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Departments of Nephrology and Medicine, Herlev Hospital, Herlev,
Denmark.
Department of Clinical Physiology, Herlev Hospital, Herlev,
Denmark.
Correspondence to Dr. Dan Wang, Department of Nephrology, Herlev Hospital, University of Copenhagen, Herlev Ringvej 75, DK-2730 Herlev, Denmark. Phone: +45 44883781; Fax: +45 44884615; E-mail: wangdan9090{at}hotmail.com
| Abstract |
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| Introduction |
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Endothelium-dependent relaxation of arteries can be induced in vitro by acetylcholine (ACh), which functions as an activator of nitric oxide synthase (NOS) by increasing intracellular calcium (6). An impaired relaxation response of resistance vessels to ACh has been demonstrated and proposed as a contributory factor to vascular disease in essential hypertension and diabetes mellitus (7,8,9,10). In Han:SPRD polycystic kidney disease rats, we have demonstrated that endothelium-dependent relaxation of resistance vessels was impaired (11). Accordingly, endothelial dysfunction of resistance vessels might be present in patients with ADPKD as well, and might contribute to the development of hypertension.
To test this hypothesis, the present study was designed to elucidate whether there is an impairment of ACh-induced endothelium-dependent relaxation in patients with ADPKD and whether this could be influenced by NOS substrate and inhibitor.
| Materials and Methods |
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Twenty-four hour ambulatory BP was measured with the Takeda 2420 monitor. GFR was measured with the 4-h one sample plasma clearance of 51Cr-ethylenediaminetetra-acetic acid (12). These measurements were done within 1 wk of the subcutaneous fat biopsy described below.
Preparation of Small Subcutaneous Vessels
All subjects arrived in the laboratory between 8 and 9 a.m. and had fasted
since the previous evening. From each individual, a biopsy of subcutaneous fat
of 1.0 x 0.5 x 0.5 cm was obtained from the gluteal region under
local anesthesia with 1% lidocaine hydrochloride. Arteries were carefully
dissected from the biopsy under a dissecting microscope (Olympus SN450). Two
segments of the same artery (about 2 mm in length with a mean diameter of
<300 µm) were isolated as described previously
(13). Vessels were mounted as
ring preparations on two 40-µm stainless steel wires in an isometric
Mulvany-Halpern small-vessel myograph (J.P. Trading, Science Park, Aarhus,
Denmark) (Figure 1) (14). One wire was attached to
a force transducer and the other was attached to a micrometer
(13,14).
This arrangement enabled the wall tension to be measured at a predetermined
internal circumference. Both dissection and mounting of the vessels were
carried out in cold (4°C) PSS solution (118 mmol/L sodium chloride, 25
mmol/L sodium bicarbonate, 4.5 mmol/L potassium chloride, 2.5 mmol/L calcium
chloride, 1.0 mmol/L magnesium sulfate, and 6.0 mmol/L glucose). The two
segments of resistance vessels from each individual were studied in parallel.
One was treated by the experimental protocol described below; the other was
used as a time control and was treated only with repeated courses of
contraction with noradrenaline (NA) (10-5 mol/L).
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Vessel Experimental Procedure
Once mounted, the resistance vessels were warmed to 37°C in PSS and
allowed to equilibrate for 30 min, with the vessels' internal circumference
set to give a wall tension of 0.2 mN/mm. The myograph chambers were bubbled
with 5% CO2 and 95% O2 to maintain a pH of 7.4.
Morphologic measurements of wall thickness were then performed with a
precalibrated filar micrometer eyepiece with a resolution of 1 µm. The
cross-sectional wall area of each vessel could then be calculated. These
measurements were made with the vessel relaxed and minimal passive stretch
(wall tension of 0.2 mN/mm), and normalized results were calculated by a
computer program (Myosight, J.P. Trading, Science Park, Aarhus, Denmark). The
resting tension/internal circumference relationship for each vessel was
determined and then the internal circumference was set to 0.9 x
L100, where L100 is the internal
circumference the vessel would have had in vivo when relaxed and
under a transmural pressure of 100 mmHg
(15). After this normalization
process, the vessels were incubated in PSS for 30 min before further study.
During this baseline period, the PSS was replaced at 10-min intervals. Vessels
were then contracted by 3 x PSS containing 10-5 mol/L NA,
followed by one exposure to high-potassium PSS (during which sodium chloride
was replaced by potassium chloride) and one exposure to PSS containing
10-5 mol/L NA, respectively. Contractions were maintained for 3 min
before rinsing with PSS back to baseline. After this stimulation procedure,
the vessels were rinsed three times with fresh PSS and left to recover at
baseline for 20 min.
Maximal contraction of the vessels was then achieved by incubation with 10-5 mol/L NA. When a plateau of contraction had been reached, relaxation was induced by adding cumulatively increasing concentrations of ACh (10-9 to 10-5 mol/L) in the presence of an unchanged concentration of NA. Afterward, the bath was rinsed with PSS three times, and the vessels were allowed to recover for at least 15 min. Then the vessels were maximally contracted with NA (10-5 mol/L) and relaxed with cumulatively increasing concentrations of the endothelium-independent vasodilator (nitric oxide [NO] donor) 3-morpholino-sydnonimine (SIN-1, 10-9 to 10-3 mol/L), again in the presence of an unchanged concentration of NA. The vessels were then rinsed to baseline and incubated with L-arginine (substrate of NOS, 10-3 mol/L) for 30 min, and subsequently the NA contraction and ACh relaxation response were studied in the presence of L-arginine. Finally, the vessels were rinsed with PSS and incubated with the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 10-4 mol/L) for 30 min, after which the NA contraction and ACh relaxation response were studied in the presence of L-NAME.
All solutions were freshly prepared 1 d before the experiment. ACh, NA, SIN-1, and L-NAME were purchased from Sigma (St. Louis, MO). All reagents were prepared in distilled water and diluted to the final bath concentration with PSS.
Statistical Analyses
All data are expressed as mean ± SD. Statistical differences were
evaluated by two-tailed t test or by Newman-Keuls statistics. ACh
sensitivity is expressed in terms of pED50, which is the
(Log) concentration of the drug required to produce 50% of the maximum
response. Values of relaxation response to ACh and SIN-1 were expressed as a
percentage decline in the maximum contractile response. Statistical
significance was defined as P < 0.05. Statistica 5.0 (StatSoft,
Tulsa, OK) was used as software.
| Results |
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Morphology of Vessels
There were no significant differences in vessel diameter, wall thickness,
wall area, and wall to lumen ratio between ADPKD patients and healthy control
subjects (Table 2).
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Endothelium-Dependent Relaxation
ACh Response. The maximum relaxation rate (Emax)
and pED50 by ACh were significantly attenuated in
maximum-contracted resistance vessels from patients with ADPKD
(Emax: 71.5% ± 12.1%; pED50: 6.72
± 0.75) compared with the healthy control subjects
(Emax: 85.2% ± 8.7%; pED50: 7.15
± 0.71) (P < 0.01)
(Figure 2).
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Effect of L-Arginine. Incubation with the substrate of NOS, L-arginine, resulted in an increased maximum relaxation to ACh of resistance vessels from healthy control subjects (from 85.2 ± 8.7 to 92.3 ± 5.7%, P < 0.05) (Figure 3A). The ACh dose-response curves of the vessels from healthy control subjects were shifted to the left in the presence of L-arginine (pED50 from 7.15 ± 0.71 to 7.45 ± 0.79; P < 0.001) (Figure 3A). A slight, nonsignificant leftward shift of the response to ACh in the presence of L-arginine was observed in vessels from patients with ADPKD (Figure 3B). The dose-response curve to ACh was significantly different in vessels from healthy control subjects and patients with ADPKD in the presence of L-arginine (Emax: 92.3 ± 5.7% versus 78.2 ± 8.1%, control versus ADPKD, P < 0.001) (Figure 4A). Incubation with L-arginine hence increased the difference in response to ACh between patients and control subjects (Figure 4A).
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Effect of L-NAME. Incubation with the NOS inhibitor L-NAME resulted in a significant decrease in the maximum relaxation response to ACh of resistance vessels from healthy control subjects (from 85.2 ± 8.7 to 68.1 ± 6.6%) (Figure 3A). The ACh dose-response curves showed a significant rightward shift in the presence of L-NAME (pED50: from 7.15 ± 0.71 to 6.56 ± 0.77, P < 0.01) (Figure 3A). Incubation with L-NAME did not influence the response to ACh in vessels from patients with ADPKD (Figure 3B). Hence, in the presence of L-NAME the dose-response curve to ACh was not significantly different in vessels from patients and control subjects (Figure 4B).
Endothelium-Independent Relaxation (Relaxation Response to
SIN-1)
The SIN-1 dose-response was identical in resistance vessels from ADPKD
patients and healthy control subjects
(Figure 5).
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Time Control of Vessel Response
The time-control studies showed no difference in vessel constriction
response to NA for the duration of the experiment. The relaxation response of
the time-control result to ACh in increasing dose was identical at start at
baseline and after incubation in PSS for 30 min. We have found previously that
the maximum response of ACh is similar between the dose-response curves
performed after incubation in PSS for 30 min and after incubation in PSS for
60 min and 120 min. Moreover, in another time-control experiment with
subcutaneous resistance arteries from healthy control subjects and ADPKD
patients, we found that NA maximum responses and sensitivity to ACh remained
unchanged within a 10-h period (data not shown).
| Discussion |
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The results of the present study demonstrated that ACh-induced endothelium-dependent relaxation was impaired in resistance vessels from patients with ADPKD. Hence, endothelial dysfunction was present in ADPKD, even though the subjects were still in the early normotensive phase of the disease. By contrast, endothelium-independent relaxation response to an NO donor (SIN-1) was similar in patients and control subjects (Figure 4), demonstrating that the impairment of endothelium-dependent relaxation was not due to a decreased ability of vascular smooth muscle to respond to exogenous NO. The role of NO was further assessed by the effect of incubation with the NOS substrate L-arginine and the NOS inhibitor L-NAME on ACh-induced vasodilation. In healthy control subjects, the effect of ACh was increased by L-arginine and impaired by L-NAME, whereas in ADPKD neither of these substances significantly influenced the ACh relaxation response. Hence, L-arginine increased the difference between the ACh response in healthy control subjects and ADPKD patients, while L-NAME-resistant or NO-independent vasodilation was the same in both groups (Figure 4). In ADPKD, dysfunction of endothelium-dependent vasorelaxation thus seemed to be associated with a defective NO release from the endothelium. Interestingly, in a recent study of blood vessels from endothelial NOS (eNOS) knockout (-/-) mice (19), isolated aorta and carotid and coronary arteries did not relax in response to ACh and endothelial-derived hyperpolarizing factor. In eNOS (+/+) control mice, the endothelium-dependent relaxation to ACh involved either NO or the combination of NO plus a product of cyclo-oxygenase. These findings demonstrate that eNOS plays an important role in endothelium-dependent relaxation, and that eNOS impairment in ADPKD endothelium may be the cause of the findings of the present study.
The changes in small artery function in ADPKD patients observed in the present study seemed to be independent of structural vascular changes, because parameters of resistance arterial structure were identical in both ADPKD patients and healthy control subjects. Similar observations have been made by our group in mesenteric arteries from young polycystic kidney disease rats (11) and suggest that endothelial dysfunction and defective endothelial NO generation may be early features in ADPKD, contributing to the development of hypertension and vascular disease well before renal function starts to decline.
A decreased endothelium-dependent relaxation response in isolated small arteries has also been demonstrated in patients with essential hypertension (20,21,22), and in patients with diabetes mellitus (9,23). In most of these studies, as in the present one, there was no change in the relaxation response to endothelium-independent vasodilators such as sodium nitroprusside. The question of whether endothelial dysfunction is an early feature of vascular disease in hypertension and diabetes mellitus, as it appears to be in ADPKD, has not been studied.
The relationship between the endothelial NO system and the well-described genetic defect in the various types of ADPKD is uncertain. Polycystin 1, which is the gene product of the most frequent form of ADPKD, has been demonstrated in the wall of aneurysms from patients, and also in large vessels from patients and control subjects (4). It is uncertain whether polycystins also are present in small resistance vessels such as those used in the present study. The functional role of vascular polycystin has not been elucidated, but it would be a likely participant in the chain of events leading to vascular disease in ADPKD, possibly with NOS dysfunction also involved.
An impairment of the NO system may also be operative in the kidneys in animal models of polycystic disease. Thus, in the kidneys of rats with polycystic disease, expression of NOS isoenzymes decreases as cyst development progresses (24). Taxol, which is an inducer of NOS, inhibits cyst growth and loss of renal function in mice with polycystic kidney disease (25,26). Interestingly, expression of endothelin receptors increases in the course of cyst growth (27).
In conclusion, the present study demonstrated impaired endothelium-dependent relaxation in resistance vessels from patients with ADPKD with a normal BP and renal function. This impairment may be a factor that contributes to the development of hypertension and vascular disease later in life. A reduced ACh response to substrate and inhibitor of NOS was also demonstrated, suggesting that an impaired function of this enzyme may be involved in the mechanism of endothelial dysfunction in ADPKD. It may be speculated that treatment with an exogenous NO donor could reduce the cardiovascular manifestations of disease in these patients.
| Acknowledgments |
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| References |
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