Ca Dynamics during Membrane Excitation of Green Alga Chara: Model Simulations and Experimental Data

Kinetic investigations of stimulus response coupling in the green alga Chara have revealed that an intermediate second messenger is formed in the process of membrane excitation. This second messenger links electrical stimulation to the mobilization of Ca from internal stores. In the present work, the experimentally based kinetic model, which describes the stimulus-dependent production of the second messenger and Ca mobilization, is combined with a model for inositol 1,4,5-trisphosphate (IP3)-and Ca-sensitive gating of a Ca-release channel in endomembranes of animal cells. The combination of models allows a good simulation of experimental data, including the all-or-none-type dependence of the Ca response on stimulus duration and complex phase locking phenomena for the dependence of the Ca response on stimulation frequency. The model offers a molecular explanation for the refractory phenomenon in Chara, assigning it to the life time of an inactive state of the Ca-release channel. The model furthermore explains the steep dependence of excitation on strength/duration of electrical stimulation as a consequence of an interplay of the dynamical variables in the model.