Cycling aggregation patterns of cytoplasmic F-actin coordinated with oscillating tension force generation.

Isometric contracting protoplasmic veins of Physarum polycephalum show cycling patterns of cytoplasmic F-actin, dependent on their oscillating contraction behaviour (minute rhythms). The process of fibrillogenesis represents a parallel arrangement of F-actin chains ("plasma filaments, microfilaments") during the isometric contraction phase. A part of the results of the present work corroborates previous results on stretch-activated veins which showed that the fibrillar form of F-actin reflects the isometric contracted state. During isometric relaxation phase, a disaggregation of the fibrillar pattern takes place and is accompanied by a deparallelisation of F-actin chains. Therefore, the isometric relaxed state of cytoplasmic actomyosin is non-fibrillar in nature. Thus, the morphologically detectable fibrillar form of cytoplasmic actomyosin, according to physiological interpretation, is solely representative of the isometric contracted state. The question whether assembly-disassembly processes, e.g. G equilibrium F-actin-transformation, play a role in the contraction-relaxation cycle is discussed.