic, carotid, and small mesenteric artery in 1K1C rats were not reduced by losartan. The internal diameter of the external spermatic artery and microvascular density of the cremaster muscle were reduced in 1K1C rats. The number of BrdU-positive nuclei per cross section did not differ between 1K1C and control arteries. PDGF-A mRNA was elevated in the arterial walls of 1K1C rats compared with controls and was hardly changed by losartan. PDGF-A protein stained strongly in the media of 1K1C arteries and was not inhibited by losartan; it appeared in the adventitia of all aortas and carotid arteries. These observations demonstrate that effects of ANG II mediated through the AT(1) receptor are not necessary for hypertrophy of the vascular wall during 1K1C hypertension or expression of PDGF-A.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Effects of angiotensin-converting enzyme inhibition on vascular remodeling of resistance vessels in hypertensive patients

Essential hypertension is known to be associated with a decrease in the lumen diameter and an increase in the wall thickness to lumen diameter ratio of the resistance vessels. Recently, it has been clarified that this alteration does not necessarily involve vascular growth, but could be due to a rearrangement of the same amount of material, a phenomenon now termed "eutrophic remodeling." These changes are found both in human essential hypertension and in animal models of genetic hypertension. Antihypertensive treatment with angiotensin-converting enzyme (ACE) inhibitors causes a dose-dependent regression of the media to lumen ratio in rats. Clinical studies have now confirmed these findings, showing that when previously untreated essential hypertensive patients are treated with the ACE inhibitor perindopril (PE), the abnormal structure of resistance vessels regresses toward normal values; in contrast, treatment with a beta-blocker does not affect the abnormal vascular structure. The available evidence thus indicates that ACE inhibitors are able to normalize the abnormal resistance vessel structure in essential hypertension, and suggest that this effect may not only be dependent on their ability to reduce blood pressure. This review summarizes these findings, and discusses the extent to which this is desirable.

Endurance exercise training increases peripheral vascular response in human fingers

The purpose of this study was to clarify whether peripheral vascular response to alteration of transmural pressure is changed by endurance exercise training. The healthy male subjects (training group; n = 6) performed endurance exercise training that consisted of cycle ergometer exercise 5 d.week-1 and 30 min.d-1 for a period of 8 weeks. Changes in the peripheral vascular response to alteration of transmural pressure in the human finger were measured by a differential digital photoplethysmogram (DeltaDPG) and blood pressure during passive movement of the arm to different vertical hand positions relative to heart level. Following 8 weeks of endurance training, percent changes in DeltaDPG from heart level in the training group increased significantly (mean +/- SD, -48.1 +/- 7. 3 to -58.7 +/- 9.3% at the lowered position, 46.1 +/- 13.4 to 84.6 +/- 8.8% at the elevated position, p<0.05). Similarly, the arterial compliance index, which was calculated from DeltaDPG-P wave amplitude and arterial pulse pressure, also significantly changed in the training group over the 8 weeks (5.6 +/- 1.3 to 2.7 +/- 1.6 mV. V-1.s-1.mmHg-1 at the lowered position, 30.0 +/- 12.4 to 54.4 +/- 18. 9 mV.V-1.s-1.mmHg-1 at the elevated position ). Maximal oxygen uptake (V.O2 max) was significantly increased in the training group. On the other hand, the control group (n = 6) showed no significant changes in all parameters for 8 weeks. Therefore these results suggest that endurance exercise training induces an increase in peripheral vascular response to alteration of transmural pressure in the human finger.

Responses to noradrenaline of tail arteries in hypertensive, hypotensive and normotensive rats under different regimens of perfusion: role of the myogenic response

The vasoconstrictor effects of noradrenaline were studied in spontaneously hypertensive rats (SHR) compared with Wistar Kyoto rats (WKY), and in Wistar rats with regional hypotension (WH) compared to control Wistar rats (WC). The abdominal aorta was ligated in WH distal to the renal arteries, lowering blood pressure in the hindquarters by 41% and tail artery wall cross-sectional area by 35% compared with WC. A cylindrical segment was dissected from the proximal part of the tail artery, cannulated at both ends and perfused with Krebs-Henseleit solution either at constant flow starting from a pressure of 120 mmHg or at a constant pressure of 120 mmHg. The cumulative dose-response relationships for noradrenaline were determined in control conditions and subsequently in the presence of gadolinium (100 microM), a non-specific blocker of mechanosensitive channels. Under constant-flow perfusion noradrenaline evoked a more prominent resistance increase in SHR compared with WKY and in WC compared with WH. Similar relations were seen in the presence of gadolinium, although responses were reduced. At constant pressure perfusion the vasoconstrictor response to noradrenaline was lower in SHR compared with WKY and in WC compared with WH. Application of gadolinium under constant-pressure perfusion reduced responses in WKY and WH, so that vasoconstriction in SHR became more pronounced than that in WKY and in WC than that in WH. It is suggested that the results can be explained by the difference in wall thickness causing different degrees of activation of the myogenic response to distension.

The impact of angiotensin converting enzyme inhibitors on the arterial wall

There is recent interest in the possibility that angiotensin converting enzyme inhibitors (ACE inhibitors) may reduce the damage inflicted on the arterial wall by common cardiovascular risk factors such as hypertension, hyperlipidaemia and ageing. The efficacy of these drugs in blood pressure reduction is accepted, but whether there is an excess benefit on arterial structure and function, conferred by use of ACE inhibitors over more traditional antihypertensives, is still under debate. There is also evidence in animal models to suggest that ACE inhibition is effective in reduction of arterial damage due to experimental hyperlipidaemia. ACE inhibitors not only reduce the conversion of angiotensin I and angiotensin II, which can interact with the sympathetic nervous system, but also prevent the degradation of bradykinin. This means that ACE inhibitors have several potential mechanisms through which they could suppress intimal hypertrophy and prevent endothelial dysfunction, which is believed to precede arteriosclerosis in man. Although much further work is needed to clarify the mechanism underlying the beneficial effects on the arterial wall of this group of drugs, they do appear to have significant potential in the effort to reduce cardiovascular mortality and morbidity, especially in high risk groups.

Hypertension and pathologic cardiovascular remodeling: a potential therapeutic role for T-type calcium antagonists

Increased myocardial mass (cardiac hypertrophy, left ventricular hypertrophy [LVH]) is an example of the widespread structural cardiovascular changes, often referred to as remodeling, that may be present in association with sustained high blood pressure. LVH strongly predicts myocardial infarction, stroke, and cardiovascular death in patients with hypertension. As a result, prevention or reversal of hypertensive LVH is widely accepted as a desirable therapeutic goal. Although the molecular mechanisms responsible for remodeling are unclear, it is believed that mechanical, endocrine, paracrine, and autocrine factors control the remodeling process. Certain antihypertensive drugs may have particularly favorable long-term effects in that they prevent and correct these structural changes in addition to reducing arterial pressure. However, the mechanism by which they achieve these effects is not well understood. It is theorized that angiotensin-converting enzyme inhibitors do so by preventing the generation of growth-promoting/mitogenic peptides and that beta-blockers interfere with the growth-promoting effects of catecholamines. In experimental models, the selective blockade of T-type calcium-ion (Ca2+) channels with mibefradil has been demonstrated to have antiproliferative effects in both the renal and cardiac vasculature; in patients with LVH, mibefradil reduced the left ventricular mass index. Therefore, blockade of T-type Ca2+ channels may be useful in the prevention or regression of cardiovascular remodeling. However, further research will be required before the clinical implications of these findings can be assessed.

Mechanical properties of small femoral arteries in spontaneously hypertensive rats

The purpose of the study was to compare the mechanical properties of small femoral arteries from spontaneously hypertensive rats (SHR) and normotensive control Wistar-Kyoto rats (WKY) to determine whether these could contribute to the narrowed lumens and thicker medial layers observed during the development of hypertension. Rats were used at either 5, 12, or 24 weeks of age. Third order branches of the right femoral artery were mounted in a myograph for morphological measurement and determination of wall mechanical properties. At 5 weeks SHR and WKY arteries were structurally similar but progressive medial thickening and hypertrophy in conjunction with lumenal narrowing was observed in SHR compared with those from WKY in the older rats. However, stress-strain and incremental elastic modulus-stress relationships were similar between strains at all three ages. These data indicate that modifications of arterial wall mechanical properties do not contribute to these progressive arterial structural modifications.

Peripheral vasculature in essential hypertension

1. Haemodynamic evidence shows that in essential hypertension minimum vascular resistance and vascular pressor response are increased and that the vascular reserve is decreased. 2. The haemodynamic changes are most easily explained in terms of a generalized narrowing of the vasculature and an increase in the ratio between the thickness of the tunica media and the lumen diameter (media: lumen ratio), with no change in the functional properties of the smooth muscle itself. 3. Histological and in vitro studies of resistance vessels confirm these predictions. Moreover, the evidence indicates that these changes are associated mainly with remodelling (rearrangement of the same amount of material) of the vessels, rather than growth. 4. Although the alteration in small artery structure is usually appropriate to the actual blood pressure, the structure appears not only to be a secondary adaptation, but is also dependent on other factors, including neurohumoral factors. 5. The available evidence shows that normalization of the resistance vessel structure (by increasing lumen diameter and decreasing the media:lumen ratio) should be achieved not by inhibition of growth but by (reverse) remodelling. Recent evidence from clinical investigations shows that this can be achieved in essential hypertensive patients treated with the angiotensin-converting enzyme inhibitor perindopril. 6. The role of the resistance vasculature as a primary determinant of blood pressure remains unclear. It is suggested that the requirement for normalization of resistance vascular structure is due to a need to increase the vascular reserve.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Low-protein diets and angiotensin-converting enzyme inhibition in progressive renal failure

Both protein restriction and administration of angiotensin-converting enzyme (ACE) inhibitors have been reported to slow the progression of chronic renal failure. Protein intake conditions the activity of the renin-angiotensin system and the renal production and excretion in the urine of eicosanoids. Both dietary protein restriction and ACE inhibitors decrease proteinuria. The effects of ACE inhibitors on the progression of renal disease may be due to hemodynamic effects (lowering of systemic or intraglomerular pressures) or to decreased activity of growth factors as a result of blockade or angiotensin II production.

Quinapril treatment and arterial smooth muscle responses in spontaneously hypertensive rats

1 The effects of long-term angiotensin-converting enzyme inhibition with quinapril on arterial function were studied in spontaneously hypertensive rats, Wistar-Kyoto rats serving as normotensive controls. 2 Adult hypertensive animals were treated with quinapril (10 mg kg-1 day-1) for 15 weeks, which reduced their blood pressure and the concentrations of atrial natriuretic peptide in plasma and ventricular tissue to a level comparable with that in normotensive rats. 3 Responses of mesenteric arterial rings in vitro were examined at the end of the study. Compared with normotensive and untreated hypertensive rats, responses to noradrenaline were attenuated in hypertensive animals on quinapril, both force of contraction and sensitivity being reduced. Quinapril also attenuated maximal contractions but not sensitivity to potassium chloride. Nifedipine less effectively inhibited vascular contractions in normotensive and quinapril-treated than in untreated hypertensive rats. 4 Arterial relaxation responses by endothelium-dependent (acetylcholine) and endothelium-independent (sodium nitrite, isoprenaline) mechanisms were similar in normotensive and quinapril-treated rats and more pronounced than in untreated hypertensive rats. 5 Cell membrane permeability to ions was evaluated by means of potassium-free solution-induced contractions of endothelium-denuded denervated arterial rings. These responses were comparable in normotensive and quinapril-treated rats and less marked than in untreated hypertensive rats. 6 Intracellular free calcium concentrations in platelets and lymphocytes, measured by the fluorescent indicator quin-2, were similar in normotensive and quinapril-treated rats and lower than in untreated hypertensive rats. 7 In conclusion, quinapril treatment improved relaxation responses and attenuated contractions in arterial smooth muscle of hypertensive rats. These changes may be explained by diminished cytosolic free calcium concentration, reduced cell membrane permeability, and alterations in dihydropyridine-sensitive calcium channels following long-term angiotensin-converting enzyme inhibition.

Structural and functional remodeling of poststenotic arteries in the rat

In rat femoral arteries situated distally from a unilateral partial iliac artery obstruction, we observed: (i) 30% reduction of media cross sectional area, without alteration of arterial DNA content, (ii) a steeper relationship between strain and circumferential wall stress at rest, (iii) 12% reduction of the diameter at which maximal active wall tension was observed, (iv) reduction (25%) of maximal active wall tension, but not maximal active wall stress, and (v) a leftward shift of the relationship between diameter and sensitivity for contractile stimuli. Chronic flow reduction at constant pressure, did not modify arterial properties. These findings indicate that (pulse) pressure, influences arterial structure and function primarily by an effect on arterial smooth muscle cell volume. Vascular remodeling may thus result from disproportionate effects on vessel wall components.

Angiotensin II, angiotensin-converting enzyme inhibitors, and blood vessel structure

Angiotensin-converting enzyme (ACE) inhibitors are effective hypotensive agents in hypertension of different types and degree. Probably, they lower pressure by reducing angiotensin II (AII); the varied timing of their hypotensive effect suggests that AII increases blood pressure in more than one way. Infusion studies show two effects of AII: at moderate dose, a rapid vasoconstrictor action; at lower dose, a slow-developing but ultimately large hypertensive effect. Vascular hypertrophy develops during the slow pressor response; at least part of the hypertrophy results from a nonpressor mechanism. In vitro studies show that AII has mitogenic and trophic actions on vascular smooth muscle cells in culture and that it stimulates synthesis of extracellular matrix proteins. One of these actions may produce the nonpressor component of vascular hypertrophy. ACE inhibitors lower pressure in the spontaneously hypertensive rat (SHR) and when given in young animals produce a hypotensive effect that endures long after the period of treatment. An action of endogenous AII, possibly a paracrine effect within the vessel wall, may cause vascular hypertrophy in young SHR with long-lasting effects on arterial pressure.