title: Sodium-hydrogen exchange in human platelets exposed to weak acid: comparison between measurements of platelet swelling and cytoplasmic sodium ion concentration.
abstract: Increased Na+-H+ exchange activity has been demonstrated in various pathological states using the rate constant of platelet swelling (ks). This indirect measure of exchange activity could, however, be influenced by conditions that alter platelet distensibility independent of Na+-H+ exchange activity. To determine the validity of ks, the change in light transmission and hence ks, was measured at 24 degrees and 37 degrees C (temperatures associated with alterations in membrane fluidity), and compared with the rate constant (kNa) for the increase in cytoplasmic sodium ion concentration ([Na+]i) and the rate constant of Na+-influx (kphiNa) under the same conditions. Both ks and kNa were dependent on Na+-H+ exchange; the rate constants differed in the order ks< kphiNa < kNa (P<0.01). Neither kNa or kphiNa changed with temperature, whereas ks was significantly lower at 24 degrees than at 37 degrees C (7.49 +/- 0.31 x 10(-3) s(-1) versus 9.96 +/- 0.42 x 10(-3) s(-1), P<0.0001). While both ks and kNa are dependent on Na+-H+ exchange, ks may underestimate the activity of the exchanger and may in part, be influenced by membrane fluidity. This could be important in interpreting data where ks has been measured in different groups of subjects.
authors.affiliation: GKT School of Biomedical Sciences, KCL London, Department of Clinical Pharmacology, St Thomas' Hospital, UK.
authors.forename: P D
authors.initials: PD
authors.lastname: Stratton
authors.forename: P J
authors.initials: PJ
authors.lastname: Chowienczyk
authors.forename: J M
authors.initials: JM
authors.lastname: Ritter
mesh_terms: D001792:Blood Platelets / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002462:Cell Membrane / Q000648:ultrastructure; D048429:Cell Size / Q000187:drug effects; D003593:Cytoplasm / Q000737:chemistry; D005454:Fluorescence Polarization; D005456:Fluorescent Dyes; D006801:Humans; D007700:Kinetics; D009995:Osmosis / Q000187:drug effects; D011422:Propionates / Q000494:pharmacology*; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism*; D013696:Temperature; D014867:Water / Q000378:metabolism
chemical_list: D005456:Fluorescent Dyes; D011422:Propionates; D017923:Sodium-Hydrogen Exchangers; D014867:Water; D012964:Sodium; C029658:propionic acid