site for graviperception. In Parameciumand other ciliates, mechano-sensitive ion chan-
nels are distributed in a characteristic, bipolar manner in the plasma membrane:
mechano-sensitive K+-channels mainly posteriorly and mechano-sensitive Ca2+-channels
mainly anteriorly (Figure 7.4; for a full discussion of mechano-sensitive channels, see
Chapter 5).
In upward-swimming cells,the mechanical load of the protoplast is supposed to induce
an outward-directed force onto the lower part of the cell membrane, thus opening
mechano-sensitive K+-channels, which hyperpolarizes the cell. In turn, membrane hyper-
polarization promotes an increase of swimming velocity, which can be measured in
upward-swimming cells. In contrast, in a downward-swimming cell, the swimming ve-
locity is reduced due to depolarization of the membrane potential by mechanical stimu-
lation of mechano-sensitive Ca2+channels. Such a distinct activation of two kinds of re-
ceptor populations delivers a reasonable explanation for gravitaxis and gravikinesis
(Baba et al. 1991; Machemer and Bräucker 1992).
Correspondingly, in Euglena, stretch-activated gravireceptor channels are proposed to
be located asymmetrically in the anterior cell membrane (Figure 7.4). In an upward-
swimming (negative gravitactic) Euglena, these channels are closed. If the longitudinal
axis of the cell deviates from the vertical or if the cell swims downward, these channels
open (Lebert and Häder 1996; Lebert et al. 1997; H ̈äder et al. 2005), thereby inducing a
depolarization of the plasma membrane, followed by activation of an intracellular signal
transduction cascade that results in a reorientation of the cell.
If graviperception in these systems occurs via gravisensory channels, it should be possi-
ble to measure a “gravireceptor potential.” This was done by intracellular electrophysiolog-
ical recording in Parameciumand in the ciliate Stylonychia. Depending on the cell’s orien-
tation with respect to the gravity vector, a hyperpolarization or a depolarization was
registered after turning a cell upside-down (Gebauer et al. 1999; Krause 2003). In the case
of Euglena,a direct measurement of electrical potentials has not been successful. However,
an involvement of the membrane potential in graviperception in these cells was supported
by experiments in which the lipophilic cation TPMP+(triphenyl-methylphosphonium) and
the ionophore calcimycin were used to alter the cellular membrane potential. Both com-
pounds resulted in a loss of gravitaxis. In another experimental approach, Richter et al.
(2001a) used voltage-sensitive dyes such as Oxonol to observe changes in membrane po-
tential depending on the Euglenacell’s orientation within the gravity field.
These experiments led to the discovery of a close relationship between precision of
orientation and membrane potential. Interestingly, the channel blocker gadolinium
blocked gravitaxis in Euglena, suggesting that stretch-activated channels function as
gravisensory ion channels in this species (Lebert et al. 1997). Corresponding experiments
with Parameciumshowed no specific effect on gravitaxis, indicating that gadolinium is
not specific for mechano-sensitive channels in this cell (Nagel and Machemer 2000).
Regardless of the nature of the mass and the gravireceptor, graviperception can only
occur when sufficient energy is provided to the system to overcome thermal noise in the
receptor. Graviresponses are found in protists of different size and cell volume, ranging
from Euglena gracilis(2.6 103 μm^3 ) to Paramecium caudatum(327 103 μm^3 )or
even the giant ciliate Bursaria truncatella(30 106 μm^3 ). Energetic considerations show
that in larger species, gating energy for gravisensory channels is clearly above the ther-
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