Calcium inward currents in internally perfused giant axons

Area of Science:

• Neuroscience
• Biophysics
• Ion Channel Physiology

Background:

• The sodium channel in squid axons is crucial for action potential propagation.
• Understanding the ion selectivity and permeability of the sodium channel is essential for comprehending neuronal excitability.

Purpose of the Study:

• To investigate the ion selectivity and permeability of the sodium channel in squid axons under specific experimental conditions.
• To elucidate the ionic basis of long-lasting action potentials observed in squid axons.

Main Methods:

• Voltage clamp experiments were performed on internally perfused squid giant axons.
• Solutions were manipulated to be sodium-free and contained high calcium or magnesium concentrations.
• Tetrodotoxin was used to differentiate between ionic currents.

Main Results:

• Inward currents were carried by calcium (Ca) ions and outward currents by cesium (Cs) ions, both traversing the sodium (Na) channel.
• Tetrodotoxin (TTX) sensitivity indicated that both Ca and Cs currents utilize the Na channel.
• Permeability ratios (P(Ca)/P(Cs), P(Ca)/P(Na), P(Cs)/P(Na)) were calculated, suggesting the Na channel retains some permeability to Na ions despite altered selectivity.

Conclusions:

• The Na channel in squid axons can conduct both Ca and Cs ions, with distinct kinetic properties.
• Slowly inactivating Ca inward currents through the Na channel contribute to long-lasting action potentials.
• These findings challenge the traditional view of the Na channel as exclusively permeable to Na ions.