Impaired Na+-dependent glutamate uptake in platelets during depolarization of their plasma membrane.

Blood platelets contain neuronal Na(+)-dependent glutamate transporters and are able to accomplish glutamate uptake. Applying high-KCl, we have demonstrated dose-dependent depolarization of the plasma membrane of rabbit platelets that was registered as an increase in the fluorescence of the potential-sensitive fluorescent dye rhodamine 6G. The initial velocity of L-[(14)C]glutamate uptake (10 microM) in platelets was decreased by 20% during 35 mM KCl-evoked depolarization and consisted of 1.2+/-0.09 pmol x min(-1) x mg(-1) protein in control and 0.96+/-0.08 pmol x min(-1) x mg(-1) protein during depolarization. Confocal laser scanning microscopy and flow cytometry revealed that these changes in glutamate uptake were not a result of platelet aggregation/activation. Also, addition of high-KCl did not change acidification of platelet secretory granules that was found with pH-sensitive fluorescent dye acridine orange, thereby showing that changes in their proton gradient could not cause glutamate uptake alteration. This malfunction of glutamate transporters has to take place under: (i) the conditions of pseudohyperkalemia or hyperkalemia, i.e. activation and clotting of platelets, haemolysis, leucocytosis, acute renal failure, hypofunction of adrenal cortex, lack of aldosterone, stroke, trauma and (ii) depolarization of the plasma membrane of platelets during their activation by ADP, thrombin, platelet-activating factor. Weak glutamate uptake might have considerable consequences for platelets per se (and thus for haemostatic system) and glutamate homeostasis in the CNS.