A role for the actin cytoskeleton in the initiation and maintenance of store-mediated calcium entry in human platelets. Evidence for conformational coupling

The nature of the mechanism underlying store-mediated Ca(2+) entry has been investigated in human platelets through a combination of cytoskeletal modifications. Inhibition of actin polymerization by cytochalasin D or latrunculin A had a biphasic time-dependent effect on Ca(2+) entry, showing an initial potentiation followed by inhibition of Ca(2+) entry. Moreover, addition of these agents after induction of store-mediated Ca(2+) entry inhibited the Ca(2+) influx mechanism. Jasplakinolide, which reorganizes actin filaments into a tight cortical layer adjacent to the plasma membrane, prevented activation of store-mediated Ca(2+) entry but did not modify this process after its activation. In addition, jasplakinolide prevented cytochalasin D-induced inhibition of store-mediated Ca(2+) entry. Calyculin A, an inhibitor of protein serine/threonine phosphatases 1 and 2 which activates translocation of existing F-actin to the cell periphery without inducing actin polymerization, also prevented activation of store-mediated Ca(2+) entry. Finally, inhibition of vesicular transport with brefeldin A inhibited activation of store-mediated Ca(2+) entry but did not alter this mechanism once initiated. These data suggest that store-mediated Ca(2+) entry in platelets may be mediated by a reversible trafficking and coupling of the endoplasmic reticulum with the plasma membrane, which shows close parallels to the events mediating secretion.