(Hypertension. 1996;27:833-837.)
© 1996 American Heart Association, Inc.
Articles |
From the Department of Medicine and Clinical Science (H.I., Y.K., T.Y., K.N.) and the Department of Cardiovascular Surgery (T.I., M.H., K.M., T.B.), Kyoto (Japan) University Graduate School of Medicine.
Correspondence to Katsuhiko Matsuda, MD, Department of Cardiovascular Surgery, Kyoto University Graduate School of Medicine, 54 Shogoin Kawahara-cho, Sakyo-ku, Kyoto 606-01, Japan.
| Abstract |
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Key Words: natriuretic peptide natriuretic peptide receptor vein graft cyclic GMP
| Introduction |
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Natriuretic peptides elicit their pharmacological actions via activation of two subtypes of the biologically active receptor, NPR-A and NPR-B, which are the particulate guanylate cyclase itself, to elevate intracellular cGMP.5 We and others have demonstrated that ANP and, to a lesser extent, BNP potently activate NPR-A, whereas CNP selectively activates NPR-B.6 7 Several studies, including ours, demonstrated that CNP infused into healthy humans has weaker vasodepressor and natriuretic effects than ANP or BNP, suggesting a difference of expression of natriuretic peptide receptors in humans and distinct roles of these natriuretic peptides in cardiovascular homeostasis.8 9 10
In the present study, to elucidate the possible differential implications of the natriuretic peptide system in human artery and vein, we examined the cGMP production in response to ANP and CNP and gene expressions of biologically active natriuretic peptide receptors in human GEA, IMA, and SV. In addition, using a rabbit model of arterialized vein graft, we further studied the alteration of the responsiveness to ANP and CNP in the jugular vein grafted into the arterial circulation by the process of arterialization.
| Methods |
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Animals
Sixteen male Japanese White rabbits weighing 2.5 to
3.0 kg were
used. Rabbits were maintained under a 12-hour light schedule and were
fed regular rabbit chow diet. All animals used in this study received
humane care in compliance with the Principles of Laboratory
Animal Care formulated by the National Society for Medical
Research and the Guide for the Care and Use of Laboratory
Animals prepared by the National Academy of Sciences and published
by the National Institutes of Health (NIH publication 85-23, revised
1985).
Surgical Technique of Rabbit Vein Graft Operation
The
operative procedures were performed under aseptic
conditions. Anesthesia was achieved by administration of 25
mg/kg pentobarbital sodium IV and 50 mg lidocaine hydrochloride SC. The
right carotid artery and jugular vein were exposed through a vertical
neck incision. After the animals were given 100 U/kg heparin sodium IV,
a 15- to 20-mm segment of the artery was isolated with vascular forceps
and removed. A 15- to 20-mm segment of the jugular vein was reversed
and interposed in the carotid artery in an end-to-end fashion.
Anastomoses were created with 10-0 nylon continuous suture at x20
magnification. After 4 weeks of observation, the vein graft, left
carotid artery, and left jugular vein were harvested from each rabbit,
and cGMP production in response to ANP and CNP in these three
vessels was examined. For histological examination,
animals were killed by an overdose of pentobarbital sodium, and neck
vessels were harvested after fixation in situ by perfusion of 10%
phosphate-buffered formalin into the ascending aorta through a
median sternotomy. Harvested arteries were subjected to usual
histological examination, and vascular wall thickness,
defined as the distance between external elastic lamina and
endothelium, was measured in at least five sections
from each vessel. Of 16 grafts, 14 were patent after 4 weeks of
observation (patency rate, 0.875); the occluded grafts were excluded
from the study.
Peptides
-Human ANP and human CNP were purchased from
the Peptide
Institute.
Determination of cGMP Production
The effects of ANP and CNP
on cGMP production in each
vessel were examined as we previously described.7 Each of
the vessel segments was cleaned of surrounding tissue and was cut
longitudinally on ice. Endothelial cells were denuded
by gentle scraping with the blade of scalpel. Then each vessel segment
was cut into fragments of approximately the same size and was subjected
to cGMP production assay for either ANP or CNP and for vehicle,
each in duplicate. Examined concentrations of the peptides were 10
nmol/L, 100 nmol/L, and 1 µmol/L in humans and 1 nmol/L, 10 nmol/L,
100 nmol/L, and 1 µmol/L in rabbits. Each fragment was preincubated
for 30 minutes at 37°C in 250 µL Dulbecco's modified
Eagle's
medium containing 0.5% fetal calf serum and 0.5 mmol/L
isobutylmethylxanthine. ANP or CNP was added to
the medium and incubated for another 30 minutes at 37°C. After the
incubation, trichloroacetic acid was added to a final concentration of
6% and the tissue fragments were mechanically homogenized
in the medium by a Teflon homogenizer. After the
trichloroacetic acid was removed by extraction three times with
water-saturated ether, the amount of cGMP in the sample was
determined by radioimmunoassay after succinylation as described
elsewhere.11 The production of cGMP was expressed
on a molar basis and was normalized by wet weight of the tissue
fragments.
Characterization of Gene Expressions of Biologically Active
Natriuretic Peptide Receptors
Gene expressions of NPR-A and NPR-B were
characterized by
the RT-PCR method. Total RNA was extracted by the guanidinium
thiocyanateCsCl method from GEA, IMA, and SV pooled from at least
three patients. The specific primers for PCR were synthesized with an
Applied Biosystems 381A DNA Synthesizer according to the
nucleotide sequence of each receptor in humans (NPR-A:
sense, 5'-GGGGATGTAGAAATGAAGGGC-3' and antisense,
5'-TCATGGTAGAAGCAAGGCATACAGG-3'; NPR-B: sense,
5'-TGACCAGCTGAGGCTACGCA-3' and antisense,
5'-CTACAACTTCCATATAAGGT-3').5 12 cDNA was
synthesized from
1 µg of total RNA by oligo (dT) priming with Molony murine leukemia
virus reverse transcriptase (Life Technologies Inc) and was subjected
to PCR using Taq DNA polymerase (Takara Shuzo). After 30
cycles of PCR, aliquots of the PCR products were
size-fractionated by agarose gel electrophoresis, and then gels
were subjected to ethidium bromide staining and Southern blot
analysis. For Southern blot analysis,
32P-labeled synthetic oligomer was used as a probe. The
nucleotide sequences of the probes were
5'-GAGCTTACAGGCTGAGCCAA-3' for NPR-A and
5'-GGTGGTAGAGGAGACATGGAT-3' for NPR-B. As an
internal control, Southern blotting of RT-PCR product for G3PDH was
also done by specific primers and oligoprobe (sense,
5'-TCAAGGCTGAGAACGGGAAGC-3'; antisense,
5'-CTTCACCACCTTCTTGATGTC-3';
oligoprobe, 5'-CTCATGACCACAGTCCAT-3').13 Every blot
was
quantified by NIH Image 1.52 software after being scanned by an HP
ScanJet 3c flatbed scanner (Hewlett-Packard) and was presented
in arbitrary units such that 1 unit equaled the intensity of blot for
G3PDH in each vessel.
To confirm the quantification by RT-PCR, we also performed PCR for NPR-A, NPR-B, and G3PDH using 0.25, 0.5, and 2 times the amount of cDNA and examined whether the intensity of blots increased according to the amount of cDNA.
Statistical Analysis
All values were expressed as
mean±SEM. When two mean values
between different groups were compared, a two-sided unpaired
Student's t test was performed. When mean values between
more than three groups were compared, factorial ANOVA followed by
Fisher's protected least significant difference was performed.
Statistical significance was defined as a value of
P<.05.
| Results |
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Gene Expressions of Biologically Active Natriuretic
Peptide Receptors in GEA, IMA, and SV
Fig 2A
shows the
results of Southern blot
analysis on PCR-amplified products of the transcripts for
two subtypes of the biologically active natriuretic peptide
receptor in GEA (n=3, pooled), IMA (n=3, pooled), and SV
(n=4, pooled).
As shown in the figure, specific signal for NPR-A gene transcript in
GEA or IMA was about four times more intense than that in SV. In
contrast, the intensity of the specific signal for NPR-B gene
transcript was approximately the same in GEA, IMA, and SV. Fig
2B
shows
the results of Southern blot analysis for NPR-A, NPR-B, and
G3PDH gene transcripts using 0.25, 0.5, 1, and 2 times the amount of
cDNA from GEA used in the analysis shown in Fig 2A
. The
intensities of blots for PCR-amplified products of NPR-A, NPR-B,
and G3PDH transcripts increased in an almost linear relation to the
amounts of cDNA used (Fig 2B
).
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Effects of ANP and CNP on cGMP Production in Rabbit Carotid
Artery, Jugular Vein, and Arterialized Vein
Graft
As shown in Fig 3
, in the arterialized
vein grafts 4 weeks after operation, wall thickness measured in
histological sections was 116±20 µm (n=5). This was
significantly greater than wall thickness of intact jugular veins
(21±1 µm, n=5, P<.0001) and comparable to that
of intact
carotid arteries (108±6 µm, n=5, P=.71).
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The effects of ANP and CNP on cGMP production in rabbit vessels
are shown in Fig 4
. Basal production of cGMP was
36.6±3.7 fmol/mg wet tissue in carotid arteries (n=15),
16.2±2.5
fmol/mg wet tissue in jugular veins (n=9), and 13.8±2.0 fmol/mg
wet
tissue in arterialized vein grafts (n=9) (carotid artery
versus jugular vein, P=.0001; carotid artery versus vein
graft, P<.0001; jugular vein versus vein graft,
P=.64). ANP stimulated cGMP production more potently
in carotid arteries than in jugular veins, but the difference of
responsiveness to ANP between the artery and the vein was smaller than
that in humans. ANP at a concentration of 1 µmol/L augmented cGMP
production by 12.6-fold in carotid arteries but 6.9-fold in
jugular veins. In arterialized vein grafts, ANP at a
concentration of 1 µmol/L caused only a 3.6-fold increase of cGMP
production. The effect of CNP on cGMP production did
not differ significantly between carotid arteries, jugular veins, and
arterialized vein grafts at any concentration. CNP 1
µmol/L elevated cGMP production by 6.0-fold in carotid
arteries, 4.9-fold in jugular veins, and 5.0-fold in
arterialized vein grafts.
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| Discussion |
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Augmentation of cGMP production in response to ANP and CNP was considered to reflect the expression of biologically active natriuretic peptide receptors in the vessel. As we have reported previously, in cultured PC12 cells, which express only NPR-A, ANP and BNP at concentrations >10 nmol/L significantly increased cGMP production and 1 µmol/L of ANP caused a 20-fold increase of cGMP generation, whereas CNP exerted almost no effect even at a concentration of 1 µmol/L.7 In contrast, in cultured rat aortic smooth muscle cells with a synthetic phenotype, which express predominantly NPR-B, CNP at concentrations >100 nmol/L significantly increased cGMP production, whereas ANP and BNP slightly increased cGMP production at a concentration of 1 µmol/L.7 From these results, it is obvious that cGMP production in response to ANP reflects the amount of NPR-A and that cGMP production in response to CNP reflects the amount of NPR-B. Therefore, the differential effects of ANP and CNP on cGMP production in human and rabbit arteries and veins suggest that NPR-B is expressed almost equally in arteries and veins in low quantity, whereas NPR-A is expressed as much as NPR-B in veins but is expressed more abundantly by one or two orders of magnitude in arteries. Indeed, the results of Southern blot analysis demonstrated that NPR-B mRNA expression was about the same in human IMA, GEA, and SV, whereas the gene expression of NPR-A was more abundant in GEA and IMA than in SV.
Another important finding in the present study is that the effects of ANP and CNP on cGMP production in the rabbit jugular veins did not change when the vein was incorporated into the arterial circulation. Although the wall thicknesses of the arterialized vein grafts were comparable to those of carotid arteries, basal cGMP production of the arterialized vein grafts was significantly lower than that of carotid arteries and almost the same as that of jugular veins. From these results, it is suggested that there are some differences in characteristics between VSMCs in intact arteries and VSMCs in neointima occurring in veins. Otherwise, it is possible that 4 weeks of observation was too short for VSMCs to alter their phenotype from the "venous" to the "arterial" type.
In the present study, the concentrations of ANP and CNP necessary to augment cGMP production in isolated vessels exceeded the levels of ANP and CNP detected in human circulation under physiological and pathophysiological conditions (at most 300 pmol/L and 50 pmol/L, respectively), as we and others previously reported.1 4 17 But we have demonstrated that the elevated plasma cGMP level of stroke-prone spontaneously hypertensive rats with a plasma ANP level of 300 pmol/L was significantly reduced by the intravenous administration of our monoclonal antibody against ANP.18 Furthermore, in our recent observations, when the CNP gene was overexpressed by adenovirus-mediated gene transfer in cultured VSMCs that expressed predominantly NPR-B, the VSMCs exhibited enhanced cGMP production and a significantly lower growth rate (unpublished observations). In this system, the CNP concentration in the culture medium was about 100 pmol/L. Exogenously administered CNP at that concentration exerts no effects on cGMP production in cultured VSMCs.19 20 Therefore, a difference exists in the effective concentration of exogenously administered natriuretic peptides and endogenously produced natriuretic peptides. It is thus possible that GEA and IMA produce more cGMP than SV in vivo in response to circulating ANP and BNP at the enhanced level.
The stimulating effect of exogenous CNP on cGMP production was not as prominent as that of ANP and was almost equal in GEA, IMA, and SV. As we and others have demonstrated, CNP is produced by vascular endothelial cells and is suggested to be a local regulator of vascular tone and growth rather than a circulating hormone.3 21 We also reported that the endothelial production of CNP is markedly augmented by several growth factors and cytokines.3 22 Therefore, the possibility still remains that CNP at the enhanced level activates NPR-B in an autocrine and paracrine manner in atherosclerotic vessels.
In conclusion, the present study demonstrated that ANP stimulates cGMP production more potently in IMA and GEA than SV in humans and in the carotid artery more than the jugular vein and the arterialized vein graft in rabbits. This is due to the preferential expression of NPR-A in these arteries. Circulating ANP thus can elicit more prominent vasodilating and antiproliferative effects in arteries than in veins. CNP is considered to exert less effect than ANP on either arteries or veins. These observations support the distinct roles of natriuretic peptides in cardiovascular homeostasis: ANP as a circulating hormone and CNP as a local regulator of vascular tone and remodeling.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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| References |
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