Blockade of potassium channels in the plasmalemma of Chara corallina by tetraethylammonium, Ba²⁺, Na⁺ and Cs⁺

The outer membranes of plant cells contain channels which are highly selective for K⁺. In the giant-celled green alga Chara corallina, K⁺ currents in the plasmalemma were measured during the action potential and when the cell was depolarized to the K⁺ equilibrium potential in high external K⁺ concentrations. Currents in both conditions were reduced by externally added tetraethylammonium (TEA⁺), Ba²⁺, Na⁺ and Cs⁺. In contrast to inhibition by TEA⁺, the latter three ions inhibited inward K⁺ current in a voltage-dependent manner, and reduced inward current more than outward. Ba²⁺ and Na⁺ also appeared to inhibit outward current in a strongly voltage-dependent manner. The blockade by Cs⁺ is studied in more detail in the following paper. TEA⁺ inhibited both inward and outward currents in a largely voltage-independent manner, with an apparent K_D of about 0.7 to 1.1 mm, increasing with increasing external K⁺. All inhibitors reduced current towards a similar linear leak, suggesting an insensitivity of the background leak in Chara to these various K⁺ channel inhibitors. The selectivity of the channel to various monovalent cations varied depending on the method of measurement, suggesting that ion movement through the K⁺-selective channel may not be independent.