Die Bedeutung von Kalzium bei der Aktivierung der Blutplättchen

Zusammenfassung

In jüngster Vergangenheit konnte gezeigt werden, daß der Freisetzung intrazellulären Kalziums bei der Aktivierung von Blutplättchen wesentliche Bedeutung zukommt. Externe Stimuli aktivieren durch Bindung an Rezeptoren, die sich an der Oberfläche der Plättchen befinden, die an der Innenseite der Plasmamembran lokalisierte Phospholipase C. Phospholipase C wandelt das ebenfalls an der Innenseite der Plasmamembran lokalisierte Phospholipid Phosphatidylinosittriphosphat (PIP3) in Inosittriphosphat (IP3) und Diacylglycerin (DAG) um. IP3 setzt höchstwahrscheinlich aus intrazellulären Speichern (dichtes tubuläres System) Kalzium in das Zytosol der Plättchen frei. DAG aktiviert die an der Innenseite der Plättchenplasmamembran befindliche C-Kinase. Die Aktivität der C-Kinase ist von der Gegenwart von Phosphatidylserin und Kalzium abhängig. Indem DAG die Sensitivität der C-Kinase gegenüber Kalzium deutlich erhöht, ist eine Aktivierung der C-Kinase auch bei fehlender intrazellulärer Kalziumfreisetzung möglich. Durch die Freisetzung intrazellulären Kalziums wird das von Kalzium abhängige Calmodulin aktiviert und katalysiert seinerseits die Phosphorylierung eines 20 kD-Proteins, die leichte Kette des Myosins, durch die Myosinleichtkettenkinase. Die CKinase phosphoryliert ein 47 kD-Protein, aber auch die leichte Kette des Myosins, allerdings an anderer Stelle als die Myosinleichtkettenkinase. Die Phosphorylierung dieser Proteine ist mit der Freisetzung von Serotonin aus den dense bodies und von sauren Hydrolasen aus den Lysosomen vergesellschaftet. Die Funktion der phosphorylierten Proteine bei der Plättchenaktivierung ist jedoch derzeit ungeklärt und sollte in Zukunft Gegenstand intensiver Forschung sein.

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