Inhibitory effect of 8-bromo cyclic GMP on an extracellular Ca2+-dependent arachidonic acid liberation in collagen-stimulated rabbit platelets.
The inhibitory effect of cyclic GMP on collagen-induced platelet activation was studied using 8-bromo cyclic GMP (8brcGMP) in washed rabbit platelets. Addition of collagen (1 micrograms/ml) to platelet suspension caused shape change and aggregation associated with thromboxane (TX) A2 formation. 8brcGMP (10-1000 microM) inhibited collagen-induced platelet aggregation and TXA2 formation in a concentration-dependent manner. 8brcGMP did not affect platelet cyclooxygenase pathways, but markedly inhibited collagen-induced arachidonic acid (AA) liberation from membrane phospholipids in [3H]AA-prelabeled platelets, indicating that the inhibitory effect of 8brcGMP on collagen-induced aggregation is due to an inhibition of AA liberation. In [32P]orthophosphate-labeled platelets, collagen stimulated phosphorylation of a 20,000 dalton (20-kD) and 40-kD proteins. 8BrcGMP stimulated phosphorylation of a specific protein having molecular weight of 46-kD and inhibited collagen-induced both 20- and 40-kD protein phosphorylation. Collagen could stimulate the AA liberation without activation of phospholipase C or Na+-H+ exchange, but could not in the absence of extracellular Ca2+. These findings suggest that cyclic GMP inhibits collagen-induced AA liberation which is mediated by an extracellular Ca2+-dependent phospholipase A2. However, cyclic GMP seems to inhibit the Ca2+-activated phospholipase A2 indirectly, since 8brcGMP had no effect on Ca2+ ionophore A23187-induced platelet aggregation or AA liberation. It is therefore suggested that cyclic GMP may regulate collagen-induced increase in an availability of extracellular Ca2+ which is responsible for phospholipase A2 activation in rabbit platelets.
Matsuoka
I
I
Department of Pharmacology, Fukushima Medical College, Japan.
Nakahata
N
N
Nakanishi
H
H
D000818:Animals; D016718:Arachidonic Acid; D001095:Arachidonic Acids / Q000097:blood*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002352:Carrier Proteins / Q000097:blood; D003094:Collagen / Q000494:pharmacology; D006152:Cyclic GMP / Q000031:analogs & derivatives* / Q000494:pharmacology; D007213:Indomethacin / Q000494:pharmacology; D008970:Molecular Weight; D010716:Phosphatidylinositols / Q000097:blood; D010741:Phospholipases A / Q000378:metabolism; D054467:Phospholipases A2; D010750:Phosphoproteins / Q000097:blood; D010974:Platelet Aggregation / Q000187:drug effects*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D011817:Rabbits; D017923:Sodium-Hydrogen Exchangers; D013929:Thromboxane B2 / Q000096:biosynthesis
D001095:Arachidonic Acids; D002352:Carrier Proteins; D010716:Phosphatidylinositols; D010750:Phosphoproteins; D010975:Platelet Aggregation Inhibitors; D017923:Sodium-Hydrogen Exchangers; D016718:Arachidonic Acid; C016276:8-bromocyclic GMP; D000001:Calcimycin; D013929:Thromboxane B2; D003094:Collagen; D010741:Phospholipases A; D054467:Phospholipases A2; D006152:Cyclic GMP; D002118:Calcium; D007213:Indomethacin