Depolarization and calcium entry in squid giant axons

Area of Science:

• Neuroscience
• Cell Physiology
• Biochemistry

Background:

• Ionized calcium (Ca2+) plays a crucial role in neuronal function, including action potential propagation and neurotransmitter release.
• Understanding calcium dynamics within axons is essential for comprehending neuronal excitability and signaling.

Purpose of the Study:

• To investigate the changes in intracellular ionized calcium concentration in giant axons using aequorin luminescence.
• To characterize the mechanisms of calcium entry and release in response to physiological and pathological conditions.

Main Methods:

• Aequorin chemiluminescence was used to monitor intracellular ionized calcium levels in giant axons.
• Calcium buffers (EGTA) and metabolic inhibitors (cyanide, oligomycin) were employed to manipulate intracellular calcium.
• Voltage-clamp techniques were utilized to analyze calcium currents during depolarization.

Main Results:

• Resting intracellular ionized calcium was approximately 0.3 μM.
• Cyanide poisoning induced rapid calcium release from internal stores, reversible by oxygen.
• Stimulation led to calcium influx, with recovery attributed to uptake into internal stores.
• Calcium entry during depolarization occurred through two components: a tetrodotoxin-sensitive early component (likely via sodium channels) and a tetrodotoxin-insensitive late component.

Conclusions:

• Intracellular calcium stores are involved in regulating axonal calcium levels.
• Axonal stimulation causes calcium influx, partly through sodium channels.
• Internal calcium stores play a role in calcium homeostasis and recovery after stimulation.