Effects of hyperlipidemia on the vascular reactivity in the Wistar-Kyoto and spontaneously hypertensive rats

European Journal of Pharmacology - Environmental Toxicology and Pharmacology Section, 248 (1993) 289-295 289 © 1993 Elsevier Science Publishers B.V. All rights reserved 0926-6917/93/$06.00

EJPTOX 40080

Effects of hyperlipidemia on the vascular reactivity in the Wistar-Kyoto and spontaneously hypertensive rats

S h e u - M e e i Y u a Z e i - S h u n g H u a n g b C h e n g - Y i W a n g b and C h e - M i n g T e n g *'c

a Department of Pharmacology, Chang Gung Medical College, Tao-Yuan, Taiwan, b Department of Internal Medicine, and c Pharmacological Institute, College of Medicine, National Taiwan University, No. 1, Jen-Ai Rd., Sect. 1, Taipei, Taiwan

Received 10 November 1992, revised MS received 7 July 1993, accepted 9 July 1993

We studied the effects of hyperlipidemia on the vascular responsiveness in aortas isolated from control rats and rats receiving a high cholesterol-high fat (HC-HF) diet (1% cholesterol and 20% olive oil). The total plasma cholesterol, very low density lipoprotein (VLDL)-, low density lipoprotein (LDL)-cholesterol, VLDL-, LDL-, high density lipoprotein (HDL)-triglyceride levels were markedly elevated in HC-HF chow fed Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) compared to the respective normal chow fed control rats. The increase in plasma cholesterol and triglyceride levels were time-dependent. Higher levels of cholesterol and triglyceride were observed in SHR compared to WKY. In the aortic arches and abdominal aortas obtained from the SHR and WKY fed the HC-HF chow for 8 weeks, evoked intimal lesions were more pronounced than those noted after 4 weeks of HC-HF chow fed. The aortic arches of SHR and WKY were significantly more affected by the intimal lesion (surface area damage and fatty streak formation) than the abdominal aortas of the respective rat strain. The damage of surface area and thickness of fatty streaks were significantly augmented with the period the rats were fed the HC-HF diet. In the denuded aortic arches of the WKY and of rats receiving HC-HF diet for 8 weeks, significantly attenuated EDs0 values and augmented maximal responses for phenylephrine (0.01-30/xM)-induced contraction were obtained. Endothelium-dependent relaxation to acetylcholine was abolished, while cndothelium-independent relaxation to nitroprusside was well preserved in the denuded aortic arches and abdominal aortas of the WKY, SHR rats with or without 8 week HC-HF diet. We conclude from these studies that in the isolated aortas from hyperlipidemic WKY and SHR: (1) the contractions induced by phenylephrine are augmented, (2) the endothelium-dependent relaxations by acetylcholine are progressively impaired and (3) the endothelium- independent relaxations by nitroprusside are not modified.

Hyperlipidemia; Vascular reactivity; Wistar-Kyoto rats (WKY); Endothelium-derived relaxing factor (EDRF); Spontaneously hypertensive rats (SHR); Aortic arch; Abdominal aorta

1. Introduct ion

Hypertension and hyperlipidemia are two of the major risk factors for atherosclerosis (Faggiotto et al., 1984). The presence of both risk factors increases significantly the risk of coronary heart disease. Several studies have illustrated that atherosclerotic blood vessels are very susceptible to the development of vasospasm (Waters et al., 1983; Heistad et al., 1984). There is accumulating evidence that atherosclerotic vessels from rabbits, monkeys, dogs and humans ex- hibit enhanced susceptibility to the constrictor effects of ergonovine (Henry and Yokoyama, 1980), histamine and serotonin (Heistad et al., 1984; Yokoyama et al., 1983) and show impaired responsiveness to endothe- l ium-dependent vasodilators including acetylcholine and substance P (Bosaller et al., 1987; Freiman et al., 1986; Verbeuren et al., 1986). These alterations of

* Corresponding author.

vascular reactivity to vasoconstrictor and vasodilator stimuli in the atherosclerotic artery may play a significant role in the pathogenesis of vasospasm. However, no studies have been performed to investigate the vascular reactivity to stimuli in hyperlipidemic Wistar- Kyoto (WKY) and spontaneously hypertensive rats (SHR).

The purpose of this study was to compare the relaxations and contractions to stimuli in the aortas from hyperlipidemic WKY and SHR with those aortas from the respective control rats.

2. Materials and methods

2.1. Rat model of hyperlipidemia

Male spontaneously hypertensive rats (SHR) weighing 230-250 g (origin: Iffa Credo Laboratories, L'Ar- bresle, France) (mean arterial pressure (MAP) = 160 + 9 mmHg) and age-matched Wistar-Kyoto (WKY) con-

290

trol rats weighing 230-250 g (origin: Biological Re- search Laboratories, Fullinsdorf, Switzerland) (MAP = 102 _+ 4 mmHg) were used in this study. The rats were housed in their own cages and given food (as follows) and water ad libitum. WKY and SHR were fed either normal rat chow (Purina rodent chow meal no. 5001) or high cholesterol-high fat (HC-HF) chow semi-syn- thetic diet containing 1% cholesterol and 20% olive oil (Quackenbush and Pawlowski, 1960). Since the 1% cholesterol and 20% olive oil diets produce a relatively mild hyperlipidemia in the rat (DeLamat re and Ro- heim, 1981; Krause and Newton, 1985), the bile acid (0.5%) is added to the diet to increase cholesterol absorption (Cohen and Krause, 1986), so that we might obtain more marked differences in plasma cholesterol between groups. The WKY or SHR was randomly assigned to one of the two following t reatment groups: normal chow (n = 15-20) and H C - H F chow (15-20). Before and at the end of the period of receiving the normal or H C - H F chow, rat tail artery was implanted with a polyethylene (PE-50) catheter and MAP and H R were continuously monitored with an electronic analyzer. No significant difference in MAP and H R of SHR and WKY were observed before and at the end of the diets (before: MAP = 160 _+ 9 mmHg in SHR, n = 20; 102 ± 4 m m H g in WKY, n = 20 vs. end of the diets: MAP = 163 + 8 m m H g in SHR, n = 20; 98_+ 6 m m H g in WKY, n = 20). After 1-8 weeks of receiving the respective diets, the rats were anesthetized with sodium pentobarbital (30 m g / k g , i.p.). A blood sample was obtained for determination of lipoprotein analysis after 16 h (overnight) fast, and the rats were then killed and aortas (aortic arch and abdominal aorta) were removed.

2.2. Measurement of plasma lipids and lipoproteins

Plasma lipoproteins were separated by a one-step density gradient ultracentrifugation technique (Dema- cker et al., 1983). The very low density lipoprotein (VLDL), low density lipoprotein (LDL), high density lipoprotein (HDL) fractions correspond to the density intervals < 1.006 g / m l , 1.006-1.063 g / m l and 1.063- 1.21 g / m l , respectively. Total plasma cholesterol and the cholesterol concentrations in each lipoprotein frac- tion were assayed by an enzymatic colorimetric technique with Boehringer Mannheim G m b H diagnostic kits (Roschlau et al., 1974) and triglycerides were measured with Biomerieux kits (Takayama et al., 1977).

2.3. Histological examinations

2.3.1 Eualuation of fatty streak formation Aortic arches and abdominal aortas (5 mm long)

were investigated histologically for the occurrence of

fatty streaks. Cross-sections of the aortic arch and abdominal aorta, stained with the hematoxylin and eosin (HE) method or with the periodic acid-Schiff method (PAS reaction) were investigated. The part of the intimal outline covered with foam cell plaques was measured and expressed as a percentage of the total intimal outline of the section; this value was taken to represent the surface area covered with lesions. The thickness of the fatty streaks was then measured and expressed as a percentage of the unaffected wall thickness; since the thickness of the fatty streaks varied over the area affected, a mean value was estimated.

2.3.2 Eualuation of the presence of the endothelium Aortic arches and abdominal aortas obtained from

the control and 1-8 weeks hyperlipidemic rats, were opened longitudinally and stained with AgNO 3. Briefly, the opened segments were mounted on pieces of cork and stained in the dark with AgNO 3 (24 raM) in the presence of glucose (233 raM) and HEPES buffer (20 mM) at pH 7.4 for 60 s. After rinsing with glucose solution, the tissues were fixed with 2.5% glutaralde- hyde in 0.1 M sodium cacodylate buffer. The fixed tissues were dehydrated, embedded in DPX, and their luminal surface was examined by light microscopy.

2.4. Pharmacological measurements

Rats were anesthetized with sodium pentobarbitone, and aortic arches or abdominal aortas were isolated and the surrounding tissue removed. Aortic rings of about 5 mm in length and mounted in organ baths containing 5 ml Krebs solution (mM composition: NaC1 118, KC1 4.0, CaC12 1.9, MgSO 4 1.2, N a H 2 P O 4 1.2, N a H C O 3 25 and glucose 11.7) and equilibrated at 37°C with 95% 02-5% CO 2 gas mixture. One of the two stainless steel hooks was inserted and fixed into the aortic lumen while the other connected to a transducer. Aortas were equilibrated in the medium for 60 min with three changes of Krebs solution and maintained under an optimal tension of 1 g before specific experimental protocols were initiated. Contractions were recorded isometrically via a force-displacement transducer connected to a Grass polygraph. The cumulative concentration-response curve was obtained with phenylephrine (0.01-30 txM) for 24 min at 3 min intervals. Aortic rings were contracted with 3 IxM phenylephrine for 15 rain and subsequently relaxed by the cumulative addition of acetylcholine or nitroprusside (0.01-30 IzM). In some experiments, the endothelium was removed mechanically by rubbing the intimal surface with filter paper moistened with the buffer. Relaxation values were expressed as percentage decreases of the phenylephrine (3 txM)-induced contraction.

2.5. Drugs

The following pharmacological agents were used: 1-phenylephrine • HC1, acetylcholine chloride and sodium nitroprusside all obtained from Sigma Chemi- cal Co. (St. Louis, MO, USA). The drugs were dis- solved in distilled water and then diluted in buffer.

2. 6. Calculations and statistical analysis

The results are expressed as means + S.E.M. The significance of the difference between group means was analyzed by one-way analysis of variance (ANOVA) and the Student's t test. P values of less than 0.05 were considered to be statistically significant.

3. Results

3.1. Body weight, plasma lipids and lipoproteins

The body weight, plasma lipid and lipoprotein pro- files of WKY and SHR fed either normal chow or HC-HF chow are summarized in table 1. Body weight, total plasma cholesterol, VLDL-, LDL-cholesterol, total plasma triglyceride, VLDL-, LDL- and HDL-triglyceride levels were markedly elevated in HC-HF chow fed WKY and SHR as compared with the respective normal chow-fed rats. However, the HDL-cholesterol was decreased in HC-HF chow rats (47% in SHR and 53% in WKY) (table 1). The increases of plasma cholesterol and triglyceride levels were t ime-dependent (fig. 1). The increase in total plasma cholesterol appeared generally to plateau out, whereas the triglyceride level appeared to increase over the whole time period of the study. Although the data also showed that high levels of cholesterol and triglyceride were achieved in SHR than in WKY, there were no statistically significant differences (fig. 1).

3.2. Histological examination of the intimal surface of the isolated blood L~essels

No visible atherosclerotic lesions were detected in any of control blood vessels studied. In the aortic arches and abdominal aortas obtained from the SHR and WKY fed the HC-HF chow for 8 weeks, evoked intimal lesions (surface area damage and fatty streak formation) were more pronounced than those noted after 4 weeks of HC-HF chow fed (table 2). The aortic arches of SHR and WKY were significantly more affected (P < 0.01) by the intimal lesion (surface area damage and fatty streak formation) than the abdominal aortas of the respective rat strain. The damage of surface area and thickness of fatty streaks were signifi-

291

TABLE 1

Body weight, total plasma-, VLDL-, LDL-, HDL-cholesterol, total plasma-, VLDL-, LDL-, HDL-triglyceride levels in normal chow-fed and HC-HF diet-fed (8 weeks) WKY and SHR

Values are means_+ S.E.M. (n = 15-20); ~ P < 0.01, b p < 0.001 as compared with the respective normal chow rats (Student 's t test).

Parameter WKY SHR

Normal HC-HF Normal HC-HF chow chow chow chow

Body weight a t s t a r t ( g ) 231+_ 6 240_+10 237+_5 241+_ 4

Body weight at 8 weeks (g) 355+_10 422+_ 9 b 370+_9 443_+11 8

Cholesterol (mg/d l ) Total 55_+ 1 218_+ 6 b 60_+1 244+_ 3 b VLDL 8+_ 2 38_+ 2 b 9_+2 40_+ 1 b LDL 28+_ 2 170+_ 4 b 36+_2 195_+ 3 8 H D L 17+_ 1 9_+ l b 19_+3 9_+ 1 ~'

Triglyceride (mg /d l ) Total 114-+14 231_+10 b 9 7 + 6 271_+12 b VLDL 91 +- 4 180+- 8 b 76+_3 200-+ 5 b LDL 12+_ 4 22_+ 5 ~ 9_+2 35_+ 7 a H D L 8_+ 3 26_+ 2 b 12-+1 30_+ 4 b

A.

A

ZSO,

0

o

"~ 150

0

o 100 . U

~ 50-

~ 0

B.

O - - O Control

A - - , ~ S H R _ , ~ , - ' - - - -

~/ ~-------o----o~O o o o o

I I I I t I I I I 0 1 2 3 4 6 8 7 8

300-

~ 250-

ILl

200.

~ 150.

0 1 0 0

T

O 1 2 3 4 5 8 7 8

W e e k s

Fig. 1. Total plasma cholesterol (A) and triglyceride (B) levels of control rats (©), HC-HF diet-fed rats (WKY • SHR z~) during experimental periods. Cholesterol and triglyceride levels are pre-

sented as means_+ S.E.M. (n = 15-20).

292

TABLE 2

Microscopic evaluation of fatty streak formation in aortas of hyperlipidemic rats

None of the control tissues examined showed any significant fatty streak formation. Surface area damaged is shown as percent of total surface area. Thickness of the fatty streak is expressed as percent of the total wall thickness. Values are means-+ S.E.M. (n = 6); a p < 0.05, h p < 0.01 and c p < 0.001 as compared to that noted in rats after 4 weeks of HC-HF diet.

Fed HC-HF chow (4 weeks) Fed HC-HF chow (8 weeks)

Surface area Thickness Surface area Thickness

WKY SHR WKY SHR WKY SHR WKY SHR

Aortic arch 45 -+ 5 65 _+ 4 21 ± 8 34 ± 4 70 ± 5 b 80 ± 6 a 74 ± 8 c 83 +_ 5 c Abdominal aorta 20 ± 3 33 ± 3 14 ± 6 25 ± 4 50 ± 5 c 66 ± 4 c 68 -+ 8 c 80 ± 4 ~

TABLE 3

Evaluation of vascular responses in the aortic arch of control and hyperlipidemic WKY rats

Values are means -+ S.E.M. (n = 6-8); value significantly different from that noted in the control tissue (endothel ium (+)) , ~'P < 0.01, b p < 0.001.

WKY fed normal chow WKY fed HC-HF chow

Endothel ium ( + ) Endothel ium ( - ) WKY (1 week) WKY (4 weeks) WKY (8 weeks)

Maximum response Phenylephrine (g) 0.92 + 0.09 1.45 ± 0.15 ~ 1.40 +_ 0.10 ~ 1.90 + 0.12 t, 2.29 +_ 0.15 h Acetylcholine (%) 74.01 ± 4.21 5.21 ± 0.19 b 49.50 + 3.00 26.25 -+ 0.74 h 15.00 ± 3.61 b Nitroprusside (%) 94.21 + 3.11 90.21 -+ 0.11 91.50 -+ 4.12 89.00 -4-_ 3.00 95.01 ± 2.22

EDs0 value (× 10 7 M) Phenylephrine 2.00 ± 0.12 0.49 ± 0.04 b 1.00 + 0.15 h 0.52 -+ 0.03 b 0.40 ± 0.08 b Acetylcholine 2.79 ± 0.05 - 5.99 -+ 0.10 h 10.52 ± 0.15 b 10.01 --4-_ 0.04 b Nitroprusside 1.59 ± 0.08 1.61 ± 0.05 1.50 -+ 0.09 1.43 ± 0.04 1.55 -+ 0.06

TABLE 4

Evaluation of vascular responses in the aortic arch of control and hyperlipidemic SHR

Values are means ± S.E,M. (n = 6-8); value significantly different from that noted in the control tissue (endothel ium intact), b p < 0.001.

SHR fed normal chow SHR fed HC-HF chow

Endothel ium intact 1 week 4 weeks 8 weeks

Maximum response Phenylephrine (g) 1.00+_0.08 1.80_+0.15 b 2.21 --+0.08 h 2.59-+0.14 h Acetylcholine (%) 70.21 + 3.65 42.50 + 5.20 b 18.17 + 1.42 h 11.00 ± 3.62 h Nitroprusside (%) 96.51 ± 3.59 90.50 + 3.40 93.22 4-_ 5.11 89.00 -+ 3.00

ED50 value (× 10 v M) Phenylephrine 1.79 -+ 0.23 0.88 -+ 0.18 b 0.48 ± 0.05 b 0.30 ± 0.03 b Acetylcholine 3.10 ± 0.06 7.00 ± 0.10 b 11.21 + 0.04 t, 11.11 -+ 0.05 b Nitroprusside 1.58 ± 0.07 1.49 ± 0.10 1.43 ± 0.09 1.70 ± 0.09

c a n t l y a u g m e n t e d w i t h t h e p e r i o d t h e r a t s w e r e f e d

H C - H F c h o w .

A f t e r s t a i n i n g t h e i n t i m a l s u r f a c e o f t h e a r t e r i a l

s e g m e n t s w i t h A g N O 3, t h e t y p i c a l m o s a i c p a t t e r n o f

s i l v e r l i n e s , c o n s i d e r e d t o r e p r e s e n t t h e b o r d e r s o f

a d j a c e n t e n d o t h e l i a l c e l l s , w a s d e t a c h e d in al l t h e

a r t e r i e s i n v e s t i g a t e d . T h e o n l y d i f f e r e n c e d e t e c t e d w a s

t h a t t h e e n d o t h e l i a l c e l l s o f t h e h y p e r l i p i d e m i c a r t e r i e s

s e e m to b e c h a n g e d in s h a p e , c o m p a r e d t o t h o s e o f

c o n t r o l a r t e r i e s .

3.3. Vascular reacti~'ity in hyperlipidemic rats

I n c r e a s i n g c o n c e n t r a t i o n s o f p h e n y l e p h r i n e ( 0 . 0 1 - 3 0

/ z M ) e v o k e d c o n c e n t r a t i o n - d e p e n d e n t c o n t r a c t i o n in

c o n t r o l s e g m e n t s o f t h e a o r t i c a r c h a n d a b d o m i n a l

a o r t a ( f ig . 2). M e c h a n i c a l r e m o v a l o f e n d o t h e l i u m o f

a o r t i c a r c h d i d s i g n i f i c a n t l y a t t e n u a t e E D s 0 v a l u e a n d

a u g m e n t m a x i m a l r e s p o n s e t o p h e n y l e p h r i n e ( t a b l e 3).

I n t h e a o r t i c a r c h e s o f W K Y a n d S H R r e c e i v i n g H C - H F

A.

g . B

" • ~,0 v

0

C 1.0

C--,

Aortic arch

T r .....-¢x

;~//• o_____o___~__o

0 ( ~ ) ~ 1 I I I I I I

B. Abdominal aorta

2.0- T I

2 °

° ~ l ~o - - - " ' ~

0.01 0 . 1 1 1 0

Phenylephrine (/.~M)

Fig. 2. Contractile responses to phenylephrine in aortic arch and abdominal aorta obtained from control rats (endothelium intact, ©) and those receiving 8 weeks of HC-HF diet (WKY, e; SHR, ~).

Tension (g) is presented as mean + S.E.M. (n = 5-8).

chow for 1-8 weeks, significantly attenuated EDs0 values and augmented maximal responses for phenylephrine were obtained (tables 3 and 4). In aortic arches from hyperlipidemic rats, the increases in maximal

A.

o-

2 0 .

"~N 0 0

100

Aortic arch

I I I I I I I t

R. Abdominal aorta

0

20

80

8o

~ .

1 0 0 ~ * 0.11 I I I I 0 0.01 1 I

Acetylcholine (/~M)

Fig. 3. Endothelium-dependent relaxation responses to acetylcholine in aortic arch (A) and abdominal aorta (B) obtained from control rats (endothelium intact, ©) and those receiving 8 weeks of HC-HF diet (WKY, e; SHR, z~). Relaxation is expressed as percent decreases in the phenylephrine (3 p.M)-induced contraction. Each

point represents the mean + S.E.M. (n = 6-8).

293

A.

0 -

20-

001 00-

100

Aortic arch

I I I I P I I

B.

0 . Abdominal a o r t a

eo

N 80- .

'~ RO. [3c~ t~ A

1 0 0 I I I I 0 . 0 1 0 . 1 1 1 0

NiLroprusside (pM)

Fig. 4. Endothelium-independcnt relaxation responses to nitroprus-

side in aortic arch (A) and abdominal aorta (B) obtained from

control rats (endothelium intact, ©) and those receiving 8 weeks of

HC-HF diet (WKY, e; SHR, Lx). Relaxation is expresscd as percent

decrcases in the phenylephrine (3 /xM)-induced contraction. Each

point represents the mean _+ S.E.M. (n = 6-8).

contraction induced by phenylephrine were time-dependent, and in SHR groups were more pronounced than in WKY.

During contractions caused by phenylephrine (3 ~M) for 15 rain, cumulative addition of acetylcholine or nitroprusside (0.01-30 ~M) evoked concentration-dependent relaxation in aortic arch and abdominal aorta of normal chow-fed WKY and SHR (figs. 3 and 4). However, whilst the concentration-dependent relaxation to acetylcholine was significantly attenuated, that to nitroprusside was unaffected in the denuded aortic arch (table 3). In the aortic arches obtained from the 1-8 week HC-HF chow-fed (hyperlipidemic) rats, acetylcholine evoked relaxation was progressively attenuated, while that of nitroprusside was unaffected during atherosclerotic progression, compared to those noted in the respective control endothelium intact aortas (tables 3 and 4). In abdominal aortas obtained from 1-8 weeks hyperlipidemic rats, contraction to phenylephrine were also enhanced and relaxation to acetylcholine were attenuated during atherosclerotic progression (data not shown).

4. D i s c u s s i o n

The results of this study indicate that when fed a

HC-HF chow, WKY and SHR have elevated total

294

plasma cholesterol, VLDL-, LDL-cholesterol, total plasma triglyceride, VLDL-, LDL- and HDL-triglyceride levels as compared with normal chow-fed rats. These data are consistent with those of Uchida et al. (1978), who reported that rats hyper-respond to HC-HF feed- ing. There is evidence from epidemiological studies that increases in plasma cholesterol, triglyceride and decreases in HDL-cholesterol levels are risk factors for atherosclerosis and coronary heart disease (Miller and Miller, 1975).

The microscopic evaluation performed on the aortas indicates that evoked surface area damage and fatty streak formation on the aortic arches and abdominal aortas from SHR and WKY fed the HC-HF chow for 8 weeks, were more pronounced than those noted after 4 weeks of HC-HF chow fed. The aortic arches of SHR and WKY were significantly more affected by intimal lession than the abdominal aortas of the respective rat strain. These results indicate that damage of surface area and thickness of fatty streak were significantly augmented with the length of time the rats were fed HC-HF chow, and the severity of intima lesions can vary depending on the anatomical localization of the blood vessel studied.

In aortic arches and abdominal aortas from hyperlipidemic rats, contraction caused by phenylephrine were enhanced during atherosclerotic progression (tables 3 and 4). Hyperlipidemia is known to enhance the sensitivity of circulatory system to adrenergic stimulus. Heistad et al. (1984) and Broderick and Tulenko (1984) reported that dose-response changes in coronary vascular resistance to noradrenaline was shifted to the left in hyperlipidemic rabbits and monkeys. They suggested that this could be due to an increase in either the number or affinity of the a-adrenoceptors (Nanda and Henry, 1982; Yokoyama et al., 1983). The results of the present study show an enhanced vasoconstrictor responsiveness to phenylephrine in rats with high levels of plasma lipids. Although the mechanism behind these responses remains to be determined, the data suggest that adrenergic vascular control is altered in the presence of high levels of plasma lipid in rats. Results from several studies have demonstrated that there is a greater sympathetic activity and norepinephrine concentration in SHR (Yu et al., 1992). Furthermore, hyperlipidemia increases the rate of migration and proliferation of smooth muscle cells (Faggiotto et al., 1984), and these lesions could be facilitated by hypertension via an increase in the wall stress (Koletsky et al., 1968). These may explain the greater contraction to phenylephrine in SHR than in WKY.

In hyperlipidemic rat aortas, endothelium-dependent relaxation to acetylcholine was decreased and was progressively attenuated during atherosclerotic progression, while relaxation to nitroprusside was well preserved. There are a number of reports which

demonstrate that endothelium-dependent relaxation is impaired in atherosclerotic arteries obtained from rabbits and man (Hirata et al., 1992). There are several possible mechanisms for the impairment of endothelium-dependent relaxation in atherosclerotic arteries. First, the production a n d / o r release of E D R F may be decreased. Second, the thickened intima may reduce the diffusion of EDRF, the half-life of which is ex- tremely short. Third, the characteristics of smooth muscle cells and their sensitivity to E D R F may be changed. The last possibility is unlikely because the relaxation to nitropursside, which induces relaxation through activation of guanylate cyclase in smooth muscle cells as does EDRF, was well maintained in atherosc[erotic arteries. Recently, Kolodgie et al. (1990) have demonstrated that endothelium-mediated relaxation is reduced in hyperlipidemic rabbits because of a loss of endothelial cells. At present, precise mechanisms for the attenuation of EDRF-mediated relaxation in atherosclerotic rat aortas are still uncertain (Yu et al., 1993).

In conclusion, the results of the present study show that in the aortas isolated from hyperlipidemic rats: (1) the contractions caused by phenylephrine are augmented, (2) the endothelium-dependent relaxations by acetylcholine are progressively impaired, and (3) the endothelium-independent relaxations by nitroprusside are not modified.

Acknowledgements

This work was supported by a research grant of the National Science Council of the Republic of China (NSC82-0412-B002-090) and CMRP 377 from Chang Gung Medical Research Foundation.

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Effects of hyperlipidemia on the vascular reactivity in the Wistar-Kyoto and spontaneously hypertensive rats