(Hypertension. 2001;37:386.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Department of Physiology, Medical College of Wisconsin, Milwaukee.
Correspondence to Andrew S. Greene, PhD, Medical College of Wisconsin, 8701 Watertown Plank Rd, PO Box 26509, Milwaukee, WI 53226-0509. E-mail agreene{at}mcw.edu
| Abstract |
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Key Words: blood vessels muscle, skeletal growth substances renin-angiotensin system chromosome 13
| Introduction |
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VEGF, a 45-kDa homodimeric glycoprotein mitogen, has been considered to be an important regulator of angiogenesis, which increases vascular permeability and promotes endothelial proliferation.6 Results from our laboratory have supported the hypothesis that VEGF is an important regulator of the angiogenesis induced by electrical stimulation, inasmuch as the angiogenic response was inhibited by a VEGF-neutralizing antibody.3
In our laboratory,7 we have derived a congenic strain of Dahl SS/Jr/Hsd/MCW (SS) rats, in which 10 cm of a region of chromosome 13 containing the renin gene of Dahl R rats was transferred onto the Dahl S genetic background (S/renRR). We have previously demonstrated that although the SS rat is unable to modulate renin activity in response to alterations in sodium intake, transfer of the resistant renin gene confers normal salt sensitivity to the S/renRR rat.7 These congenic rats provide a model to evaluate the importance of the modulation of the RAS in VEGF expression and angiogenesis in skeletal muscle. In the present study, we have used the chromosome 13 congenic rat to test the hypothesis that VEGF expression and physiological angiogenesis in skeletal muscle induced by electrical stimulation can be modulated by the RAS. A map of chromosome 13 is shown in Figure 1.
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| Methods |
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The effectiveness of chronic inhibition of ACE by oral lisinopril was determined by an intravenous bolus of angiotensin I (Ang I, 0.1 mL at 1 ng/mL IV).3
Tissue Harvest and Morphological
Analysis of Vessel Density
After 7 days of stimulation, the animals were
euthanized by an overdose of Beuthanasia solution (Sigma Chemical Co),
and the stimulated and contralateral unstimulated muscles were removed,
rinsed in physiological salt solution, and weighed.
A 100-mg section was taken from TA muscle and immediately frozen in
liquid nitrogen for Western analysis. The remaining TA and the
EDL were lightly fixed overnight in 0.25% formalin solution and
sectioned.8 From every
animal, sections of EDL and TA were immersed in a solution of 25
µg/mL rhodamine-labeled Griffonia
simplicifolia I lectin (Sigma). After a 2-hour exposure to
G simplicifolia I lectin, the
muscles were rinsed and mounted on microscope
slides.9 The sections were
visualized by using a video fluorescent microscope system with
epi-illumination (Olympus ULWD CD Plan, x20 objective). Ten
representative fields were selected for study from each
muscle section. Images were digitized and quantified with an automated
computer vessel counting as previously
described.10
Plasma Renin Activity
The method used for measurements of plasma renin
activity (PRA) was previously described by Rieder et
al.11 Arterial
blood was collected in tubes containing K3EDTA
and immediately centrifuged at
1500g and 4°C. The samples
were thawed on ice, and neomycin sulfate (0.1%), phenylmethylsulfonyl
fluoride (0.25%), and maleic anhydrate (0.2 mol/L) were added
to 50 µL of sample to inhibit converting enzyme and protease
activity. After generation, the Ang I concentration was determined by
radioimmunoassay.11
Western Blot Analysis to Detect the
Presence of VEGF Protein
The 100 mg of TA was homogenized, and the
protein was suspended in 0.1 mol/L potassium buffer
(KPO4). Five micrograms of protein (as
determined by protein assay kit, Bio-Rad) from each muscle and a tumor
cell line, known to express VEGF at high levels
(C6, American Type Culture Collection, 107-CCL),
were separated on a 12% denaturing polyacrylamide gel. The
gels were transferred to a nitrocellulose membrane, which was blocked
for 2 hours in 5% nonfat dry milk diluted in Tris-buffered saline
(50 mmol/L Tris and 750 mmol/L NaCl, pH 8) with 0.08% Tween
20 (Bio-Rad). The blots were then incubated with a polyclonal antibody
to a peptide derived from the human VEGF sequence (1:1000 dilution,
clone G143-850, Pharmingen) for 2 hours at room temperature. Washed
blots were then incubated with goat anti-mouse secondary antibody at a
dilution of 1:1000 for 1 hour at room temperature and then subjected to
a SuperSignal West Dura chemiluminescence substrate (Pierce) detection
system. Membranes were exposed to x-ray film (Fuji Medical) for 30
seconds and developed by using a Kodak M35 X-Omat processor. For the
quantitative VEGF analysis, film was always exposed for a
period of time that ensured that all signals were within the linear
range of the detection of the film. The VEGF band intensity was
quantified by using a Scion Image Beta 4.02 for Windows (Scion Corp),
and values were expressed as a percentage of the
C6 tumor cell standard.
Data Analysis and Statistics
For each muscle, the vessel counts of all the
selected fields were averaged to a single-vessel density. Vessel
density was expressed in terms of mean number of vessel-grid
intersections per microscope field (0.224
mm2 ). For each experimental
group, the measured vessel density of the stimulated muscle was
compared with its unstimulated counterpart. All values are
presented as mean±SEM. The significance of differences in
values measured in the same animal was evaluated by a 2-factor ANOVA
(dietxstimulation) with repeated measures on 1 factor (stimulation).
To evaluate the significance of differences in vessel density between
stimulated and unstimulated sides, a 1-way ANOVA was performed.
Significant differences were further investigated by a post hoc test
(Tukey).
| Results |
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The mean arterial pressure of the S/renRR-LS group was significantly higher than that of the SS-LS group (148.67±1.12 versus 113.3±2.25 mm Hg, respectively). When the high salt diet was introduced to the S/renRR animals (S/renRR-HS), the mean arterial pressure increased to 172.0±4.78 mm Hg. The blockade of ACE in S/renRR-LS rats, by lisinopril, effectively lowered the mean arterial pressure to 100.76±7.46 mm Hg (S/renRR-LSLIS). The control value of PRA in the SS-LS group was (1.69±0.50 ng Ang I/mL per hour), and this value was not affected by increasing dietary sodium intake (1.27±0.52 ng Ang I/mL per hour for the SS-HS group). Transfer of the region containing the renin gene restored the PRA responses. The renin activity was high in the S/renRR-LS group (4.59±1.51 ng Ang I/mL per hour) and was suppressed by 72% with a high salt diet (1.29±0.22 ng Ang I/mL per hour for S/renRR-HS).
In contrast to previous findings in other strains of rats,3 8 7 days of electrical stimulation resulted in no increase in vessel density in either the TA (Figure 2, top left) or EDL (Figure 2, top right) of SS rats on a low salt diet. Transfer of the region of chromosome 13 containing the normally functioning renin gene from the R rat onto the S background (S/renRR) restored the angiogenic response to electrical stimulation in both TA (Figure 2, top left) and EDL (Figure 2, top right). In the S/renRR group, the suppression of RAS by high salt or blockade of the RAS by lisinopril restored the antiangiogenic phenotype observed in the SS rats in both muscles (Figure 2). Neither sodium intake nor ACE blockade had any effect on vessel density in the unstimulated muscles of either rat strain. Vessel density analysis on sham-operated animals showed that stimulation alone had no effect on vessel density.
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Because we have previously shown that the angiogenesis induced by electrical stimulation is largely mediated by VEGF, Western blot analysis was performed to detect the presence of VEGF protein in stimulated and control TA muscles of congenic rats (Figure 3, top). A tumor cell line, known to be rich in VEGF protein, was used as a positive control. Figure 3 (top) shows that the intensity of the bands was relatively high for tumor cells (VEGF, lane 1) and stimulated S/renRR-LS rats (VEGF, lane 5). In contrast, the contralateral unstimulated muscle had very faint band intensity (lane 4). Treatment with high salt diet and lisinopril did not alter the intensity of the bands of unstimulated muscles (lanes 2, 4, and 8) and also blocked the presence of VEGF protein in rats stimulated for 7 days (lanes 7 and 9). Quantitative densitometry was used to compare the responses of VEGF after 7 days of stimulation in all of the animals. As shown in Figure 3 (bottom), VEGF protein levels were significantly increased by stimulation in only the S/renRR-LS group (P<0.05).
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| Discussion |
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We have previously transferred in our laboratory7 10 cM of a region of chromosome 13 containing the renin gene from the Dahl R rat onto the Dahl S background and derived a congenic strain of Dahl SS/Jr/HsD/MCW rat (S/renRR). Results from the present study confirmed that in SS rats, PRA is lower and unresponsive to known physiological stimuli of the RAS. In contrast, the congenic S/renRR group exhibited the expected changes in PRA. In the present study, we investigated the effects of electrical stimulation on VEGF protein expression and angiogenesis in these rats by use of 2 different designs, physiological and pharmacological manipulations of the RAS.
Physiological angiogenesis in skeletal muscle as a result of electrical stimulation3 8 12 13 14 15 and physical exercise16 17 has been extensively studied. However, the mechanisms responsible for angiogenesis in adult animals remain unclear. Angiogenesis in adult skeletal muscle does not necessarily follow the standard description: capillary growth may occur without the breakage of the basement membrane and without proliferation of endothelial cells, whereas proliferation may precede and not follow migration.14 Several growth factors have been postulated to play an important role in the vasculogenesis and differentiation of the cells in the vascular wall, including smooth muscle cells. Among them, VEGF has been considered to be a potent angiogenic factor and a specific mitogen for endothelial cells.6 18 Our previous results3 support the hypothesis that VEGF plays an important role in regulating the angiogenesis induced by electrical stimulation, inasmuch as the angiogenic response was inhibited by treatment with VEGF-neutralizing antibody.
Increases in VEGF have been found after electrical stimulation and exercise. Breen et al19 and, more recently, Gavin et al20 have demonstrated that 1 hour of exercise promotes a 3- to 4-fold increase in VEGF mRNA, and Hang et al21 have shown a 6-fold increase in VEGF mRNA in TA and EDL after 4 days of electrical stimulation. Although these authors have shown increases in VEGF mRNA expression, ours is the first study to document increases in VEGF protein and vessel density in the same animals.
The regulation of VEGF expression is not completely understood. VEGF can be induced by several factors, such as adenosine,22 cytokines,6 hypoxia,6 23 NO,15 20 growth factors (such as platelet-derived growth factor and basic fibroblast growth factor),24 and Ang II.1 25 In the present study, we speculated on the involvement of the RAS in angiogenesis and VEGF protein expression in electrically stimulated skeletal muscle. The hypothesis that Ang II can induce VEGF has been shown in cell culture. Chua et al1 and Pupilli et al26 have demonstrated increases in VEGF mRNA in response to Ang II and have shown that this response was mediated by Ang II type 1 (AT1) receptors. Previous data from our laboratory have confirmed this hypothesis also in vivo and have demonstrated that Ang II, when infused continuously in conscious rats, produces increases in the vessel density of the cremaster muscle and that this action also is mediated by the AT1 receptor.4 More recently, we have demonstrated one pathway involving AT1 receptors of Ang II and VEGF protein expression in electrically stimulated angiogenesis in skeletal muscle.3 Taken together, these results suggest that the RAS may be involved in the regulation of VEGF protein in vivo as well as angiogenesis.
Results from the present study have confirmed the hypothesis that the RAS has a role in angiogenesis and that VEGF expression implicates a defect in the renin gene as the causative factor for impaired angiogenesis in Dahl S rats. After 7 days of electrical stimulation, both increases in VEGF expression and angiogenesis in skeletal muscle were absent in SS rats. In contrast, the transfer of 10 cM of chromosome 13, containing the renin gene (S/renRR), restored the angiogenic response in EDL (16% increase) and TA (30% increase), despite significant higher blood pressure in the S/renRR group. To confirm the importance of the RAS in VEGF expression and angiogenesis in these strains, we used pharmacological suppression and pharmacological inhibition. Although the angiogenic response induced by electrical stimulation was still present in the TA of the S/renRR-HS group, we observed that suppression of the RAS by a high salt diet significantly attenuated the increases in vessel density when we compared the differences between the stimulated and unstimulated side of the S/renRR-LS and S/renRR-HS groups. The blockade of RAS by lisinopril abolished this response. On the other hand, both suppression and blockade of RAS by high salt diet and lisinopril, respectively, restored the responses observed in the EDL of the SS group. These effects were likely due to inhibition of the RAS, although the possibility of other angiogenic factors arising from electrical stimulation cannot be completely eliminated.
We have shown previously2 that inhibition of ACE effectively blocked the angiogenesis induced by a short-term exercise program, and more recently, Amaral et al3 have suggested that the VEGF expression and angiogenesis, induced by electrical stimulation, could be modulated by Ang II. In these studies, a causative link between VEGF expression and vessel growth was made through the use of a VEGF-neutralizing antibody.
These findings regarding Ang II have important implications for the role of RAS in the normal physiology of blood vessels in vivo. Although Ang II has been classically defined as an endocrine substance acting on blood pressure regulation, many tissues express endogenous RAS activity, implying that locally generated Ang II is involved in autocrine/paracrine regulatory mechanisms. Blood vessels contain all components of RAS27 ; thus, Ang II could act as an autocrine hormone to regulate VEGF as well as vascular permeability factor, allowing the vascular permeability factor to act as a paracrine hormone to regulate the permeability of the overlying endothelium.28 Although we did not measure the tissue Ang II in these animals, previous studies using the same model have suggested that the circulating RAS is not responsible for the electrically induced increases in vessel density.
In summary, our data showed that VEGF expression and angiogenesis in skeletal muscle were absent in the SS-LS group and reestablished in the S/renRR-LS group. We have also shown that the suppression of RAS completely blocked the VEGF expression and attenuated the angiogenesis. These results are consistent with the hypothesis that genetic or pharmacological manipulations of the RAS may contribute to the angiogenesis and VEGF protein expression in skeletal muscle induced by electrical stimulation.
| Acknowledgments |
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Received October 24, 2000; first decision November 30, 2000; accepted December 18, 2000.
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