Aquaporins in human platelets: intracellular localization and possible role in granule and lysosome secretion

This study was undertaken to establish the presence and the role of aquaporins (AQPs) in human platelets. Immunodetection with polyclonal antibodies and fluorescent microscopy suggest the presence of AQP isoforms - 0-7 and 9-12 - localized (in resting platelets) in the plasma membrane and in the dense and alpha granules. In thrombin- or monensin-treated platelets, the granules' AQPs become visible in the whole cell body, indicating the granules' swelling. In our studies on the role of AQPs in platelet responses we used tetrachloroauric acid (HAuCl4), a classical water channel blocker. We found that 10-100 µM of Au(III) inhibited the hypotonicity-, monensin (simulating the action of Na+/H+ exchanger)-, and collagen-evoked platelet swelling and reduced tritiated water uptake by platelets treated by collagen or monensin, indicating its ability to block water channels in these cells. HAuCl4, at the concentrations reducing water influx, did not induce cell lysis, alter the plasma membrane shape or the -SH group content. The inhibitor also failed to affect Na+ and Cl--related osmotic gradient formation and protein kinase D2 phosphorylation. In platelets activated by threshold concentrations of collagen, the thrombin receptor activating peptide, ADP, calcium ionophore A23187, phorbol ester and arachidonic acid, HAuCl4 (100 µM) completely inhibited secretion of ATP from dense granules but failed to reduce platelet aggregation. In collagen-stimulated platelets, HAuCl4 (10-100 µM) reduced secretion from dense and alpha granules, as well as lysosomes, in a dose-dependent manner. We conclude that human platelets possess numerous AQPs subtypes localized in the plasma and granule membranes. AQP-mediated water fluxes may be crucial for platelet volume regulation as well as secretion from dense and alpha granules and lysosomes.