Patch-clamp study on ion channels in the tonoplast of nitellopsis obtusa

Cytoplasmic drops covered with the tonoplast were prepared from internodal cells of Nitellopsis grown in fresh water. Applying the patch-clamp technique and the microinjection technique to such drops, we characterized the ion channels in the tonoplast. Both in cell-free patches and in the cytoplasmic-drop-attached patches, the tonoplast K⁺ channel was identified. The permeability ratio between Na⁺ and K⁺ was calculated to be 0.2. This channel would provide a molecular basis for the Na⁺/K⁺ exchange at the tonoplast. In cell-free patches, the K⁺channel was not activated by Ca²⁺. However, in the case of attached patches, microinjection of Ca²⁺ into a drop activated the K⁺ channel with a lag of a few seconds, suggesting that some cytoplasmic factor(s) may mediate the activation of the K⁺ channel by Ca²⁺. The conductance of this channel was not changed by cytoplasmic Ca²⁺, but the probability of opening increased markedly. In addition to the K⁺ channel, a second type of channel was also identified in cell-free patches. This channel may be the Cl^- channel.