Graviperception in ciliates: thresholds and gravireceptor potentials

Due to their clear gravity-induced behavioral responses (gravitaxis and gravikinesis) ciliates represent suitable model systems to study the mechanisms of graviperception and signal transduction. The cytoplasm is proposed to act as a "statolith" which stimulates polarily distributed mechanosensitive ion channels in the cell membrane. This results in a change of the membrane potential which in turn controls the beating activity of the cilia and thus the behaviour of the cell. In order to test this hypothesis, the membrane potential of isolated Stylonchia mytilus cells was measured during a defined change of the cell's spatial orientation. The data agree with the proposed changes (de- or hyperpolarization) depending on the orientation of the cell. The behaviour of Paramecium and Stylonchia was also analyzed during variable acceleration conditions of parabolic flights (5th German Parabolic Flight Campaign, 2003). The corresponding data confirm the relaxation of the graviresponses in microgravity as well as the existence of thresholds of graviresponses [0.4 x g (gravikinesis) and 0.6 x g (gravitaxis)]