Hemmersbach R, Häder D P
Institute of Aerospace Medicine, German Aerospace Research Establishment, 51170 Köln Institute for Botany and Pharmaceutical Biology, University of Erlangen, Germany.
FASEB J. 1999;13 Suppl:S69-75. doi: 10.1096/fasebj.13.9001.s69.
Protozoa are eukaryotic cells and represent suitable model systems to study the mechanisms of gravity perception and signal transduction due to their clear gravity-induced responses (gravitaxis and gravikinesis). Among protists, parallel evolution for graviperception mechanisms have been identified: either sensing by distinct stato-organelles (e.g., the Müller vesicles of the ciliate Loxodes) or by sensing the density difference between the whole cytoplasm and the extracellular medium (as proposed for Paramecium and Euglena). These two models are supported by experiments in density-adjusted media, as the gravitaxis of Loxodes was not affected, whereas the orientation of Paramecium and Euglena was completely disturbed. Both models include the involvement of ion channels in the cell membrane. Diverse experiments gave new information on the mechanism of graviperception in unicellular systems, such as threshold values in the range of 10% of gravity, relaxation of the responses after removal of the stimulus, and no visible adaptation phenomena during exposure to hypergravity or microgravity conditions for up to 12 days.
原生动物是真核细胞，由于其明显的重力诱导反应（趋重力性和重力运动），是研究重力感知和信号转导机制的合适模型系统。在原生生物中，已确定重力感知机制存在平行进化：要么通过独特的平衡细胞器（如纤毛虫Loxodes的米勒囊泡）感知，要么通过感知整个细胞质与细胞外介质之间的密度差异（如草履虫和眼虫所提出的）。密度调节介质中的实验支持了这两种模型，因为Loxodes的趋重力性不受影响，而草履虫和眼虫的定向则完全受到干扰。这两种模型都包括细胞膜中离子通道的参与。各种实验提供了关于单细胞系统中重力感知机制的新信息，如重力阈值在重力的10%范围内，去除刺激后反应的松弛，以及在高达12天的超重力或微重力条件下暴露期间没有明显的适应现象。
