Pertussis toxin-sensitive GTP-binding proteins may regulate phospholipase A2 in response to thrombin in rabbit platelets.
Incubation of rabbit platelets with thrombin resulted in rapid accumulations of inositol trisphosphate (IP3) in [3H]inositol-labeled platelets, increases of [3H]arachidonic acid [( 3H]AA) release, and [3H]serotonin secretion from the platelets prelabeled with these labeled compounds. The experiments using phospholipase A2 or C inhibitor suggested that not only phospholipase C but also phospholipase A2 activity plays an important role in serotonin secretion. We then studied the regulatory mechanisms of phospholipase A2 activity. Guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), guanyl-5'-(beta,gamma-iminio)triphosphate), or AlF4- caused a significant liberation of AA in digitonin-permeabilized platelets but not in intact platelets. Thrombin-stimulated AA release was not observed in permeabilized platelets, whereas thrombin acted synergistically with GTP or GTP analogs to stimulate AA release. GTP analog-stimulated AA release was inhibited by guanosine 5'-(2-O-thio)diphosphate) and was also inhibited by decreased Mg2+ concentrations. Thrombin-induced, GTP-dependent AA release, but not IP3 formation, was diminished by 100 ng/ml of pertussis toxin, associated with ADP-ribosylation of membrane 41-kDa protein(s). Thrombin-stimulated AA release from intact platelets and GTP gamma S-stimulated release from permeabilized platelets were both markedly dependent on Ca2+. However, Ca2+ addition could not enhance AA release without GTP gamma S even when Ca2+ was increased up to 10(-4) M in permeabilized platelets. The results show that thrombin-stimulated AA release from rabbit platelets is mainly mediated by phospholipase A2 activity, not by phospholipase C activity, and that Ca2+ is an important factor to the activation of phospholipase A2 but is not the sole factor to the regulation. GTP-binding protein(s) is involved in receptor-mediated activation of phospholipase A2.
Kajiyama
Y
Y
Department of Pharmacology, Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo, Japan.
Murayama
T
T
Nomura
Y
Y
D000818:Animals; D001095:Arachidonic Acids / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D002118:Calcium / Q000494:pharmacology; D004072:Digitonin / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000097:blood* / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000096:biosynthesis / Q000097:blood; D007700:Kinetics; D009638:Norepinephrine / Q000494:pharmacology; D037342:Pertussis Toxin; D010740:Phospholipases / Q000097:blood*; D010741:Phospholipases A / Q000097:blood*; D054467:Phospholipases A2; D010972:Platelet Activating Factor / Q000494:pharmacology; D011817:Rabbits; D012701:Serotonin / Q000097:blood; D013873:Thionucleotides / Q000494:pharmacology; D013917:Thrombin / Q000502:physiology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
D001095:Arachidonic Acids; D007295:Inositol Phosphates; D010972:Platelet Activating Factor; D013873:Thionucleotides; D010566:Virulence Factors, Bordetella; D012701:Serotonin; D006165:Guanylyl Imidodiphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D015544:Inositol 1,4,5-Trisphosphate; D006160:Guanosine Triphosphate; D037342:Pertussis Toxin; D010740:Phospholipases; D010741:Phospholipases A; D054467:Phospholipases A2; D013917:Thrombin; D019204:GTP-Binding Proteins; D004072:Digitonin; D002118:Calcium; D009638:Norepinephrine