A novel alpha-type fibrinogenase from Agkistrodon rhodostoma snake venom

By means of CM-Sephadex C-50 column chromatography, gel-filtration on sephadex G-75 and Sephacryl S-200 columns, a purified fibrinogenase, kistomin, was obtained from venom of Agkistrodon rhodostoma. It was a single peptide-chain with a molecular mass of about 21,800 Da containing about 202 amino-acid residues as revealed by amino acid analysis. Kistomin preferentially cleaved A alpha- and subsequently the gamma-chain of fibrinogen, leaving the B beta-chain unaffected. Its fibrinogenolytic activity was estimated to be 36.6 +/- 4.5 mg/min per mg protein and was inhibited by the pretreatment of EDTA, suggesting that it is a metalloproteinase. Its fibrinogenolytic activity in platelet-poor plasma is much less potent as compared to that in purified fibrinogen solution. It inhibited ristocetin-induced aggregation of human platelets in a dose-dependent manner in the presence of von Willebrand factor.

Triflavin, an RGD-containing antiplatelet peptide, binds to GpIIIa of ADP-stimulated platelets

Triflavin, an Arg-Gly-Asp-containing snake venom peptide, inhibits platelet aggregation through the blockade of fibrinogen binding to the activated platelets. It binds to fibrinogen receptors associated with the glycoprotein IIb/IIIa complex with a Kd value of 7 x 10(-8) M. In this report, a chemical cross-linking approach was used to further characterize the binding components of triflavin on platelet membrane. 125I-triflavin binding was performed with the aid of a chemical cross-linking reagent, DTSSP. Analysis of the cross-linked products by SDS-PAGE (7.5% gel) and subsequent autoradiogram revealed that 125I-triflavin was cross-linked specifically to a protein with an apparent molecular weight of 1.1 x 10(5), and this reaction was inhibited by GRGDS and excess of non-labeled triflavin. This 110 KDa component was identified to be GpIIIa, recognized by AP3, a mAb against GpIIIa, by immunoblotting technique. These results indicate that the triflavin-binding sites on platelets reside at a site in close proximity to GpIIIa.

Haemodynamic effects of dicentrine, a novel alpha 1-adrenoceptor antagonist: comparison with prazosin in spontaneously hypertensive and normotensive Wistar-Kyoto rats

1. The haemodynamic effects of dicentrine, an aporphine derivative isolated from the plant Lindera megaphylla, were investigated and compared with prazosin in rats. 2. In anaesthetized normotensive Wistar-Kyoto (WKY) rats, i.v. administration of dicentrine (0.1, 0.5, 1.0 mg kg-1) and prazosin (0.01, 0.05, 0.1 mg kg-1) induced a dose-related reduction of mean arterial pressure (MAP) which reached a maximal effect 5-10 min after injection and persisted for 2 h. 3. In anaesthetized WKY rats, a higher dose of dicentrine (1.0 mg kg-1, i.v.) did not cause any significant changes in heart rate (HR), cardiac output (CO) and stroke volume (SV) but markedly increased tail blood flow. In contrast, a higher dose of prazosin (0.1 mg kg-1, i.v.) produced a decrease in HR which paralleled the time course of the hypotensive response. 4. The hypotensive activity of dicentrine was completely abolished by alpha-adrenoceptor blockade. Both dicentrine and prazosin significantly attenuated pressor responses to noradrenaline but failed, even at maximal hypotensive doses, to impair the pressor effects of angiotensin II or vasopressin. These observations suggest that dicentrine appears to exert its hypotensive action through alpha 1-adrenoceptor blockade. 5. In conscious normotensive and spontaneously hypertensive (SH) rats, dicentrine (0.5-2.0 mg kg-1, i.v.) and prazosin (0.05-0.2 mg kg-1, i.v.) also evoked dose-related decreases in MAP which were of greater magnitude in SH rats. Oral administration of dicentrine (5 and 8 mg kg-1) to conscious SH rats caused a hypotensive effect which persisted for over 15 h.6. These results suggest that dicentrine may have therapeutic potential as an oral antihypertensive drug via alpha-adrenoceptor blockade.

An antiplatelet peptide, gabonin, from Bitis gabonica snake venom

Interaction of fibrinogen with its receptors (glycoprotein IIb/IIIa complex) on platelet membranes leads to platelet aggregation. By means of gel filtration, CM-Sephadex C-50, and reverse-phase HPLC, an antiplatelet peptide, gabonin, was purified from the venom of Bitis gabonica. The purified protein migrates as a 21,100-Da polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions and as a 11,000-Da peptide in the presence of beta-mercaptoethanol, indicating that gabonin is a disulfide-linked dimer. It is a polypeptide consisting of about 84 amino acid residues, rich in Asp, Pro, and half-cystine. Gabonin dose-dependently inhibited human platelet aggregation stimulated by ADP, collagen, U46619, or thrombin in preparations of platelet-rich plasma and platelet suspension (IC50 = 340-1600 nM). It also blocked platelet aggregation of whole blood. However, it apparently did not affect the initial shape change and only slightly reduced ATP release caused by aggregation agonists. Gabonin did not inhibit the rise of cytosolic calcium in Quin-2-loaded platelets stimulated by thrombin. In addition, gabonin dose-dependently inhibited fibrinogen-induced aggregation of elastase-treated platelets. In conclusion, gabonin inhibits platelet aggregation mainly through the blockade of fibrinogen binding toward fibrinogen receptors of the activated platelets.

Characterization of snake venom components acting on blood coagulation and platelet function

Snake venoms can affect blood coagulation and platelet function in various ways. The physicochemical properties and the mechanisms of actions of the snake venom components affecting blood coagulation and platelet function are discussed.

Vasorelaxation of rat thoracic aorta caused by osthole isolated from Angelica pubescens

The pharmacological effects of osthole on isolated rat thoracic aorta were examined. Osthole inhibited norepinephrine (NE, 3 microM)-induced phasic and tonic contractions in rat thoracic aorta in a concentration-dependent manner (40-200 microM). The tonic contraction elicited by NE was also relaxed by the addition of osthole. This relaxing effect of osthole was not affected by indomethacin (20 microM) and was still observed in endothelium-denuded rat aorta. Methylene blue (50 microM) partially antagonized this relaxing effect of osthole. In high-K+ medium (80 mM), the Ca2+ (0.03-3 mM)-induced vasocontraction was inhibited concentration dependently by osthole (20-100 microM). Addition of osthole (100 microM) at the plateau of the K+ (80 mM)-induced contraction caused relaxation. Methylene blue (50 microM) did not antagonize this relaxation. In Ca(2+)-free medium, the caffeine (10 mM)-induced phasic contraction was also suppressed by osthole in a concentration-dependent manner. Although the cAMP level was not changed by osthole, the cGMP level of rat aorta was increased by osthole in a concentration-dependent manner. The increase in cGMP level caused by osthole was completely blocked by methylene blue. [3H]Inositol monophosphate formation caused by NE was not affected by osthole at a concentration of 200 microM. The 45Ca2+ influx elicited by either NE or high K+ was inhibited by osthole in a concentration-dependent manner. It is concluded that osthole relaxes rat thoracic aorta by virtue of its Ca(2+)-channel blocking properties and by elevating cGMP levels in vascular smooth muscle.

Triflavin, an Arg-Gly-Asp-containing antiplatelet peptide inhibits cell-substratum adhesion and melanoma cell-induced lung colonization

Triflavin, an Arg-Gly-Asp (RGD) containing peptide purified from Trimeresurus flavoviridis snake venom, inhibits human platelet aggregation by blocking fibrinogen binding to fibrinogen receptors associated with glycoprotein IIb/IIIa complex. In this study, we show that triflavin (1-30 micrograms/mouse) inhibits B16-F10 melanoma cell-induced lung colonization in C57BL/6 mice in a dose-dependent manner. In vitro, triflavin dose-dependently inhibits adhesion of B16-F10 melanoma cells to extracellular matrices (ECMs; i.e., fibronectin, fibrinogen, vitronectin, and collagen type I). Triflavin is approximately 600-800 times more potent than GRGDS at inhibiting cell adhesion. In addition, triflavin dose-dependently inhibits B16-F10 cell-induced platelet aggregation. These results imply that the inhibitory effect of triflavin on the adhesion of tumor cells to ECMs (e.g., fibronectin, vitronectin and collagen type I) and/or tumor cell-induced platelet aggregation may be partially responsible for its antimetastatic activity in C57BL/6 mice.

PAF antagonism in vitro and in vivo by aglafoline from Aglaia elliptifolia Merr

Aglafoline, isolated from Aglaia elliptifolia Merr, inhibited in a selective and concentration-dependent manner the aggregation and ATP release reaction induced in washed rabbit platelets by PAF (platelet-activating factor). The IC50 values of aglafoline, BN52021 and kadsurenone on PAF (3.6 nM)-induced platelet aggregation were about 50, 12 and 18 microM, respectively. Aglafoline also inhibited [3H]PAF (3.6 nM) binding to washed rabbit platelets with an IC50 value of 17.8 +/- 2.6 microM. The concentration-response curve of PAF-induced platelet aggregation was shifted to the right by aglafoline with pA2 and pA10 values of 5.97 and 5.04, respectively. Although thromboxane B2 formation caused by collagen and thrombin was partially suppressed by aglafoline, thromboxane B2 formation caused by ionophore A23187 and arachidonic acid was not affected. Aglafoline inhibited the [3H]inositol monophosphate formation caused by PAF but not that caused by collagen or thrombin in the presence of indomethacin (20 microM). The cAMP content of washed rabbit platelets was not affected by aglafoline. Rat femoral intravenous administration of aglafoline (10 mg/kg) did not affect blood pressure. However, aglafoline (10 mg/kg) both prophylactically and therapeutically antagonized PAF (2.5 micrograms/kg)-induced hypotensive shock in rats. Intravenous PAF (30 ng/kg) caused severe bronchoconstriction in guinea pigs. This effect was completely blocked by aglafoline. This implies aglafoline is an effective PAF antagonist not only in vitro, but also in vivo.

Triflavin, an Arg-Gly-Asp containing snake venom peptide, inhibits aggregation of human platelets induced by human hepatoma cell line

Triflavin, an Arg-Gly-Asp (RGD)-containing peptide, purified from snake venom of Trimeresurus flavoviridis, inhibits human platelet aggregation through the blockade of fibrinogen binding to fibrinogen receptors associated with glycoprotein IIb/IIIa complex. In this report, we examined the effect of triflavin on tumor cells (human hepatoma J-5)-induced platelet aggregation (TCIPA) of heparinized platelet-rich plasma (PRP). ADP-scavenger agents, apyrase (10 U/ml) and creatine phosphate (5 mM)/creatine phosphokinase (5 U/ml) did not inhibit TCIPA while hirudin (5 U/ml) completely inhibited it. J-5 cells initially induced platelet aggregation, then blood coagulation occurred. J-5 cells concentration-dependently shortened the recalcification time of normal as well as Factor VIII, IX-deficient human plasmas, while it was inactive at shortening the recalcification time of Factor VII-deficient plasma, suggesting J-5 cells induced platelet aggregation through activation of extrinsic pathway, leading to thrombin formation as evidenced by the amidolytic activity on s-2238 by expressing tissue factor-like activity. Triflavin inhibited TCIPA in a dose-dependent manner (IC50, 0.02 microM). When compared on molar ratio, triflavin was approximately 30,000 times more potent than GRGDS (IC50, 0.58 mM). On the other hand, GRGES showed no significant effect on TCIPA, even its concentration was raised to 4 mM. Additionally, the monoclonal antibodies, raised against glycoprotein IIb/IIIa complex (i.e., 7E3 and 10 E5) inhibited J-5 TCIPA. In conclusion, we suggest the inhibitory effect of triflavin on J-5 TCIPA may be chiefly mediated by the binding of triflavin to the fibrinogen receptor associated with glycoprotein IIb/IIIa complex on platelet surface membrane.

Antiplatelet actions of some coumarin compounds isolated from plant sources

Xanthyletin, xanthoxyletin, suberosin (all from Citrus grandis), aurapten (from Severinia huxifolia) and poncitrin (from Poncirus trifoliata) were isolated and their chemical structures were characterized to be coumarin compounds. All these coumarin compounds except xanthyletin inhibited the aggregation and ATP release of rabbit platelets induced by arachidonic acid, collagen, ADP, platelet-activating factor (PAF) or U46619 (a thromboxane A2 analog). Thrombin-induced ATP release, but not the aggregation, was also inhibited by these compounds. Xanthyletin inhibited only platelet aggregation induced by arachidonic acid and collagen, while poncitrin inhibited that caused by PAF more markedly than other coumarin compounds. The thromboxane B2 formation in washed platelets caused by arachidonic acid and collagen was suppressed by these coumarin compounds. The phosphoinositide breakdown caused by collagen and PAF was also inhibited by these compounds. They did not affect fibrinogen-induced aggregation of elastase-treated platelets. These antiplatelet actions were immediate, reversible by washout and independent on the incubation time (except suberosin). Antiaggregating effect was also studied by an electrical impedance method and the inhibitory effect of coumarin compounds on the whole blood and platelet-rich plasma was much less than that of platelet suspension in the aggregation induced by collagen, PAF and ADP. It is concluded that the antiplatelet actions of these coumarin compounds are due to the inhibition on thromboxane A2 formation and phosphoinositides breakdown.

HA-29: an inhibitor of thromboxane A2 formation with antagonism of thromboxane A2/prostaglandin endoperoxide receptor in rabbit platelets

HA-29, 2-[(3-methoxyphenyl)methyl]-pyrano[2,3-c]pyrazol-6(1H)-one, was investigated for its inhibitory mechanism of action in washed rabbit platelets. This compound inhibited the aggregation and ATP release of rabbit platelets induced by arachidonic acid and collagen in a concentration-dependent manner, without affecting those induced by ADP, PAF and thrombin. Prolongation of the incubation time of HA-29 with platelets did not cause further inhibition and the aggregability of the agent-treated platelets could be restored after washing of platelets. The concentration-response curve of U-46619-induced platelet aggregation was shifted to the right by HA-29 in a concentration-dependent manner, but the maximal aggregation was suppressed by HA-29. The pA2 and pA10 values of HA-29 on U-46619-induced platelet aggregation were 4.26 and 3.58, respectively, with a slope value of -1.4. The U-46619-induced aggregation was markedly disaggregated by HA-29 even it was added 5 min after U-46619. HA-29 inhibited the secondary aggregation and ATP release, but not the primary aggregation of human platelet-rich plasma induced by ADP and epinephrine. Thromboxane B2 formation caused by arachidonic acid, collagen and thrombin was markedly suppressed by HA-29. HA-29 also inhibited the formation of prostaglandin D2 caused by arachidonic acid. HA-29 inhibited almost completely the formation of inositol monophosphate caused by U-46619, but not that by collagen or thrombin. HA-29 did not affect U-46619-induced contraction of rat aorta. It is concluded that the antiplatelet effect of HA-29 is due to the inhibition of thromboxane A2 formation and blockade of thromboxane A2/prostaglandin endoperoxide receptor.

Vasorelaxing effect in rat thoracic aorta caused by fraxinellone and dictamine isolated from the Chinese herb Dictamnus dasycarpus Turcz: comparison with cromakalim and Ca2+ channel blockers

The components of Dictamnus dasycarpus Turcz were tested for their vasorelaxing effect on the rat aorta, and fraxinellone and dictamine were shown to be effective vasorelaxants. In high K+ (60 mmol/l) medium, Ca2+ (0.03 to 3 mmol/l)-induced vasoconstriction was inhibited concentration-dependently by both agents. The IC50 for fraxinellone and dictamine were calculated to be about 25 mumol/l and 15 mumol/l (for Ca2+ concentration of 1 mmol/l), respectively. Cromakalim (0.2-10 mumol/l) relaxed aortic rings precontracted with 15 but not 60 mmol/l of K+. Fraxinellone and verapamil were more potent and effective in producing relaxation in 60 mmol/l than in 15 mmol/l K(+)-induced contraction. However, dictamine was more potent in producing relaxation in 15 mmol/l K(+)-induced contraction. Nifedipine (1 mumol/l), dictamine (100 mumol/l) and fraxinellone (100 mumol/l) relaxed the aortic contraction caused by KCl or Bay K 8644. The tonic contraction elicited by noradrenaline (NA, 3 mumol/l) was also relaxed by dictamine (500 mumol/l), but not by fraxinellone (500 mumol/l) in the nifedipine (1 mumol/l)-treated aorta. This relaxing effect of dictamine persisted in endothelium-denuded aorta. Glibenclamide (10 mumol/l) shifted the concentration-relaxation curve of cromakalim, but not that of dictamine, to the right in rat aortic rings precontracted with NA. Dictamine (500 mumol/l) did not affect tonic contraction of NA which are reduced by H-7 (1 mumol/l) in Ca(2+)-depleted medium. In conclusion, fraxinellone is a selective blocker of voltage-dependent Ca2+ channel, while dictamine relaxed the rat aorta by suppressing the Ca2+ influx through both voltage-dependent and receptor-operated Ca2+ channels.

Dicentrine, a novel antiplatelet agent inhibiting thromboxane formation and increasing the cyclic AMP level of rabbit platelets

Dicentrine is an antiplatelet agent isolated from the Chinese herb Lindera megaphylla. We examined the in vitro effects of dicentrine on various aspects of platelet reactivity. Dicentrine inhibited the aggregation and ATP release of washed rabbit platelets induced by arachidonic acid (AA), collagen, ADP, platelet-activating factor (PAF), thrombin and U46619. Dicentrine also inhibited the thromboxane B2 formation caused by AA, collagen and thrombin in washed intact platelets or that induced by AA in lysed platelet homogenate, while prostaglandin D2 formation caused by AA was not increased. The generation of inositol monophosphates (in the presence of indomethacin) caused by thrombin, collagen and PAF was not suppressed significantly, nor did dicentrine suppress fibrinogen-induced aggregation of elastase-treated platelets. Dicentrine inhibited the intracellular Ca2+ increase in quin-2/AM-loaded platelets caused by thrombin, PAF, collagen and AA. The cyclic AMP level was elevated by dicentrine in a concentration-dependent manner. These data indicate that the inhibitory effect of dicentrine on platelet aggregation and ATP release was due to the inhibition of thromboxane formation and the elevation of the level of cyclic AMP.

Ca(2+)-channel blockade in rat thoracic aorta by protopine isolated from Corydalis tubers

The pharmacological properties and mechanism of the action of protopine on isolated rat thoracic aorta were examined. It inhibited norepinephrine (NE, 3 microM)-induced tonic contraction in rat thoracic aorta in a concentration-dependent manner (25-100 micrograms/ml). The phasic contraction caused by NE was inhibited only by a high concentration of protopine (100 micrograms/ml). At the plateau of NE-induced tonic contraction, the addition of protopine also caused relaxation. This relaxing effect of protopine was not antagonized by indomethacin (20 microM) or methylene blue (50 microM), and it still existed in denuded rat aorta or in the presence of nifedipine (2-100 microM). Protopine also inhibited high potassium (60 mM)-induced, calcium-dependent (0.03-3 mM) contraction of rat aorta in a concentration-dependent manner. Neither cAMP nor cGMP level was changed by protopine. Both the formation of inositol monophosphate caused by NE and the phasic contraction induced by caffeine were also not affected by protopine. 45Ca2+ influx caused by either NE or K+ was inhibited by protopine concentration-dependently. It is concluded that protopine relaxed the rat thoracic aorta mainly by suppressing the Ca2+ influx through both voltage- and receptor-operated calcium channels.

A common precursor for a putative hemorrhagic protein and rhodostomin, a platelet aggregation inhibitor of the venom of Calloselasma rhodostoma: molecular cloning and sequence analysis

Rhodostomin is a platelet aggregation inhibitor secreted by the venom gland of Calloselasma rhodostoma. We report here the isolation of a 1.67-kilobase (kb) lambda gt11 cDNA clone using degenerate oligonucleotide probe based on a partial amino acid sequence of rhodostomin. The amino acid sequence deduced from an open reading frame of the cDNA indicates that (i) the 68-amino acid sequence of rhodostomin is located at the carboxyl terminus of the precursor polypeptide and (ii) a peroxisomal targeting sequence (ser.his.ala.) exists between the stop codon and the rhodostomin sequence of the precursor. Since the amino-terminal segment of the deduced sequence shows a high degree of identity with hemorrhagic proteins, which are zinc-metalloproteinases, found in the venom of some crotalid and viperid snakes, our results also predict the existence of at least one such hemorrhagic protein in the venom of Calloselasma rhodostoma. The derivation of a platelet aggregation inhibitor and a hemorrhagic protein from the same precursor protein is consistent with the fact that these proteins may be synergistic in function.

Comparison of the actions of some platelet-activating factor antagonists on platelets and aortic smooth muscles

The pharmacological actions of five platelet-activating factor (PAF) antagonists were compared in rabbit platelets and rat thoracic aorta. In PAF (2 ng/ml)-induced aggregation of washed rabbit platelets, WEB 2086 and WEB 2170 much were more potent inhibitors than BN 52021, kadsurenone and denudatin B, and the IC50 values were calculated to be 0.1, 0.3, 5, 8 and 10 micrograms/ml, respectively. WEB 2086, WEB 2170 and BN 52021 did not affect the platelet aggregation caused by collagen (10 micrograms/ml), ADP (20 microM), arachidonic acid (100 microM) or thrombin (0.1 U/ml). Kadsurenone and denudatin B suppressed ATP release, thromboxane B2 formation and the rise in intracellular calcium of washed rabbit platelets caused by collagen and thrombin, while WEB 2086, WEB 2170 and BN 52021 did not have an effect. Norepinephrine (3 microM) induced a sustained contraction in rat thoracic aorta. Pretreatment with these PAF antagonists (20-100 micrograms/ml) caused inhibition of the aortic contraction in the following order: kadsurenone greater than denudatin B greater than WEB 2086 greater than BN 52021 greater than WEB 2170. In high potassium (60 mM)-induced contraction of rat aorta, kadsurenone and denudatin B caused marked relaxation, while WEB 2086, WEB 2170 and BN 52021 had only a slight effect. It is concluded that WEB 2086, WEB 2170 and BN 52021 are specific PAF antagonists in rabbit platelets, and weak relaxants in rat aorta. Two other PAF antagonists, kadsurenone and denudatin B, may inhibit some aspects of signal transduction, e.g., thromboxane formation or intracellular Ca2+ mobilization in rabbit platelets, and cause vasorelaxation in rat aorta by inhibiting calcium influx.

Vasodilatory action mechanisms of apigenin isolated from Apium graveolens in rat thoracic aorta

The effect of apigenin, isolated from Apium graveolens, on the contraction of rat thoracic aorta was studied. Apigenin inhibited the contraction of aortic rings caused by cumulative concentrations of calcium (0.03-3 mM) in high potassium (60 mM) medium, with an IC50 of about 48 microM. After pretreatment it also inhibited norepinephrine (NE, 3 microM)-induced phasic and tonic contraction in a concentration (35-140 microM)-dependent manner with an IC50 of 63 microM. At the plateau of NE-induced tonic contraction, addition of apigenin caused relaxation. This relaxing effect of apigenin was not antagonized by indomethacin (20 microM) or methylene blue (50 microM), and still existed in endothelial denuded rat aorta or in the presence of nifedipine (2-100 microM). Neither cAMP nor cGMP levels were changed by apigenin. Both the formation of inositol monophosphate caused by NE and the phasic contraction induced by caffeine in the Ca(2+)-free solution were unaffected by apigenin. 45Ca2+ influx caused by either NE or K+ was inhibited by apigenin concentration-dependently. It is concluded that apigenin relaxes rat thoracic aorta mainly by suppressing the Ca2+ influx through both voltage- and receptor-operated calcium channels.

Dicentrine, a natural vascular alpha 1-adrenoceptor antagonist, isolated from Lindera megaphylla

1. The pharmacological activity of dicentrine, isolated from Lindera megaphylla, was determined in rat isolated thoracic aorta, guinea-pig isolated trachea and human platelet-rich plasma. 2. Dicentrine was found to be a potent alpha 1-adrenoceptor blocking agent in rat thoracic aorta as revealed by its competitive antagonism of noradrenaline- (pA2 = 8.19 +/- 0.09) or phenylephrine (pA2 = 9.01 +/- 0.10)-induced vasoconstriction. These effects still persisted in denuded aorta. It was less potent than prazosin (pA2 = 10.60 +/- 0.10), but was more potent than phentolamine (pA2 = 7.53 +/- 0.10) or yohimbine (pA2 = 6.20 +/- 0.05). 3. Inositol monophosphate formation induced by noradrenaline (3 microM) in rat thoracic aorta was suppressed by dicentrine (3-10 microM) and prazosin (3 microM). 4. A high concentration of dicentrine (30 microM) did not affect the aortic contraction induced by the thromboxane receptor agonist U-46619 (1 microM), angiotensin II (1 microM), high potassium (60 mM) or carbachol (3 microM). 5. Contraction of guinea-pig trachea caused by histamine or carbachol was slightly inhibited by dicentrine (30 microM), while beta-adrenoceptor relaxation to isoprenaline in trachea was not affected. 6. Aggregation in human platelet-rich plasma induced by adrenaline (10 microM) was blocked by yohimbine (5 microM). A high concentration of dicentrine (greater than 30 microM) caused slight inhibition of aggregation, the release reaction and thromboxane formation. Complete blockade was obtained with 150 microM dicentrine. 7. It is concluded that dicentrine is a potent, selective alpha 1-adrenoceptor antagonist in vascular smooth muscle.

Mechanism of action of a potent antiplatelet peptide, triflavin from Trimeresurus flavoviridis snake venom

Triflavin, an antiplatelet peptide from Trimeresurus flavoviridis snake venom, inhibits aggregation of human platelets stimulated by a variety of agonists. However, triflavin does not affect the shape change and release reaction of platelets stimulated by thrombin and collagen. In this paper, we further investigate its effect on the intracellular events occurring after the activation of platelets. Triflavin does not inhibit the intracellular free calcium rise of Quin 2-AM loaded platelets stimulated by thrombin and it also has no significant effect on thromboxane B2 formation of platelets stimulated by thrombin. Triflavin does not affect the 3(H)-inositol monophosphate formation of the 3(H)-myoinositol loaded platelets. However, triflavin dose-dependently inhibits fibrinogen-induced aggregation and 125I-fibrinogen binding of ADP-stimulated platelets. In addition, triflavin dose-dependently blocks fibrinogen-induced aggregation of elastase-treated platelets. It is concluded that triflavin specifically inhibits fibrinogen binding to fibrinogen receptors associated with glycoprotein IIb/IIIa complex on platelet membrane surface without any inhibitory effect on the platelet-activation process.

Antihaemostatic and antithrombotic effect of some antiplatelet agents isolated from Chinese herbs

Five antiplatelet agents have been isolated from Chinese herbs. Apigenin and magnolol are inhibitors of thromboxane synthesis, while osthole, protopine and norathyriol are inhibitiors of phosphoinositide breakdown. Thirty min after intraperitoneal (i.p.) administration of these drugs, tail bleeding time of mice was prolonged markedly in a dose-dependent manner by norathyriol, protopine, osthole and magnolol, but not by apigenin. However, the antiplatelet agents (up to 200 mg kg-1, i.p.) could not prevent acute thromboembolic death in mice. In endotoxin-induced experimental disseminated intravascular coagulation in rats, norathyriol (50-100 mg kg-1, i.p.) prevented the decrease in platelet counts and fibrinogen, and the prolongation of plasma prothrombin time. Norathyriol (100 mg kg-1, i.p.) also suppressed ex-vivo platelet aggregation induced by collagen and ADP in rat plasma.

Halysin, an antiplatelet Arg-Gly-Asp-containing snake venom peptide, as fibrinogen receptor antagonist

By means of Sephadex G-75 and CM-Sephadex C-50 column chromatography and reverse-phase HPLC, a low molecular weight (Mr = 7500), cysteine-rich peptide, halysin, was purified from Agkistrodon halys (mamushi) snake venom. Halysin is a potent platelet aggregation inhibitor that concentration-dependently inhibited human platelet aggregation stimulated by ADP, thrombin and collagen (IC50 = 0.16 to 0.36 microM) without affecting platelet secretion. It was active in inhibiting platelet aggregation of platelet-rich plasma and whole blood. Halysin had no effect on thromboxane B2 formation of platelets or intracellular Ca2+ mobilization of Quin 2-AM loaded platelets stimulated by thrombin. It inhibited the fibrinogen-induced aggregation of elastase-treated platelets. Halysin concentration-dependently inhibited the 125I-fibrinogen binding to ADP-stimulated platelets in a competitive manner (IC50 = 0.16 microM). 125I-Halysin bound to resting platelets (Kd = 1.6 x 10(-7) M) and to ADP-stimulated platelets (Kd = 3.4 x 10(-8) M) in a saturable manner. EDTA, the Arg-Gly-Asp (RGD)-containing snake venom peptides trigamin and rhodostomin, Arg-Gly-Asp-Ser (RGDS), and Gly-Gln-Gln-His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val blocked both 125I-fibrinogen binding and 125I-halysin binding to ADP-stimulated platelets. The monoclonal antibody, 7E3, raised against glycoprotein IIb-IIIa complex blocked both 125I-fibrinogen and 125I-halysin binding, whereas 10E5 had no significant effect on halysin binding to ADP-stimulated platelets, indicating that 7E3 and halysin bind to an epitope which is different from that of 10E5. RGDS concentration-dependently inhibited 125I-halysin binding in a competitive manner. We determined the primary structure of halysin which is a single peptide chain of 71 amino acid residues. An RGD sequence appeared in the carboxy-terminal domain of halysin. Halysin showed about an 85% identical sequence with trigamin which is a specific antagonist of fibrinogen receptor associated with glycoprotein IIb-IIIa complex. In conclusion, halysin inhibited platelet aggregation by interfering with fibrinogen binding to the fibrinogen receptor of the activated platelets. The RGD sequence of halysin plays an important role in the expression of its biological activity.

A potent antiplatelet peptide, triflavin, from Trimeresurus flavoviridis snake venom

The interaction of fibrinogen with its receptors on platelet surfaces leads to platelet aggregation. A snake-venom peptide, trigramin, has previously been demonstrated to inhibit platelet aggregation by acting as a fibrinogen-receptor antagonist. By means of gel filtration, ionic-exchange chromatography and reverse-phase h.p.l.c., a potent platelet-aggregation inhibitor, triflavin, has now been purified from the venom of Trimeresurus flavoviridis. The purified triflavin is a single-chain polypeptide, consisting of about 71 amino acid residues with a molecular mass of 7600 Da, and its N-terminal sequence is Gly-Glu-Glu-Cys-Asp. Triflavin dose-dependently inhibited human platelet aggregation stimulated by ADP, adrenaline, collagen, thrombin or prostaglandin endoperoxide analogue U46619 in preparations of platelet-rich plasma, platelet suspension and whole blood. Its IC50 ranged from 38 to 84 nM, depending on the aggregation inducer used and the platelet preparation. However, triflavin apparently did not affect the platelet shape change and ATP-release reactions caused by these agonists. Triflavin inhibited fibrinogen-induced aggregation of human elastase-treated platelets in a dose-dependent manner, indicating that it directly interferes with the binding of fibrinogen to its receptors on platelet membranes exposed by elastase treatment. Additionally, triflavin dose-dependently blocked 125I-labelled fibrinogen binding to ADP-activated platelets. In conclusion, triflavin inhibits platelet aggregation through the blockade of fibrinogen binding to fibrinogen receptors on platelet membranes.

Purification and characterization of an antiplatelet peptide, arietin, from Bitis arietans venom

By means of Fractogel TSK-50, CM-Sephadex C-50 column chromatography, gel filtrations on Sephadex G-75 and Sephacryl S-200 columns and reverse-phase HPLC, an antiplatelet peptide, arietin, was purified from venom of Bitis arietans. Arietin was shown to be an Arg-Gly-Asp-containing peptide with a NH2-terminus, Ser-Pro-Pro-Val-Cys-Gly-Asn-Lys- (Mr 8500). Arietin dose-dependently inhibited aggregation of human platelet suspension stimulated by ADP, thrombin, collagen and U46619 with IC50, 1.3-2.7.10(-7) M, while it had no effect on the initial shape changes and only slightly affected ATP release of platelets caused by thrombin and collagen. Arietin also blocked platelet aggregation in platelet-rich plasma and whole blood, and inhibited thrombin-induced clot retraction of platelet-rich plasma. Furthermore, arietin (6.5.10(-8) M) completely blocked the fibrinogen-induced aggregation of elastase-treated platelets, indicating that arietin interferes with the fibrinogen binding to fibrinogen receptors on platelet membranes. In conclusion, arietin, an Arg-Gly-Asp-containing peptide, inhibits platelet aggregation probably through the blockade of fibrinogen binding to the activated platelets.

Mechanism of action of the antiplatelet peptide, arietin, from Bitis arietans venom

Arietin, an Arg-Gly-Asp containing peptide from venom of Bitis arietans, inhibited aggregation of platelets stimulated by a variety of agonists with a similar IC50, 1.3-2.7.10(-7) M. It blocked aggregation through the interference of fibrinogen binding to fibrinogen receptors on platelet surface. In this paper, we further demonstrated that arietin had no significant effect on the intracellular mobilization of Ca2+ in Quin2-AM-loaded platelets stimulated by thrombin. It inhibited 125I-fibrinogen binding to ADP-stimulated platelets in a competitive manner (IC50, 1.1.10(-7) M). 125I-arietin bound to unstimulated, ADP-stimulated and elastase-treated platelets in a saturable manner and its Kd values were estimated to be 3.4.10(-7), 3.4.10(-8) and 6.5.10(-8) M, respectively, while the corresponding binding sites were 46,904, 48,958 and 34,817 per platelet, respectively. Arg-Gly-Asp-Ser (RGDS) inhibited 125I-arietin binding to ADP-stimulated platelets in a competitive manner. RGD-containing peptides, including trigramin and rhodostomin, EDTA and monoclonal antibody, 7E3, raised against glycoprotein IIb-IIIa complex, inhibited 125I-arietin binding to ADP-stimulated platelets, indicating that the binding sites of arietin appear to be located at or near glycoprotein IIb-IIIa complex. In conclusion, arietin and other RGD-containing trigramin-like peptides preferentially bind to the fibrinogen receptors associated with glycoprotein IIb-IIIa complex of the activated platelets, thus leading to the blockade of fibrinogen binding to its receptors and subsequent aggregation. The presence of RGD of arietin is essential for the expression of its biological activity. Its binding sites are overlapped with those of trigramin, rhodostomin and the monoclonal antibody, 7E3.