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The tracheal relaxant activities and action mechanisms of
flavonoids, including various classes such as flavones (i.e.
flavone and apigenin), flavonols (i.e. flavonol), isoflavones
(i.e. genistein), flavanones (i.e. naringenin) and chalcones (i.
e. chalcone) were analyzed to understand their structure-
activity relationship (SAR). The above six flavonoids dose-
dependently relaxed the histamine (30 μM)-, carbachol (0.2 μ
M)-, KCl (30 mM)-, and leukotriene D4 (10 nM)-induced
precontractions of isolated guinea-pig trachea. Roughly,
according to their IC50s, the order of their relaxant activity
was flavone, apigenin, genistein > flavonol > naringenin >
chalcone. The SAR was concluded as follows: (a) The substitution
of -OH group at position 3 on flavones to form flavonols, for
example flavone to flavonol, reduced their relaxant activity ;
(b) The saturation of the double bond between position 2 and 3
on flavones to form flavanones, such as apigenin to naringenin,
also reduced their relaxant activity; (c) The opening of C-ring
from ether linkage on flavones to form chalcones, such as
flavone to chalcone, largely reduced their relaxant activity;
and (d) Change from flavones to isoflavones, such as apigenin to
genistein, did not affect their relaxant activity. The
preincubation of these three more potent flavonoids, flavone,
genistein and apigenin among the above six, non-competitively
inhibited contraction induced by cumulatively adding histamine,
carbachol or KCl in isolated guinea-pig trachea. In general,
their pD2' values were significantly less than their -logIC50s.
Therefore, their inhibitory abilities on calcium release from
calcium stores may be less potent than their suppression of
calcium influx from extracellular fluid. They also non-
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 μM)-induced precontraction by nifedipine (10μM),
they caused further relaxation of the trachealis. The result
suggests that they may have other relaxing mechanisms in
addition to inhibiting voltage (VOC) and/or receptor operated
calcium channels (ROC) in the trachealis. However, their
relaxant responses were not affected by the removal of
epithelial cells or by the presences of propranolol (1 μM),
glibenclamide (10 μM), methyleneblue (25 μM) and
2',5'-dideoxyadenosine (10 μM). Therefore , their relaxing
effects may not be related to epithelium derived relaxing
factor(s), activation of β-adrenoreceptor, opening of ATP-
sensitive potassium channels, or activation of guanylate cyclase
or adenylate cyclase. Similar to IBMX (3-6 μM), flavone
(12.5-25μM), genistein(17.5-35μM) and apigenin (15-30μM)
dose-dependently and parallelly to the left shifted the log
dose-response curve of forskolin or nitroprusside, and reduced
the IC50s and dose ratios of forskolin or nitroprusside. The
dose ratios of nitroprusside in the pressence of flavone or
apigenin, but not genistein, were signifcantly less than those
of forskolin. It suggests that the former two may selectively
inhibit cGMP-phosphodiesterase (PDE). Moreover, these three
flavonoids (150 μM) significantly enhanced cAMP and cGMP
contents in the trachealis determined by using enzyme
immunoassy. Flavone or apigenin, but not genistein,
significantly enhanced more cGMP content than that of cAMP. The
result supports the context that flavones (flavone or apigenin)
but not isoflavones (genistein), may selectively inhibit cGMP-
PDE.Keywords : flavonoids, structure-activity relationship,
action mechanism, calcium influx, calcium release,
phosphodiesterase.
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