title: Human platelet activation is inhibited by the occupancy of glycoprotein IIb/IIIa receptor.
abstract: We studied the effect of glycoprotein GPIIb/IIIa (integrin alpha IIb beta 3) receptor occupancy by adenosine 5',1-thiotriphosphate (ATP alpha S), a competitive inhibitor of the ADP receptor, by fibrinogen, and by peptides containing the RGD (Arg-Gly-Asp) sequence as RGDW (Arg-Gly-Asp-Trp), RGDS (Arg-Gly-Asp-Ser), or the negative control RGGW (Arg-Gly-Gly-Trp) on human platelet physiological functions: aggregation, ATP secretion, and [Ca2+]in. As the presence of a nucleotide binding site on GPIIb alpha has been demonstrated in platelets [N. J. Greco, N. Yamamoto, B. W. Jackson, N. N. Tandon, M. Moos, and G. A. Jamieson (1991) J. Biol. Chem. 266, 13627-13633], we studied the effect of ATP alpha S, which specifically binds to this site, on platelet activation. We observed that ATP alpha S inhibited aggregation by thrombin, ADP, PMA, and ionophore A23187. Moreover, ATP alpha S dose dependently inhibited ATP secretion by ionophore A23187 and Ca2+ transients by thrombin and vasopressin in both the presence and absence of external Ca2+. Fibrinogen, although induced by a potentiation of platelet aggregation, inhibited ATP secretion and [Ca2+]in elevation induced by low thrombin concentrations or by vasopressin, interfering with both Ca2+ entry and Ca2+ release by the intracellular stores. RGD peptides, which specifically bind to GPIIb/IIIa, inhibited aggregation, secretion, and Ca2+ transients by thrombin, whereas the negative control RGGW did not exert any effect. We conclude that the occupancy of the GPIIb/IIIa receptor binding sites modulates platelet function by giving an inhibitory outside-in signal in platelets, particularly effective in platelets stimulated with low agonist doses. We suggest that ATP alpha S, fibrinogen, or RGD compounds, by interacting with GPIIb/IIIa receptor, prime some intracellular negative feedback mechanisms, which prevent further activation of circulating platelets by low-intensity stimuli and intravascular aggregation.
authors.affiliation: Institute of Human Physiology, Faculty of Medicine and Surgery, University of Padova, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Padoin
authors.forename: A
authors.initials: A
authors.lastname: Alexandre
authors.forename: L
authors.initials: L
authors.lastname: Cavallini
authors.forename: P
authors.initials: P
authors.lastname: Polverino de Laureto
authors.forename: G H
authors.initials: GH
authors.lastname: Rao
authors.forename: M G
authors.initials: MG
authors.lastname: Doni
mesh_terms: D000255:Adenosine Triphosphate / Q000097:blood; D000595:Amino Acid Sequence; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000097:blood / Q000494:pharmacology; D005340:Fibrinogen / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D009842:Oligopeptides / Q000494:pharmacology*; D015539:Platelet Activation; D010974:Platelet Aggregation / Q000187:drug effects; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000187:drug effects / Q000502:physiology*; D013329:Structure-Activity Relationship; D013873:Thionucleotides / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology
chemical_list: D009842:Oligopeptides; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D013873:Thionucleotides; C017817:adenosine 5'-(1-thio)triphosphate; C101629:arginyl-glycyl-glycyl-tryptophan; D000001:Calcimycin; C047981:arginyl-glycyl-aspartic acid; D000255:Adenosine Triphosphate; D005340:Fibrinogen; C046617:arginyl-glycyl-aspartyl-serine; D013917:Thrombin; D002118:Calcium