Re-modeling <em>Chara</em> action potential: I. from Thiel model of Ca<sup>2+</sup>transient to action potential form

Mary Jane Beilby; Sabah Al Khazaaly; Mary Jane Beilby; Sabah Al Khazaaly

doi:10.3934/biophy.2016.3.431

AIMS Biophysics

2016, Volume 3, Issue 3: 431-449. doi: 10.3934/biophy.2016.3.431

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

Mary Jane Beilby ^{1
,
,},
Sabah Al Khazaaly ²

1.
School of Physics, University of NSW, Kensington, NSW 2052, Australia
2.
Mathematics and Statistics, Western Sydney University, Bankstown, NSW 2200, Australia

Received: 20 August 2016 Accepted: 22 September 2016 Published: 26 September 2016

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.
- action potential,
- Chara,
- cytoplasmic Ca²⁺ concentration,
- Ca²⁺ channels on internal stores,
- inositol triphosphate,
- Ca²⁺-activated Cl^– channels,
- TRP channels,
- Ca²⁺ pump
Citation: Mary Jane Beilby, Sabah Al Khazaaly. Re-modeling Chara action potential: I. from Thiel model of Ca2+transient to action potential form[J]. AIMS Biophysics, 2016, 3(3): 431-449. doi: 10.3934/biophy.2016.3.431

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Abstract

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.

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