Studies on the Ca2+-induced lysis of platelet alpha-granules

Platelet alpha-granules have been reported to lyse upon addition of submillimolar Ca2+ (J. Van der Meulen and S. Grinstein, J. Biol. Chem. 257, 5190). Similar observations in parotid granules have been attributed to extensive lipid hydrolysis. Experiments were performed to assess the role of lipases and proteases in Ca2+-induced lysis of alpha-granules. No differences were detected between lipids of Ca2+-treated and control granules by two-dimensional thin-layer chromatography. Moreover, several phospholipase inhibitors were without effect on Ca2+-induced lysis. Similarly, the polypeptide patterns of control and treated granules were identical and protease inhibitors failed to prevent lysis. In contrast, lysis could be suppressed by increasing the osmolarity of the medium or by substitution with nonpermeating ions. Lysis was unaffected by quinine, amiloride, furosemide, or tetraethylammonium but was inhibited by 4,4'-diisothiocyano-2,2'-stilbene disulfonate (DIDS), a powerful inhibitor of anion transport. The data suggest that Ca2+-induced lysis of alpha-granules does not result from wholesale hydrolysis of either lipids or proteins. Instead, the results are consistent with a Ca2+-mediated change in membrane permeability. In the presence of permeating ions, this leads to entry of salt and osmotically obliged water with consequent swelling and eventual lysis.