(Hypertension. 1995;26:401-405.)
© 1995 American Heart Association, Inc.
Articles |
From the First Department of Internal Medicine, Osaka City (Japan) University Medical School.
Correspondence to Miwako Ikeda, MD, Division of Hypertension and Atherosclerosis, First Department of Internal Medicine, Osaka City University Medical School, 1-5-7 Asahi-machi, Abeno-ku, Osaka 545, Japan.
| Abstract |
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Key Words: natriuretic peptides vascular endothelium cell movement rats
| Introduction |
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ANP was originally isolated from mammalian hearts.6 A second type of natriuretic peptide has been identified in the porcine brain.7 Designated as BNP, it has subsequently also been isolated from mammalian heart.8 9 It has been demonstrated that ANP and BNP exert an antiproliferative effect on bovine aortic endothelial cells via a guanylate cyclasecoupled mechanism.10 However, the roles of the natriuretic peptides on vascular endothelial cell migration remain to be elucidated despite the existence of a large number of gyanylate cyclasecoupled natriuretic peptide receptors in these cells.11 12 13
Accordingly, we designed the present study to examine the possible effects of rat ANP-(1-28) and rat BNP-45, which are the major circulating forms of ANP and BNP in rats,14 15 on rat vascular endothelial cell migration after stimulation with FCS. In addition, the effect of rat ANP-(5-25), which is much weaker biologically than rat ANP-(1-28) or rat BNP-45, on migration was also examined in these cells.
| Methods |
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Endothelial Cell Culture
Vascular endothelial cells were grown
from the explants of the thoracic aorta of male Wistar rats weighing 50
to 100 g as previously described.16 Briefly, small
segments of aortic tissue were placed endothelial-side
down on the surface of culture dishes and maintained in DMEM containing
7% FCS, endothelial cell growth supplement (50
µg/mL), penicillin (50 µg/mL), and streptomycin (50 µg/mL) with
atmospheric air and 5% CO2. The cells that migrated from
the explants were removed by trypsinization and seeded in culture
flasks. Endothelial cells were identified by their
"cobblestone" appearance by phase-contrast microscopy and
positive fluorescence with factor VIII antigen and negative
fluorescence with antibodies against
-smooth muscle actin.
Contamination by cells with the morphological features of smooth muscle
cells was not found. Cells after two to five passages were used for the
experiment.
Migration Assay
Endothelial cell migration was assayed by
a modification of Boyden's chamber method with the use of a
microchemotaxis chamber (Neuro Probe, Inc) and polycarbonate filters
(Nucrepore Corp) with pores 5.0 µm in diameter, as previously
reported by Koyama et al.17 Cultured
endothelial cells were trypsinized and suspended with a
concentration of 5.0x105 cells per milliliter in DMEM
supplemented with 0.5% bovine serum albumin. The cell number
was counted with an electronic cell counter (model ZB1, Coulter
Electronica). A 200-µL volume of cell suspension was placed in the
upper chamber, and 35 µL of DMEM containing 5% FCS with or without
various concentrations of rat ANP-(1-28), rat BNP-45, rat ANP-(5-25)
(10-12 to 10-6 mol/L), or 8-bromo-cGMP
(10-7 to 10-3 mol/L) was placed in the lower
chamber. The chamber was incubated at 37°C with atmospheric air and
5% CO2 for 4 hours. The cells on the upper side of the
filter were scraped off, and the filter was removed. Cells that had
migrated to the lower side of the filter were fixed in ethanol, stained
with Diff-Quick staining solution (The Green Cross Corp), and counted
under a microscope (magnification x400) for quantitation of
endothelial cell migration. Migratory activity is
expressed as the sum of the number of cells that were observed in four
high-power fields.
cGMP Measurement
After preincubation the cell monolayers were washed twice with
phosphate-buffered saline and then stimulated for 30 minutes with
different concentrations of rat ANP-(1-28), rat BNP-45, or rat
ANP-(5-25) dissolved in DMEM that contained 5% FCS and 0.5 mmol/L
IBMX. The reaction was stopped by rapid aspiration and the addition of
2 mL ice-cold 65% ethanol. After evaporation by a centrifuge
evaporator, the dry residue was dissolved in an assay buffer. cGMP
levels were determined by radioimmunoassay done with a cGMP assay kit
as previously described.12 13
Calculations and Statistical Analysis
The statistical significance of differences in the
studies was evaluated by ANOVA, and P values were calculated
by Scheffé's method.18 Values are expressed as
mean±SD.
| Results |
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Effects of Natriuretic Peptides on
Endothelial Cell Migration
Effects of various concentrations
(10-6, 10-8,
10-10, and 10-12 mol/L) of rat
ANP-(1-28), rat BNP-45, and rat ANP-(5-25) on
endothelial cell migration after stimulation with 5%
FCS are shown in Fig 2. Rat ANP-(1-28) significantly
inhibited migration at concentrations of 10-8 and
10-6 mol/L and tended to inhibit migration at a
concentration of 10-10 mol/L. Rat BNP-45 significantly
inhibited migration at concentrations between 10-10 and
10-6 mol/L. In parallel with the inhibition by rat
ANP-(1-28) and rat BNP-45 on FCS-stimulated migration, cellular cGMP
increased after treatment with rat ANP-(1-28) and rat BNP-45 (Fig 3). There was a significant inverse correlation between
the increase in cellular cGMP levels and the decrease in migratory
activity by ANP and BNP (n=12, r=.76, P<.01)
(Fig 4).
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Rat ANP-(5-25) was much less effective than rat ANP-(1-28) or rat BNP-45 with respect to inhibiting migration and increasing cGMP levels (Figs 2 and 3). Rat ANP-(5-25) significantly inhibited migration only at the highest concentration (10-6 mol/L).
To confirm the inhibitory effect of natriuretic peptides on endothelial cell migration, we examined the effects of 10-6 mol/L rat ANP-(1-28), rat BNP-45, and rat ANP-(5-25) on endothelial cell migration stimulated by various concentrations of FCS (5%, 10%, and 20%) (Fig 5). Both rat ANP-(1-28) and rat BNP-45 at a concentration of 10-6 mol/L significantly inhibited migration stimulated by 5% to 20% FCS. Rat ANP-(5-25) at 10-6 mol/L also inhibited migration stimulated by 5% to 20% FCS, but this inhibition was significantly weaker than that of rat ANP-(1-28) or BNP-45.
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Effect of 8-Bromo-cGMP on Endothelial Cell
Migration
To determine whether the inhibitory effects of
natriuretic peptides on endothelial cell
migration after stimulation with FCS are causally linked to the
increase in cellular cGMP, we examined the effect of a cGMP analogue,
8-bromo-cGMP, on 5% FCSstimulated endothelial cell
migration (Fig 6). 8-Bromo-cGMP inhibited FCS-stimulated
migration in a concentration-dependent manner between 10-7
and 10-3 mol/L.
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| Discussion |
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We also showed that rat ANP-(1-28) and rat BNP-45, which are the major circulating forms of ANP and BNP in rats,14 15 inhibit the migration of cultured rat vascular endothelial cells after stimulation with FCS in a concentration-dependent manner between 10-10 and 10-6 mol/L. To our knowledge this is the first demonstration concerning the relationship of cardiac natriuretic peptides and vascular endothelial cell migration.
The concentrations of ANP and BNP used in the present experiment are higher than the normal concentrations of plasma ANP and BNP in rats and humans.19 20 21 22 23 24 25 26 27 However, during high sodium intake28 29 30 31 or dynamic exercise,32 33 34 the levels of these peptides increase. In addition, several investigators, including our group, have previously shown that these natriuretic peptide levels are markedly high in the malignant or severe stage of rat and human hypertension.19 20 25 28 In particular, plasma BNP levels are found to be higher than plasma ANP levels in patients with severe congestive heart failure and acute myocardial infarction.35 ANP and BNP are secreted through the coronary sinus from the heart,19 20 but ANP is secreted mainly from the atria and BNP predominantly from the ventricles.14 15 36 Furthermore, vascular endothelial cells are shown to express a large number of biologically active receptors11 12 13 of ANP and BNP as well as clearance receptors. Taken together with these observations, our data raise the possibility that ANP and BNP secreted from the heart circulate into the general circulation and bind natriuretic peptide receptors in vascular endothelial cells, thereby inhibiting migration of these cells in certain pathological conditions. It remains to be further elucidated whether these natriuretic peptides inhibit endothelial cell migration in vivo in the arterial wall.
We have obtained three pieces of evidence for a causal link between cGMP production and inhibition of vascular endothelial cell migration after stimulation with FCS. First, rat ANP-(1-28) and rat BNP-45 increased cGMP levels, and these effects paralleled the inhibition of migration. Second, rat ANP-(5-25) had much weaker effects than rat ANP-(1-28) or rat BNP-45 with respect to inhibiting migration and increasing cGMP in cells stimulated with FCS. Third, a cGMP analogue inhibited FCS-stimulated migration in a concentration-dependent manner. These results suggest that ANP and BNP inhibit FCS-stimulated vascular endothelial cell migration probably through a cGMP-dependent process. Indeed, there was a significant inverse correlation between the percent increase in cellular cGMP levels and the percent decrease in migratory activity. However, further studies are necessary to elucidate the involvement of cGMP and its related systems in the inhibition by ANP and BNP of vascular endothelial cell migration.
Overall, the present work suggests that both ANP and BNP may inhibit vascular endothelial cell migration and that the increase in cellular cGMP levels is likely to be involved in this inhibition. This action may be added to natriuresis, vasodilation, and antiproliferation37 38 39 40 as yet another effect of cardiac natriuretic peptides.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received December 27, 1994; first decision March 14, 1995; accepted May 30, 1995.
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