Mechanosensory ion channels in Chara: the influence of cell turgor pressure on touch-activated receptor potentials and action potentials

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.