- Email: [email protected]
Structural remodeling of resistance arteries in uremic hypertension
Kidney International, Vol. 65 (2004), pp. 1818–1825
Structural remodeling of resistance arteries in uremic hypertension DAVID I. NEW, ALISTAIR M. CHESSER, RAJ C. THURAISINGHAM, and MAGDI M. YAQOOB Anthony Raine Research Laboratories, St Bartholomew’s Hospital, London, United Kingdom
Structural remodeling of resistance arteries in uremic hypertension. Background. Structural remodeling of the resistance vasculature is present in many forms of human and experimental hypertension. In particular, an increase in the ratio of wall thickness to lumen diameter develops, and might in itself maintain hypertension by increasing vascular resistance. Because uremia is associated with raised peripheral resistance, hypertension, and histologic changes suggestive of vascular remodeling, we sought to formally examine the structural and mechanical (elastic) properties of isolated pressurized resistance arteries in uremic hypertension. Methods. Cremaster, cerebral and mesenteric arteries from subtotally nephrectomised Wistar-Kyoto rats, normotensive control Wistar-Kyoto rats, and spontaneously hypertensive rats were mounted on a pressure myograph and relaxed in calciumfree buffer. Wall thickness and lumen diameter were measured at increasing lumen pressures from 10 to 200 mm Hg, and from this wall:lumen ratio, wall cross-sectional area, and an index of elasticity were derived. Results. In uremic hypertensive animals increased wall:lumen ratio and decreased lumen diameter was seen in cremaster and mesenteric arteries, although no significant changes were observed in cerebral arteries, compared to normotensive controls. In spontaneously hypertensive animals increased wall thickness and wall:lumen ratio was seen in cerebral and mesenteric arteries, decreased lumen diameter in cremaster and mesenteric arteries, and increased wall cross-sectional area in cerebral arteries, compared to normotensive controls. Elasticity of the arterial wall in uremic and spontaneously hypertensive animals did not differ from normotensive controls. Conclusion. Cremaster and mesenteric resistance arteries undergo predominantly eutrophic inward remodeling in uremic hypertension, broadly similar to that seen in spontaneous hypertension.
Changes in the structural properties of resistance vessels have been observed in various forms of human
Key words: uremia, hypertension, remodeling, elasticity, resistance artery. Received for publication January 19, 2003 and in revised form July 28, 2003, and October 18, 2003 Accepted for publication December 16, 2003 C
2004 by the International Society of Nephrology
[1–3] and experimental [4–6] hypertension. Structural (dimensional) changes manifest as alterations in wall thickness and lumen diameter, and therefore in wall:lumen ratio; remodeling is the term used to describe this process. Eutrophic inward remodeling implies reorganization of existing vascular wall material around a reduced lumen, such that wall cross-sectional area is not altered (no vascular growth); hypertrophic inward remodeling indicates lumen reduction due to increased wall cross-sectional area (vascular growth). These processes are depicted schematically in Figure 1. In general, hypertension is associated with either eutrophic or hypertrophic inward remodeling; eutrophic remodeling being seen, for example, in essential hypertension [1, 7, 8], and hypertrophic remodeling in renovascular hypertension [9]. Both forms frequently coexist in different vascular beds of the same individual, such as in the spontaneously hypertensive rat [5, 10, 11]. Arterial diameter is affected by lumen pressure (or more strictly the transmural pressure) and smooth muscle activation. Comparisons of arterial structure are commonly performed at the standard pressure of 100 mm Hg [5], with active smooth muscle tension abolished by calcium-free buffer. Under such circumstances, an increase in wall:lumen ratio could be due to structural remodeling [12] or to decreased arterial elasticity [4]. Hence, before attributing changes in arterial dimensions to structural remodeling it is necessary to exclude a difference in elasticity [1, 12]. It follows that full assessment of vascular structure requires study under increasing degrees of distention in order to measure both structural and elastic parameters, and visualization of isolated arteries cannulated and pressurized on a myograph allows such measurement. If inward remodeling or decreased elasticity of an artery is present, then the resistance to flow through that artery will be increased since, according to the Poiseuille equation, resistance is proportional to the fourth power of diameter. This might be expected to manifest as raised peripheral vascular resistance causing hypertension, and as impaired vasodilator reserve limiting the response to exercise or ischemia. Because uremia is associated with 1818
New et al: Resistance arteries in uremic hypertension
B
A D
WT C Fig. 1. Diagramatic representation of remodeling. (A) Normal artery, with lumen diameter (D) and wall thickness (WT) labeled. (B) Hypertrophic inward remodeling has occurred; the wall cross-sectional area (shaded) has increased and wall:lumen ratio is elevated, due to addition of material to the inner surface of the arterial wall. (C) Eutrophic inward remodeling has occurred; wall cross-sectional area is unchanged but the wall:lumen ratio is increased, due to the re-organization of existing wall material around a narrower lumen (not drawn to scale).
hypertension and raised vascular resistance [13], decreased forearm post-ischemic hyperemia [14], and histologic evidence suggestive of vascular remodeling [15], we sought to establish the structural and elastic properties of isolated pressurized resistance arteries in uremic hypertension. The subtotal nephrectomy model of uremic hypertension was employed using Wistar-Kyoto (WKY) rats, and compared with nonuremic normotensive controls and nonuremic spontaneously hypertensive animals. The purpose of including nonuremic hypertensive animals (spontaneously hypertensive) in this study was to attempt to control for the effect of hypertension in uremic animals [16, 17], since vascular changes may occur in response to either uremia or to hypertension or both. We studied cremaster and mesenteric arteries because the skeletal muscle and splanchnic circulations control a substantial proportion of the peripheral resistance [18], and cerebral arteries because hypertensive vascular end-organ damage is prevalent in uremia [19]. METHODS Animals All procedures had the prior approval of the Home Office (Project License number 70/5014), and were performed in accordance with the Animals Scientific Procedures Act, 1986. Twelve to 14-week-old male WKY and spontaneously hypertensive rats were obtained from Harlan Ltd. (Bicester, Oxfordshire, UK). General anesthesia was induced for both stages of the subtotal
1819
nephrectomy procedure. This was achieved by intramuscular injection of HypnormTM 1 mL/kg body weight (comprising fentanyl citrate 0.315 mg/mL plus fluanisone 10 mg/mL) (Janssen-Cilag Ltd., Buckinghamshire, UK) and intraperitoneal injection of diazepam 2.5 mg/kg (CP Pharmaceuticals Ltd., Wrexham, UK). Postoperative analgesia was provided by subcutaneous injection of buprenorphine 50 lg/kg (Schering-Plough Ltd., Welwyn Garden City, Hertfordshire, UK). Left subtotal nephrectomy through a midline abdominal incision removed approximately three-fourths of that kidney, with total right nephrectomy via a flank incision 1 week later. Control WKY and spontaneously hypertensive animals underwent sham surgery in which the appropriate kidney was stripped of its capsule only. Animals were maintained for an average 15 to 16 weeks during which time WKY contol and spontaneously hypertensive rats were pair fed with their uremic WKY partner. Then, under halothane general anesthesia, an abdominal aortic catheter was inserted retrogradely through the femoral artery, heparinized, and exteriorized through the skin at the nape of the neck [20]. Two to three days later, with the rat was conscious and free to move in its cage, mean arterial pressure was measured by attaching the catheter to a pressure transducer (model number 60–3003) Harvard Apparatus, Cambridge, MA, USA). Blood pressure was recorded for at least 20 minutes until a stable value was obtained. Blood was drawn for analysis, and the animal killed by cervical dislocation. Cremaster muscle, brain, and mesentery were placed in ice-cold physiologic salt solution (PSS). The 1A branch of the cremaster [21], the middle cerebral artery at its most lateral position over the hemisphere, and the first order branch of the mesenteric artery [22] were studied. Drugs and solutions PSS had the following composition (mmol/L): 119 NaCl, 4.7 KCl, 25 NaHCO 3 , 1.17 KH 2 PO 4 , 1.17 MgSO 4 , 2.0 CaCl 2 , and 5.5 glucose. Calcium-free PSS contained ethyleneglycol–bis (b-aminoethylether)-N, N tetraacetic acid (EGTA) 1 mmol/L and sodium nitroprusside 100 lmol/L. Perfusion myograph The myograph method has been described [4, 20]. Briefly, an artery was mounted between the two glass microcannulas of a myograph (Living Systems Instrumentation, Burlington, VT, USA) and viewed through a microscope. Calcium-free PSS was gassed (95% O 2 /5% CO 2 ) and heated in a reservoir, and circulated at a rate of 40 mL/min to the vessel chamber. Temperature was maintained at 33◦ C for cremaster (its temperature in vivo) and 37◦ C for cerebral and mesenteric arteries. Active smooth muscle tension was lost during equilibration in
1820
New et al: Resistance arteries in uremic hypertension
calcium-free PSS. Passive lumen diameter and wall thickness were then measured at lumen pressures from 10 to 200 mm Hg using a video dimension analyser. Because the vessel was immersed in less than 1 cm of PSS within the chamber, lumen pressure was negligibly different from transmural pressure. Measurements performed with this myograph system are accurate to 1 lm. All measurements were performed by the same investigator, and intraobserver variation was <1 lm. Calculations The following calculations were used: Creatinine clearance(mL/min) = UCr · V/PCr · t
Table 1. General data
Animal data Time of uremia/sham uremia weeks Mean arterial pressure mmHg Blood hemoglobin g/dL Plasma creatinine lmol/L Creatinine clearance mL/min Weight g
2
Wall cross-sectional area (CSA)(lm ) =1/4 .p.{(D + 2.WT)2 − D2 } where D is lumen diameter (lm) and WT is wall thickness (lm) at a given pressure. Wall:lumen ratio R (%) = 100.WT/D. Circumferential stress r (N/m2 ) = P.D/2.WT. (P is converted to N/m2 by the factor 1 mm Hg = 133.4 N/m2 ). Circumferential strain e = (D – D o )/D o , where D o is the “original” diameter at 10 mm Hg (it is difficult to accurately determine internal diameter of resistance arteries at pressures lower than this [23]). The data for each individual artery can be fitted to a curve r = r o .eb·e using least-squares analysis, where r o is the circumferential stress at 10 mm Hg, and b is a constant. Furthermore, the elastic characteristic of an artery can be described by the tangential elastic modulus of its wall material, E, where E = dr/de [4]. It follows that dr/de = b.r o .eb·e = b.r. The constant b therefore describes the linear slope of the tangential elastic modulus versus stress, and is used as an index of elasticity [4, 24]. The higher the value of b, the lower the elasticity of the wall material. Structural changes in small arteries are frequently due to the dual processes of remodeling and growth (hypertrophy) [3], although one process frequently dominates. The relative contribution of each can be assessed by inspection of the calculated “remodeling index” and “growth index.” Here, remodeling index is defined as the percentage of the observed difference in the internal diameter of hypertensive (uremic or spontaneous) and normotensive arteries that could be accounted for by remodeling of the normotensive vessel. If the remodeling index is not equal to 100%, then there must have been growth, and growth index is defined as the percent increase in wall cross-sectional area of the hypertensive (uremic or spontaneous) relative to normotensive arteries. The remodeling and growth indices were calculated by standard methods [3], which are not elaborated here, when a statistically significant difference in one of the ar-
WistarKyoto controls (N = 33)
Spontaneously hypertensive rats (N = 28)
15.1 (0.4)
15.2 (0.5)
15.1 (0.5)
166 (5)a,b
139 (2)
194 (3)b,c
7.9 (0.3)a,b
13.3 (0.3)
14.1 (0.4)
249 (25)a,b
45 (3)
47 (3)
0.41 (0.09)a,b
2.98 (0.43)
2.61 (0.32)
303 (6)a,b
360 (3)
384 (5)
P < 0.001 compared to Wistar-Kyoto controls; P < 0.001 compared to spontaneously hypertensive rats; c P < 0.001 compared to uremic Wistar-Kyoto. a
where U Cr is urinary creatinine and P Cr is plasma creatinine concentration (lmol/L), and V is the volume of urine (mL) collected in time t (minutes).
Uremic WistarKyoto (N = 23)
b
terial structural (dimensional) parameters was observed between experimental groups. Statistical analyses Values are represented as mean (standard error). In all comparisons, there were three separate groups; WKY uremic rats, WKY controls, and spontaneously hypertensive rats. Therefore one-way analysis of variance (ANOVA) was chosen for comparison of means. The Bonferroni correction was applied to allow comparisons between each combination of two groups. The primary comparisons were between uremic WKY and WKY controls, and between spontaneously hypertensive rats and WKY controls, although, when appropriate, reference is made to comparison between uremic WKY and spontaneously hypertensive rats. Although comparisons were made at the standard lumen pressure of 100 mm Hg [5], the data for diameter and wall thickness are also plotted across the full pressure range to demonstrate the consistency of the changes. In all analyses the significance level was taken as P < 0.05. RESULTS General animal data Animal groups had very similar periods of uremia/sham uremia (Table 1). Mean arterial pressure was raised in both uremic WKY and spontaneously hypertensive rats compared to WKY controls, and spontaneously hypertensive rat pressure was also significantly higher than uremic WKY. Uremic WKY animals were anemic, with substantially raised plasma creatinine, decreased creatinine clearance, and lower body mass compared to both WKY controls and spontaneously hypertensive rats. Cremaster arteries Cremaster arteries from uremic WKY demonstrated significantly decreased lumen diameter and increased
New et al: Resistance arteries in uremic hypertension
Table 2. Characteristics of branch 1A cremaster arteries at 100 mm Hg lumen pressure
Cremaster artery Wall thickness lm Lumen diameter lm Wall to lumen ratio % Wall cross-sectional area lm2 b value Remodeling index % Growth index %
Uremic WistarKyoto (N = 19)
WistarKyoto controls (N = 20)
Spontaneously hypertensive rats (N = 14)
15.4 (1.9) 149 (3)a 10.6 (1.4)c 8020 (1064)
11.1 (0.9) 168 (3) 6.6 (0.5) 6260 (552)
13.4 (1.2) 151 (5)b 9.0 (0.9) 6926 (739)
8.2 (0.6) 62 28
6.7 (0.6) — —
5.7 (0.4) 85 11
1821
WKY was not significantly different to WKY controls. Arteries from spontaneously hypertensive rats showed increase in wall thickness, reduced lumen diameter, and increased wall:lumen ratio compared to WKY controls. Wall cross-sectional area was not significantly increased in spontaneously hypertensive rats compared to WKY controls. The remodeling index was 84% while the growth index was 23%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that uremic WKY and spontaneously hypertensive mesenteric arteries undergo predominantly eutrophic inward remodeling.
a < P 0.005 compared to Wistar-Kyoto controls; b P < 0.01 compared to WistarKyoto controls; c P < 0.02 compared to Wistar-Kyoto controls.
wall:lumen ratio, with no significant difference in wall cross-sectional area compared to WKY controls (Table 2) (Figs. 2 and 3). The remodeling index was 62% whereas the growth index was 28%. The elasticity (b value) for uremic WKY was not significantly different to WKY controls. Arteries from spontaneously hypertensive rats showed reduced lumen diameter, with no significant difference in wall cross-sectional area compared to WKY controls. The remodeling index was 85% and the growth index 11%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that the changes in uremic WKY and spontaneously hypertensive rat cremaster arteries are predominantly due to eutrophic inward remodeling. Cerebral arteries Cerebral arteries from uremic WKY showed no significant structural changes compared to WKY controls (Table 3) (Figs. 2 and 3). The elasticity (b value) for uremic WKY did not differ from WKY controls. Arteries from spontaneously hypertensive rats showed increase in wall thickness, wall:lumen ratio, and wall cross-sectional area compared to WKY controls. The growth index was 46% and the remodeling index −3%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that uremic WKY cerebral arteries undergo no significant remodeling, while spontaneously hypertensive rat cerebral arteries display hypertrophic remodeling. Mesenteric arteries Mesenteric arteries from uremic WKY demonstrated significantly decreased lumen diameter and increased wall:lumen ratio compared to WKY controls (Table 4) (Figs. 2 and 3). Wall cross-sectional area was not significantly different in uremic WKY compared to WKY controls. The remodeling index was 95% compared to the growth index of 11%. The elasticity (b value) for uremic
DISCUSSION The important finding of this study is that cremaster and mesenteric arteries from uremic hypertensive rats display predominantly eutrophic inward remodeling with no change in elasticity, when compared to vessels from normotensive control animals. No significant changes were seen in cerebral arteries from uremic hypertensive animals. In a classic autopsy study [25], renal small arteries from patients with Bright’s (chronic renal) disease were shown to have increased wall thickness relative to lumen. This finding was later extended to other vascular beds [26]. These observations could not, however, distinguish between structural change and greater vascular contraction by arteries from uremic subjects during the fixation process. Recent thorough histologic studies using procaine to relax vascular smooth muscle prior to fixation at a controlled lumen pressure show that myocardial arterioles in uremia display wall thickening and increased wall:lumen ratio [15]. Histologic assessment, however, cannot distinguish between remodeling and change in arterial elasticity. To our knowledge, this paper provides the first systematic assessment of the structural and elastic properties of isolated pressurized resistance arteries in uremic hypertension; the findings support previous histological studies, and confirm that the basis is predominantly structural remodeling rather than change in elasticity. Several factors might cause structural cardiovascular changes in uremia. These include uremic hyperparathyroidism [27], abnormal activity of the renin-angiotensin system [15], and accumulation of endothelin [28]. However, increased wall:lumen ratio is a common finding in many forms of hypertension [2] and it is conceivable that structural (dimensional) changes are simply a consequence of raised arterial pressure [29]. The structural changes reported here in uremic hypertensive cremaster and mesenteric arteries were comparable in degree to those from spontaneously hypertensive animals even though the magnitude, and probably also the duration [30], of hypertension were substantially higher in the latter. Since there is histologic evidence of myocardial
1822
New et al: Resistance arteries in uremic hypertension
Cremaster
180
260
440
Mesenteric
400
240
160 Diameter, µm
Cerebral
360 140
220 320 200
120
280 180
100
240
160
80 0
100
200
Cremaster
24
200 100 0 200 0 Lumen pressure, mm Hg
24
Cerebral
34
100
200
Fig. 2. Artery passive internal diameter plotted against lumen pressure. Uremic WistarKyoto (WKY) (), WKY control (), and spontaneously hypertensive rats (). At 100 mm Hg lumen pressure, cremaster arteries from both uremic WKY and spontaneously hypertensive rats demonstrated significantly decreased lumen diameter compared to WKY controls (P < 0.005 and P < 0.01, respectively). Cerebral arteries from neither uremic WKY nor spontaneously hypertensive rats showed significant change in lumen diameter compared to WKY controls. Mesenteric arteries from both uremic WKY and spontaneously hypertensive rats demonstrated significantly decreased lumen diameter compared to WKY controls (P < 0.005 and P < 0.02, respectively).
Mesenteric
30 Wall thickness, µm
20
20 26
16
16
12
12
22
18 14
8
10 0 200 0 100 Lumen pressure, mm Hg 8
0
100
200
arteriolar wall thickening in uremia even in the absence of hypertension [27], it is possible to contemplate that uremia and hypertension act synergistically to effect vascular changes. Alternatively, because in uremia the distensibility of large (conduit) arteries is decreased [31], it is possible that the distal arteries are exposed to a greater proportion of the pulse pressure that speculatively might enhance the small artery structural changes. A likely functional consequence of arterial remodeling reported here in uremic hypertension is raised peripheral resistance, because geometrical reasoning dictates that a smaller lumen diameter would follow from any given degree of smooth muscle shortening [32]. The result would be increased vascular resistance, which is known to be
100
200
Fig. 3. Wall thickness plotted against lumen pressure. Uremic Wistar-Kyoto (WKY) (), WKY controls (), and spontaneously hypertensive rats (). At 100 mm Hg lumen pressure, cremaster arteries from neither uremic WKY nor spontaneously hypertensive rats showed significant increase in wall thickness compared to WKY controls. Cerebral arteries from spontaneously hypertensive rats but not uremic WKY showed an increase in wall thickness compared to WKY controls (P < 0.001 for spontaneously hypertensive rats). Mesenteric arteries from spontaneously hypertensive rats but not uremic WKY demonstrated a significant increase in wall thickness compared to WKY controls (P < 0.01 for spontaneously hypertensive rats).
present in uremic [13] and essential [33] hypertension. Although the matter is debated [34, 35], this so-called “amplifier hypothesis” provides a possible means by which vascular resistance is increased without requiring any increase in vascular smooth muscle tension generation. It is therefore an attractive hypothesis, since most studies find normal vascular contractility in many types of hypertension [24]. Additionally, inward remodeling would also be expected to limit the maximum attainable arterial diameter [29]. This may explain why postischemic forearm blood flow is decreased in uremia [14] and in essential hypertension [36], and why cardiac ischemia may occur in the absence of discrete coronary stenosis in uremic [37] as well as nonuremic hypertension [38]. Nevertheless,
1823
New et al: Resistance arteries in uremic hypertension
Fig. 4. Wall stress plotted against lumen pressure. Uremic Wistar-Kyoto (WKY) (), control WKY (), and spontaneously hypertensive rats (). Table 3. Characteristics of middle cerebral arteries at 100 mm Hg lumen pressure
Cerebral artery
Uremic WistarKyoto (N = 17)
WistarKyoto controls (N = 16)
Spontaneously hypertensive rats (N = 14)
Wall thickness lm 11.6 (0.7) 10.2 (0.5) 15.0 (1.1)a Lumen diameter lm 234 (5) 241 (7) 232 (6) Wall to lumen ratio % 4.98 (0.31) 4.24 (0.18) 6.44 (0.41)a Wall cross-sectional area lm2 8896 (588) 8161 (544) 11884 (1094)b b value 12.0 (0.7) 9.9 (0.5) 9.5 (0.6) Remodeling indexc % NA NA −3 Growth indexc % NA NA 46 NA, not applicable. P < 0.001 compared to Wistar-Kyoto controls; b P < 0.005 compared to WistarKyoto controls; c Remodeling index and growth index not calculated as no significant structural (dimensional) differences were present compared to Wistar-Kyoto controls. a
factors other than remodeling, such as microvascular rarefaction [39, 40], might also cause or contribute to the observed hemodynamic changes. Finally, the changes seen in both uremic and spontaneous hypertension serve to decrease the passive wall stress of arteries at any given lumen pressure or, when blood pressure is high, they tend to normalize wall stress. The relation of pressure to wall stress (which is itself derived from the parameters pressure, diameter, and wall thickness) for arteries in each group is shown by Figure 4. This might represent an adaptation to preserve microvascular integrity under conditions of raised arterial pressure. In this respect the absence of significant remodeling of the cerebral artery in uremic hypertension, together with impairment in contractile response to lumen distension [41, 42], may contribute to the association of uremia with hemorrhagic stroke in humans [19] and animals [42]. We report that spontaneously hypertensive cremaster and mesenteric arteries undergo eutrophic inward re-
Table 4. Characteristics of first order mesenteric arteries at 100 mm Hg lumen pressure
Mesenteric artery Wall thickness lm Lumen diameter lm Wall to lumen ratio % Wall cross-sectional area lm2 b value Remodeling index % Growth index %
Uremic WistarKyoto (N = 8)
WistarKyoto controls (N = 12)
Spontaneously hypertensive rats (N = 12)
20.7 (2.0) 342 (12)b 6.0 (0.4)d 24034 (3138)
15.9 (0.9) 411 (16) 3.9 (0.2) 21640 (1810)
22.2 (1.4)a 359 (9)c 6.2 (0.4)e 26600 (1899)
6.1 (0.6) 95 11
5.2 (0.4) — —
5.8 (0.3) 84 23
a P < 0.01 compared to Wistar-Kyoto controls; b P < 0.005 compared to Wistar-Kyoto controls; c P < 0.02 compared to Wistar-Kyoto controls; d P = 0.001 compared to Wistar-Kyoto controls; e P < 0.001 compared to Wistar-Kyoto controls.
modeling consistent with previous studies [10, 43, 44]. We demonstrated that spontaneously hypertensive cerebral vessels display hypertrophic remodeling, also conforming with earlier reports [5, 11]. Cerebral artery hypertrophy appears principally to be a response to elevated blood pressure [6, 45, 46], and other models of hypertension in which blood pressure is not raised to the same extent as in the spontaneously hypertensive rats do not display cerebral hypertrophy [4, 47]. This might explain why we found no hypertrophy of uremic WKY cerebral arteries. Small arteries in vivo have smooth muscle tone which is modified by the metabolic demands of the tissue they supply [24]. In anemia for example, arteries may dilate in response to tissue hypoxia [48], with blood flow through them increasing as a consequence of both vasodilation and lowered blood viscosity. Because chronic elevation of blood flow through a small artery results in outward hypertrophic remodeling [49], it is possible that chronic
1824
New et al: Resistance arteries in uremic hypertension
anemia in the uremic hypertensive group affected the remodeling process. CONCLUSION We have shown that cremaster and mesenteric resistance arteries of uremic hypertensive rats manifest inward remodeling without vascular hypertrophy and without change in elasticity. Such remodeling might contribute to the maintenance of raised vascular resistance, and decrease blood flow reserve, in uremic hypertension. Reprint requests to David New, Department of Renal Medicine, Hope Hospital, Salford, Manchester M6 8HD, United Kingdom. E-mail: [email protected]
REFERENCES 1. THYBO NK, MULVANY MJ, JASTRUP B, et al: Some pharmacological and elastic characteristics of isolated subcutaneous small arteries from patients with essential hypertension. J Hypertens 14:993–998, 1996 2. RIZZONI D, PORTERI E, CASTELLANO M, et al: Endothelial dysfunction in hypertension is independent from the etiology and from vascular structure. Hypertension 31:335–341, 1998 3. HEAGERTY AM, AALKJAER C, BUND SJ, et al: Small artery structure in hypertension. Dual processes of remodeling and growth. Hypertension 21:391–397, 1993 4. DUNN WR, GARDINER SM: Differential alteration in vascular structure of resistance arteries isolated from the cerebral and mesenteric vascular beds of transgenic [(mRen-2)27], hypertensive rats. Hypertension 29:1140–1147, 1997 5. OSOL G, HALPERN W: Myogenic properties of cerebral blood vessels from normotensive and hypertensive rats. Am J Physiol 249:H914– H921, 1985 6. BAUMBACH GL, SIEMS JE, HEISTAD DD: Effects of local reduction in pressure on distensibility and composition of cerebral arterioles. Circ Res 68:338–351, 1991 7. COOPER A, HEAGERTY AM: Blood pressure parameters as determinants of small artery structure in human essential hypertension. Clin Sci (Colch) 92:551–557, 1997 8. KORSGAARD N, AALKJAER C, HEAGERTY AM, et al: Histology of subcutaneous small arteries from patients with essential hypertension. Hypertension 22:523–526, 1993 9. RIZZONI D, PORTERI E, GUEFI D, et al: Cellular hypertrophy in subcutaneous small arteries of patients with renovascular hypertension. Hypertension 35:931–935, 2000 10. BUND SJ: Spontaneously hypertensive rat resistance artery structure related to myogenic and mechanical properties. Clin Sci (Lond) 101:385–393, 2001 11. BAUMBACH GL, HAJDU MA: Mechanics and composition of cerebral arterioles in renal and spontaneously hypertensive rats. Hypertension 21:816–826, 1993 12. BUND SJ, OLDHAM AA, HEAGERTY AM: Mechanical properties of porcine small coronary arteries in one-kidney, one-clip hypertension. J Vasc Res 33:175–180, 1996 13. ZIMLICHMAN RR, CHAIMOVITZ C, CHAICHENCO Y, et al: Vascular hypersensitivity to noradrenaline: A possible mechanism of hypertension in rats with chronic uraemia. Clin Sci 67:161–166, 1984 14. BRADLEY JR, EVANS DB, COWLEY AJ: Abnormalities of the peripheral circulation in patients with chronic renal failure. Nephrol Dial Transplant 3:412–416, 1988 15. TORNIG J, AMANN K, RITZ E, et al: Arteriolar wall thickening, capillary rarefaction and interstitial fibrosis in the heart of rats with renal failure: The effects of ramipril, nifedipine and moxonidine. J Am Soc Nephrol 7:667–675, 1996 16. THURAISINGHAM RC, RAINE AE: Maintenance of normal agonistinduced endothelium-dependent relaxation in uraemic and hypertensive resistance vessels. Nephrol Dial Transplant 14:70–75, 1999
17. AMANN K, BREITBACH M, RITZ E, MALL G: Myocyte/capillary mismatch in the heart of uremic patients. J Am Soc Nephrol 9:1018– 1022, 1998 18. SCHMIDT RF, THEWS G: Human Physiology, 20th ed, Berlin, Heidelberg, New York, Springer-Verlag, 1983 19. ISEKI K, FUKIYAMA K: Clinical demographics and long-term prognosis after stroke in patients on chronic haemodialysis. The Okinawa Dialysis Study (OKIDS) Group. Nephrol Dial Transplant 15:1808– 1813, 2000 20. IZZARD AS, BUND SJ, HEAGERTY AM: Myogenic tone in mesenteric arteries from spontaneously hypertensive rats. Am J Physiol 270:H1–H6, 1996 21. FALCONE JC, GRANGER HJ, MEININGER GA: Enhanced myogenic activation in skeletal muscle arterioles from spontaneously hypertensive rats. Am J Physiol 265:H1847–H1855, 1993 22. DUNN WR, GARDINER SM: Structural and functional properties of isolated, pressurized, mesenteric resistance arteries from a vasopressin-deficient rat model of genetic hypertension. Hypertension 26:390–396, 1995 23. BAUMBACH GL, HEISTAD DD: Remodeling of cerebral arterioles in chronic hypertension. Hypertension 13:968–972, 1989 24. MULVANY MJ, AALKJAER C: Structure and function of small arteries. Physiol Rev 70:921–961, 1990 25. JOHNSON G: On certain points in the anatomy of Bright’s disease of the kidney. Trans R Med Chir Soc 51:57–58, 1868 26. GULL WW, SUTTON HG: Chronic Bright’s disease with contracted kidney. Med Chir Trans 55:273–326, 1872 27. AMANN K, TORNIG J, FLECHTENMACHER C, et al: Blood-pressureindependent wall thickening of intramyocardial arterioles in experimental uraemia: Evidence for a permissive action of PTH. Nephrol Dial Transplant 10:2043–2048, 1995 28. DEMUTH K, BLACHER J, GUERIN AP, et al: Endothelin and cardiovascular remodelling in end-stage renal disease. Nephrol Dial Transplant 13:375–383, 1998 29. MULVANY MJ: Are vascular abnormalities a primary cause or secondary consequence of hypertension? Hypertension 18:I52–57, 1991 30. GRIFfiN KA, PICKEN M, BIDANI AK: Method of renal mass reduction is a critical modulator of subsequent hypertension and glomerular injury. J Am Soc Nephrol 4:2023–2031, 1994 31. LONDON GM, MARCHAIS SJ, SAFAR ME, et al: Aortic and large artery compliance in end-stage renal failure. Kidney Int 37:137–142, 1990 32. FOLKOW B: Physiological aspects of primary hypertension. Physiol Rev 62:347–504, 1982 33. LUND-JOHANSEN P: Haemodynamics in early essential hypertension—Still an area of controversy. J Hypertens 1:209–213, 1983 34. IZZARD AS, HEAGERTY AM, LEENEN FH: The amplifier hypothesis: permission to dissent? J Hypertens 17:1667–1669, 1999 35. KORNER PI, ANGUS JA, WRIGHT CE: Structure and the resistance amplifier in hypertension: reply to the dissenters. J Hypertens 18:235– 239, 2000 36. SCHULTE KL, BRAUN J, MEYER-SABELLEK W, et al: Functional versus structural changes of forearm vascular resistance in hypertension. Hypertension 11:320–325, 1988 37. ROSTAND SG, KIRK KA, RUTSKY EA: Dialysis-associated ischemic heart disease: Insights from coronary angiography. Kidney Int 25:653–659, 1984 38. OPHERK D, MALL G, ZEBE H, et al: Reduction of coronary reserve: A mechanism for angina pectoris in patients with arterial hypertension and normal coronary arteries. Circulation 69:1–7, 1984 39. AMANN K, TYRALLA K: Cardiovascular changes in chronic renal failure—Pathogenesis and therapy. Clin Nephrol 58:S62–S72, 2002 40. ANTONIOS TF, SINGER DR, MARKANDU ND, et al: Structural skin capillary rarefaction in essential hypertension. Hypertension 33:998– 1001, 1999 41. NEW DI, CHESSER AM, THURAISINGHAM RC, YAQOOB MM: Cerebral artery responses to pressure and flow in uremic hypertensive and spontaneously hypertensive rats. Am J Physiol Heart Circ Physiol 284:H1212–H1216, 2003 42. SMEDA JS: Cerebral vascular changes associated with hemorrhagic stroke in hypertension. Can J Physiol Pharmacol 70:552–564, 1992 43. LI JS, SCHIFFRIN EL: Effect of chronic treatment of adult
New et al: Resistance arteries in uremic hypertension
spontaneously hypertensive rats with an endothelin receptor antagonist. Hypertension 25:495–500, 1995 44. INTENGAN HD, THIBAULT G, LI JS, SCHIFFRIN EL: Resistance artery mechanics, structure, and extracellular components in spontaneously hypertensive rats: Effects of angiotensin receptor antagonism and converting enzyme inhibition. Circulation 100:2267–2275, 1999 45. HAJDU MA, HEISTAD DD, GHONEIM S, BAUMBACH GL: Effects of antihypertensive treatment on composition of cerebral arterioles. Hypertension 18:II15–II21, 1991 46. CHILLON JM, BAUMBACH GL: Effects of an angiotensin-converting
1825
enzyme inhibitor and a beta- blocker on cerebral arterioles in rats. Hypertension 33:856–861, 1999 47. DUNN WR, WALLIS SJ, GARDINER SM: Remodelling and enhanced myogenic tone in cerebral resistance arteries isolated from genetically hypertensive Brattleboro rats. J Vasc Res 35:18–26, 1998 48. BLITZER ML, LEE SD, CREAGER MA: Endothelium-derived nitric oxide mediates hypoxic vasodilation of resistance vessels in humans. Am J Physiol 271:H1182–H1185, 1996 49. POURAGEAUD F, DE MEY JG: Structural properties of rat mesenteric small arteries after 4-wk exposure to elevated or reduced blood flow. Am J Physiol 273:H1699–H1706, 1997
Structural remodeling of resistance arteries in uremic hypertension DAVID I. NEW, ALISTAIR M. CHESSER, RAJ C. THURAISINGHAM, and MAGDI M. YAQOOB Anthony Raine Research Laboratories, St Bartholomew’s Hospital, London, United Kingdom
Structural remodeling of resistance arteries in uremic hypertension. Background. Structural remodeling of the resistance vasculature is present in many forms of human and experimental hypertension. In particular, an increase in the ratio of wall thickness to lumen diameter develops, and might in itself maintain hypertension by increasing vascular resistance. Because uremia is associated with raised peripheral resistance, hypertension, and histologic changes suggestive of vascular remodeling, we sought to formally examine the structural and mechanical (elastic) properties of isolated pressurized resistance arteries in uremic hypertension. Methods. Cremaster, cerebral and mesenteric arteries from subtotally nephrectomised Wistar-Kyoto rats, normotensive control Wistar-Kyoto rats, and spontaneously hypertensive rats were mounted on a pressure myograph and relaxed in calciumfree buffer. Wall thickness and lumen diameter were measured at increasing lumen pressures from 10 to 200 mm Hg, and from this wall:lumen ratio, wall cross-sectional area, and an index of elasticity were derived. Results. In uremic hypertensive animals increased wall:lumen ratio and decreased lumen diameter was seen in cremaster and mesenteric arteries, although no significant changes were observed in cerebral arteries, compared to normotensive controls. In spontaneously hypertensive animals increased wall thickness and wall:lumen ratio was seen in cerebral and mesenteric arteries, decreased lumen diameter in cremaster and mesenteric arteries, and increased wall cross-sectional area in cerebral arteries, compared to normotensive controls. Elasticity of the arterial wall in uremic and spontaneously hypertensive animals did not differ from normotensive controls. Conclusion. Cremaster and mesenteric resistance arteries undergo predominantly eutrophic inward remodeling in uremic hypertension, broadly similar to that seen in spontaneous hypertension.
Changes in the structural properties of resistance vessels have been observed in various forms of human
Key words: uremia, hypertension, remodeling, elasticity, resistance artery. Received for publication January 19, 2003 and in revised form July 28, 2003, and October 18, 2003 Accepted for publication December 16, 2003 C
2004 by the International Society of Nephrology
[1–3] and experimental [4–6] hypertension. Structural (dimensional) changes manifest as alterations in wall thickness and lumen diameter, and therefore in wall:lumen ratio; remodeling is the term used to describe this process. Eutrophic inward remodeling implies reorganization of existing vascular wall material around a reduced lumen, such that wall cross-sectional area is not altered (no vascular growth); hypertrophic inward remodeling indicates lumen reduction due to increased wall cross-sectional area (vascular growth). These processes are depicted schematically in Figure 1. In general, hypertension is associated with either eutrophic or hypertrophic inward remodeling; eutrophic remodeling being seen, for example, in essential hypertension [1, 7, 8], and hypertrophic remodeling in renovascular hypertension [9]. Both forms frequently coexist in different vascular beds of the same individual, such as in the spontaneously hypertensive rat [5, 10, 11]. Arterial diameter is affected by lumen pressure (or more strictly the transmural pressure) and smooth muscle activation. Comparisons of arterial structure are commonly performed at the standard pressure of 100 mm Hg [5], with active smooth muscle tension abolished by calcium-free buffer. Under such circumstances, an increase in wall:lumen ratio could be due to structural remodeling [12] or to decreased arterial elasticity [4]. Hence, before attributing changes in arterial dimensions to structural remodeling it is necessary to exclude a difference in elasticity [1, 12]. It follows that full assessment of vascular structure requires study under increasing degrees of distention in order to measure both structural and elastic parameters, and visualization of isolated arteries cannulated and pressurized on a myograph allows such measurement. If inward remodeling or decreased elasticity of an artery is present, then the resistance to flow through that artery will be increased since, according to the Poiseuille equation, resistance is proportional to the fourth power of diameter. This might be expected to manifest as raised peripheral vascular resistance causing hypertension, and as impaired vasodilator reserve limiting the response to exercise or ischemia. Because uremia is associated with 1818
New et al: Resistance arteries in uremic hypertension
B
A D
WT C Fig. 1. Diagramatic representation of remodeling. (A) Normal artery, with lumen diameter (D) and wall thickness (WT) labeled. (B) Hypertrophic inward remodeling has occurred; the wall cross-sectional area (shaded) has increased and wall:lumen ratio is elevated, due to addition of material to the inner surface of the arterial wall. (C) Eutrophic inward remodeling has occurred; wall cross-sectional area is unchanged but the wall:lumen ratio is increased, due to the re-organization of existing wall material around a narrower lumen (not drawn to scale).
hypertension and raised vascular resistance [13], decreased forearm post-ischemic hyperemia [14], and histologic evidence suggestive of vascular remodeling [15], we sought to establish the structural and elastic properties of isolated pressurized resistance arteries in uremic hypertension. The subtotal nephrectomy model of uremic hypertension was employed using Wistar-Kyoto (WKY) rats, and compared with nonuremic normotensive controls and nonuremic spontaneously hypertensive animals. The purpose of including nonuremic hypertensive animals (spontaneously hypertensive) in this study was to attempt to control for the effect of hypertension in uremic animals [16, 17], since vascular changes may occur in response to either uremia or to hypertension or both. We studied cremaster and mesenteric arteries because the skeletal muscle and splanchnic circulations control a substantial proportion of the peripheral resistance [18], and cerebral arteries because hypertensive vascular end-organ damage is prevalent in uremia [19]. METHODS Animals All procedures had the prior approval of the Home Office (Project License number 70/5014), and were performed in accordance with the Animals Scientific Procedures Act, 1986. Twelve to 14-week-old male WKY and spontaneously hypertensive rats were obtained from Harlan Ltd. (Bicester, Oxfordshire, UK). General anesthesia was induced for both stages of the subtotal
1819
nephrectomy procedure. This was achieved by intramuscular injection of HypnormTM 1 mL/kg body weight (comprising fentanyl citrate 0.315 mg/mL plus fluanisone 10 mg/mL) (Janssen-Cilag Ltd., Buckinghamshire, UK) and intraperitoneal injection of diazepam 2.5 mg/kg (CP Pharmaceuticals Ltd., Wrexham, UK). Postoperative analgesia was provided by subcutaneous injection of buprenorphine 50 lg/kg (Schering-Plough Ltd., Welwyn Garden City, Hertfordshire, UK). Left subtotal nephrectomy through a midline abdominal incision removed approximately three-fourths of that kidney, with total right nephrectomy via a flank incision 1 week later. Control WKY and spontaneously hypertensive animals underwent sham surgery in which the appropriate kidney was stripped of its capsule only. Animals were maintained for an average 15 to 16 weeks during which time WKY contol and spontaneously hypertensive rats were pair fed with their uremic WKY partner. Then, under halothane general anesthesia, an abdominal aortic catheter was inserted retrogradely through the femoral artery, heparinized, and exteriorized through the skin at the nape of the neck [20]. Two to three days later, with the rat was conscious and free to move in its cage, mean arterial pressure was measured by attaching the catheter to a pressure transducer (model number 60–3003) Harvard Apparatus, Cambridge, MA, USA). Blood pressure was recorded for at least 20 minutes until a stable value was obtained. Blood was drawn for analysis, and the animal killed by cervical dislocation. Cremaster muscle, brain, and mesentery were placed in ice-cold physiologic salt solution (PSS). The 1A branch of the cremaster [21], the middle cerebral artery at its most lateral position over the hemisphere, and the first order branch of the mesenteric artery [22] were studied. Drugs and solutions PSS had the following composition (mmol/L): 119 NaCl, 4.7 KCl, 25 NaHCO 3 , 1.17 KH 2 PO 4 , 1.17 MgSO 4 , 2.0 CaCl 2 , and 5.5 glucose. Calcium-free PSS contained ethyleneglycol–bis (b-aminoethylether)-N, N tetraacetic acid (EGTA) 1 mmol/L and sodium nitroprusside 100 lmol/L. Perfusion myograph The myograph method has been described [4, 20]. Briefly, an artery was mounted between the two glass microcannulas of a myograph (Living Systems Instrumentation, Burlington, VT, USA) and viewed through a microscope. Calcium-free PSS was gassed (95% O 2 /5% CO 2 ) and heated in a reservoir, and circulated at a rate of 40 mL/min to the vessel chamber. Temperature was maintained at 33◦ C for cremaster (its temperature in vivo) and 37◦ C for cerebral and mesenteric arteries. Active smooth muscle tension was lost during equilibration in
1820
New et al: Resistance arteries in uremic hypertension
calcium-free PSS. Passive lumen diameter and wall thickness were then measured at lumen pressures from 10 to 200 mm Hg using a video dimension analyser. Because the vessel was immersed in less than 1 cm of PSS within the chamber, lumen pressure was negligibly different from transmural pressure. Measurements performed with this myograph system are accurate to 1 lm. All measurements were performed by the same investigator, and intraobserver variation was <1 lm. Calculations The following calculations were used: Creatinine clearance(mL/min) = UCr · V/PCr · t
Table 1. General data
Animal data Time of uremia/sham uremia weeks Mean arterial pressure mmHg Blood hemoglobin g/dL Plasma creatinine lmol/L Creatinine clearance mL/min Weight g
2
Wall cross-sectional area (CSA)(lm ) =1/4 .p.{(D + 2.WT)2 − D2 } where D is lumen diameter (lm) and WT is wall thickness (lm) at a given pressure. Wall:lumen ratio R (%) = 100.WT/D. Circumferential stress r (N/m2 ) = P.D/2.WT. (P is converted to N/m2 by the factor 1 mm Hg = 133.4 N/m2 ). Circumferential strain e = (D – D o )/D o , where D o is the “original” diameter at 10 mm Hg (it is difficult to accurately determine internal diameter of resistance arteries at pressures lower than this [23]). The data for each individual artery can be fitted to a curve r = r o .eb·e using least-squares analysis, where r o is the circumferential stress at 10 mm Hg, and b is a constant. Furthermore, the elastic characteristic of an artery can be described by the tangential elastic modulus of its wall material, E, where E = dr/de [4]. It follows that dr/de = b.r o .eb·e = b.r. The constant b therefore describes the linear slope of the tangential elastic modulus versus stress, and is used as an index of elasticity [4, 24]. The higher the value of b, the lower the elasticity of the wall material. Structural changes in small arteries are frequently due to the dual processes of remodeling and growth (hypertrophy) [3], although one process frequently dominates. The relative contribution of each can be assessed by inspection of the calculated “remodeling index” and “growth index.” Here, remodeling index is defined as the percentage of the observed difference in the internal diameter of hypertensive (uremic or spontaneous) and normotensive arteries that could be accounted for by remodeling of the normotensive vessel. If the remodeling index is not equal to 100%, then there must have been growth, and growth index is defined as the percent increase in wall cross-sectional area of the hypertensive (uremic or spontaneous) relative to normotensive arteries. The remodeling and growth indices were calculated by standard methods [3], which are not elaborated here, when a statistically significant difference in one of the ar-
WistarKyoto controls (N = 33)
Spontaneously hypertensive rats (N = 28)
15.1 (0.4)
15.2 (0.5)
15.1 (0.5)
166 (5)a,b
139 (2)
194 (3)b,c
7.9 (0.3)a,b
13.3 (0.3)
14.1 (0.4)
249 (25)a,b
45 (3)
47 (3)
0.41 (0.09)a,b
2.98 (0.43)
2.61 (0.32)
303 (6)a,b
360 (3)
384 (5)
P < 0.001 compared to Wistar-Kyoto controls; P < 0.001 compared to spontaneously hypertensive rats; c P < 0.001 compared to uremic Wistar-Kyoto. a
where U Cr is urinary creatinine and P Cr is plasma creatinine concentration (lmol/L), and V is the volume of urine (mL) collected in time t (minutes).
Uremic WistarKyoto (N = 23)
b
terial structural (dimensional) parameters was observed between experimental groups. Statistical analyses Values are represented as mean (standard error). In all comparisons, there were three separate groups; WKY uremic rats, WKY controls, and spontaneously hypertensive rats. Therefore one-way analysis of variance (ANOVA) was chosen for comparison of means. The Bonferroni correction was applied to allow comparisons between each combination of two groups. The primary comparisons were between uremic WKY and WKY controls, and between spontaneously hypertensive rats and WKY controls, although, when appropriate, reference is made to comparison between uremic WKY and spontaneously hypertensive rats. Although comparisons were made at the standard lumen pressure of 100 mm Hg [5], the data for diameter and wall thickness are also plotted across the full pressure range to demonstrate the consistency of the changes. In all analyses the significance level was taken as P < 0.05. RESULTS General animal data Animal groups had very similar periods of uremia/sham uremia (Table 1). Mean arterial pressure was raised in both uremic WKY and spontaneously hypertensive rats compared to WKY controls, and spontaneously hypertensive rat pressure was also significantly higher than uremic WKY. Uremic WKY animals were anemic, with substantially raised plasma creatinine, decreased creatinine clearance, and lower body mass compared to both WKY controls and spontaneously hypertensive rats. Cremaster arteries Cremaster arteries from uremic WKY demonstrated significantly decreased lumen diameter and increased
New et al: Resistance arteries in uremic hypertension
Table 2. Characteristics of branch 1A cremaster arteries at 100 mm Hg lumen pressure
Cremaster artery Wall thickness lm Lumen diameter lm Wall to lumen ratio % Wall cross-sectional area lm2 b value Remodeling index % Growth index %
Uremic WistarKyoto (N = 19)
WistarKyoto controls (N = 20)
Spontaneously hypertensive rats (N = 14)
15.4 (1.9) 149 (3)a 10.6 (1.4)c 8020 (1064)
11.1 (0.9) 168 (3) 6.6 (0.5) 6260 (552)
13.4 (1.2) 151 (5)b 9.0 (0.9) 6926 (739)
8.2 (0.6) 62 28
6.7 (0.6) — —
5.7 (0.4) 85 11
1821
WKY was not significantly different to WKY controls. Arteries from spontaneously hypertensive rats showed increase in wall thickness, reduced lumen diameter, and increased wall:lumen ratio compared to WKY controls. Wall cross-sectional area was not significantly increased in spontaneously hypertensive rats compared to WKY controls. The remodeling index was 84% while the growth index was 23%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that uremic WKY and spontaneously hypertensive mesenteric arteries undergo predominantly eutrophic inward remodeling.
a < P 0.005 compared to Wistar-Kyoto controls; b P < 0.01 compared to WistarKyoto controls; c P < 0.02 compared to Wistar-Kyoto controls.
wall:lumen ratio, with no significant difference in wall cross-sectional area compared to WKY controls (Table 2) (Figs. 2 and 3). The remodeling index was 62% whereas the growth index was 28%. The elasticity (b value) for uremic WKY was not significantly different to WKY controls. Arteries from spontaneously hypertensive rats showed reduced lumen diameter, with no significant difference in wall cross-sectional area compared to WKY controls. The remodeling index was 85% and the growth index 11%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that the changes in uremic WKY and spontaneously hypertensive rat cremaster arteries are predominantly due to eutrophic inward remodeling. Cerebral arteries Cerebral arteries from uremic WKY showed no significant structural changes compared to WKY controls (Table 3) (Figs. 2 and 3). The elasticity (b value) for uremic WKY did not differ from WKY controls. Arteries from spontaneously hypertensive rats showed increase in wall thickness, wall:lumen ratio, and wall cross-sectional area compared to WKY controls. The growth index was 46% and the remodeling index −3%. The elasticity (b value) for spontaneously hypertensive rats was not significantly different to WKY controls. These data confirm that uremic WKY cerebral arteries undergo no significant remodeling, while spontaneously hypertensive rat cerebral arteries display hypertrophic remodeling. Mesenteric arteries Mesenteric arteries from uremic WKY demonstrated significantly decreased lumen diameter and increased wall:lumen ratio compared to WKY controls (Table 4) (Figs. 2 and 3). Wall cross-sectional area was not significantly different in uremic WKY compared to WKY controls. The remodeling index was 95% compared to the growth index of 11%. The elasticity (b value) for uremic
DISCUSSION The important finding of this study is that cremaster and mesenteric arteries from uremic hypertensive rats display predominantly eutrophic inward remodeling with no change in elasticity, when compared to vessels from normotensive control animals. No significant changes were seen in cerebral arteries from uremic hypertensive animals. In a classic autopsy study [25], renal small arteries from patients with Bright’s (chronic renal) disease were shown to have increased wall thickness relative to lumen. This finding was later extended to other vascular beds [26]. These observations could not, however, distinguish between structural change and greater vascular contraction by arteries from uremic subjects during the fixation process. Recent thorough histologic studies using procaine to relax vascular smooth muscle prior to fixation at a controlled lumen pressure show that myocardial arterioles in uremia display wall thickening and increased wall:lumen ratio [15]. Histologic assessment, however, cannot distinguish between remodeling and change in arterial elasticity. To our knowledge, this paper provides the first systematic assessment of the structural and elastic properties of isolated pressurized resistance arteries in uremic hypertension; the findings support previous histological studies, and confirm that the basis is predominantly structural remodeling rather than change in elasticity. Several factors might cause structural cardiovascular changes in uremia. These include uremic hyperparathyroidism [27], abnormal activity of the renin-angiotensin system [15], and accumulation of endothelin [28]. However, increased wall:lumen ratio is a common finding in many forms of hypertension [2] and it is conceivable that structural (dimensional) changes are simply a consequence of raised arterial pressure [29]. The structural changes reported here in uremic hypertensive cremaster and mesenteric arteries were comparable in degree to those from spontaneously hypertensive animals even though the magnitude, and probably also the duration [30], of hypertension were substantially higher in the latter. Since there is histologic evidence of myocardial
1822
New et al: Resistance arteries in uremic hypertension
Cremaster
180
260
440
Mesenteric
400
240
160 Diameter, µm
Cerebral
360 140
220 320 200
120
280 180
100
240
160
80 0
100
200
Cremaster
24
200 100 0 200 0 Lumen pressure, mm Hg
24
Cerebral
34
100
200
Fig. 2. Artery passive internal diameter plotted against lumen pressure. Uremic WistarKyoto (WKY) (), WKY control (), and spontaneously hypertensive rats (). At 100 mm Hg lumen pressure, cremaster arteries from both uremic WKY and spontaneously hypertensive rats demonstrated significantly decreased lumen diameter compared to WKY controls (P < 0.005 and P < 0.01, respectively). Cerebral arteries from neither uremic WKY nor spontaneously hypertensive rats showed significant change in lumen diameter compared to WKY controls. Mesenteric arteries from both uremic WKY and spontaneously hypertensive rats demonstrated significantly decreased lumen diameter compared to WKY controls (P < 0.005 and P < 0.02, respectively).
Mesenteric
30 Wall thickness, µm
20
20 26
16
16
12
12
22
18 14
8
10 0 200 0 100 Lumen pressure, mm Hg 8
0
100
200
arteriolar wall thickening in uremia even in the absence of hypertension [27], it is possible to contemplate that uremia and hypertension act synergistically to effect vascular changes. Alternatively, because in uremia the distensibility of large (conduit) arteries is decreased [31], it is possible that the distal arteries are exposed to a greater proportion of the pulse pressure that speculatively might enhance the small artery structural changes. A likely functional consequence of arterial remodeling reported here in uremic hypertension is raised peripheral resistance, because geometrical reasoning dictates that a smaller lumen diameter would follow from any given degree of smooth muscle shortening [32]. The result would be increased vascular resistance, which is known to be
100
200
Fig. 3. Wall thickness plotted against lumen pressure. Uremic Wistar-Kyoto (WKY) (), WKY controls (), and spontaneously hypertensive rats (). At 100 mm Hg lumen pressure, cremaster arteries from neither uremic WKY nor spontaneously hypertensive rats showed significant increase in wall thickness compared to WKY controls. Cerebral arteries from spontaneously hypertensive rats but not uremic WKY showed an increase in wall thickness compared to WKY controls (P < 0.001 for spontaneously hypertensive rats). Mesenteric arteries from spontaneously hypertensive rats but not uremic WKY demonstrated a significant increase in wall thickness compared to WKY controls (P < 0.01 for spontaneously hypertensive rats).
present in uremic [13] and essential [33] hypertension. Although the matter is debated [34, 35], this so-called “amplifier hypothesis” provides a possible means by which vascular resistance is increased without requiring any increase in vascular smooth muscle tension generation. It is therefore an attractive hypothesis, since most studies find normal vascular contractility in many types of hypertension [24]. Additionally, inward remodeling would also be expected to limit the maximum attainable arterial diameter [29]. This may explain why postischemic forearm blood flow is decreased in uremia [14] and in essential hypertension [36], and why cardiac ischemia may occur in the absence of discrete coronary stenosis in uremic [37] as well as nonuremic hypertension [38]. Nevertheless,
1823
New et al: Resistance arteries in uremic hypertension
Fig. 4. Wall stress plotted against lumen pressure. Uremic Wistar-Kyoto (WKY) (), control WKY (), and spontaneously hypertensive rats (). Table 3. Characteristics of middle cerebral arteries at 100 mm Hg lumen pressure
Cerebral artery
Uremic WistarKyoto (N = 17)
WistarKyoto controls (N = 16)
Spontaneously hypertensive rats (N = 14)
Wall thickness lm 11.6 (0.7) 10.2 (0.5) 15.0 (1.1)a Lumen diameter lm 234 (5) 241 (7) 232 (6) Wall to lumen ratio % 4.98 (0.31) 4.24 (0.18) 6.44 (0.41)a Wall cross-sectional area lm2 8896 (588) 8161 (544) 11884 (1094)b b value 12.0 (0.7) 9.9 (0.5) 9.5 (0.6) Remodeling indexc % NA NA −3 Growth indexc % NA NA 46 NA, not applicable. P < 0.001 compared to Wistar-Kyoto controls; b P < 0.005 compared to WistarKyoto controls; c Remodeling index and growth index not calculated as no significant structural (dimensional) differences were present compared to Wistar-Kyoto controls. a
factors other than remodeling, such as microvascular rarefaction [39, 40], might also cause or contribute to the observed hemodynamic changes. Finally, the changes seen in both uremic and spontaneous hypertension serve to decrease the passive wall stress of arteries at any given lumen pressure or, when blood pressure is high, they tend to normalize wall stress. The relation of pressure to wall stress (which is itself derived from the parameters pressure, diameter, and wall thickness) for arteries in each group is shown by Figure 4. This might represent an adaptation to preserve microvascular integrity under conditions of raised arterial pressure. In this respect the absence of significant remodeling of the cerebral artery in uremic hypertension, together with impairment in contractile response to lumen distension [41, 42], may contribute to the association of uremia with hemorrhagic stroke in humans [19] and animals [42]. We report that spontaneously hypertensive cremaster and mesenteric arteries undergo eutrophic inward re-
Table 4. Characteristics of first order mesenteric arteries at 100 mm Hg lumen pressure
Mesenteric artery Wall thickness lm Lumen diameter lm Wall to lumen ratio % Wall cross-sectional area lm2 b value Remodeling index % Growth index %
Uremic WistarKyoto (N = 8)
WistarKyoto controls (N = 12)
Spontaneously hypertensive rats (N = 12)
20.7 (2.0) 342 (12)b 6.0 (0.4)d 24034 (3138)
15.9 (0.9) 411 (16) 3.9 (0.2) 21640 (1810)
22.2 (1.4)a 359 (9)c 6.2 (0.4)e 26600 (1899)
6.1 (0.6) 95 11
5.2 (0.4) — —
5.8 (0.3) 84 23
a P < 0.01 compared to Wistar-Kyoto controls; b P < 0.005 compared to Wistar-Kyoto controls; c P < 0.02 compared to Wistar-Kyoto controls; d P = 0.001 compared to Wistar-Kyoto controls; e P < 0.001 compared to Wistar-Kyoto controls.
modeling consistent with previous studies [10, 43, 44]. We demonstrated that spontaneously hypertensive cerebral vessels display hypertrophic remodeling, also conforming with earlier reports [5, 11]. Cerebral artery hypertrophy appears principally to be a response to elevated blood pressure [6, 45, 46], and other models of hypertension in which blood pressure is not raised to the same extent as in the spontaneously hypertensive rats do not display cerebral hypertrophy [4, 47]. This might explain why we found no hypertrophy of uremic WKY cerebral arteries. Small arteries in vivo have smooth muscle tone which is modified by the metabolic demands of the tissue they supply [24]. In anemia for example, arteries may dilate in response to tissue hypoxia [48], with blood flow through them increasing as a consequence of both vasodilation and lowered blood viscosity. Because chronic elevation of blood flow through a small artery results in outward hypertrophic remodeling [49], it is possible that chronic
1824
New et al: Resistance arteries in uremic hypertension
anemia in the uremic hypertensive group affected the remodeling process. CONCLUSION We have shown that cremaster and mesenteric resistance arteries of uremic hypertensive rats manifest inward remodeling without vascular hypertrophy and without change in elasticity. Such remodeling might contribute to the maintenance of raised vascular resistance, and decrease blood flow reserve, in uremic hypertension. Reprint requests to David New, Department of Renal Medicine, Hope Hospital, Salford, Manchester M6 8HD, United Kingdom. E-mail: [email protected]
REFERENCES 1. THYBO NK, MULVANY MJ, JASTRUP B, et al: Some pharmacological and elastic characteristics of isolated subcutaneous small arteries from patients with essential hypertension. J Hypertens 14:993–998, 1996 2. RIZZONI D, PORTERI E, CASTELLANO M, et al: Endothelial dysfunction in hypertension is independent from the etiology and from vascular structure. Hypertension 31:335–341, 1998 3. HEAGERTY AM, AALKJAER C, BUND SJ, et al: Small artery structure in hypertension. Dual processes of remodeling and growth. Hypertension 21:391–397, 1993 4. DUNN WR, GARDINER SM: Differential alteration in vascular structure of resistance arteries isolated from the cerebral and mesenteric vascular beds of transgenic [(mRen-2)27], hypertensive rats. Hypertension 29:1140–1147, 1997 5. OSOL G, HALPERN W: Myogenic properties of cerebral blood vessels from normotensive and hypertensive rats. Am J Physiol 249:H914– H921, 1985 6. BAUMBACH GL, SIEMS JE, HEISTAD DD: Effects of local reduction in pressure on distensibility and composition of cerebral arterioles. Circ Res 68:338–351, 1991 7. COOPER A, HEAGERTY AM: Blood pressure parameters as determinants of small artery structure in human essential hypertension. Clin Sci (Colch) 92:551–557, 1997 8. KORSGAARD N, AALKJAER C, HEAGERTY AM, et al: Histology of subcutaneous small arteries from patients with essential hypertension. Hypertension 22:523–526, 1993 9. RIZZONI D, PORTERI E, GUEFI D, et al: Cellular hypertrophy in subcutaneous small arteries of patients with renovascular hypertension. Hypertension 35:931–935, 2000 10. BUND SJ: Spontaneously hypertensive rat resistance artery structure related to myogenic and mechanical properties. Clin Sci (Lond) 101:385–393, 2001 11. BAUMBACH GL, HAJDU MA: Mechanics and composition of cerebral arterioles in renal and spontaneously hypertensive rats. Hypertension 21:816–826, 1993 12. BUND SJ, OLDHAM AA, HEAGERTY AM: Mechanical properties of porcine small coronary arteries in one-kidney, one-clip hypertension. J Vasc Res 33:175–180, 1996 13. ZIMLICHMAN RR, CHAIMOVITZ C, CHAICHENCO Y, et al: Vascular hypersensitivity to noradrenaline: A possible mechanism of hypertension in rats with chronic uraemia. Clin Sci 67:161–166, 1984 14. BRADLEY JR, EVANS DB, COWLEY AJ: Abnormalities of the peripheral circulation in patients with chronic renal failure. Nephrol Dial Transplant 3:412–416, 1988 15. TORNIG J, AMANN K, RITZ E, et al: Arteriolar wall thickening, capillary rarefaction and interstitial fibrosis in the heart of rats with renal failure: The effects of ramipril, nifedipine and moxonidine. J Am Soc Nephrol 7:667–675, 1996 16. THURAISINGHAM RC, RAINE AE: Maintenance of normal agonistinduced endothelium-dependent relaxation in uraemic and hypertensive resistance vessels. Nephrol Dial Transplant 14:70–75, 1999
17. AMANN K, BREITBACH M, RITZ E, MALL G: Myocyte/capillary mismatch in the heart of uremic patients. J Am Soc Nephrol 9:1018– 1022, 1998 18. SCHMIDT RF, THEWS G: Human Physiology, 20th ed, Berlin, Heidelberg, New York, Springer-Verlag, 1983 19. ISEKI K, FUKIYAMA K: Clinical demographics and long-term prognosis after stroke in patients on chronic haemodialysis. The Okinawa Dialysis Study (OKIDS) Group. Nephrol Dial Transplant 15:1808– 1813, 2000 20. IZZARD AS, BUND SJ, HEAGERTY AM: Myogenic tone in mesenteric arteries from spontaneously hypertensive rats. Am J Physiol 270:H1–H6, 1996 21. FALCONE JC, GRANGER HJ, MEININGER GA: Enhanced myogenic activation in skeletal muscle arterioles from spontaneously hypertensive rats. Am J Physiol 265:H1847–H1855, 1993 22. DUNN WR, GARDINER SM: Structural and functional properties of isolated, pressurized, mesenteric resistance arteries from a vasopressin-deficient rat model of genetic hypertension. Hypertension 26:390–396, 1995 23. BAUMBACH GL, HEISTAD DD: Remodeling of cerebral arterioles in chronic hypertension. Hypertension 13:968–972, 1989 24. MULVANY MJ, AALKJAER C: Structure and function of small arteries. Physiol Rev 70:921–961, 1990 25. JOHNSON G: On certain points in the anatomy of Bright’s disease of the kidney. Trans R Med Chir Soc 51:57–58, 1868 26. GULL WW, SUTTON HG: Chronic Bright’s disease with contracted kidney. Med Chir Trans 55:273–326, 1872 27. AMANN K, TORNIG J, FLECHTENMACHER C, et al: Blood-pressureindependent wall thickening of intramyocardial arterioles in experimental uraemia: Evidence for a permissive action of PTH. Nephrol Dial Transplant 10:2043–2048, 1995 28. DEMUTH K, BLACHER J, GUERIN AP, et al: Endothelin and cardiovascular remodelling in end-stage renal disease. Nephrol Dial Transplant 13:375–383, 1998 29. MULVANY MJ: Are vascular abnormalities a primary cause or secondary consequence of hypertension? Hypertension 18:I52–57, 1991 30. GRIFfiN KA, PICKEN M, BIDANI AK: Method of renal mass reduction is a critical modulator of subsequent hypertension and glomerular injury. J Am Soc Nephrol 4:2023–2031, 1994 31. LONDON GM, MARCHAIS SJ, SAFAR ME, et al: Aortic and large artery compliance in end-stage renal failure. Kidney Int 37:137–142, 1990 32. FOLKOW B: Physiological aspects of primary hypertension. Physiol Rev 62:347–504, 1982 33. LUND-JOHANSEN P: Haemodynamics in early essential hypertension—Still an area of controversy. J Hypertens 1:209–213, 1983 34. IZZARD AS, HEAGERTY AM, LEENEN FH: The amplifier hypothesis: permission to dissent? J Hypertens 17:1667–1669, 1999 35. KORNER PI, ANGUS JA, WRIGHT CE: Structure and the resistance amplifier in hypertension: reply to the dissenters. J Hypertens 18:235– 239, 2000 36. SCHULTE KL, BRAUN J, MEYER-SABELLEK W, et al: Functional versus structural changes of forearm vascular resistance in hypertension. Hypertension 11:320–325, 1988 37. ROSTAND SG, KIRK KA, RUTSKY EA: Dialysis-associated ischemic heart disease: Insights from coronary angiography. Kidney Int 25:653–659, 1984 38. OPHERK D, MALL G, ZEBE H, et al: Reduction of coronary reserve: A mechanism for angina pectoris in patients with arterial hypertension and normal coronary arteries. Circulation 69:1–7, 1984 39. AMANN K, TYRALLA K: Cardiovascular changes in chronic renal failure—Pathogenesis and therapy. Clin Nephrol 58:S62–S72, 2002 40. ANTONIOS TF, SINGER DR, MARKANDU ND, et al: Structural skin capillary rarefaction in essential hypertension. Hypertension 33:998– 1001, 1999 41. NEW DI, CHESSER AM, THURAISINGHAM RC, YAQOOB MM: Cerebral artery responses to pressure and flow in uremic hypertensive and spontaneously hypertensive rats. Am J Physiol Heart Circ Physiol 284:H1212–H1216, 2003 42. SMEDA JS: Cerebral vascular changes associated with hemorrhagic stroke in hypertension. Can J Physiol Pharmacol 70:552–564, 1992 43. LI JS, SCHIFFRIN EL: Effect of chronic treatment of adult
New et al: Resistance arteries in uremic hypertension
spontaneously hypertensive rats with an endothelin receptor antagonist. Hypertension 25:495–500, 1995 44. INTENGAN HD, THIBAULT G, LI JS, SCHIFFRIN EL: Resistance artery mechanics, structure, and extracellular components in spontaneously hypertensive rats: Effects of angiotensin receptor antagonism and converting enzyme inhibition. Circulation 100:2267–2275, 1999 45. HAJDU MA, HEISTAD DD, GHONEIM S, BAUMBACH GL: Effects of antihypertensive treatment on composition of cerebral arterioles. Hypertension 18:II15–II21, 1991 46. CHILLON JM, BAUMBACH GL: Effects of an angiotensin-converting
1825
enzyme inhibitor and a beta- blocker on cerebral arterioles in rats. Hypertension 33:856–861, 1999 47. DUNN WR, WALLIS SJ, GARDINER SM: Remodelling and enhanced myogenic tone in cerebral resistance arteries isolated from genetically hypertensive Brattleboro rats. J Vasc Res 35:18–26, 1998 48. BLITZER ML, LEE SD, CREAGER MA: Endothelium-derived nitric oxide mediates hypoxic vasodilation of resistance vessels in humans. Am J Physiol 271:H1182–H1185, 1996 49. POURAGEAUD F, DE MEY JG: Structural properties of rat mesenteric small arteries after 4-wk exposure to elevated or reduced blood flow. Am J Physiol 273:H1699–H1706, 1997