title: Aggregation efficiency of activated normal or fixed platelets in a simple shear field: effect of shear and fibrinogen occupancy.
abstract: Shear rate can affect protein adsorption and platelet aggregation by regulating both the collision frequency and the capture efficiency (alpha). These effects were evaluated in well defined shear field in a micro-couette for shear rate G = 10 - 1000 s-1. The rate of protein binding was independent of G, shown for adsorption of albumin to latex beads and PAC1 to activated platelets. The initial aggregation rate for ADP-activated platelets in citrated platelet-rich plasma followed second order kinetics at the initial platelet concentrations between 20,000 and 60,000/microliters. alpha values, which dropped nearly fivefold for a 10-fold increase in G, were approximately proportional to G-1, contrary to a minor drop predicted by the theory that includes protein cross-bridging. Varying ADP concentration did not change alpha of maximally activated platelet subpopulations, suggesting that aggregation between unactivated and activated platelets is negligible. Directly blocking the unoccupied but activated GPIIb-IIIa receptors without affecting pre-bound Fg on "RGD"-activated, fixed platelets (AFP) by GRGDSP or Ro 43-5054 eliminated aggregation, suggesting that cross-bridging of GPIIb-IIIa on adjacent platelets by fibrinogen mediates aggregation. Alpha for AFP remained maximal (approximately 0.24) over 25-75% Fg occupancy, otherwise decreasing rapidly, with a half-maximum occurring at around 2% occupancy, suggesting that very few bound Fg were required to cause significant aggregation.
authors.affiliation: Department of Physiology, McGill University, Montreal, Quebec, Canada.
authors.forename: Z
authors.initials: Z
authors.lastname: Xia
authors.forename: M M
authors.initials: MM
authors.lastname: Frojmovic
mesh_terms: D001792:Blood Platelets / Q000502:physiology*; D005340:Fibrinogen / Q000378:metabolism*; D016650:Fluorescein-5-isothiocyanate; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D009842:Oligopeptides / Q000494:pharmacology; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D011485:Protein Binding
chemical_list: D009842:Oligopeptides; D010980:Platelet Membrane Glycoproteins; C078174:Ro 43-5054; D005340:Fibrinogen; C058161:glycyl-arginyl-glycyl-aspartyl-seryl-proline; D016650:Fluorescein-5-isothiocyanate