Interaction between calcium ions and surface charge as it relates to calcium currents

Area of Science:

• Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• Calcium ions (Ca²⁺) are crucial for regulating neuronal excitability by affecting the gating of Ca²⁺ currents.
• The precise extent to which surface charge influences Ca²⁺ channel gating remains incompletely understood.

Purpose of the Study:

• To investigate the role of surface charge in modulating Ca²⁺ channel gating properties.
• To quantify the effects of various extracellular divalent cations on Ca²⁺ channel activation and inactivation.

Main Methods:

• Utilized the combined microelectrode-suction pipette method for voltage-clamp experiments on isolated Helix aspersa nerve cell bodies.
• Performed internal perfusion to manipulate intracellular ion concentrations and assess Ca²⁺ current-dependent inactivation.
• Applied the Gouy-Chapman theory for diffuse double layer with binding to model surface charge effects.

Main Results:

• Barium (Ba²⁺) and Strontium (Sr²⁺) currents activated at less positive potentials than Ca²⁺ currents.
• Magnesium ions (Mg²⁺) do not permeate Ca²⁺ channels, and changes in extracellular Mg²⁺ concentration ([Mg²⁺]o) shifted activation curves similarly to Ba²⁺ or Sr²⁺.
• Extracellular divalent cation effects on activation and inactivation voltage shifts were accurately described by diffuse double-layer theory, revealing equivalent Ca²⁺, Ba²⁺, and Sr²⁺ permeabilities after surface charge correction.

Conclusions:

• Surface charge significantly impacts Ca²⁺ channel gating, necessitating its consideration when evaluating channel properties.
• The observed voltage shifts are predictable using diffuse double-layer theory, highlighting the importance of surface potential.
• While surface charge effects explain relative permeabilities of Ca²⁺, Ba²⁺, and Sr²⁺, the presence of Cobalt ions (Co²⁺) suggests a potential channel binding site influencing Ca²⁺ permeability.