Abstract:We investigated the mechanisms of action of S-peta-sin and S-isopetaS-peta-sin, from Petasites formosanus Kitamura which is used as a folk medicine for treating hypertension, tu-mors, and asthma in Taiwan. The tension changes of tracheal segments were isometrically recorded on a polygraph. S-Peta-sin and S-isopetaS-Peta-sin non-competitively inhibited cumulative histamine-, and carbachol-induced contractions with an ex-ception that S-isopetasin produced a parallel, rightward shift of the concentration-response curve of carbachol in a compet-itive manner. S-Petasin also non-competcompet-itively inhibited cu-mulative Ca2+-induced contractions in depolarized (K+, 60 mM; histamine, 100mmM; or carbachol, 10mmM) guinea-pig tracheas. S-Isopetasin did in depolarized (K+, 60 mM) trachea too. The nifedipine (10mmM)-remaining tension of carbachol (0.2mmM)-induced precontraction was further relaxed by S-pe-tasin or S-isopeS-pe-tasin, suggesting that no matter whether ei-ther blocked VDCCs or not, S-petasin or S-isopetasin may have other mechanisms of relaxant action. The relaxant effect of S-petasin or S-isoS-petasin was unaffected by the presence of pro-pranolol (1mmM), 2¢¢,5¢¢-dideoxyadenosine (10mmM), methylene blue (25mmM), glibenclamide (10mmM), Nww-nitro-L-arginine (20mmM), or aa-chymotrypsin (1 U/ml). However, S-petasin (100 ± 300mmM), but not S-isopetasin, significantly inhibited cAMP-, but not cGMP-dependent PDE activity of the trachea-lis. The above results reveal that the mechanisms of relaxant action of S-petasin and S-isopetasin may be primarily due to its non-specific antispasmodic and antimuscarinic effects, respectively.
Key words:S-Petasin, S-isopetasin, Petasites formosanus, Aster-aceae, guinea-pig trachea, calcium release, calcium influx, cAMP-dependent PDE.
Abbreviations:
ROCCs: receptor-operated calcium channels VDCCs: voltage dependent calcium channels cAMP: adenosine 3¢¢,5¢¢-cyclic monophosphate cGMP: guanosine 3¢¢,5¢¢-cyclic monophosphate
PDE: phosphodiesterase
IBMX: 3-isobutyl-1-methylxanthine
Introduction
In 1993, Brune et al. (1) reported the extract of Petasites hy-bridus L. (Compositae), a therapeutically spasmolytic agent for gastrointestinal tract spasm and for asthmatic attacks in the late middle ages in Europe, to have gastro-protective ef-fects. Bickel et al. (2) identified two main compounds, petasin and isopetasin, in this species, and reported isopetasin and oxopetasan esters to have inhibitory effects on the biosynthe-sis of vasoconstrictive peptido-leukotrienes. However, petasin has been known to have a spasmolytic effect, although its ac-tion mechanism remains unclear, and has been quantitatively analyzed from this plant by Wildi et al. (3). Petasites formosa-nus Kitamura, a perennial herb and the only indigenous Peta-sites species in Taiwan, is used as a folk medicine for treating hypertension, tumors, and asthma in Taiwan (4). Recently, Lin et al. (5), (6) have reported that it contains several new ere-mophilane-type sesquiterpenes, together with six known compounds, including S-petasin, S-isopetasin, petasin, and isopetasin. The contents of S-petasin, S-isopetasin, petasin, and isopetasin in the aerial part of the plant have been report-ed to be 0.068%, 0.024%, 0.026%, and 0.005%, respectively (6). The content of S-petasin is the most abundant among these four. S-Petasin (IC50< 10mM) has been proven to be the most potent in relaxing guinea-pig trachea precontracted by hista-mine, carbachol, KCl, or leukotriene D4, although S-isopetasin (IC50~= 10mM) has a similar relaxing potency on carbachol and KCl, but almost has no effect on histamine and leukotriene D4 (7). In the present study, we investigated the mechanisms of action of S-petasin and S-isopetasin.
Materials and Methods Reagents and drugs
S-Petasin and S-isopetasin (Fig.1) were isolated as previously described (5) from the aerial parts of Petasites formosanus Ki-tamura, and identified by spectral methods, including IR, MS, 1D- and 2D-NMR spectroscopic techniques. The purity of S-pe-tasin or S-isopeS-pe-tasin was over 99%. The optical rotation values of S-petasin and S-isopetasin were [a]25
D+58.08 (c 1.0, MeOH) and [a]25
D+38.58 (c 1.0, CHCl3), respectively. Atropine, amino-phylline, carbachol, histamine, propranolol, 2¢,5¢ -dideoxyade-nosine, methylene blue, glibenclamide, Nw-nitro-L-arginine
(L-NNA),a-chymotrypsin, nifedipine, indomethacin, ethylene
gly-Mechanisms of Relaxant Action of S-Petasin and S-Isopetasin,
Sesquiterpenes of Petasites formosanus,
in Isolated Guinea Pig Trachea
Wun-Chang Ko
1,*, Chien-Bang Lei
1, Yun-Lian Lin
2, Chieh-Fu Chen
21Graduate Institute of Medical Sciences, Taipei Medical College, Taipei, Taiwan, R.O.C. 2National Research Institute of Chinese Medicine, Taipei, Taiwan, R.O.C.
Received: March 7, 2000; Accepted: July 3, 2000
Planta Med 67 (2001) 224±229
Georg Thieme Verlag Stuttgart·New York
ISSN: 0032-0943
col-bis(b-aminoethyl ether) N,N,N¢,N¢-tetraacetic acid (EGTA), Trizma base,DL-dithiothreitol,b-mercaptoethanol, cyclic AMP,
cyclic GMP, calmodulin, Dowex resin, and Ophiophagus hannah snake venom, etc. were purchased from Sigma Chemical, St. Louis, MO, USA. [3H]cAMP and [3H]cGMP were purchased from DuPont, Boston, MA, USA. 3-Isobutyl-1-methylxanthine (IBMX) was purchased from Aldrich Chem., Milwaukee, WI, USA. All reagents, including KCl, were of analytical grade. Gli-benclamide was dissolved in dimethyl sulfoxide (DMSO), S-pe-tasin or nifedipine was dissolved in ethyl alcohol:DMSO (1:1), indomethacin was dissolved in ethyl alcohol, and other drugs were dissolved in distilled water. The final concentration of DMSO or ethyl alcohol was less than 0.1% and did not signifi-cantly affect the contraction of the trachea.
Guinea-pig trachea
Male Hartley guinea pigs weighing 250 to 450 g were killed by cervical dislocation and the tracheas were removed. Each tra-chea was cut into six segments. Each segment consisted of three cartilage rings. All segments were cut open opposite the trachealis. After the segments were randomized to minimize regional variability, they were tied at one end to holders via silk suture, placed in 5 ml of normal or Ca2+-free Krebs solution containing indomethacin (2.8mM), gassed with a 95% O2±5% CO2mixture at 37 8C, and attached by the other end of each segment to force displacement transducers (Grass FT03) for the isometric recording of tension changes on a polygraph (Gould RS3200). The composition of the normal Krebs solution was (mM): NaCl 118, KCl 4.7, MgSO41.2, KH2PO41.2, CaCl22.5, NaHCO325, and dextrose 10.1. The isotonic high K+, Ca2+-free Krebs solution consisted of the above composition without CaCl2, but 60 mM NaCl was replaced by 60 mM KCl. There were three Ca2+-free Krebs solutions prepared by omitting CaCl
2 with 2 mM or 0.02 mM EGTA, and without EGTA. The tissues were suspended in normal Krebs solution under an initial ten-sion of 1.5 g and allowed to equilibrate for at least 1 h with washing at 15-min intervals. Either histamine or carbachol was then cumulatively added to the normal or to the Ca2+-free Krebs solution with 0.02 mM EGTA, and the procedure was re-peated until the contraction reached constancy after washout. Then, cumulative concentration-response curves were con-structed. Maximal contractions of the tracheas without incu-bation of drugs or their vehicles were set as 100%. After the tis-sues were preincubated with S-petasin (10±200mM), S-isope-tasin (10±200mM) or their vehicles for 15 min, these two con-tractile agonists were also cumulatively added again in normal Krebs solution. When the antagonistic effects of petasin or S-isopetasin on these cumulative concentration-response curves were measured in a non-competitive manner, aminophylline was used as a positive control, and their antagonistic potencies were expressed as pD2¢ values. In contrast, when the antago-nistic effect of S-isopetasin on the cumulative concentration-response curves of carbachol was measured in a competitive manner, atropine was used as a positive control, and their
an-tagonistic potencies were expressed as pA2values. In the case of isotonic high K+(60 mM)-, histamine (100mM)-, or carba-chol (10mM)-depolarized tracheal preparations, normal Krebs solution was replaced after equilibration by Ca2+-free Krebs solution without EGTA, and washed with the Ca2+-free solution with 2 mM EGTA after tracheal contraction reached constancy and then incubated for 5 min. After repeating the above proce-dure until no contraction was observed, cumulative Ca2+ (0.01±10 mM) was added and contractions were elicited in the depolarized trachealis. The maximal contractile response elicit-ed by Ca2+(10 mM) was taken as 100%, and the cumulative concentration-response curve was constructed. The inhibitory effects of S-petasin or S-isopetasin on cumulative Ca2+-induced contractions in isotonic high K+(60 mM)-, histamine (100mM)-, or carbachol (10mM)-depolarized tracheas were expressed by -log IC50values. The tracheal relaxant effects of cumulative S-petasin (0.1±300mM) or S-isopetasin (0.1±300mM) to hista-mine (10mM)-induced precontraction were allowed to reach a steady state at each concentration. At the end of the experi-ment without washout, 1 mM of aminophylline was added to standardize the maximal tissue relaxation (100%). All antago-nists or their vehicles were incubated after the precontraction reached a steady state for 15 min prior to the first addition of S-petasin or S-isopetasin. In a similar manner, nifedipine (10mM) was added after carbachol (0.2mM)-induced precon-traction reached a steady state, at 15 min prior to the addition of S-petasin (100mM), S-isopetasin (100mM) or their vehicle. At the end of the experiment, 1 mM of aminophylline was also added to standardize maximal tissue relaxation.
Phosphodiesterase activity
The isolated trachealis was homogenized with a glass/teflon homogenizer (Glas-Col, Terre Haute, IN, USA) in 20 volumes of cold medium (pH 7.4) containing 100 mM Tris-HCl, 2 mM MgCl2, and 1 mM dithiothreitol, cAMP- and cGMP-dependent phosphodiesterase (PDE) activities in the homogenate were measured by a modification of the method of Cook et al. (8). The homogenate was centrifuged at 9500 rpm for 15 min, and the upper layer was decanted. Twenty-five microliters of the upper layer were taken for determination of enzyme activity in a final volume of 100ml containing 40 mM Tris-HCl (pH 8.0), 2.5 mM MgCl2, 3.75 mM mercaptoethanol, 0.1 unit calmo-dulin (PDE activator), 10mM CaCl2, and either 1mM cAMP with 0.2mCi [3H]-cAMP or 1mM cGMP with 0.2mCi [3H]-cGMP. In tests of enzyme inhibition, the reaction mixture contained various concentrations of S-petasin (30±300mM), S-isopeta-sin (30±300mM) or IBMX (100±300mM), a positive control. The reagents and homogenate were mixed on ice, and the re-action was initiated by transferring the mixture to a water bath at 37 8C. Following a 30-min incubation, the reaction was stopped by transferring the reaction vessel to a bath of boiling water for 3 min. After cooling on ice, 20ml of a 1 mg/ml solu-tion of Ophiophagus hannah venom were added to the reac-tion mixture, and the mixture was incubated at 37 8C for Fig.1 Chemical structures of S-petasin and S-isopetasin.
10 min. Unreacted [3H]-cAMP or [3H]-cGMP was removed by the addition of 500ml of 1-in-1 Tris-HCl (40 mM) buffer sus-pension of Dowex resin (1 8±200) with incubation on ice for 30 min. Each tube was then centrifuged for 2 min at 6000 rpm, and 150ml of the supernatant was removed for liq-uid scintillation counting. Less than 15% of the tritiated cyclic nucleotide was hydrolyzed in this assay.
Statistical analysis
The antagonistic effects of S-petasin or S-isopetasin on these cumulative concentration-response curves were calculated and expressed as pA2or pD2¢values, according to the method described by Arins and van Rossum (9), when the antago-nism was competitive or non-competitive, respectively. Ac-cordingly, pA2= pAx+ log (x ± 1), where pAxis negative loga-rithm of the molar concentration of S-isopetasin and x is ratio between concentration of agonist in the presence of S-isope-tasin and that in the absence of S-isopeS-isope-tasin; whereas pD2¢= pDx¢+ log (x ± 1), where pDx¢is negative logarithm of the mo-lar concentration of S-petasin or S-isopetasin and x is ratio be-tween maximal effect of agonist in the absence of S-petasin or S-isopetasin and that in the presence of S-petasin or S-isope-tasin (10). The ±logIC50value was considered to be equal to the negative logarithm of the molar concentrations of S-peta-sin or S-isopetaS-peta-sin at which a half-inhibitory effect on Ca2+ (10 mM)-induced contraction was observed. The IC50 value was calculated by linear regression. All values are shown as means SEM. The differences among these values were statis-tically calculated by one-way analysis of variance (ANOVA), then determined by least significant difference (LSD). The dif-ference between two values, however, was determined by use of Student©s unpaired t-test. The differences were considered statistically significant if the P-value was less than 0.05.
Results
S-Petasin (20±200mM) concentration-dependently, but S-iso-petasin (100±200mM) concentration-independently, inhibit-ed concentration-response curves of cumulative histamine in a non-competitive manner (Figs. 2A, C). The pD2¢values were
4.10 0.08 (n = 18), and 3.15 0.11 (n = 14), respectively which are significantly different from each other (Table 1). S-Petasin (10±200mM) concentration-dependently inhibited concen-tration-response curves of cumulative carbachol in a non-competitive manner (Fig. 2B). However, S-isopetasin (50± 200mM) produced a parallel, rightward shift of the concentra-tion-response curve of carbachol in a competitive manner Table 1 pD2¢, pA2and ±log IC50values of S-petasin and S-isopetasin in non-depolarized and depolarized guinea-pig trachealis
Non-depolarized preparation Depolarized preparation
Normal Ca2+(2.5 mM) Ca2+-free (0.02 mM EGTA) K+(60 mM) His (100mM) CCh (10mM)
His CCh His CCh Ca2+ Ca2+ Ca2+ S-petasin pD2¢ 4.10 0.08 (18) 3.95 0.11 (20)### 4.20 0.17 (12)# 4.74 0.16 (14) ±log IC50 4.50 0.31 (6) 3.76 0.32 (6) 4.05 0.07 (5)# S-isopetasin pA2 5.36 0.09 (25)*** pD2¢ 3.15 0.11 (14)*** ND ND ±log IC50 4.82 0.15 (6) ND ND Atropine pA2 8.92 0.08 (7)$$$ Aminophylline pD2¢ 3.76 0.10 (12)** 3.57 0.12 (17)* Values are presented as means SEM (n); n is the number of experiments.
*P < 0.05, **P < 0.01, ***P < 0.001 when compared with the corresponding pD2¢value of
S-petasin.
#P < 0.05,###P < 0.001 when compared with the pD
2¢value of S-petasin against CCh
in Ca2+-free Krebs solution with 0.02 mM EGTA.
$$$P < 0.001 when compared with the corresponding pA
2value of S-isopetasin.
ND: not determined. His: histamine. CCh: carbachol.
Fig. 2 The inhibitory effects of S-petasin (A, B) and S-isopetasin (C, D) (*, vehicle; ~, 10mM; &, 20mM; l, 50mM; ~, 100mM; n, 200mM) on cumulative histamine (A, C)-, and carbachol (B, D)-in-duced contractions in guinea-pig trachealis in normal Krebs solution. Each point represents the mean SEM of 4±11 experiments. The relationship between ±log concentration of S-isopetasin and log (DR-1), where DR is the dose ratio, is shown in the inset.
(Fig. 2D). The pD2¢value of S-petasin was 3.95 0.11 (n = 20), but the pA2 value of S-isopetasin was 5.36 0.09 (n = 25), respectively, which are significantly different from each other (Table 1). The Schild regression equation for S-isopetasin is y = 6.57 + 1.30x (r = 0.9626). The slopes [1.299 0.232 (n = 6)] of Schild plots were not significantly different from unity. The pA2 value of atropine, a positive control, against carbachol was 8.92 0.08 (n = 7), which was significantly greater than that of S-isopetasin (Table 1).
In Ca2+-free Krebs solution with 0.02 mM EGTA, S-petasin (50±200mM) also inhibited concentration-response curves of cumulative histamine and carbachol in a non-competitive manner (Fig. 3). The pD2¢values were 4.20 0.17 (n = 12) and 4.74 0.16 (n = 14), respectively, which significantly differ from each other (Table 1). The pD2¢value against carbachol in Ca2+-free Krebs solution was also significantly greater than that in normal Krebs solution (Table 1).
In isotonic Ca2+-free high K+-, histamine- and carbachol-depo-larized tracheas, S-petasin concentration-dependently inhibit-ed concentration-response curves of cumulative Ca2+(0.01± 10 mM) in a non-competitive manner (Figs. 4A, B, C). The ±log IC50values were 4.50 0.31 (n = 6), 3.76 0.32 (n = 6) and 4.05 0.07 (n = 5), respectively, which are not significantly different from each other (Table 1). The ±log IC50value of S-isopetasin against cumulative Ca2+-induced contractions in isotonic Ca2+-free high K+-depolarized tracheas was 4.82 0.15 (n = 6) (Fig. 4D, Table 1), which was not significantly dif-ferent from the corresponding value of S-petasin.
Nifedipine (10mM) only relaxed 24 10 % (n = 6) of carbachol (0.2mM)-elicited submaximal precontraction [1.22 0.16 g (n = 6)] in normal Krebs solution. Similarly, nifedipine (10mM) relaxed 28 9% (n = 6) of the precontraction [2.01 0.16 g (n = 6)]. The nifedipine-remaining tension was further relaxed by S-petasin (100mM) or S-isopetasin (100mM) to 80 9% (n = 6) or 72 8% (n = 6), respectively. Finally, aminophylline (1 mM) completely relaxed the trachea (Fig. 5).
However, none of the antagonists used, such as propranolol (1mM), 2¢,5¢-dideoxyadenosine (10mM), methylene blue (25mM), glibenclamide (10mM), L-NNA (20mM), anda -chy-motrypsin (1 U/ml), affected the log concentration-relaxing response curves of cumulative S-petasin or S-isopetasin to histamine (10mM)-induced precontraction in normal Krebs solution (data not shown).
S-Petasin at 100 and 300mM, but not S-isopetasin, significant-ly inhibited 33.9 6.1% (n = 5) and 33.2 4.4% (n = 6) of cAMP-, but not cGMP-dependent PDE activity, respectively. The comparative drug, IBMX (30±300mM) as a positive con-trol, however, inhibited both enzyme activities except IBMX (30mM) on cGMP-PDE activity (Fig. 6).
Fig. 4 The inhibitory effects of S-petasin (A, B, C, D) and S-isopeta-sin (D) (*, vehicle; ~, 10mM; &, 20mM;l, 50mM;~, 100mM; n, 200mM) on cumulative calcium-induced contractions in guinea pig trachealis depolarized by (A) histamine 100mM, (B) carbachol 10mM, and (C, D) KCl 60 mM in Ca2+-free medium without EGTA. Each point represents the mean SEM of 4±15 experiments. Fig. 3 The inhibitory effects of S-petasin (*, vehicle;l, 50mM;~,
100mM, n, 200mM) on cumulative (A) histamine- and (B) carba-chol-induced contractions in guinea pig trachealis in Ca2+-free medi-um with 0.02 mM EGTA. Each point represents the mean SEM of 4±6 experiments.
Discussion
The log concentration-relaxing response curves of cumulative S-petasin and S-isopetasin to histamine (10mM)-induced pre-contraction was not affected by propranolol (1mM), a non-se-lectiveb-adrenoceptor blocker (12), suggesting that the relax-ant effect of both is not via the activation ofb-adrenoceptor. 2¢,5¢-Dideoxyadenosine, an adenylate cyclase inhibitor (13), (14) and methylene blue, a soluble guanylate cyclase inhibitor (15), also did not affect the log concentration-response curves of S-petasin and S-isopetasin. This reveals that the relaxant ef-fect of both is neither via the activation of adenylate cyclase nor via that of guanylate cyclase. Glibenclamide, an ATP-sen-sitive potassium channel blocker (16), also did not affect the log concentration-response curves of S-petasin and S-isopeta-sin, suggesting that the relaxant effect of both is not via the opening of ATP-sensitive potassium channels (17). L-NNA
(20mM), a nitric oxide (NO) synthase inhibitor (18), did not af-fect the log concentration-response curves of petasin and S-isopetasin, suggesting that the relaxant effect of both is unre-lated to NO formation.a-Chymotrypsin (1 U/ml), a peptidase, also did not affect the log concentration-response curves of S-petasin and S-isoS-petasin, suggesting that the relaxant effect of both is unrelated to the neuropeptides.
S-Petasin (20±200mM) and S-isopetasin (10±200mM) con-centration-dependently and non-competitively inhibited cu-mulative Ca2+-induced contractions in the depolarized (K+, 60 mM) trachealis. At the highest concentration, S-petasin and S-isopetasin almost blocked these contractions, therefore they may inhibit Ca2+influx via voltage-dependent calcium channels (VDCCs) opened by 60 mM KCl. For example, nifedi-pine, a selective VDCCs blocker (19), at concentrations below 1mM, also inhibits those contractions in a non-competitive manner. Nifedipine at 1mM can further completely inhibit those contractions (11). In the present study, nifedipine (10mM) only (24±28%) relaxed the carbachol-induced pre-contraction in normal Krebs solution. The nifedipine-remain-ing tension was further (72±80%) relaxed by petasin or S-isopetasin a 100mM suggesting that no matter whether either blocked the VDCCs or not, either may have other relaxant ac-tion mechanisms.
S-Isopetasin (30±300mM) did not significantly inhibit either cAMP- or cGMP-dependent PDE activity. Therefore, the tra-cheal relaxant action mechanisms of S-isopetasin may be due to its antimuscarinic and VDCCs blocking effects on the tra-chealis. The antimuscarinic effect of S-isopetasin is signifi-Fig. 6 The log concentration-inhibitory effects of petasin (A), S-isopetasin (B) (~,~) and IBMX (l,*) on cAMP (~,l)- and cGMP (~, *)-dependent phosphodiesterase activities. The inhibitory ef-fects do not include those of their vehicle. Each point represents the mean SEM of 4±7 experiments. *P < 0.05, ***P < 0.001 when analyzing the difference between drugs and their vehicles by Stu-dent©s unpaired t-test.
Fig. 5 The tracing graph of relaxant effects of S-petasin and S-isope-tasin on carbachol (CCh, 0.2mM)-induced precontraction in guinea-pig trachealis in normal Krebs solution. S-Petasin (100mM) or S-isope-tasin (100mM), compared to their vehicle, further relaxed nifedipine (Nif, 10mM)-remaining tension. At the end of the experiment, amino-phylline (AP, 1 mM) was added to completely relax the trachealis.
cantly less than that of atropine in potency, but significantly greater than the non-specific antispasmodic effect of S-peta-sin against carbachol in potency (Table 1).
S-Petasin concentration-dependently relaxed the histamine (10mM)-, carbachol (0.2mM)-, KCl (30 mM)-, and leukotriene D4 (10 nM)-induced precontractions. Their ±log IC50 values did not significantly differ from each other (7). This suggests that the relaxant effects of S-petasin are equally effective to any of these four contractile agents, and that S-petasin non-selectively and non-specifically inhibits calcium influx via VDCCs and/or receptor-operated calcium channels (ROCCs) in-duced by these four contractile agents. The non-specific anti-spasmodic effects of S-petasin, like some well known phos-phodiesterase (PDE) inhibitors such as aminophylline and pa-paverine, may be due to its inhibitory effect on the activity of PDE. S-Petasin, in this present study, at 100 and 300mM sig-nificantly inhibited 34% and 33% of cAMP-, but not cGMP-de-pendent PDE activity, respectively. Although the inhibitory ef-fect on this enzyme was slight, the content of cAMP may in-crease. The increased cAMP subsequently activates cAMP-de-pendent protein kinase which may phosphorylate and inhibit myosin light-chain kinase, thus inhibiting contraction (20). The precise mechanism by which relaxation is produced by this second-messenger pathway is not known, but it may re-sult from decreased intracellular Ca2+([Ca2+]
i). The decrease of [Ca2+]
imay be due to reduced influx of Ca2+, enhanced Ca2+ uptake into the sarcoplasmic reticula, or enhanced Ca2+ extru-sion through the cell membrane (20). The decreasing effect of S-petasin on [Ca2+]
i was also preliminarily reported in rat thoracic aorta (21).
In conclusion, therefore, the mechanisms of tracheal relaxant action of S-petasin and S-isopetasin may be primarily due to its non-specific antispasmodic and antimuscarinic effects, respectively.
Acknowledgements
This work was supported by a grant (NSC 89-2320-B038-003) from the National Council of Science, ROC.
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Prof. Wun-Chang Ko
Graduate Institute of Medical Sciences Taipei Medical College 250 Wu-Hsing St.
Taipei 110 Taiwan, R.O.C.
E-mail: [email protected] Fax: +886-2-2377-7639
