Selective Release of Serotonin from Human Platelets by X-537 A, Acting as an Ionophore for Serotonin
The ionophore X-537 A facilitates the permeation of calcium through membranes. Human platelets exposed to X-537 A undergo a release of serotonin, which has been attributed to an increase in the concentration of cytoplasmic calcium, mediated by X-537 A. However, at a low concentration of X-537 A, platelets release serotonin but one cannot observe a release of heparin-neutralizing activity (PF4) and adenine nucleotides, an activation of clot-promoting activity (PF3), or a significant uptake of 45calcium into platelets. Electron micrographs of platelets after exposure to X-537A do not exhibit the alterations known to be coupled with the release reaction, despite a considerable release of serotonin. Two-phase partition studies demonstrate that X-537A is able to extract serotonin into an organic phase. In summary, these results indicate that X-537A causes a selective release of serotonin by transporting it through membranes. Only at high concentrations of X-537A can one establish an uptake of 45calcium by platelets and a release of PF4 and adenine nucleotides in addition to that of serotonin.(Supported by “Deutsche Forschungsgemeinschaft, Bonn”, Sonderforschungsbereich 90, “Cardiovasculares System”.)