Block I, Briegleb W
DFVLR--Institute for Aerospace Medicine, Cologne, FRG.
Adv Space Res. 1989;9(11):75-8. doi: 10.1016/0273-1177(89)90058-6.
Recently a gravisensitivity of the acellular slime mold Physarum polycephalum, which possesses no specialized gravireceptor, could be established by conducting experiments under simulated and under real near weightlessness. In these experiments macroplasmodia showed a modulation of their contraction rhythm followed by regulation phenomena. Until now the perception mechanism for the gravistimulus is unknown, but several findings indicate the involvement of mitochondria: A) During the impediment of respiration the 0g-reaction is inhibited and the regulation is reduced. B) The response to a light stimulus and the following regulation phenomena strongly resemble the behavior during exposure to 0g, the only difference is that the two reactions are directed into opposite directions. In the blue-light reaction a flavin of the mitochondrial matrix seems to be involved in the light perception. C) The contraction rhythm as well as its modulations are coupled to rhythmic changes in the levels of ATP and calcium ions, involving the mitochondria as sites of energy production and of Ca(++)-storage. So the mitochondria could be the site of the regulation and they possibly are the receptor sites for the light and gravity stimuli. Also the observation of a morphologic polarity of the slime mold's plasmodial strands has to be considered: Cross-sections reveal that the ectoplasmic wall surrounding the streaming endoplasm is much thinner on the physically lower side than on the upper side of the strand--this applies to strands lying on or hanging on a horizontal surface. So, in addition to the mitochondria, also the morphologic polarity may be involved in the perception mechanism of the observed gravisensitivity and of the recently established geotaxis. The potential role of the nuclei and of the contractile elements in the perception of gravity is also discussed.
最近，通过在模拟和实际近失重条件下进行实验，发现了无专门重力感受器的多核变形虫黏菌对重力敏感。在这些实验中，大变形体显示出其收缩节律的调节，随后出现调节现象。到目前为止，重力刺激的感知机制尚不清楚，但一些发现表明线粒体参与其中：A）在呼吸受阻时，零重力反应受到抑制，调节作用减弱。B）对光刺激的反应以及随后的调节现象与暴露于零重力时的行为非常相似，唯一的区别是这两种反应方向相反。在蓝光反应中，线粒体基质中的黄素似乎参与了光感知。C）收缩节律及其调节与ATP和钙离子水平的节律性变化相关，线粒体作为能量产生和钙离子储存的场所参与其中。因此，线粒体可能是调节的部位，它们可能是光和重力刺激的感受器部位。还必须考虑到黏菌变形体丝的形态极性观察结果：横截面显示，围绕流动内质的外质壁在丝的物理下侧比上侧薄得多——这适用于位于水平表面或悬挂在水平表面上的丝。所以，除了线粒体之外，形态极性也可能参与了所观察到的重力敏感性和最近确定的趋地性的感知机制。文中还讨论了细胞核和收缩元件在重力感知中的潜在作用。
