Transient currents and Ca2+ gradient relaxation in characean algae cells: Theory and experiment

The transient Ca²⁺ and Ca²⁺-dependent Cl^? currents in the plasma membrane of voltage-clamped cells of the freshwater alga Chara corallina were studied. We used our own earlier proposed method, which utilized a rapid (～10 ms) injection of Ca²⁺ ions into the cell during the deactivation period of calcium channels after their activation with a positive voltage pulse (injection with a “tail” Ca²⁺ current). This procedure makes it possible to determine the amplitude of the Ca²⁺ component in the transient current as well as the amplitude and kinetics of the Cl^? component, dependent on the Ca²⁺ submembrane concentration. The calculated results, which used a cell model that takes the diffusion of Ca²⁺, the Ca²⁺-buffering properties of the cytoplasm, and the nonlinear dependence of i _Cl on Ca²⁺]_cyt, as well as the presence of chloroplasts into account, were in good agreement with the actual behavior of transient current in the experiments. It was demonstrated that the duration of the slow stage of Ca²⁺]_cyt relaxation to the resting level (～10^?7 M) (which is related to the function of Ca²⁺-ATPases), was ～10² s. This suggests that the slow stage determines the duration of the refractory period after generation of the action potential.