Quercetin 3,7-dimethyl ether: A vasorelaxant flavonoid isolated from Croton schiedeanus Schlecht

Article abstract of DOI:10.1211/002235702760345455

The vasorelaxant profile of quercetin 3,7-dimethyl ether, a flavonoid isolated from Croton schiedeanus Schlecht (Euphorbiaceae), was assessed in aortic rings isolated from Wistar rats. To gain insight into its structure-activity relationship, we compared this substance with quercetin 3,4′,7-trimethyl ether (ayanin), another flavonoid isolated from this plant, quercetin 3,3′,4′,7-tetramethyl ether, a flavonoid synthesized by us, and quercetin. In addition we examined the interaction of quercetin 3,7-dimethyl ether with the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway. According to their pEC50 values (concentration producing a 50% inhibition of the maximal contractile response) to phenylephrine-induced precontraction in rat isolated aorta, the potency order was quercetin 3,7-dimethyl ether > quercetin > quercetin 3,4′,7-trimethyl ether > quercetin 3,3′,4′,7-tetramethyl ether (4.70 ± 0.18; 3.96 ± 0.07; 3.64 ± 0.02; 3.11 ± 0.16). The relaxant effect of quercetin 3,7-dimethyl ether was significantly decreased by the removal of endothelium as well as by methylene blue, an inhibitor of guanylyl cyclase, and by N^G-nitro-L-arginine methyl ester hydrochloride (L-NAME), an NO-synthase inhibitor. Therefore, quercetin 3,7-dimethyl ether has a NO/cGMP pathway-related profile, with increased vasorelaxant activity due to hydroxylation at positions 3 and 4 of the B ring. In addition, methylation at positions 3 and 7 with respect to quercetin of the C and A rings, respectively, seems to further enhance the vasorelaxant activity of quercetin 3,7-dimethyl ether.

Full text of DOI:10.1211/002235702760345455

Journal of
JPP 2002, 54: 1373–1378
# 2002 The Authors
Received April 24, 2002
Accepted June 14, 2002
ISSN 0022-3573
Quercetin 3,7-dimethyl ether: a vasorelaxant  avonoid
isolated from Schlecht
Croton schiedeanus
õ
M. F. Guerrero, P. Puebla, R. Carro´ n, M. L. Mart´n and L. San Roma´n
Abstract
The vasorelaxant proŽ le of quercetin 3,7-dimethyl ether,
a  avonoid isolated from Croton
schiedeanus Schlecht (Euphorbiaceae), was assessed in aortic rings isolated from Wistar rats. To gain
insight into its structure–activity relationship, we compared this substance with quercetin 3,4«,7-
trimethyl ether (ayanin), another  avonoid isolated from this plant, quercetin 3,3«,4«,7-tetramethyl
ether, a  avonoid synthesized by us, and quercetin. In addition we examined the interaction of
quercetin 3,7-dimethyl ether with the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP)
pathway. According to their pEC50 values (concentration producing a 50% inhibition of the maximal
contractile response) to phenylephrine-induced precontraction in rat isolated aorta, the potency
"
"
"
order was quercetin 3,7-dimethyl ether quercetin quercetin 3,4«,7-trimethyl ether quercetin
3,3«,4«,7-tetramethyl ether (4.70³0.18; 3.96³0.07; 3.64³0.02; 3.11³0.16). The relaxant effect of
quercetin 3,7-dimethyl ether was signiŽ cantly decreased by the removal of endothelium as well as by
methylene blue, an inhibitor of guanylyl cyclase, and by NG-nitro-L-arginine methyl ester hy-
drochloride (L-NAME), an NO-synthase inhibitor. Therefore, quercetin 3,7-dimethyl ether has a
NO/cGMP pathway-related proŽ le, with increased vasorelaxant activity due to hydroxylation at
positions 3 and 4 of the B ring. In addition, methylation at positions 3 and 7 with respect to quercetin
of the C and A rings, respectively, seems to further enhance the vasorelaxant activity of quercetin 3,7-
dimethyl ether.
Laboratorio de Farmacognosia y
´
õ
Farmacolog a, Facultad de
Farmacia, Universidad de
Salamanca, E-37007 Salamanca,
Spain
´
M. F. Guerrero, R. Carron, M. L.
´
Martin, L. San Roman
õ
Departamento de Qu mica
´
´
Farmaceutica, Facultad de
Farmacia, Universidad de
Salamanca, E-37007 Salamanca,
Spain
M. F. Guerrero, P. Puebla
Introduction
Departamento de Farmacia,
Facultad de Ciencias,
Universidad Nacional de
There is increasing interest in the cardioprotective eŒects of ¯ avonoid compounds in
disorders such as hypertension and coronary artery disease (Chan et al 2000 ; Middleton
et al 2000). The antioxidant properties of these compounds seems to be the main reason
for their cardiovascular eŒects since they act as scavengers of reactive oxygen species
such as superoxide anions, hydroxyl radicals and hydrogen peroxide, which play a role
in the genesis of such diseases (Nakazono et al 1991 ; Duarte et al 1993 ; Griendling &
Alexander 1997 ; Givertz & Colucci 1998). In addition, enhanced synthesis of NO may
also play an important role (Lemos et al 1999).
´
Colombia, AA 11430 Bogota,
Colombia
M. F. Guerrero
Correspondence: M. L. Martin,
Laboratorio de Farmacognosia y
In previous work we reported the hypotensive and vasorelaxant eŒects in rats of
Croton schiedeanus
Schlecht (Guerrero et al 2001), a medicinal plant used in Colombian
folk medicine for hypertension. From this plant, we isolated ayanin, a bio¯ avonoid
that, in-vitro, induces relaxation in Wistar rat aorta mediated by endothelium-
dependent NO production and subsequent activation of guanylate cyclase (unpublished
data) and that, in-vivo, exerts protective cardiovascular eŒects against the increase in
õ
Farmacolog a, Facultad de
´
Farmacia, Universidad de
Salamanca, E-37007 Salamanca,
Spain. E-mail: marisam!usal.es
Acknowledgment and funding:
Project ALFA-RELAPLAMED.2
and Subprogram X, ‘‘Qu mica
´
õ
}
blood pressure and heart rate also through a mechanism that depends on the NO cyclic
´
Fina Farmaceutica’’ of the
guanosine monophosphate (cGMP) pathway (Guerrero et al 2002). Further phyto-
chemical fractionating of this plant led us to the identi®cation of quercetin 3,7-dimethyl
ether (QDME).
CYTED supported this work. We
thank the School of Pharmacy of
the University of Salamanca and
´
the project ‘‘Busqueda de
In searching for structure±activity relationships, we wished to compare the vaso-
Principios Bioactivos en Plantas
Medicinales Colombianas ’’
developed at the Department of
Pharmacy, School of Sciences,
National University of Columbia.
«
relaxant pro®le of this ¯ avonoid with those of ayanin (quercetin 3,4 ,7-trimethyl ether),
quercetin 3,3 ,4 ,7-tetramethyl ether (QTME), a ¯ avonoid synthesized in our lab-
« «
oratory, and with quercetin, a well-known ¯ avonoid. We also wished to asses the
}
interaction of QDME with the nitric oxide (NO) cGMP pathway.
1373

1374
M. F. Guerrero et al
Extraction and isolation of ayanin and QDME
Materials and Methods
The aerial plant (5 kg) was dried and soaked in 96% EtOH
(20 L) at room temperature for 3 days. The EtOH was
removed under vacuum to yield a dark residue (80 g),
which was partitioned between CHCl3 and H2O to give a
CHCl3-soluble fraction (45 g). The residue of the CHCl3
extract was fractionated with 4% NaOH, yielding an acid
part (9 g) and a neutral part (32 g). The acid part was
subjected to silica-gel column chromatography ; gradient
Drugs and solutions
The following salts and drugs were used: NaCl, CaCl2,
NaHPO4, NaHCO3, glucose (Panreac); KCl, MgCl2,
KH2PO4 and MgSO4 (Probus); ascorbic acid (Prolabo);
phenylephrine hydrochloride, methylene blue (Sigma); L-
NAME and dimethyl sulfoxide (DMSO; Merck). All ¯ avo-
NG
l
n
elution with 0±100% EtOAc± -hexane aŒorded 24 frac-
tions. Fractions 14±21, eluting with 15±20% EtOAc± -
noids, methylene blue and -nitro- -arginine methyl ester
(L-NAME) were dissolved in dimethylsulfoxide (DMSO).
This vehicle was used at a ®nal concentration in the bath
n
hexane, were recrystallized from acetone±MeOH to give
.
ayanin (1 7 g) as a yellow solid.
The neutral part was separated by column chromatog-
raphy over silica gel; gradient elution with 0±100%
.
(see Pharmacology, below) of 0 1% .
n
EtOAc± -hexane aŒorded 68 fractions. Fractions 40±43,
eluting with 40% EtOAc -hexane, were obtained as an
Chemistry
}
n
oil after removal of the solvent; further puri®cation over
Croton
Croton schiedeanus
Schlecht
QDME and ayanin (Figure 1) were isolated from
n
Sephadex LH-20 using MeOH±CH2Cl2± -hexane (1:1:1)
as eluent yielded 0 5 g of ayanin as an amorphous solid,
schiedeanus
. The aerial part of
.
(Euphorbiaceae) was collected from the Tocaima region,
Cundinamarca, Colombia in November 1998. Its identity
was con®rmed by Jose Luis Fernandez and a voucher
specimen has been deposited (No. 432164) in the Her-
barium of Natural Sciences Institute of National University
of Colombia.
which was puri®ed by crystallization from acetone±MeOH.
n
Fractions 44±45, eluting with 40% EtOAc± -hexane,
were obtained as an oil after removal of the solvent.
Further puri®cation over Sephadex LH-20 using
n
MeOH±CH2Cl2± -hexane (1:1:1) as eluent yielded 0 3 g of
QDME, a ¯ avonoid previously isolated from
.
Notholaena
QTME was synthesized according to the method de-
scribed by Rao & Owoyale (1976).
californica
(Wollenweber et al 1981).
Melting points were determined on a Buchi 510-K melt-
ing point apparatus and are uncorrected. 1H and 13C NMR
were recorded on Bruker WP 200 SY (200 MHz and
50 MHz) and Bruker 400 DRX (400 MHz and 100 MHz)
1
m.p., H and 13C NMR spectra are identical to
Ayanin
those reported (Malan & Roux 1979 ; Agrawal 1989).
.
d
instruments using CDCl3 or DMSO- and tetramethyl
silane (TMS)as internal standard. Sephadex LH-20 (Fluka,
°
25±100 m) and silica gel 60 (Merck, 230±400 mesh) were 220±222 C. H NMR (DMSO, 400 MHz):
used for column chromatography ; precoated silica gel OMe-3), 3,81 (3H, s, OMe-7), 6,29 (1H, d,
.
plates (Merck, Kieselgel 60 F254, 0 25 mm) were used for 6), 6 62 (1H, d,
TLC analysis.
6
QDME
.
Colourless powder from acetone±MeOH, m.p.
1
¯ .
3 77 (3H, s,
J ¯ .
2 1 Hz, H-
J ¯ .
8 4 Hz,
.
J ¯ .
.
2 1 Hz, H-8), 6 90 (1H, d,
«
H-5 ), 7 45 (1H, dd,
.
J ¯ .
J ¯ .
« .
2 1 Hz, 8 4 Hz, H-6 ), 7 57
1
2
Figure 1 The chemical structures of quercetin methyl ethers.

1375
Quercetin 3,7-dimethyl ether: a vasorelaxant compound
J ¯ .
(DMSO, 100 MHz):
«
.
. . .
56 0 (OMe-7), 59 5 (OMe-3), 92 1 rings was removed by gently rubbing the internal surface
(1H, d,
2 1 Hz, H-2 ), 9 70 (1H, br s, OH). 13C NMR of quercetin 3,7-dimethyl ether. The endothelium of some
¯
.
.
.
«
.
.
(C-8), 97 6 (C-6), 105 1 (C-10), 115 5 (C-2 ), 115 7 (C-5 ), with the tip of small forceps. Removal of the endothelium
120 6 (C-6 ), 120 7 (C-1 ), 137 8 (C-3), 145 2 (C-3 ), 148 8 was con®rmed by lack of relaxation to 10 acetylcholine.
«
.
5 m
-
.
«
.
«
.
.
«
.
«
(C-4 ), 155 9 (C-2), 156 1 (C-9), 160 9 (C-5), 165 0 (C-7),
178 0 (C-4).
.
.
All the provisions concerning the protection of animals
used for experiments stipulated by Spanish law and Euro-
pean Community (EEC 1986) speci®cations were applied.
.
Synthesis of QTME
A solution of quercetin (1 g, Aldrich) in acetone (30 mL)
Data analysis and statistics
was boiled under re¯ ux with methyl sulfate (0 96 mL, 3 The response of aortic rings was expressed as a percentage
.
equivalents )and potassium carbonate (1 4 g, 3 equivalents ) of the initial contraction to KCl (80 m ) or phenylephrine
.
for approximately 1 h (Rao & Owoyale 1976). After ®ltra- (10
tion and concentration of the solvent, a solid was obtained give the negative logarithm of ¯ avonoid concentration
m
). Concentration±response curves were analysed to
6 m
-
.
and crystallization from acetone±MeOH yielded 0 6 mg of producing a 50% inhibition of the maximal contractile
tetra- -methyl quercetin as a yellow powder, 150±152 C. response (pEC50) by sigmoidal curve-®tting analysis. The
O
°
1H NMR (CDCl3, 200 MHz):
3 84 (6H, s, 2 OMe), maximum relaxation (Rmax) values corresponded to the
¯ .
¬
.
3 94 (6H, s, 2 OMe), 6 30 (1H, d,
(1H, d,
¬
.
J ¯ .
.
2 2 Hz, H-6), 6 44 percentage of relaxation obtained with the maximum ¯ avo-
«
J ¯ .
.
J ¯ .
2 2 Hz, H-8), 6 96 (1H, d,
8 4 Hz, H-5 ), noid concentration dissolved and assayed.
J
.
J ¯ .
«
2 5 Hz, H-2 ), 7 70 (1H, dd,
.
J ¯ .
2 5 Hz,
³
All results are expressed as means the standard error of
55 8 the mean (s.e.m.) of n 8±11. DiŒerences in the concen-
7 66 (1H, d,
1
2
8 4 Hz, H-6 ). 13C NMR (CDCl3, 50 MHz):
¯ .
«
¯
.
¯
.
(OMe-7), 56 0 (OMe-3 and 4 ), 60 1 (OMe-3), 92 2 (C-8), tration±response curves were analysed by one-way analysis
97 8 (C-6), 106 0 (C-10), 110 2 (C-2 ), 111 2 (C-5 ), 122 1 of variance, followed by the Dunett post-hoc test with a
!
«
(C-6 ), 122 3 (C-1 ), 138 9 (C-3), 148 8 (C-3 ), 151 4 (C-4 ), criterion set for statistical signi®cance at
155 8 (C-2), 156 7 (C-9), 162 0 (C-5), 165 4 (C-7), 178 7 (C- and SPSS 7 5 software was used for data analysis.
«
«
.
.
.
.
.
«
.
.
«
.
«
.
«
.
«
.
P
.
0 05. Excel 97
.
.
.
.
.
.
4).
Results
Pharmacology
Effect of QDME on the sustained contraction
induced by phenylephrine or KCl in
endothelium intact aorta rings
Rat aortic ring preparation and experimental procedure
Male Wistar rats (320±470 g) from the Animal House of
the University of Salamanca (P.A.E.-SA001) were anaes-
thetized with ether and sacri®ced. The descending thoracic
aorta was dissected and placed in an oxygenated Krebs
The maximal contractile responses to phenylephrine
6 m
-
m
³
) and KCl (80 m ) were 3825 45 and 4042
³
(10
110 mg (n 99 and 32), respectively.
m
solution with the following composition (in m ): NaCl,
118 0; KCl, 4 75; CaCl2, 1 8; MgSO4, 1 2; KH2PO4, 1 2;
¯
.
.
.
.
.
Figures 2 and 3 show the relaxant responses to the tested
¯ avonoids in KCl- and phenylephrine-precontracted aortic
rings, respectively. QDME showed a concentration-
.
NaHCO3, 25; glucose, 11 and ascorbic acid 0 1.
Rings of thoracic aorta (5±6 mm in length) were carefully
excised and submerged in Allhin organ chambers con-
taining 5 mL of Krebs solution of bathing medium main-
°
tained at 37 C and bubbled with a 95% O2 and 5% CO2
gas mixture (pH 7 4). The rings were mounted by means
¯ .
of two parallel L-shaped stainless-steel holders inserted
into the lumen. One holder served as an anchor while the
other was connected to a force-displacement transducer
(Harvard UF1) to measure isometric contractile force
}
recorded by a MacLab 8-computer system (A.D. Instru-
Ments Ltd, London, UK). A basal tension of 2 g was
applied. Each preparation was allowed to equilibrate for
60±90 min in Krebs solution before initiating the exper-
imental procedures, and during this period the incubation
media were changed every 15 min.
Afterequilibration, intact aortic rings incubated in Krebs
m
solution were exposed to KCl (80 m ) or phenylephrine
) until the contractile response reached a steady
6 m
-
(10
tension. Then, QDME, ayanin, QTME or quercetin were
-
Figure 2 Concentration±response curves to quercetin 3,7-dimethyl
ether (QDME), quercetin, quercetin 3,4«,7-trimethyl ether (ayanin)
and quercetin 3,3«,4«,7-tetramethylether (QTME) in rat endothelium-
6
added cumulatively to the bath (10 to 10
3 m
-
). In some
rings, DMSO was added as control.
In another set of experiments, L-NAME (10
4 m
-
) or
) were added 15 min before phenyl-
m
intact aortic rings precontracted with 80 m KCl. Each point repre-
6 m
ephrine-induced contraction and the cumulative addition
-
!
!
. P
0 05, **
methylene blue (10
P
sents the mean³s.e.m. of n¯ 8±11 experiments. *
!
.
.
0 01, ***
P
0 001 vs control (DMSO) rings.

1376
M. F. Guerrero et al
Figure 3 Concentration±response curves to quercetin 3,7-dimethyl
ether (QDME), quercetin, quercetin 3,4«,7-trimethyl ether (ayanin)
and quercetin 3,3«,4«,7-tetramethylether (QTME) in rat endothelium-
Figure 4 Concentration±response curves to quercetin 3,7-dimethyl
ether (QDME) in rat endothelium-denuded and endothelium-intact
6 m
6
-
aortic rings precontracted with 10
or presence of methylene blue (10
phenylephrine, in the absence
4 m
-
m
intact aortic rings precontracted with 10
phenylephrine. Each
!
6 m
-
-
) or L-NAME (10
). Each
!
P
point represents the mean³s.e.m. of n ¯ 8±11 experiments. *
P
point represents the mean³s.e.m. of n ¯ 8±11 experiments. *
!
!
.
0 001 vs control (DMSO) rings.
.
0 05, **
P
.
0 01, ***
P
!
!
.
0 001 vs control (DMSO) rings.
.
0 05, **
P
.
0 01, ***
P
Table 1 Comparison of the relaxant potency (pEC50) and maximal
relaxation (Rmax,% ) values obtained with quercetin 3,7-dimethyl
ether (QDME), quercetin, quercetin 3,4«,7-trimethyl ether (ayanin)
and quercetin 3,3«4«7-tetramethyl ether (QTME) in Wistar-rat
Furthermore, the maximum relaxation in methylene blue-
and L-NAME-preincubated rings was prevented by only
38% and 17% , respectively.
m
endothelium-intact aortic rings precontracted with KCl (80 m ) or
6 m
-
phenylephrine (10
)
6
-
M
M
Flavonoids
n
KCl 80 m
Phenylephrine 10
Discussion
pEC50
Rmax
pEC50
Rmax
We investigated the structure±activity relationship of two
Croton schiedeanus
¯ avonoids isolated from
± QDME and
.
.
.
.
QDME
Ayanin
QTME
16
35
15
12
3 98³0 04
72³3
45³3
18³6
81³6
4 70³0 18 94³3
ayanin ± as compared with QTME, a ¯ avonoid obtained
by synthesis, and with quercetin, a well-known ¯ avonoid,
in causing vasorelaxation in rat isolated aorta. This may be
helpful for the synthesis of active compounds for the
treatment of hypertension and coronary artery disease.
The mechanism of action of QDME was also explored.
Our data show that QDME is able to elicit an important
vasorelaxant response, producing close to 100% relaxation
in aortic rings precontracted with phenylephrine. Accord-
ing to the pEC50 values for phenylephrine-induced precon-
.
.
. .
3 64³0 02 67³4
. .
3 11³0 16 43³7
. .
3 96³0 07 85³6
2 76³0 05
.
.
1 81³0 59
.
.
Quercetin
4 18³0 01
dependent relaxation, the maximum response being ob-
"
.
served against phenylephrine ( 95% ). DMSO (0 1% ),
the vehicle used for the ¯ avonoids, had no eŒect on vascular
tone.
"
traction, the potency order was QDME quercetin
ayanin QTME. Since QDME and quercetin elicited the
"
The pEC50 and Rmax values obtained with the ¯ avonoids
are compared in Table 1. QDME had a signi®cantly higher
potency in causing vasorelaxation as compared with quer-
cetin, ayanin and QTME (5-, 11- and 38-fold shifts in the
curve to the right, respectively, in phenylephrine-precon-
tracted rings). Its eŒects were more pronounced against
phenylephrine- than KCl-precontracted rings. Quercetin
showed a slightly greater response in KCl-precontracted
rings than QDME (less than a 2-fold shift in the curve).
"
«
«
most marked eŒects it can be assumed that 3 and 4
hydroxylation of the B ring improves the relaxant potency.
Although QDME and quercetin showed similar responses
in KCl-precontracted rings, QDME showed a greater
response than quercetin in phenylephrine-precontracted
rings. Thus, methylation of positions 3 and 7 of the C and
A rings, respectively, could add some degree of greater
activity.
However, when methylation occurs at the A, B and C
rings at the same time, the eŒects of these substitutions may
oŒset each other (Ko et al 1999). This point of view is
supported by the intermediate and rather slight response
Effect of endothelium denudation and the
NO/cGMP pathway in the relaxation induced by
QDME
«
bited by previous removal of the endothelium as well as by and 4 methylations of the B ring seem to decrease the
The relaxation induced by QDME was signi®cantly inhi- obtained with ayanin and QTME, respectively, in which 3
«
) (Figure 4). response.
4 m
6 m
-
-
L-NAME, 10
and methylene blue (10

1377
Quercetin 3,7-dimethyl ether: a vasorelaxant compound
The cardiovascular eŒects of ¯ avonoid compounds such others ¯ avonoids (Ko et al 1991 ; Chan et al 2000). Ac-
as reduction of high blood pressure, a decrease in cardiac cording to the inhibition of potassium contraction induced
and renal hypertrophy and direct vasodilatation are associ- by this ¯ avonoid, at least at high concentrations, QDME
ated with the antioxidant properties of these substances may interfere with calcium-dependent and -independent
(Duarte et al 2001a, b). It is known that the presence of a contractile mechanisms. Further studies are needed to
4-oxo substitution in the C ring, especially in association establish the exact mechanism of action of this quercetin-
with the C2-C3 double bond, increases scavenger activity derived compound.
of ¯ avonoids by delocalizing electrons from the B ring
(Middleton et al 2000). Likewise, such a con®guration
Conclusions
could increase the cardioprotective eŒects of these com-
pounds. Nevertheless, substitutions of the A and C rings
also seem to play an important role in this.
In our study we observed a correlation between the struc-
ture and mechanisms of action of quercetin 3,7-dimethyl
Flavones and ¯ avonols are structurally similar, ¯ avonols
having an extra hydroxyl substitution at the carbon 3
position (Figure 1). This hydroxyl substitute seems to be
particularly sensitive for ¯ avonoid compounds (Chan et al
2000). The C3 methyl substitution could further increase
the relaxant potency. Such could also be the case of QDME.
Therefore, hydroxylation at position 3 of the C ring may
not be the best combination with the C2-C3 double bond of
C, whereas the extra methyl substitution at position C7
increases the vasodilator actions of the ¯ avonoids. Al-
though other authors have found that ¯ avonols with an
extra hydroxyl substitution at the carbon 3 position, such
as quercetin and kaempferol, can elicit a greater relaxant
response (Duarte et al 1993), others have shown that
¯ avones and ¯ avonols have similar eŒects (Chan et al
2000). In view of our results, we may assume that 3,4
hydroxylation of the B ring must be preserved and that C3
and C7 methyl substitutions of the C and A rings further
increase the relaxant activity.
«
ether, quercetin, 3,4 ,7-trimethyl ether (ayanin) and quer-
cetin 3,3 ,4 ,7-tetramethyl ether. Extra hydroxyl substitu-
« «
«
«
tions at the C3 and C4 positions of the B ring increased
the vasorelaxant potency. In addition, methyl substitutions
at positions C3 and C7 of the C and A rings can further
extend theresponse. The vasorelaxation induced by QDME
}
is partially NO cGMP pathway-dependent, although in-
teractions with contractile mechanisms of endothelium-
independent origin could also contribute to the increase in
relaxation. Such actions may also add to the reported
cardioprotective eŒects of ¯ avonoid compounds.
References
Carbon-13 NMR of ¯ avonoids
. Elsevier,
Agrawal, P. K. (ed.) (1989)
Amsterdam, pp 158±168
Burke, T. M., Wolin, M. S. (1987) Hydrogen peroxide elicits pul-
Am J
monary arterial relaxation and guanylatecyclase activation.
Physiol
. .
. 252: H721±H732
Removal of the endothelium induced an important de-
crease in the relaxant response by QDME (20-fold shift in
the curve to the right). Therefore, the role of endothelium
factors in the vasodilator eŒect of this compound is clear.
Indeed, the relaxant response to QDME was inhibited
after the inhibition of NO and cGMP synthesis using L-
NAME, a nitric oxide synthase inhibitor, and methylene
blue, a drug that interferes directly with soluble guanylyl
cyclase activated by NO (Gruetter et al 1981 ; Wang et al
1995) and inhibits vasorelaxation induced by hydrogen
peroxide (Burke & Wolin 1987) or organic peroxide
(Thomas & Ramwell 1986). However, at higher concentra-
tions QDME relaxation is only aŒected to a certain extent.
}
Accordingly, the NO cGMP pathway might be involved in
causing the vasorelaxation to QDME but another mech-
anism of action must also contribute to the response.
-
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