Chara cells produce receptor potentials (RPDs) in
response to mechanical stimulation. We have used a mechanostimulatory device
to compare characteristics of touch-activated RPDs and action potentials (APs)
when cell turgor pressure was changed. The device delivered a series of
mechanical stimulations of increasing energy (F0.5, F1, F2, F3, F4, F5 and
F6). Cells were alternately stimulated in artificial pondwater (APW) and a
sorbitol series, in long-term experiments, involving up to six solution
changes. The calculated cell turgor pressures were about 0.6 MPa (APW), and
0.49 MPa, 0.37 MPa, 0.24 MPa and 0.12 MPa in 50, 100, 150 and 200 mM
sorbitol–APW, respectively. In other experiments, cells were
pre-conditioned in the sorbitol solutions, and then transferred to APW. All
cells were allowed long recovery periods (40–60 min) after APs or
solution transfers. Only small changes in cell conductance were observed in
I–V and
G–V analysis of unstimulated
cells after reducing turgor pressure from 0.59 MPa to 0.24 MPa. In APW, the
RPDs increased in amplitude and duration with increased stimulus energy until
the threshold RPD was reached, and an AP was triggered, usually between
stimulus F4 and F5. Cells with decreased turgor pressure became more sensitive
to stimulation, giving threshold RPDs or APs with smaller stimulus (e.g.
between F0.5 and F3). Conversely, an increase in cell turgor pressure (return
to APW) led to a decrease in sensitivity to stimulus. When turgor pressure was
greatly decreased (to 0.12 MPa), some cells became unresponsive or gave
unusual responses. However, only the mechanical part of the touch response was
affected by changing the cell turgor pressure. The mean amplitudes of the
subthreshold and threshold RPD (that triggers the AP), and of the
touch-activated APs, were independent of cell turgor pressure, although action
potentials had smaller amplitude when turgor was reduced to about 0.12 MPa.
The amplitude of the subthreshold RPD was close to 20 mV, and the amplitude of
the threshold RPD was close to 50 mV, in all cells. If tension of the cell
wall–plasma membrane–cytoskeleton complex decreased along with
decreased cell turgor pressure, a given stimulus could stretch the complex to
a greater extent, resulting in activation of more mechanosensory channels. The
effect on the RPD of changes in cell turgor pressure is discussed in relation
to the mechanical properties of the cell wall–plasma
membrane–cytoskeleton complex.
Force requirements of endocytic vesicle formation
