Membrane Stretching Triggers Mechanosensitive Ca2+ Channel Activation in Chara

In order to confirm that mechanosensitive Ca2+ channels are activated by membrane stretching, we stretched or compressed the plasma membrane of Chara by applying osmotic shrinkage or swelling of the cell by varying the osmotic potential of the bathing medium. Aequorin studies revealed that treatments causing membrane stretching induced a transient but large increase in cytoplasmic concentration of Ca2+ (Delta[Ca2+](c)). However, the observed Delta[Ca2+](c) decreased during the treatments, resulting in membrane compression. A second experiment was carried out to study the relationship between changes in membrane potential (Delta E (m)) and stretching or compression of the plasma membrane. Significant Delta E (m) values, often accompanied by an action potential, were observed during the initial exchange of the bathing medium from a hypotonic medium to a hypertonic one (plasmolysis). Delta E (m) appears to be triggered by a partial stretching of the membrane as it was peeled from the cell wall. After plasmolysis, other exchanges from hypertonic to hypotonic media, with their accompanying membrane stretching, always induced large Delta E (m) values and were often accompanied by an action potential. By contrast, action potentials were scarcely observed during other exchanges from hypotonic to hypertonic solutions (=membrane compression). Thus, we concluded that activation of the mechanosensitive channels is triggered by membrane stretching in Chara.