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The tracheal relaxant activities and action mechanisms of
quercetin methyl ether derivatives, including quercetin
4'-methyl ether (tamarixetin), quercetin 3,4',7-trimethyl ether
(ayanin), quercetin 3,3',4',7-tetramethyl ether (QTME) and
quercetin 3,3',4',5,7-pentamethyl ether (QPME) were analyzed and
compared with our previous studies of quercetin and quercetin
3-methyl ether to understand their structure-activity
relationship (SAR). The above querce\9?methyl ether derivatives
concentration-dependently reled the histamine (30 mM)-,
carbachol (0.2 mM)- and KCl (30 mM)-induced precontractions of
isolated guinea-pig trachea. Roughly, according to their IC25s,
the order of their relaxant activity was QPME, quercetin
3-methyl ether > quercetin, ayanin > tamarixetin > QTME. The SAR
was concluded as follows: (a) Methylation at position 3 or 5 on
quercetin, for example quercetin to quercetin 3-methyl ether and
QTME to QPME, largely increased their relaxant activity. (b)
Methylation at position 3' or 4' on quercetin,uch as ayanin to
QTME and quercetin to tamarixetin, reduced their relaxant
activity; and (c) The relaxant activity does not increase with
the number of methoxyl group because QTME has the lowest
relaxant effect and ayanin has lower relaxant activity than
quercetin 3-methyl ether.
The preincubation of the more potent quercetin methyl ether
derivative, QPME, non-competitively inhibited contraction
induced by cumulatively adding histamine, carbachol or KCl in
isolated guinea-pig trachea. In carbachol and KCl, the pD2'
values were significantly less than their -logIC50s. Therefore,
the inhibitory ability of QPME on calcium release from calcium
stores may be less potent than the suppression of calcium influx
from extracellular fluid in carbachol- and KCl- induced
contraction. QPME also n-competitively inhibited contractions of
the trachealis induced by cumulatively adding calcium into high
potassium (60 mM)-Ca2+ free medium in the trachealis. After
maximal relaxation on histamine (30 mM)-induced precontraction
by nifedipine (10 mM), QPME caused further relaxation of the
trachealis. The result suggests that QPME may have other
relaxant mechanisms in addition to inhibiting voltage (VOC) and/
or receptor operated calcium channels (ROC) in the trachealis.
However, its relaxant response was not fected by the removal of
epithelial cells or by the presences of propranolol (1 mM),
glibenclamide (10 mM), methyleneblue (25 mM) and
2',5'-dideoxyadenosine (10 mM). Therefore, its relaxing effect
may not be related to epithelium derived relaxing factor(s),
activation of b-adrenoreceptor, opening of ATP-sensitive
potassium channels, or activation of guanylate cyclase or
adenylate cyclase.
Similar to IBMX (3-6 mM), QPME (10, 20 mM) parallelly leftward
shifted the log concentration-response curve of forskolin, and
at 20 mM, QPME also leftward shifted the log concentration-
response curve of nitroprusside, and reduced the pD2 values of
forskolin or nitroprusside. It suggests that QPME may inhibit
phosphodiesterase (PDE). In the assay of PDE activity, QPME
(50-300 mM) inhibited both cAMP- and cGMP-PDE. In the presence
of QPME 50 and 100 mM, the cAMP-PDE activity was significantly
lower than cGMP-E, suggesting that QPME has stronger inhibition
in cAMP-PDE than in cGMP-PDE activity at 50 and 100 mM.
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