Time course changes of the mechanical properties of the carotid artery in renal hypertensive rats

Peripheral and Cerebral Resistance Arteries in the Spontaneously Hypertensive Heart Failure Rat: Effects of Stilbenoid Polyphenols

Hypertension is associated with aberrant structure and mechanical properties of resistance arteries. We determined the effects of resveratrol, a non-flavonoid polyphenol found in foods such as red grapes, and structurally-similar analogues (pterostilbene and gnetol) on systolic blood pressure (SBP) and resistance arteries from the spontaneously hypertensive heart failure (SHHF) rat. SBP was elevated in 17-week-old SHHF vs. Sprague-Dawley rats (normotensive control; 194 ± 3 vs. 142 ± 6 mmHg, p < 0.01) and was unaffected by resveratrol, pterostilbene, or gnetol (2.5 mg/kg/d). Geometry and mechanical properties of pressurized mesenteric resistance arteries and middle cerebral arteries were calculated from media and lumen dimensions measured at incremental intraluminal pressures. SHHF arteries exhibited remodeling which consisted of augmented media-to-lumen ratios, and this was attenuated by stilbenoid treatment. Compliance was significantly reduced in SHHF middle cerebral arteries but not mesenteric arteries vis-à-vis increased wall component stiffness; stilbenoid treatment failed to normalize compliance and wall component stiffness. Our data suggest that neither AMPK nor ERK mediate stilbenoid effects. In conclusion, we observed arterial bed-specific abnormalities, where mesenteric resistance arteries exhibited remodeling and cerebral arteries exhibited remodeling and stiffening. Resveratrol, pterostilbene, and gnetol exhibited similar abilities to attenuate vascular alterations.

Different biomechanical properties of medial and adventitial layers of thoracic aorta in Wistar-Kyoto and spontaneously hypertensive rats

AIM

To evaluate the biomechanical properties of thoracic aorta with or without adventitia, and to determine whether there are corresponding changes with hypertension.

METHODS

Normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) at the age of 16 and 32 weeks were used. Thoracic aortic adventitial layer was mechanically separated from thoracic aorta and the adventitia-denuded artery ring was viewed as thoracic media. A load-strain curve was obtained by stretching the ring-shaped intact thoracic aorta or thoracic media with a tensile testing machine. Then, the slope of the load-stain curve at 30%-40% strains was viewed as the elastic stiffness at physiological load, whereas the slope near the breaking point was calculated as maximum stiffness. The maximum load is the load at the breaking point.

RESULTS

There was no significant difference in elastic stiffness and maximum stiffness of intact thoracic aorta between SHR and age-matched WKY. The elastic stiffness of intact thoracic aorta showed no significant difference from that of thoracic media in WKY and SHR at both ages. In contrast, both maximum stiffness and maximum load were reduced in thoracic media compared with intact thoracic aorta in SHR and WKY at both ages.

CONCLUSION

These results indicated that vascular adventitia contributes to maximum stiffness, but not elastic stiffness in both SHR and WKY.

An angiotensin II- and NF-kappaB-dependent mechanism increases connexin 43 in murine arteries targeted by renin-dependent hypertension

AIMS

Connexins (Cxs) play a role in the contractility of the aorta wall. We investigated how connexins of the endothelial cells (ECs; Cx37, Cx40) and smooth muscle cells (SMCs; Cx43, Cx45) of the aorta change during renin-dependent and -independent hypertension.

METHODS AND RESULTS

We subjected both wild-type (WT) mice and mice lacking Cx40 (Cx40(-/-)), to either a two-kidney, one-clip procedure or to N-nitro-l-arginine-methyl-ester treatment, which induce renin-dependent and -independent hypertension, respectively. All hypertensive mice featured a thickened aortic wall, increased levels of Cx37 and Cx45 in SMC, and of Cx40 in EC (except in Cx40(-/-) mice). Cx43 was up-regulated, with no effect on its S368 phosphorylation, only in the SMCs of renin-dependent models of hypertension. Blockade of the renin-angiotensin system of Cx40(-/-) mice normalized blood pressure and prevented both aortic thickening and Cx alterations. Ex vivo exposure of WT aortas, carotids, and mesenteric arteries to physiologically relevant levels of angiotensin II (AngII) increased the levels of Cx43, but not of other Cx. In the aortic SMC line of A7r5 cells, AngII activated kinase-dependent pathways and induced binding of the nuclear factor-kappa B (NF-kappaB) to the Cx43 gene promoter, increasing Cx43 expression.

CONCLUSION

In both large and small arteries, hypertension differently regulates Cx expression in SMC and EC layers. Cx43 is selectively increased in renin-dependent hypertension via an AngII activation of the extracellular signal-regulated kinase and NF-kappaB pathways.

The macrocirculation and microcirculation of hypertension

Changes in vascular structure that accompany hypertension may contribute to hypertensive end-organ damage. Both the macrovascular and microvascular levels should be considered, as interactions between them are believed to be critically important. Regarding the macrocirculation, the article first reviews basic concepts of vascular biomechanics, such as arterial compliance, arterial distensibility, and stress-strain relationships of arterial wall material, and then reviews how hypertension affects the properties of conduit arteries, particularly examining evidence that it accelerates the progressive stiffening that normally occurs with advancing age. High arterial stiffness may increase central systolic and pulse pressure by two different mechanisms: 1) Abnormally high pulse wave velocity may cause pressure waves reflected in the periphery to reach the central aorta in systole, thus augmenting systolic pressure; 2) In the elderly, the interaction of the forward pressure wave with high arterial stiffness is mostly responsible for abnormally high pulse pressure. At the microvascular level, hypertensive disease is characterized by inward eutrophic or hypertrophic arteriolar remodeling and capillary rarefaction. These abnormalities may depend in part on the abnormal transmission of highly pulsatile blood pressure into microvascular networks, especially in highly perfused organs with low vascular resistance, such as the kidney, heart, and brain, where it contributes to hypertensive end-organ damage.

Elastin-insufficient mice show normal cardiovascular remodeling in 2K1C hypertension despite higher baseline pressure and unique cardiovascular architecture

Mice heterozygous for the elastin gene (ELN(+/-)) show unique cardiovascular properties, including increased blood pressure and smaller, thinner arteries with an increased number of lamellar units. Some of these properties are also observed in humans with supravalvular aortic stenosis, a disease caused by functional heterozygosity of the elastin gene. The arterial geometry in ELN(+/-) mice is contrary to the increased thickness that would be expected in an animal demonstrating hypertensive remodeling. To determine whether this is due to a decreased capability for cardiovascular remodeling or to a novel adaptation of the ELN(+/-) cardiovascular system, we increased blood pressure in adult ELN(+/+) and ELN(+/-) mice using the two-kidney, one-clip Goldblatt model of hypertension. Successfully clipped mice have a systolic pressure increase of at least 15 mmHg over sham-operated animals. ELN(+/+) and ELN(+/-)-clipped mice show significant increases over sham-operated mice in cardiac weight, arterial thickness, and arterial cross-sectional area with no changes in lamellar number. There are no significant differences in most mechanical properties with clipping in either genotype. These results indicate that ELN(+/+) and ELN(+/-) hearts and arteries remodel similarly in response to adult induced hypertension. Therefore, the cardiovascular properties of ELN(+/-) mice are likely due to developmental remodeling in response to altered hemodynamics and reduced elastin levels.

Dietary magnesium intake can affect mechanical properties of rat carotid artery

The purpose of the present study was to determine the effects of Mg deficiency and supplementation on the mechanical properties of the rat common carotid artery. The internal diameter and intra-arterial pressure of carotid artery were measured continuously using an echo-tracking device. Systolic, diastolic and mean intra-arterial pressures were not significantly different in Mg-deficient, -supplemented or control rats. Histological examination showed a larger cross-sectional area, increased intima-media thickness and a greater media:lumen value in carotid artery of Mg-deficient rats, indicating that Mg deficiency may directly stimulate growth and/or proliferation of arterial wall components. In addition, we observed a negative linear relationship between intima-media thickness and plasma Mg concentration, suggesting that increased Mg intake may counteract arterial wall hypertrophy. Neither Mg deficiency nor supplementation modified the arterial distensibilityv.intra-arterial pressure curve or the Eincv.wall stress curve, indicating that dietary Mg intake did not modify wall stiffness in young rats. At mean intra-arterial pressure, the stress and Eincvalues were, however, significantly lower in Mg-deficient rats (P<0·05 in both cases); this finding could be related to the alteration in the geometry of the carotid artery. In conclusion, these findings suggest that Mg deficiency modifies the mechanical properties of the common carotid artery in young rats. Since Mg deficiency is considered a risk factor, these mechanical alterations could contribute to the development of atherosclerosis, hypertension and cardiovascular diseases.

Altered structure and distensibility of arteries in salt-fed rats

BACKGROUND AND OBJECTIVES

Habitual high-sodium diet may cause stiffening of arteries. The aim of this study was to investigate the long-term effects of physiologically relevant high-sodium diet on the structure and distensibility of arteries in rats.

METHODS

Adult male Sprague-Dawley rats were fed 2% NaCl diet for 3 or 6 months; rats fed 0.7% NaCl diet were controls. Pressure-volume (distensibility) relationships were measured in the presence and absence of calcium in excised, in-vitro perfused segments of right carotid artery and of second order mesenteric arteries. The left carotid artery and the remaining mesenteric arteries of rats were perfused in situ with papaverine followed by fixative at 100 mmHg, and then embedded in epoxy for morphometric measurements.

RESULTS

The tail systolic blood pressure (SBP), and in subgroups of rats, the directly measured mean arterial pressure (MAP), of salt-fed rats at 3 and 6 months were unchanged. At 3 months, there was dilatation (increased lumen area) of both carotid and mesenteric arteries of salt-fed rats, without a change in distensibility. At 6 months, the lumen area of carotid arteries of salt-fed rats returned to control value (inward remodeling), and carotid artery distensibility remained unchanged. At 6 months, there was further dilatation (P <0.01) and reduced distensibility (P =0.01) of mesenteric arteries in salt-fed rats.

CONCLUSIONS

A three-fold increase in dietary sodium intake leads to dilatation of arteries in normotensive rats. When there is compensatory remodeling, the distensibility of arteries remains unchanged; when compensation is lacking, unopposed dilatation is associated with reduced distensibility.

Radial artery compliance in response to mental stress in normotensive offspring of hypertensive parents

OBJECTIVES

Compared to normal subjects hypertensive patients have an increased radial artery isobaric distensibility, contrasting with a decrease in elasticity of large arteries and systemic compliance. To address the question whether elasticity is increased in response to long-standing elevated blood pressure or is present at an early stage of the disease, we compared normotensive offspring of hypertensive parents with control subjects. Furthermore, enhanced sympathetic response to mental stress was demonstrated in individuals predisposed to hypertension and might contribute to the elevation of blood pressure via a peripheral mechanism. Thus, an abnormal vasoconstrictive response of the radial artery to psychological stress was sought in these subjects.

DESIGN

The geometry and the elastic porperties of the radial artery were assessed in normotensive offspring of hypertensive and normotensiven parents at baseline and during mental stress.

METHODS

A high-precision echo-tracking ultrasound device was combined with photoplethysmography for continuous measurement of radial artery diameter and isobaric distensibility in 18 normotensive offspring of parents with essential hypertension and 18 control subjects under resting conditions and during a 3-minute mental stress test.

RESULTS

Baseline arterial distensibility and compliance were comparable in offspring of hypertensive and normotensive parents. During mental stress, blood pressure and heart rate increased similarly in both groups. Adrenergic activation did not alter the elastic properties of the radial artery in the individuals with a genetic predisposition to essential hypertension.

CONCLUSIONS

There was no alteration in elastic properties of the radial artery in normotensiven individuals at genetic risk to develop arterial hypertension. Furthermore, mental stress did not abnormally increase the vascular tone of this medium-sized muscular artery in these subjects as compared to controls. This indicates that functional and/or structural vascular alterations do not precede a distinct rise in blood pressure or abnormal blood pressure reactivity in subjects prone to develop essential hypertension.

Small artery remodeling is the most prevalent (earliest?) form of target organ damage in mild essential hypertension

BACKGROUND

The heart and blood vessels are exposed to elevated blood pressure (BP) in hypertensive patients, but their changes in response to BP or non-hemodynamic stimuli may be different, and occur with different time-courses. To evaluate this, we studied the prevalence of structural and functional alterations of resistance arteries and cardiac hypertrophy in patients with mild essential hypertension.

METHODS

Resistance arteries were dissected from gluteal subcutaneous tissue from 38 hypertensive patients (47 +/- 1 years; 71% male; BP 148 +/- 2/99 +/- mmHg), studied on a pressurized myograph, and compared to those from 10 normotensives (44 +/- 3 years; 40% male; BP 113 +/- 4/76 +/- 2 mmHg).

RESULTS

The prevalence of abnormal structure (media-to-lumen ratio, M/L) and impaired endothelial function (maximal acetylcholine response) was 97 and 58% (abnormal was defined as greater than mean + 1 SD of normotensives), or 63 and 34% (abnormal defined as greater than mean +/- 2SD). Thirty four percent of hypertensive patients exhibited left ventricular hypertrophy by echocardiography. When grouped into tertiles according to increasing ambulatory systolic BP (SBP), the highest BP tertile showed increased M/L (P< 0.01) and left ventricular mass index (LVMI, P < 0.05) and marginally decreased endothelial function (P= 0.07). LVMI was greatest in the tertile of patients with highest M/L (P< 0.05). Endothelial function was decreased in the tertile with greatest vascular stiffness (P< 0.01). By multivariate analysis, M/L correlated with ambulatory SBP (beta = 0.40, P= 0.02), and LVMI correlated with ambulatory SBP (beta = 0.41, P = 0.001) and body mass index (beta = 0.30, P< 0.05). Female sex influenced endothelial function negatively (beta = -0.63, P< 0.01).

CONCLUSION

Structural alterations of resistance arteries were demonstrated in most hypertensive patients, followed by endothelial dysfunction and cardiac hypertrophy in a smaller number of hypertensives. Small artery structural remodeling may precede most clinically relevant manifestations of target organ damage in mild essential hypertension.

Effects of antihypertensive therapy on hypertensive vascular disease

Hypertension is associated with alterations in the structure, function, and mechanical properties of large and small arteries. Changes in the endothelium, smooth muscle cell, extracellular matrix, and possibly the adventitia, contribute to complications of hypertension. In large arteries, vascular hypertrophy is found, often with increased stiffness of media components. In small arteries, particularly in mild hypertension, rearrangement of smooth muscle cells around a smaller lumen without changes in media volume (eutrophic remodeling) occurs; in more severe hypertension, hypertrophic remodeling with increased vascular stiffness can be found. Vascular remodeling is accompanied by an increase in the extracellular matrix, particularly collagen deposition. Recent studies have demonstrated that vascular remodeling and endothelial dysfunction of small and large vessels may be normalized by treatment with some antihypertensive agents (angiotensin converting enzyme inhibitors, angiotensin AT(1) receptor antagonists, and long-acting calcium channel blockers). Angiotensin converting enzyme inhibitors have now been shown to improve outcomes in hypertensive patients, an effect that may in part be related to the vascular protective effects reviewed here.

Chronic hypertension alters the expression of Cx43 in cardiovascular muscle cells

Connexin43 (Cx43), the predominant gap junction protein of muscle cells in vessels and heart, is involved in the control of cell-to-cell communication and is thought to modulate the contractility of the vascular wall and the electrical coupling of cardiac myocytes. We have investigated the effects of arterial hypertension on the expression of Cx43 in aorta and heart in three different models of experimental hypertension. Rats were made hypertensive either by clipping one renal artery (two kidney, one-clip renal (2K,1C) model) by administration of deoxycorticosterone and salt (DOCA-salt model) or by inhibiting nitric oxide synthase with N G-nitro-L-arginine methyl ester (L-NAME model). After 4 weeks, rats of the three models showed a similar increase in intra-arterial mean blood pressure and in the thickness of the walls of both aorta and heart. Analysis of heart mRNA demonstrated no change in Cx43 expression in the three models compared to their respective controls. The same 2K,1C and DOCA-salt hypertensive animals expressed twice more Cx43 in aorta, and the 2K, 1C rats showed an increase in arterial distensibility. In contrast, the aortae of L-NAME hypertensive rats were characterized by a 50% decrease in Cx43 and the carotid arteries did not show increased distensibility. Western blot analysis indicated that Cx43 was more phosphorylated in the aortae of 2K,1C rats than in those of L-NAME or control rats, indicating a differential regulation of aortic Cx43 in different models of hypertension. The data suggest that localized mechanical forces induced by hypertension affect Cx43 expression and that the cell-to-cell communication mediated by Cx43 channels may contribute to regulating the elasticity of the vascular wall.

Reduction of resistance artery stiffness by treatment with the AT(1)-receptor antagonist losartan in essential hypertension

In spontaneously hypertensive rats resistance artery structure, endothelial dysfunction and geometry-independent wall stiffness were reduced by an angiotensin AT(1)-receptor antagonist. In previous studies of human hypertension, interruption of the renin-angiotensin system corrected small artery structure and endothelial dysfunction, whereas the beta-blocker atenolol did not. We hypothesized that the AT(1)R antagonist losartan, but not the beta-blocker atenolol, would reduce stiffness of gluteal subcutaneous small arteries in essential hypertensive patients. Seventeen untreated mild essential hypertensive patients (47+/-2 years; 75% male) were randomly assigned in double-blind fashion to losartan or atenolol treatment for one year. Small, resistance size arteries were studied on pressurized myographs. Blood pressure (mmHg) was reduced (p<0.01) from 145 +/- 4/101 +/- 2 and 147 +/- 6/98 +/- 2 to 128 +/- 4/86 +/- 2 and 131 +/- 3/84 +/- 1 by losartan and atenolol, respectively. The media/lumen ratio of small arteries was unaffected by atenolol (8.3+/-0.3% before and 8.8+/-0.5% after treatment). In contrast, losartan reduced media/lumen ratio from 8.4+/-0.4% to 6.7+/-0.3% (p<0.01). Whereas isobaric elastic modulus was unaffected by either treatment, geometry-independent stiffness (slope of elastic modulus vs. stress) was reduced from 9.7+/-1.2 to 6.1+/-0.9 (P<0.05) under losartan treatment, but was unchanged by atenolol (8.2+/-1.3 to 7.8+/-0.6). In conclusion, treatment with losartan reduced stiffness and structural alterations of subcutaneous resistance arteries of previously untreated essential hypertensive patients, whereas atenolol failed to do so.

Pressure but not angiotensin II-induced increases in wall mass or tone influences static and dynamic aortic mechanics

OBJECTIVE

To distinguish between static (due to slow changes in pressure) and dynamic (due to pressure pulsatility) components of aortic compliance over a large pressure range in vivo and to examine the effects of increased vascular mass and smooth muscle tone on these components.

METHODS

Using ultrasound wall tracking, aortic lumen area-pressure curves were generated in anaesthetized rats over a broad range of pressures by altering blood volume. The compliance coefficient calculated at each mean pressure was considered the dynamic compliance at that pressure; the slope of the diastolic lumen area-pressure curve represents static compliance. Experiments were performed in control rats and rats treated with angiotensin II (ANG II) acutely (500 ng/kg per min intravenously) to modify vascular tone or chronically (250 ng/kg per min subcutaneously for 2 weeks) to modify vascular mass.

RESULTS

The dynamic compliance-pressure curve approximated a parabola. Maximal dynamic compliance (0.272+/-0.026 mm2/kPa in control rats) was achieved at near-normotensive pressure (+/-105 mm Hg). The diastolic lumen area-pressure curve showed an exponential relationship within a physiological range (30-130 mm Hg). ANG II-induced increases in aortic wall mass or smooth muscle tone did not modify the relationship between static or dynamic compliance and pressure.

CONCLUSIONS

These findings demonstrate that static and dynamic mechanics of the rat thoracic aorta depend differently on blood pressure. Static compliance increases slightly with pressure in a physiological range, while dynamic compliance is auto-regulated around normotensive pressures. Neither static nor dynamic compliance of the rat thoracic aorta are influenced by ANG II-induced increases in aortic wall mass or smooth muscle tone.

Age-related changes of the mechanical properties of the carotid artery in spontaneously hypertensive rats

BACKGROUND

We had previously demonstrated that the distensibility of the carotid artery in spontaneously hypertensive rats (SHR) aged 18 weeks does not differ from that of the carotid artery in normotensive animals for common pressure levels, despite vascular hypertrophy in SHR.

OBJECTIVE

To examine the time-course effects of hypertension on the geometry and the mechanical properties of the carotid artery in SHR.

METHODS

The mechanical behavior of the carotid arteries of anesthetized SHR, stroke-prone SHR (SHRSP), and Wistar-Kyoto (WKY) rats aged 4, 8, 12, 16, and 32 weeks was examined by simultaneously measuring the internal diameter with an A-mode ultrasonic echo-tracking device and the intra-arterial pressure with a computerized data-acquisition system. Histometric measurements of the carotid artery were performed after death of rats.

RESULTS

Blood pressure increased with time in rats of the two genetic hypertensive models. However, it rose earlier and to higher levels in the SHRSP. Cardiac hypertrophy was comparable in the two hypertensive groups whereas vascular hypertrophy was less pronounced in the SHRSP than it was in the SHR. There was an age-related decrease in arterial distensibility in rats of all groups that was more pronounced in the SHRSP than it was in the SHR compared with that in WKY rats (decreases of 57 and 36%, respectively, versus WKY rats aged 32 weeks; P < 0.05). For rats of all ages studied, although aging affected differently the vascular properties of the distinct animal strains, arterial distensibility was increased in the SHR and SHRSP compared with that in control animals for similar blood pressure levels, implying a rightward shift of the distensibility-pressure curves in the two hypertensive models. However, there was a significant reduction in arterial distensibility in rats of the two hypertensive strains at their respective mean blood pressures, compared with that in control animals.

Differential alterations of common carotid and femoral artery distensibility in 12-week-old spontaneously hypertensive rats

OBJECTIVE

In essential hypertension, the mechanical properties of the radial artery have been shown to be largely unaltered, whereas more controversial and less reliable data have been obtained for the common carotid artery. We therefore examined the distensibility/pressure relationships of the predominantly elastic common carotid artery and of the predominantly muscle-type femoral artery in 12-week-old normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

METHODS

Eleven 12-week-old SHR and 10 age-matched WKY rats were anesthetized with sodium pentobarbitone. Blood pressure and pulse rate were measured by catheters inserted in the common carotid and in the femoral arteries, while contralateral arterial diameter was continuously recorded via an echo-tracking device. Arterial compliance was derived according to the Langewouters formula, and its values were normalized for the diameter, to obtain distensibility/pressure curves and to calculate the distensibility index. The Peterson elastic modulus was also calculated in order to obtain a pressure-independent estimate of arterial mechanical properties.

RESULTS

Femoral artery distensibility/pressure curves and distensibility index were similar in the two groups of rats, the latter being 1.13+/-0.13 mm/mmHg10(-3) in SHR and 1.28+/-0.15 mm/mmHg10(-3) in WKY rats (means+/-SEM; NS). In contrast, in SHR, common carotid artery mechanical properties were clearly impaired, as shown by a marked reduction in distensibility index (2.55+/-0.16 mm/mmHg10(-3) in SHR versus 3.4+/-0.3 mm/mmHg10(-3) in WKY rats; P< 0.05), and by a significant increase in the Peterson elastic modulus.

CONCLUSIONS

In the SHR model, high blood pressure alters the mechanics of large arteries even in the relatively early stage of the disease; however, the alterations are not homogeneous inasmuch elastic-type vessels are affected to a much greater extent than muscle-type vessels.