Re-modeling Chara action potential: I. from Thiel model of Ca2+ transient to action potential form

Thiel and colleagues demonstrated that the all-or-none nature of Chara action potential (AP) is determined by formation of a second messenger, probably inositol triphosphate (IP3), which in turn releases Ca2+ from internal stores. The Ca2+-activated Cl- channels are the main agent of the depolarization phase of the AP. Once the Ca2+ is re-sequestered by the calcium pumps, the chloride conductance drops and depolarization-activated outward rectifier current, the background current and the proton pump current return the membrane potential difference (PD) to resting level. Departing from the Thiel model of transient increase of Ca2+ concentration, we set up membrane PD rate of change equation to calculate the AP form by numerical integration. Compared to data, this model AP depolarized more gradually. We introduced a prompt Ca2+ transient from the outside, achieving a good correspondence with the experimental AP. In Chara cells subjected to 50 mM NaCl/0.1 mM Ca2+ medium, the AP duration increased from 2 s to up to 50 s and the APs were often spontaneous. The lack of stimulating pulse revealed a sharp positive spike at the beginning of each AP, confirming that Chara plasma membrane may contain transient receptor potential (TRP)-like channels, possibly activated by another second messenger diacylglycerol (DAG) formed at the same time as IP3. The long duration of the saline AP can be modeled by decreasing the coefficients in the Hill equation describing the Ca2+ pumps on the internal stores. The model provides new insights into the characean AP and suggests a range of experiments.