THE DIHYDROPYRIDINE CALCIUM CHANNEL AGONIST, BAY K 8644, AND THE ANTAGONIST, NIFEDIPINE, INHIBIT U46619-INDUCED HUMAN PLATELET ACTIVATION BY COMPETITIVE BINDING TO THE THROMBOXANE A22/PGH2 RECEPTOR

1987 ◽  
Author(s):  
G J Johnson ◽  
P C Dunlop ◽  
M J Rabiet ◽  
L A Leis ◽  
AH L From
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Competitive Inhibition ◽  
Human Platelet ◽  
Ex Vivo ◽  
Competitive Binding ◽  
Bay K 8644 ◽  
Voltage Dependent ◽  
Ca2 Channels

The dihydropyridine (DP) Ca2+ channel antagonist, nifedipine (NF), inhibits platelet aggregation .in vitro and ex vivo by an undefined mechanism. Inhibition of Ca2+ influx via Ca2+ channels is a postulated mechanism, but voltage-dependent Ca2+ channels have not been demonstrated in platelets. We previously observed that NF blocked thromboxane A2 (TXA2)-induced platelet aggregation and secretion. In order to further evaluate the mechanism of DP inhibition of platelet activation, we studied the effects of NF and BAY K 8644, (BAY), a DP with opposite (agonist) effects on muscle cells, on human platelet aggregation and secretion induced by the TXA2 mimic, U46619. We also observed the effects of DP on biochemical consequences of platelet activation: cytoplasmic ionized Ca2+ ([Ca2+]i) by fura-2 fluorescence; phosphorylation of 40,000 Dalton protein (40KP) substrate of protein kinase C by SDS-PAGE and [32p] counting; TXA2 formation by RIA of TXB2. 1μM BAY and 10μM NF inhibited the 2nd wave of platelet aggregation and secretion induced by ADP or epinephrine and blocked aggregation and secretion induced by U46619. A Schild plot gave a slope of -1 indicating competitive inhibition of U46619 by BAY (K1[=0.7μM).BAY and NF also blocked U46619-induced phosphorylation of 40KP, rise in [Ca2+]i and TXB2 formation. The (+)-(R) enantiomer of BAY (BAY+) was responsible for BAY inhibition. BAY, BAY(+), and the R enantiomer of another DP, 202-791, all functioned as competitive antagonists of [3H]-U4661 9 binding (K1[ for BAY=2.8 μM-comparable to known receptor antagonists, 13-azaprostanoic acid and BM 13.177; K1 for BAY(+)=0.69μM). Neither BAY nor NF inhibited[3H]-yohimbine binding to α adrenergic receptors.NF, BAY, BAY(+) and BAY(-) in nM concentrations slightly stimulated platelet aggregation,secretion and biochemical events induced by U46619 similar to their effects on muscle. Therefore, DP's do not inhibit platelet activation by blocking voltage-dependent Ca2+ channels. The mechanism of DP inhibition of TXA2-induced platelet activation is stereoselective, competitive binding to the TXA2/PGH2 receptor. DP's may exert similar effects on TXA2-induced vascular smooth muscle contraction.

In vitro and ex vivo effect of cimetidine on human platelet aggregation

1986 ◽  
Vol 42 (1) ◽  
pp. 113-114
Author(s):  
B. Gachályi ◽  
K. Tihanyi ◽  
Á. Vas ◽  
B. Nádas ◽  
A. Káldor
Keyword(s):  
Platelet Aggregation ◽  
Human Platelet ◽  
Ex Vivo ◽  
Human Platelet Aggregation

The pharmacology of L-670,596, a potent and selective thromboxane/prostaglandin endoperoxide receptor antagonist

1989 ◽  
Vol 67 (9) ◽  
pp. 989-993 ◽  
Author(s):  
A. W. Ford-Hutchinson ◽  
Y. Girard ◽  
A. Lord ◽  
T. R. Jones ◽  
M. Cirino ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Guinea Pig ◽  
Receptor Antagonist ◽  
Competitive Inhibition ◽  
Ex Vivo ◽  
Platelet Rich Plasma ◽  
Prostaglandin Endoperoxide ◽  
Induced Aggregation

L-670,596 ((−)6,8-difluoro-9-p-methylsulfonyl benzyl-1,2,3,4-tetrahydrocarbazol-1-yl-acetic acid) has been shown to be a potent receptor antagonist as evidenced by the inhibition of the binding of 125I-labeled PTA-OH to human platelets (IC50, 5.5 × 10−9 M), inhibition of U-44069 induced aggregation of human platelet rich plasma (IC50, 1.1 × 10−7 M), and competitive inhibition of contractions of the guinea pig tracheal chain induced by U-44069 (pA2,9.0). The compound was also active in vivo as shown by inhibition of arachidonic acid and U-44069 induced bronchoconstriction in the guinea pig (ED50 values, 0.04 and 0.03 mg/kg i.v., respectively), U-44069 induced renal vasoconstriction in the pig (ED50, 0.02 mg/kg i.v.), and inhibition of ex vivo aggregation of rhesus monkey platelets to U-44069 (active 1–5 mg/kg p.o.). The selectivity of the compound was indicated by the failure to inhibit, first, ADP-induced human or primate platelet aggregation and, second, bronchoconstriction in the guinea pig in vivo and contraction of the guinea pig tracheal chain in vitro to a variety of agonists. It is concluded that L-670,596 is a potent, selective, orally active thromboxane A2/prostaglandin endoperoxide receptor antagonist.Key words: thromboxane A2, thromboxane antagonist, prostaglandin endoperoxides, platelet aggregation.

Studies on the Effect of Platelet Inhibitors on Platelet Adhesion to Collagen and Collagen-Induced Human Platelet Activation

1985 ◽  
Vol 53 (03) ◽  
pp. 337-342 ◽  
Author(s):  
S Krishnamurthi ◽  
V V Kakkar
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Platelet Adhesion ◽  
Human Platelet ◽  
Platelet Rich Plasma ◽  
Antiplatelet Agents ◽  
Calmodulin Antagonist ◽  
Release Reaction ◽  
Low Concentrations

SummaryThe effect of pyridoxal 5’-phosphate (PALP) and trifluoperazine (TFPZ), the calmodulin antagonist, on in vitro platelet adhesion to collagen and collagen-induced platelet activation was studied using platelet-rich-plasma (PRP) or washed platelets (WPL). Platelet aggregation and [14C]-5HT release induced by “threshold” or low concentrations of collagen (0.6 μg/ ml) in PRP were completely abolished by PALP (24 mM), TFPZ (250 μM) as well as indomethacin (10 μM). At higher concentrations of collagen (10–15 μg/ml) in PRP and WPL, the use of stirred and unstirred platelets treated with collagen enabled a distinction to be made between aggregation and adhesion- mediated release reaction. Platelet aggregation and the aggregation-mediated release reaction induced by these concentrations of collagen in stirred platelets were completely abolished by PALP, TFPZ and indomethacin although neither adhesion to collagen nor the adhesion-mediated release reaction of unstirred platelets was significantly affected by these inhibitors. Interestingly, both adhesion and the adhesion-mediated release reaction were abolished by concentrations of PALP 10–40 fold higher than those required to abolish aggregation. Collagen-induced platelet aggregation, but not platelet adhesion, was inhibited in resuspended platelets pretreated with PALP and NaBH4 indicating a separation in the membrane sites involved in aggregation and adhesion. The results further emphasize the distinction between adhesion and aggregation-mediated events with regards to collagen with the latter being more susceptible to inhibition by antiplatelet agents such as PALP and TFPZ.

SELECTIVE ANTAGONISTS OF PAF: INTERFERENCE WITH PLATELET FUNCTION AND EFFECT ON THROMBOGENESIS INDUCED BY SUBENDOTHELIUM.

1987 ◽  
Author(s):  
P Hadvary ◽  
H R Baumgartner
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Ex Vivo ◽  
Platelet Rich Plasma ◽  
Thrombus Formation ◽  
Blood Platelets ◽  
Rabbit Aorta ◽  
Constant Infusion ◽  
Induced Aggregation

Platelet activating factor (PAF) is a very potent excitatory agonist of blood platelets but the physiological importance of this mediator in platelet thrombus formation is not known. We investigated the effect of two chemically unrelated selective inhibitors of PAF-induced platelet aggregation on thrombogenesis induced by rabbit aorta subendothelium (SE) using an ex vivo perfusion system.Ro 19-3704 is a highly potent inhibitor structurally related to PAF. This compound inhibits PAF-induced aggregation of rabbit platelets in platelet rich plasma in vitro competitively. Against 4 nM PAF, a concentration resulting in submaximal platelet aggre-gregation velocity, the IC50 was 70 nM. Inhibition was highly selective for PAF-induced aggregation, since aggregation induced by collagen (HORM, 5 yg/ml), ADP (1 yM) or thrombin (0.4 U/ml) was not inhibited even at a concentration as high as 10 yM. Bro-tizolam, a triazolobenzodiazepine reported to be a selective inhibitor of PAF-induced platelet activation, had in our system an IC50 of 200 nM. The selective benzodiazepine antagonist Ro 151788 was without effect on inhibition of PAF-induced platelet activation by brotizolam.Ro 19-3704 was given intravenously to rabbits as a bolus of 0.2 mg/kg followed by constant infusion of 0.02 mg/kg/min. This dosage provoked ex vivo a constant right shift ratio of the dose response curve for PAF-induced aggregation (RSR[PAF]) by a factor of 25 to 35. Brotizolam was given orally at a dose of 100 mg/ kg together with 300 mg/kg of Ro 15-1788 (to antagonize the central effects) 90 minutes before starting the perfusion experiment, resulting in a RSR[PAF] of 35 to 135. ADP induced platelet aggregation was not impaired by either compound. SE was exposed to the non-anticoagulated blood withdrawn from the carotid artery for 3 min at 2600 s-1 and for 20 min at 200 s-1 shear rate. Quantitative morphometric evaluation showed that SE coverage by platelets and by fibrin, thrombus area and thrombus height were all unchanged by the PAF antagonists at low and at high shear rates despite a very substantial inhibition of PAF-induced platelet aggregation. Therefore a major role of PAF in SE-induced thrombogenesis seems unlikely.

Reversible Inhibition of Human Platelet Activation by Hypothermia In Vivo and In Vitro

1994 ◽  
Vol 71 (05) ◽  
pp. 633-640 ◽  
Author(s):  
Alan D Michelson ◽  
Hollace MacGregor ◽  
Marc R Barnard ◽  
Anita S Kestin ◽  
Michael J Rohrer ◽  
...  
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Whole Blood ◽  
Peripheral Blood ◽  
Bleeding Time ◽  
Human Platelet ◽  
Platelet Surface ◽  
Shed Blood

SummaryA hypothermia-induced hemorrhagic diathesis is associated with cardiopulmonary bypass, major surgery, and multiple trauma, but its pathophysiological basis is not well understood. We examined the hypothesis that hypothermia reversibly inhibits human platelet activation in vitro and in vivo. Platelet activation was studied in normal volunteers by whole blood flow cytometric analysis of modulation of platelet surface GMP-140 and the glycoprotein (GP) Ib-IX complex in: a) shed blood emerging from a standardized in vivo bleeding time wound; b) peripheral blood activated in vitro with either thrombin (in the presence of gly-pro-arg-pro, an inhibitor of fibrin polymerization) or the stable thromboxane (TX) A2 analogue U46619. Platelets in peripheral whole blood were activated at temperatures between 22° C and 37° C. the forearm skin temperature was maintained at temperatures between 22° C and 37° C prior to and during the bleeding time incision. Platelet aggregation was studied in shed blood by flow cytometry and in peripheral blood by aggregometry. Generation of TXB 2 (the stable metabolite of TXA 2) was determined by radioimmunoassay. In vitro, hypothermia inhibited both thrombin- and U46619-induced upregulation of GMP-140, downregulation of the GPIb-IX complex, platelet aggregation, and TXB2 generation. These inhibitory effects of hypothermia were all completely reversed by rewarming the blood to 37° C. In vivo, platelet activation was inhibited by hypothermia as shown by 5 independent assays of shed blood: upregulation of GMP-140, downregulation of the GPIb-IX complex, platelet aggregate formation, TXB 2 ggeneration, and the bleeding time. In summary, by a combination of immunologic, biochemical, and functional assays, we demonstrate that hypothermia inhibits human platelet activation in whole blood in vitro and in vivo. Rewarming hypothermic blood completely reverses the activation defect. These results suggest that maintaining normothermia or rewarming a hypothermic bleeding patient may reduce the need for platelet transfusions.

Human Platelet Aggregation In Response To Multiple Agonists In Plasma Anticoagulated With Heparin

1981 ◽  
Author(s):  
H A Culliver ◽  
N G Ardlie
Keyword(s):  
Platelet Aggregation ◽  
Platelet Activation ◽  
Human Platelet ◽  
Platelet Rich Plasma ◽  
Adenosine Diphosphate ◽  
Human Platelet Aggregation

The lowest concentrations at which epinephrine and vasopressin have been reported to interact positively in causing platelet aggregation in vitro are at least two orders of magnitude greater than the physiological concentrations of these hormones in blood. The aim of this study was to examine the interaction between several agonists of human platelet aggregation. The aggregating agents used were adenosine diphosphate (ADP), epinephrine, norepinephrine, 5-hydroxytryptamine and vasopressin. Platelet-rich plasma (PRP) was prepared from blood anticoagulated with minimal concentrations of heparin in an attempt to more closely reflect the in vivo situation.Aggregation caused by ADP was potentiated by epinephrine at a concentration exceeding the level obtained in circulating blood. When a third agonist (vasopressin) was used in combination with ADP and epinephrine, aggregation was enhanced at concentrations of vasopressin and epinephrine obtained in blood. When used as a fourth agonist norepinephrine and 5-hydroxytryptamine potentiated aggregation at physiological concentrations. The response to multiple agonists was greater in heparinized PRP than citrated PRP. Hirudin decreased the extent of aggregation in heparinized PRP caused by multiple agonists suggesting that thrombin may be involved.Since the concentrations of combined agonists required to induce in vitro platelet aggregation can be obtained in circulating blood these findings may explain why platelet activation occurs in certain pathological states.

scholarly journals The role of polyphenolic compounds in the diet as inhibitors of platelet function

2003 ◽  
Vol 62 (2) ◽  
pp. 469-478 ◽  
Author(s):  
Gary P. Hubbard ◽  
Siegfried Wolffram ◽  
Julie A. Lovegrove ◽  
Jonathan M. Gibbins
Keyword(s):  
Cardiovascular Disease ◽  
Platelet Aggregation ◽  
Platelet Activation ◽  
Platelet Function ◽  
Dietary Intervention ◽  
Ex Vivo ◽  
Polyphenolic Compounds ◽  
Dietary Fats

Platelets play a substantial role in cardiovascular disease, and for many years there has been a search for dietary components that are able to inhibit platelet function and therefore decrease the risk of cardiovascular disease. Platelets can be inhibited by alcohol, dietary fats and some antioxidants, including a group of compounds, the polyphenols, found in fruits and vegetables. A number of these compounds have been shown to inhibit platelet function both in vitro and in vivo. In the present study the effects of the hydroxycinnamates and the flavonoid quercetin on platelet activation and cell signalling in vitro were investigated. The hydroxycinnamates inhibited platelet function, although not at levels that can be achieved in human plasma by dietary intervention. However, quercetin inhibited platelet aggregation at levels lower than those previously reported. Quercetin was also found to inhibit intracellular Ca mobilisation and whole-cell tyrosine protein phosphorylation in platelets, which are both processes essential for platelet activation. The effect of polyphenols on platelet aggregation in vivo was also investigated. Twenty subjects followed a low-polyphenol diet for 3 d before and also during supplementation. All subjects were supplemented with a polyphenol-rich meal every lunchtime for 5 d. Platelet aggregation and plasma flavonols were measured at baseline and after 5 d of dietary supplementation. Total plasma flavonoids increased significantly after the dietary intervention period (P=0·001). However, no significant changes in ex vivo platelet aggregation were observed. Further investigation of the effects of individual polyphenolic compounds on platelet function, both in vitro and in vivo, is required in order to elucidate their role in the relationship between diet and the risk of cardiovascular disease.

ICI 170809, A selective 5-hydroxytryptamine antagonist, inhibits human platelet aggregation in vitro and ex vivo

1990 ◽  
pp. 459-463 ◽  
Author(s):  
Thomas P. Blackburn ◽  
Stephen J. Haworth ◽  
Carol L. Jessup ◽  
Pamela B. Morton ◽  
Christine Williams
Keyword(s):  
Platelet Aggregation ◽  
Human Platelet ◽  
Ex Vivo ◽  
Human Platelet Aggregation

Role Of Platelet cAMP And Prostaglandin Synthesis In Platelet Aggregation Inhibition By Ticlopidine Hydrochloride

1981 ◽  
Author(s):  
J J Bruno ◽  
D Yang ◽  
L A Taylor ◽  
C Feamster
Keyword(s):  
Platelet Aggregation ◽  
Human Platelet ◽  
Ex Vivo ◽  
Antiplatelet Agent ◽  
Camp Phosphodiesterase ◽  
Pde Inhibitors ◽  
Aggregation Inhibition ◽  
Induced Aggregation

Ticlopidine hydrochloride (T), 5-[(2-chlorophenyl) methyl]-4,5,6,7-tetrahydrothieno [3,2-c] pyridine hydrochloride, is a potent antiplatelet agent of unknown mechanism of action. T does not inhibit human platelet low Km, cAMP phosphodiesterase (PDE) in vitro, (IC50>>10-3M), but does have some activity versus the low Km, cGMP-PDE (IC503-4×10-4). Equivalent values for theophylline, a weak PDE inhibitor, are 1.6×10-4M and 4.2×10-4M, respectively. T has no synergistic effect in vitro on platelet aggregation inhibited by PGE1. PDE inhibitors do show synergism with PGE1 and other antiaggregatory PGs. Ex vivo in humans and animals, T inhibits ADP-induced aggregation. This inhibition is not altered by addition of SQ-22536 (f.c.-10-4M), a purported inhibitor of adenylate cyclase activity, whereas drugs whose platelet inhibitory activity depend on elevation of platelet cAMP, e.g., PGE1, adenosine and dipyridamole, have this inhibition partially reversed by SQ-22536.PG synthesis is not required for the antiplatelet activity of T. Addition of aspirin (f.c.-5×10-4M) in vitro to PRP from humans treated with T (500 mg/day) did not modify the aggregation response to ADP. In addition, PGI2 is not essential for the antiaggregation effect of T.The inhibition of ADP-induced aggregation of PRP prepared from blood of T-treated humans just after collection and more than 30 minutes after, to allow for inactivation of PGI2 were identical.We conclude that neither elevated cAMP nor PG synthesis is essential for T activity.

Sign in / Sign up

Export Citation Format

Share Document