THE TOUCH-INDUCED ACTION-POTENTIAL IN CHARA - INQUIRY INTO THE IONIC BASIS AND THE MECHANORECEPTOR

We developed an assay to characterize the mechanoreceptor for the touch-induced action potential of Chara internodal cells in order to compare it with the mechanoreceptor involved in sensing the gravitational pressure that induces a polarity of cytoplasmic streaming in the same cells. We show that there are at least two classes of mechanoreceptors in characean internodal cells. One class contains a RGD-binding, integrin-like protein that is localized at the end of the cells. This class of mechanoreceptors requires a turgid plasma membrane for sensing-competence and does not exhibit adaptation. We propose that this type of mechanoreceptor senses gravitational and hydrostatic pressure by sensing the magnitude and sign of its displacement relative to the extracellular matrix. The second class of mechanoreceptors most likely does not contain RGD-binding, integrin-like proteins and does not require a turgid plasma membrane. This class is uniformly localized throughout the length of the cell and displays adaptation. We propose that this class of mechanoreceptors senses touch by sensing changes in tension which is a result of the magnitude but not the sign of the displacement of the membrane.