Inhibition of phospholipase A2 by tiaramide in rabbit platelets.
The mechanism by which tiaramide inhibited platelet aggregation was investigated using phospholipid labelling techniques by 14C-arachidonic acid (AA) and thin-layer chromatography. Tiaramide did not affect cyclo-oxygenase nor thromboxane synthetase, because TXB2 was detected in tiaramide-treated platelets, unlike aspirin-treated ones, and PGE2 and PGD2 did not increase, unlike in platelets treated with OKY-1581 (an inhibitor of thromboxane synthetase). Total phospholipid radioactivity was 82.5% of radioactivity recovered before aggregation, and this decreased to 49.0% (n = 5, P less than 0.05) after aggregation by collagen (30 micrograms/ml). AA radioactivity was 9.6% before aggregation and 40.0% after. Tiaramide (100 microM) restored total phospholipid and AA levels to those before aggregation. Tiaramide decreased the amount of AA liberated from 2-(3H-arachidonyl)phosphatidylcholine by whole platelet phospholipase A2 (PLA2). Tiaramide at 10 microM inhibited collagen-induced aggregation, but not that by AA. Tiaramide did not affect 45Ca-uptake by itself nor collagen-induced 45Ca-uptake from the external medium. Tiaramide did not inhibit intracellular Ca mobilization, and it did not affect the calmodulin-dependent cyclic nucleotide phosphodiesterase of rabbit brain. These facts suggest that tiaramide inhibits platelet PLA2 through mechanisms other than the blockade of Ca-influx and intracellular Ca mobilization or antagonism to calmodulin.
D000818:Animals; D000893:Anti-Inflammatory Agents / Q000494:pharmacology*; D052160:Benzothiazoles; D001792:Blood Platelets / Q000201:enzymology; D001921:Brain / Q000201:enzymology; D002118:Calcium / Q000097:blood; D002132:Calcium Radioisotopes; D002147:Calmodulin / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D010740:Phospholipases / Q000037:antagonists & inhibitors*; D010741:Phospholipases A / Q000037:antagonists & inhibitors*; D054467:Phospholipases A2; D010727:Phosphoric Diester Hydrolases / Q000302:isolation & purification; D010879:Piperazines / Q000494:pharmacology*; D010974:Platelet Aggregation / Q000187:drug effects*; D011451:Prostaglandin-Endoperoxide Synthases / Q000097:blood; D011817:Rabbits; D012701:Serotonin / Q000097:blood; D013930:Thromboxane-A Synthase / Q000097:blood
D000893:Anti-Inflammatory Agents; D052160:Benzothiazoles; D002132:Calcium Radioisotopes; D002147:Calmodulin; D010879:Piperazines; D012701:Serotonin; C003468:tiaramide; D011451:Prostaglandin-Endoperoxide Synthases; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; D010727:Phosphoric Diester Hydrolases; D013930:Thromboxane-A Synthase; D002118:Calcium