Facilitation of calcium-dependent potassium current

Area of Science:

• Neuroscience
• Cell Physiology
• Ion Channel Function

Background:

• The activation of calcium-dependent potassium current (Ic) plays a crucial role in neuronal excitability.
• Facilitation, an increase in response upon repeated stimulation, is well-documented in synaptic transmission but less understood for ion currents.

Purpose of the Study:

• To investigate the mechanism underlying the facilitation of calcium-dependent potassium current (Ic) in molluscan neurons.
• To determine the role of intracellular calcium dynamics in Ic facilitation.

Main Methods:

• Macropatch recordings from molluscan neurons to measure Ic.
• Voltage-clamp pulses to induce and study current facilitation.
• Cytoplasmic microinjection of Ca2+ buffers and manipulation of Na/Ca exchange.
• Fluorescence imaging with Fluo-3 to monitor intracellular Ca2+ ([Ca]i) near the membrane.
• Numerical simulations of Ca2+ dynamics.

Main Results:

• Repeated depolarizing pulses led to a facilitation of Ic, characterized by increased amplitude and altered kinetics.
• Ic facilitation was reduced by Ca2+ buffering and elevated temperatures, confirming its Ca2+-dependent nature.
• Blocking Na/Ca exchange potentiated Ic facilitation, suggesting a role for Ca2+ extrusion mechanisms.
• Fluorescence imaging and simulations demonstrated that Ic facilitation correlates with the accumulation of Ca2+ near the inner membrane face.
• Spatial heterogeneity in Ic facilitation strength was observed, indicating non-uniform Ca2+ accumulation.

Conclusions:

• The facilitation of Ca-dependent K+ current is primarily driven by the accumulation of intracellular Ca2+ at the membrane's inner surface during repetitive neuronal depolarization.
• These findings highlight the dynamic regulation of ion channel function by local Ca2+ signaling microdomains.
• The study suggests that spatial gradients in Ca2+ concentration contribute to variations in neuronal excitability.