Contractile and structural reactions to impediments of Ca2+-homeostasis in Physarum polycephalum

A combined application of 5 mM KCN and 19 microM Ca++-ionophore A-23187 leads to pronounced contractures of plasmodial strands of Physarum polycephalum. The appearance of the contractures is independent of the amount of Ca++ in the external medium. Tensiometric registrations of longitudinal contraction activity (isometric regime) reveal an average tension increase of 50 mp compared with the preceding tension level before the addition of KCN and ionophore. This high force output during the contracture coincides with a pronounced increase in the number of cytoplasmic actomyosin fibrils. Their ultrastructure is seen as a high lateral density of strictly parallel arranged F-actin filaments; the state of cytoplasmic actomyosin during this isometric contracture corresponds to the ultrastructure of isometrically contracted fibrils during the normal contraction-relaxation cycle of this organism. A simultaneous impediment of respiration and Ca++ homeostasis strongly favours a shift of the actin equilibrium to the high polymeric side in the form of fibrils and may thus be used as a preparatory step improving the specimens in the context of other investigations, e.g., for immunocytochemical investigations or for the preparation of cell-free models to be reactivated after extraction procedures.