Introduction
Portulaca oleracea is a grassy plant with small-yellow flowers and
height of 10-30 cm, which grows in different areas of the world
including north and northwest of Iran. It contains water
(92-95%), mucilage, pectin and lipids (0.3-0.4%). Several
therapeutic effects including anti-pyretic, anti-scurvy,
anthelmentic, pain relief in migraine headaches and
gastrointestinal tract and anti-inflammatory have been
described for Portulaca oleracea in Iranian folk medicine (1, 2). There is
evidence for relaxant effect of the aqueous extract of this
plant on skeletal and smooth muscles (3-7), analgesic and
anti-inflammatory effects (8), antioxidant effect (9), and
neuropharmacological activity (10). Our laboratory findings
also showed a relaxant effect of the boiled aqueous extract of
this plant in airways of asthmatic patients and also on
isolated guinea pig tracheal chains. In the present study
the antitussive effect of the boiled extract (2.5 and 5%, w/v)
of Portulaca oleracea was evaluated.
Experimental
Plant and extracts
The whole plant was collected from the
farm of school of Pharmacy, Mashhad University of Medical
Sciences, identified by Mr. Ahi, dried and powdered. The
extracts were prepared as follows: ten g of the dried plant
powder was added to 100 ml boiled water for 15 min and then
filtered. The solvent was then removed under reduced pressure
until the extracts volume reached to 10 ml. The concentration
in the final extracts was 10% w/v.
Protocol
The Dunkin-Hartley guinea pigs (Razi
Institute, Mashhad, Iran) of both sexes (body weight 500-600g)
were used in this study. The method used has been described
previously (11). Unanaesthetized unrestrained animals were
placed individually in a transparent Perspex chamber,
dimensions 30×20×20 cm and exposed to a nebulized
aqueous solution of 0.1 g/ml citric acid for 7 min. The aerosol
was produced by an air flow of 8 L/min through a Wright
nebulizer (producing an aerosol with particles with an
aerodynamic mass median diameter of 0.9 mm) nebulizing
0.65±0.04 ml solution per minute. During the exposure, a
trained observer continuously watched the animals, and the
number of coughs produced during the last 5 min of exposure was
counted. Coughs could easily be distinguished from sneeze,
since there is a clear difference in sound as well as in
behaviour of the animal (11).
The above method was performed 10 min
after exposing animals to 4 different aerosols of following
solutions (n=6 for each solution):
i) Normal saline (baseline measurements)
ii) Boiled extract (2.5% w/v)
iii) Boiled extract (5% w/v)
iv) Codeine solution (0.03 g/ml, positive
control)
All of the experiments were performed
randomly with 2 h resting period between eachexperiment. The
effective concentration of extract causing 50% reduction of
cough number (EC50) was also determined.
Statistical analysis
All data were expressed as
mean±SEM. Comparison of baseline data with number of
cough obtained in the presence of plant extracts and codeine
were made using ANOVA. Comparison of data obtained in the
presence of two different concentrations of boiled extracts was
made using paired "t" test. Significance was accepted
at p<0.05.
Results and Discussion
Both concentrations of boiled extract and
codeine caused significant reduction in number of citric
acid-induced coughs compared to saline (p<0.001). However,
the antitussive effect of 5% concentration of boiled extract
was significantly different with that of codeine (p<0.01),
but 2.5% extract showed no significant difference from codeine.
There was also significant difference between antitussive
effects of both boiled extracts (p<0.001) (Fig. 1). EC50 of
the plant extract was 4.5%.
In the present study the antitussive
effect of boiled extract of Portulaca
oleracea was evaluated using a
standard method applied previously by several investigators
(11, 12). Data showed relatively a potent antitussive effect
for boiled extract of the plant that was even greater than the
effect of codeine. Codeine at concentration of 3% reduced
number of coughs from 14.8 to 10.2 and caused 31.1% protection,
while the 2.5% and 5% solutions of plant extract reduced number
of coughs to 10.7 and 6.7 and caused 27.7% and 54.7%
protection, respectively. The EC50 for plant extract was 4.5%.
It means that effect of 3% codeine solution is almost
equivalent to 2.5% extract.Opioids such as morphine and codeine
are generally considered to be the most potent and effective
antitussive drugs available and are believed to inhibit cough
through suppression of cough centre in the CNS (13, 14).
Although the antitussive effect of plant extracts was similar
to that of codeine, the mechanism(s) of antitussive effect of
this plant cannot be concluded from the results of the present
study.
Cough may be caused by several mechanisms
including; mechanical stimulation of normal afferent system,
chemicals, inflammatory mediators, and neurotransmitter
stimulation of normal afferent system (15). Although the
modulating effects of inflammatory mediators and
neurotransmitters on the cough reflex are likely to be very
important but it needs further investigations (16-18). Results
of studies to date attest the complexity of this issue. For
example, in human, prostaglandin F2 potentiates cough
responses to both inhalation of low chloride solution and
capsaicin, but prostaglandin E2 potentiates cough resulting
from capsaicin only (18). In another study, it has been shown
that iv injection of serotonin, decreases cough in response to
inhaled low chloride solution but not capsaicin (17). In
another study, Richard et al., (2001) have tabulated chemicals
that could induce cough. Some of these agents are acetic acid,
capsaicin, acetylcholine, citric acid, histamine, HCl, Tear
gas, phosphoric acid, sulfuric acid, bradykinin, ether,
lobeline, ammonia vapor and tobacco smoke (19).
Morphine was recently shown to reduce a
vagally mediated bronchoconstriction produced by inhaled
distilled water in asthmatics (20) and in healthy human
subjects. The mechanism behind this inhibitory effect is
unknown, but suppression of neurotransmitter release has been
suggested (21). Some experimental data indicated that opioids
might interact with the peripheral nervous system of the
tracheobronchial tree. A partial antagonism of a noncholinergic
neurogenic bronchoconstriction in the guinea pig by opioid
agonists has been reported (22-24). Kerlsson et al., (1990)
also showed that nebulized codeine and morphine could inhibit
bronchoconstriction and cough induced by citric acid using a
method similar to that of the present study (12).
Therefore, the similar antitussive effect of extracts
from Portulaca oleracea and codeine may indicate that this plant
may have bronchodilator activity that has been shown in our
previous studies both in human (25) and experimental animals
(26).
In conclusion the results of this study
indicated that the antitussive effect of Portulaca oleracea was
comparable to that of codeine. The probable antitussive
mechanism(s) of this plant, may be due to its central effect,
and/or suppression of neurotransmitter release or may be due to
its relaxant effect, but the exact mechanism(s), should be
clarified in future studies. Finally, since the 2.5% extract
showed equivalent effect with 3% codeine solution and based of
our previous data on the airways of asthmatic patients (25) and
its fantastic relaxant effect on tracheal chain in guinea pig
(26), it is suggested that the aqueous extract of this plant is
a good candidate to be used in clinic.
Acknowledgment
This study was financially supported by
the Vice Presidency of Research of Mashhad University of
Medical sciences.
References