Signaling mechanisms mediated by G-protein coupled receptors in human platelets.
OBJECTIVE
The present study deals with the investigation of mechanisms involved in the synergistic interaction between epinephrine and arachidonic acid (AA).


METHODS
Venous blood was taken from healthy human volunteers reported to be free of medications for one week. Platelet aggregation was monitored at 37 degree using Dual-channel Lumi-aggregometer. The resulting aggregation was recorded for 5 min by the measurement of light transmission as a function of time.


RESULTS
The data show that a synergism in platelet aggregation mediated by subthreshold concentrations of epinephrine (1 micromol/L) and AA (0.2 micromol/L) was inhibited by the alpha2-receptor antagonist (yohimbine, IC50)=0.6 micromol/L) and an inhibitor of AA-cyclooxygenase (COX), indomethacin (IC50=0.25 micromol/L). In examining receptor influence on intraplatelet signalling pathways, it was found that the synergistic effect was inhibited by calcium channel blockers, verapamil (IC50=0.4 micromol/L) and diltiazem (IC50=2.5 micromol/L), as well as by low concentrations of inhibitors of phospholipase C (PLC) (U73122; IC50=0.2 micromol/L) and mitogens activated protein kinase (MAPK) (PD 98059; IC50=3.8 micromol/L). Herbimycin A, a specific inhibitor of tyrosine light chain kinase (TLCK), showed inhibition at IC50 value of 15 micromol/L, whereas chelerythrine, a protein kinase C (PKC) inhibitor, had no effect up to 20 micromol/L.


CONCLUSIONS
These data suggest that synergism between epinephrine and AA in platelet aggregation is triggered through receptors coupled to G-protein, which in turn, activate PLC, COX, and MAP kinase-signaling pathways.
Saeed
Sheikh Arshad
SA
Department of Biological and Biomedical Sciences, The Aga Khan University, Karachi-74800, Pakistan.
Rasheed
Huma
H
Fecto
Faisal A Wahed
FA
Achakzai
Mohammad Ilyas
MI
Ali
Rahmat
R
Connor
John Dennis
JD
Gilani
Anwar-ul-Hassan
AU
D000322:Adrenergic Agonists / Q000494:pharmacology; D058669:Adrenergic alpha-2 Receptor Antagonists; D000328:Adult; D016718:Arachidonic Acid / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D004357:Drug Synergism; D004837:Epinephrine / Q000494:pharmacology*; D005260:Female; D005419:Flavonoids / Q000494:pharmacology; D006801:Humans; D007213:Indomethacin / Q000494:pharmacology; D008297:Male; D010974:Platelet Aggregation / Q000187:drug effects*; D043562:Receptors, G-Protein-Coupled / Q000502:physiology*; D015398:Signal Transduction; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors; D014700:Verapamil / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
D000322:Adrenergic Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D002121:Calcium Channel Blockers; D016861:Cyclooxygenase Inhibitors; D005419:Flavonoids; D043562:Receptors, G-Protein-Coupled; D016718:Arachidonic Acid; D015016:Yohimbine; D014700:Verapamil; D010738:Type C Phospholipases; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one; D007213:Indomethacin; D004837:Epinephrine