(Hypertension. 1995;25:1030-1033.)
© 1995 American Heart Association, Inc.
Articles |
From the Department of Pharmacology and the Cardiovascular Center, The University of Iowa College of Medicine, Iowa City.
| Abstract |
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Key Words: argipressin aorta blood vessels RNA, messenger
| Introduction |
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Over the last decade, a number of peripheral tissues in several species have been shown to contain and produce AVP. For example, AVP is present in significant quantities in the ovaries of rats, cows, and humans2 3 4 and is synthesized by isolated bovine ovarian cells.5 Immunoreactive AVP has also been found in both human and rat testis6 7 as well as adrenal6 8 and thymus.9 Several of these tissues have also been shown to contain AVP mRNA,10 11 thus providing additional support that these peptides are locally produced. Interestingly, Brattleboro rats, which have minimal amounts of central nervous system and circulating AVP, have normal levels of AVP in peripheral tissues,4 12 13 suggesting that peripheral and central AVP systems function independently.
We have recently reported the presence of immunoreactive AVP in various blood vessels of the rat and cow.14 This immunoreactive material appears indistinguishable from authentic AVP when analyzed by immunologic, biochemical, and physiochemical means. Burnstock and colleagues (Lincoln et al15 and Loesch et al16 17 ) have provided immunohistochemical data also indicating the presence of AVP in vascular endothelial cells of several species. These observations, plus the fact that blood vessels of Brattleboro rats and hypophysectomized Sprague-Dawley rats contain levels of immunoreactive AVP that are no different from those in normal animals,14 led us to suggest that the vascular peptide was not derived from the circulation but was produced locally.14 To provide further support for this hypothesis, we performed the present studies to determine whether blood vessels contained mRNA for AVP.
| Methods |
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Ribonuclease Protection Assay
RNA was analyzed for AVP mRNA by a ribonuclease (RNase)
protection assay following the method of Liu et al.19 RNA
samples (0.1 to 180 µg) dissolved in 200 µL H2O were
combined with 5x105 cpm 32P-labeled cRNA probe
(prepared as described below) and precipitated with 0.3 mol/L sodium
acetate and 2.5 vol ethanol for 15 minutes at -70°C. Precipitated
RNA and probe were collected by centrifugation (12 000g at
4°C, 10 minutes), washed once with 70% ethanol, and dried for 15
minutes in a vacuum oven at 55°C. Samples were resuspended in 30 µL
hybridization solution (800 mL/L deionized formamide, 400 mmol/L NaCl,
1 mmol/L EDTA, and 40 mmol/L
piperazine-N-N'-bis[2-ethanesulfonic acid] [PIPES], pH
6.4), heated at 85°C for 10 minutes, and incubated overnight at
55°C. Single-stranded RNA was digested by addition of 270 µL
digestion buffer (300 mmol/L NaCl, 5 mmol/L EDTA, 10 mmol/L Tris-HCl,
pH 7.4) containing RNase T1 (500 U/mL, GIBCO-BRL) and RNase A (10
mg/mL, Sigma Chemical Co) and then incubated for 1 hour at 22°C.
Next, 20 µL sodium dodecyl sulfate (100 g/L) and 10 µL proteinase K
(10 mg/mL, Sigma) were added and the samples incubated at 37°C for 30
minutes. After digestion, nucleic acids were extracted once with
phenol/chloroform and once with chloroform, precipitated with ethanol
at -70°C for 15 minutes, and pelleted by centrifugation as above.
Pellets were washed with 70% ethanol, dried, and resuspended in 5 µL
H2O and 10 µL stop buffer (Promega). The protected RNA
fragments were separated by electrophoresis on an acrylamide (60
g/L)/urea (7.5 mol/L) gel. Gels were then fixed for 30 minutes in 10%
methanol/12% acetic acid, dried, and exposed to Kodak XAR-5 film for 2
to 10 days. Probe fragment sizes were determined by including a lane
containing a 100-bp DNA ladder (GIBCO-BRL) that was end-labeled with
[
-32P]ATP (3000 Ci/mmol, DuPont-NEN) using T4
polynucleotide kinase (GIBCO-BRL),20 and input probe size
was determined by including a lane containing 5x103 cpm of
32P-labeled cRNA probe.
Probe Preparation
A plasmid containing 622 bp of the AVP sequence (designated
AVP8c and provided by Dr Thomas Sherman, University of Pittsburgh) was
used to generate an AVP-specific cRNA probe. The plasmid was linearized
by digestion with HindIII, and a cRNA probe was transcribed
using DNA-dependent SP6 RNA polymerase and [
-32P]CTP
(800 Ci/mmol, DuPont-NEN).20 This procedure produced a
674-nucleotide cRNA probe specific for AVP mRNA. Typically, specific
activities were 108 cpm/µg RNA.
AVP Radioimmunoassay
Tissues were extracted and assayed as previously
described.14 Briefly, tissues were removed, rinsed free of
blood, and homogenized in 10 vol of 1.0 mol/L HCl. Homogenates were
centrifuged to remove precipitated protein, and the neutralized
supernatants were assayed for AVP immunoreactivity in a specific AVP
radioimmunoassay. Cross-reactivity of the assay was 100% for lysine
vasopressin, 0.7% for arginine vasotocin, and less than 0.1% for
oxytocin, angiotensin II, and endothelin-1. The intra-assay and
interassay coefficients of variation were 20% and 5%, respectively.
The sensitivity of this short, overnight assay was 7 pg per tube.
| Results |
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Vascular Localization of AVP and AVP mRNA
Burnstock and colleagues15 16 17 have previously
demonstrated that between 10% and 50% of endothelial cells from rat
vessels, including the aorta, mesenteric, and pulmonary arteries, stain
positive for AVP by immunohistochemistry. However, they were unable to
detect AVP immunoreactivity in any other vascular structures, including
smooth muscle and adventitia. From their findings, we reasoned that if
AVP were produced in endothelial cells, endothelium removal should
deplete the vessels of AVP mRNA. To test this hypothesis, isolated rat
aortas were first mechanically denuded of endothelium by dissecting the
vessels longitudinally and scraping the endothelial surface with a
blunt spatula. The effectiveness of this procedure was assessed by
comparing scanning electron micrographs of the luminal surfaces of the
intact and endothelium-denuded vessels. As shown in Fig 2, the smooth appearance of the intact aorta (A)
indicates the presence of vascular endothelium, whereas the rough,
textured appearance of the denuded aorta (B) confirms the removal of
endothelium. Subsequently, an experiment was performed in which aortas
were collected from 40 rats; half were denuded of endothelium and half
were left intact. The two groups were then extracted for RNA and
examined for AVP mRNA by RNase protection assay. Fig 3
shows results from this experiment, which was replicated three separate
times and revealed 622-bp protected probe fragments when RNA from
either endothelium-intact or
endothelium-denuded vessels was used. Again, the
autoradiographic intensities of the protected probe fragments from both
intact and denuded vessels were approximately equivalent to the
intensity obtained with 0.1 µg hypothalamic RNA. Since these studies
indicated that endothelium denudation had little if any effect on the
level of AVP mRNA present in the vessels, we next compared the
level of AVP peptide in intact and endothelium-denuded
vessels using a specific radioimmunoassay for AVP. These studies (n=3)
showed that the level of AVP immunoreactivity in
endothelium-denuded aorta (4.9±1.0 ng/g tissue) was
not significantly different from that in intact aorta (4.5±1.7 ng/g
tissue).
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| Discussion |
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Previous immunocytochemical studies performed by Burnstock's group15 16 17 detected the presence of AVP only in vascular endothelium. When attempting to verify these findings in our studies, we were surprised to find no difference in the levels of AVP mRNA or AVP immunoreactivity between intact and endothelium-denuded aorta. These data clearly indicate that subendothelial structures such as vascular smooth muscle or perivascular adventitia possess both AVP message and peptide and may thus represent significant sites of AVP production. However, because endothelial cells constitute such a small proportion of total aortic cell mass (probably <1%), it is possible, and perhaps likely, that the decrease in total AVP peptide and message levels produced by the removal of these cells is insufficient to be detected in our assays. Thus, the present data do not address the issue of whether endothelial cells contain AVP message. However, the data of Loesch et al,16 showing that primary cultures of rabbit aortic endothelial cells contain AVP, support the possibility that endothelial cells do in fact contain, and probably produce, AVP. If so, endothelial cells may in fact contain AVP message. The present observations are not the first to indicate local vascular production of a vasoactive peptide. Several groups have previously demonstrated that endothelial cells synthesize a number of vasoactive peptides, including endothelin,24 angiotensin II,25 and C-type natriuretic peptide.26 In fact, endothelin was originally characterized, sequenced, and cloned from cultured porcine aortic endothelial cells. In addition to these vasoactive peptides, vascular cells have also been shown to contain other components of the angiotensin IIproducing system, including angiotensin-converting enzyme,27 renin,22 23 and angiotensinogen.28
In conclusion, these studies have demonstrated the presence of AVP mRNA in rat aorta. This observation, coupled with the previous identification and characterization of AVP immunoreactivity in aorta (and other blood vessels) as well as the presence of AVP in the vasculature of AVP-deficient Brattleboro and hypophysectomized rats, provides compelling evidence that blood vessels are capable of synthesizing AVP. Although the functions of this locally produced peptide are not yet clear, it may have important regulatory functions within the vasculature, including regulation of local vascular tone through its vasoconstricting effects as well as regulation of vascular regrowth and remodeling through its growth-promoting effects.29 30
| Acknowledgments |
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| Footnotes |
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Received May 30, 1994; first decision September 21, 1994; accepted January 3, 1995.
| References |
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