title: Kinetic analysis of a TxA2 receptor antagonist. Evidence for the existence of an uptake system.
abstract: The kinetics of the binding of TxA2 and the characteristics of its putative uptake by platelets (plt) were investigated with a radiolabelled structural TxA2 analogue, 125I-PTA-OH, in platelets from 6 male healthy volunteers. The kinetic constants (Kobs, K1 and K-1) were calculated by time-course experiments according to Weiland and Molinoff. The affinity constant (Km) and maximal uptake velocity (Vmax) values for each subject were determined by the double-reciprocal plot of Lineweaver-Burk. The kinetically determined dissociation constant Kd was 10 nmol/l. In the uptake experiments a saturability of the association rate of the non-displaceable binding was observed at increased ligand concentration. Michaelis-Menten type kinetics were obtained and the Vmax was 32.44 fmol/10(8) plt x hour with a Km of 6.39 nmol/l. These observations suggest that the uptake was a carrier-mediated active transport.
authors: Clinica Medica I, University of Florence, Florence, Italy.
authors: P A
authors: PA
authors: Modesti
authors: R
authors: R
authors: Abbate
authors: G F
authors: GF
authors: Gensini
authors: A
authors: A
authors: Colella
authors: G G
authors: GG
authors: Serneri
mesh_terms: D000328:Adult; D001792:Blood Platelets / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008297:Male; D011982:Receptors, Prostaglandin / Q000037:antagonists & inhibitors*; D017482:Receptors, Thromboxane; D013928:Thromboxane A2 / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D011982:Receptors, Prostaglandin; D017482:Receptors, Thromboxane; D013928:Thromboxane A2; C046957:I-PTA-OH