Stimulation of non-selective cation channels providing Ca2+ influx into platelets by platelet-activating factor and other aggregation inducers.
To elucidate the mechanism of the receptor-stimulated Ca2+ entry into human platelets, the influence of Ca(2+)-mobilizing agonists on plasma membrane potential (Em) has been studied. Em changes were registered using potentiometric probe 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide. The agonist effect on Em varied from hyperpolarization to slight and slow rise. On the contrary, after loading of platelets with intracellular Ca2+ indicator quin2, platelet-activating factor (PAF), thrombin, vasopressin, ADP and thromboxane-A2-mimetic U46619 cause substantial transient membrane depolarization. Similar effects were observed after platelet loading with other Ca2+ chelators fura-2 and indo-1. Agonist-induced depolarization considerably reduced if quin2-loaded platelets were suspended in isoosmotic choline-containing medium. Using Ba2+ as a substitute of Ca2+, we have demonstrated that in choline-containing medium PAF-induced Ba2+ entry into platelets results in membrane depolarization. Dependence on Ba2+ concentration and depolarization kinetics correlates with the dose dependence and kinetics of Ba2+ entry detected by quin2 fluorescence. The agonists also stimulate considerable Na+, Li+ and Cs+ inward currents into platelets. Na(+)-dependent depolarization is 2-5-fold suppressed by extracellular Ca2+ [median inhibitory concentration (IC50) approximately 0.3 mM]. Ni2+ and Cd2+ at similar concentrations block Ca2+ entry and agonist-induced Na2+ current (IC50 for both cations approximately 50 microM). Agonist-induced depolarization is blocked by the adenylate cyclase stimulator prostaglandin E1 and the protein kinase C stimulator phorbol ester. It is concluded that agonists stimulate Ca2+ entry into human platelets via receptor-operated channels which are not strictly selective toward divalent cations and are permeable to Na+, Li+ and Cs+.
Avdonin
P V
PV
Laboratory of Molecular Endocrinology, Institute of Experimental Cardiology, USSR Cardiology Research Center, Moscow.
Cheglakov
I B
IB
Tkachuk
V A
VA
D000634:Aminoquinolines; D001464:Barium / Q000378:metabolism; D001692:Biological Transport; D001792:Blood Platelets / Q000378:metabolism* / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D002412:Cations / Q000378:metabolism; D005456:Fluorescent Dyes; D007473:Ion Channels / Q000187:drug effects*; D008564:Membrane Potentials; D010972:Platelet Activating Factor / Q000494:pharmacology*; D010974:Platelet Aggregation / Q000187:drug effects*
D000634:Aminoquinolines; D002412:Cations; D005456:Fluorescent Dyes; D007473:Ion Channels; D010972:Platelet Activating Factor; D001464:Barium; C035739:Quin2; D002118:Calcium