Re-modeling <em>Chara</em> 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), whichin turn releases Ca²⁺ from internal stores. The Ca²⁺-activated Cl^– channels are the main agent of thedepolarization phase of the AP. Once the Ca²⁺ is re-sequestered by the calcium pumps, the chlorideconductance drops and depolarization-activated outward rectifier current, the background current andthe proton pump current return the membrane potential difference (PD) to resting level. Departingfrom the Thiel model of transient increase of Ca²⁺ concentration, we set up membrane PD rate ofchange equation to calculate the AP form by numerical integration. Compared to data, this model APdepolarized more gradually. We introduced a prompt Ca²⁺ transient from the outside, achieving agood correspondence with the experimental AP. In Chara cells subjected to 50 mM NaCl/0.1 mMCa²⁺ 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, confirmingthat Chara plasma membrane may contain transient receptor potential (TRP)-like channels, possiblyactivated by another second messenger diacylglycerol (DAG) formed at the same time as IP₃. Thelong duration of the saline AP can be modeled by decreasing the coefficients in the Hill equationdescribing the Ca²⁺ pumps on the internal stores. The model provides new insights into the characeanAP and suggests a range of experiments