come more pronounced (up to 9 μgin the case of Paramecium) (Hemmersbach et al.
1996b; Bräucker et al. 2001).
When considering likely elements of the gravitactic signal-transduction pathway, it
was assumed that second messengers such as Ca2+, cAMP, cGMP, and calmodulin might
play a role in this process, as they are clearly involved in the regulation of ciliary and fla-
gellary activity; in the case of cAMP, a coupling to hyperpolarizing cellular events has
been shown (Bonini et al. 1986; Schultz and Schönborn 1994). Application of phospho-
diesterase inhibitors (IBMX, caffeine, 8-bromo-cyclo-AMP) to Euglenacultures, with
the aim of increasing intracellular cAMP levels, resulted in an increase in the precision
of negative gravitaxis, whereas decreasing cAMP levels inhibited the capacity of the cells
for gravitactic orientation. Determination of the cAMP levels of Paramecium and
Euglena under different acceleration conditions revealed significant acceleration-
dependent changes (Tahedl et al. 1998; Häder et al. 2005). Indeed, experiments in hyper-
gravity and microgravity showed changes in cAMP in Parameciumaccording to the pred-
ication of the “statocyst” (protoplast-pressure) hypothesis: a decrease in cAMP was
found in microgravity where no stimulation of the lower membrane should occur and an
increase was found under hypergravity conditions due to an increased mechanical load
on the lower membrane compared to 1 gconditions (Häder et al. 2005).
Direct visualization of second messenger changes in living cells are promising ap-
proaches to clarify the exact underlying mechanism and the time course of events. Using
the chlorophyll-free flagellate Astasia longa loaded with calcium Crimson dextran and
excited by laser light, researchers have investigated a possible correlation between the cy-
tosolic calcium concentration and the orientation of the cell. In this context, a 180-degree
turn of a negative-gravitactic Astasiaculture induced an increase in the calcium signal,
with a maximum after 30 seconds correlated with the reorientation of the cells. When
performed in space, this experiment revealed that microgravity-adapted Astasiacells
showed an increase in calcium-dependent fluorescence signal when accelerated above
their threshold for gravitaxis. An image analysis system established a correlation between
the calcium-dependent fluorescence signal and the swimming direction: cells moving in
parallel to the acceleration vector showed a low signal and cells moving perpendicular to
the vector displayed a high signal (Richter et al. 2001b). Acceleration-dependent changes
in intracellular calcium levels in Euglenawere also observed in a recent parabolic flight
experiment onboard the Airbus A300 Zero-G (Richter et al. 2002). Together, these data
support the existence of a calcium influx during reorientation of misaligned cells (Richter
et al. 2001b, 2002).
7.15 Conclusions
The multitude of significant findings that have increased our knowledge of gravity-
sensing and gravity-response mechanisms strongly emphasizes the usefulness of unicel-
lular model systems for this field of research. Thus, the details of the molecular and cel-
lular processes of gravity sensing are not obscured by a multiplicity of systems or by re-
dundant processes, as appears to be the case for gravitropism in higher plants (Barlow
1995; Kiss 2000). Although the gravity-dependent responses of single-celled systems are