thromboxane-a2 and kadsurenone

Trimucytin is a potent platelet aggregation inducer isolated from Trimeresurus mucrosquamatus snake venom. Similar to collagen, trimucytin has a run of (Gly-Pro-X) repeats at the N-terminal amino acids sequence. It induced platelet aggregation, ATP release and thromboxane formation in rabbit platelets in a concentration-dependent manner. The aggregation was not due to released ADP since it was not suppressed by creatine phosphate/creatine phosphokinase. It was not either due to thromboxane A2 formation because indomethacin and BW755C did not have any effect on the aggregation even thromboxane B2 formation was completely abolished by indomethacin. Platelet-activating factor (PAF) was not involved in the aggregation since a PAF antagonist, kadsurenone, did not affect. However, RGD-containing peptide triflavin inhibited the aggregation, but not the release of ATP, of platelets induced by trimucytin. Indomethacin, mepacrine, prostaglandin E1 and tetracaine inhibited the thromboxane B2 formation of platelets caused by collagen and trimucytin. Forskolin and sodium nitroprusside inhibited both platelet aggregation and ATP release, but not the shape change induced by trimucytin. In quin-2 loaded platelets, the rise of intracellular calcium concentration caused by trimucytin was decreased by 12-O-tetradecanoyl phorbol-13 acetate, imipramine, TMB-8 and indomethacin. In the absence of extracellular calcium, both collagen and trimucytin caused no thromboxane B2 formation, but still induced ATP release which was completely blocked by R 59022. Inositol phosphate formation in platelets was markedly enhanced by trimucytin and collagen. MAB1988, an antibody against platelet membrane glycoprotein Ia, inhibited trimucytin- and collagen-induced platelet aggregation and ATP release.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Adenosine Triphosphate; Amino Acid Sequence; Animals; Antibodies, Monoclonal; Antigen-Antibody Reactions; Benzofurans; Calcium; Collagen; Crotalid Venoms; Lignans; Molecular Sequence Data; Peptides; Platelet Aggregation; Platelet Aggregation Inhibitors; Platelet Membrane Glycoproteins; Proteoglycans; Rabbits; Sequence Homology, Amino Acid; Thromboxane A2

Platelet-activating factor (PAF) has been implicated as a potential mediator of inflammatory processes. In this study, we quantified the effects of PAF on vessel diameter in a microvascular bed and investigated the biochemical pathways of this compound. The hamster cheek pouch microcirculation was observed with intravital microscopy. Experiments were video-recorded and analyzed with an image shearing device. Vasoconstriction was the predominant vasomotor response to PAF. PAF (10(-10) -10(-5) M) was applied topically to the pouch for 3 minutes. Arterioles ranging in size from 8 to 15 micron were the most sensitive, and they constricted completely in response to PAF 10(-7) and 10(-5) M. Arterioles 21-40 micron in diameter constricted to 12-17% of control after PAF at 10(-7) and 10(-5) M, respectively; they reopened to about 70% of their control value after a few minutes and remained near that size throughout the experiment. Arterioles 41-60 micron in diameter constricted to about 20% control size in response to 10(-7) and 10(-5) M PAF, and by the end of the experiment, these vessels had returned to about 90% control size. To determine the pathways of PAF actions, inhibitors of the arachidonic acid cascade and receptor blockers were used. Dexamethasone, indomethacin, OKY-046 (a thromboxane A2 synthetase inhibitor), and kadsurenone (a PAF-receptor blocker) blocked the vasoconstrictor response to PAF. Our experiments demonstrate that PAF-produced arteriolar constriction in a microvascular bed is 1) dose-related, 2) dependent upon vessel size, 3) largely due to thromboxane A2 activity, and 4) mediated by PAF-receptor interactions.

Topics: Animals; Arterioles; Benzofurans; Blood Pressure; Cheek; Cricetinae; Cyclooxygenase Inhibitors; Dose-Response Relationship, Drug; Hematocrit; Lignans; Male; Mesocricetus; Microcirculation; Platelet Activating Factor; Platelet Membrane Glycoproteins; Receptors, Cell Surface; Receptors, G-Protein-Coupled; Thromboxane A2; Vasoconstriction