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Calcium entry in squid axons during voltage clamp pulses
J Requena1, J Whittembury, L J Mullins
1Centro de Biociencias, Instituto Internacional de Estudios Ananzados (IDEA), Caracas, Venezuela.
Cell Calcium
|August 1, 1989
Summary
Calcium entry into squid giant axons depends on internal sodium concentration and membrane voltage. This suggests a voltage-sensitive sodium-calcium exchange mechanism is crucial for regulating intracellular calcium levels.
Area of Science:
- Neuroscience
- Cellular Physiology
- Biophysics
Background:
- The squid giant axon is a model system for studying neuronal ion transport.
- Intracellular calcium levels ([Ca2+]i) are critical for neuronal function.
- Sodium-calcium exchange is a key mechanism for regulating [Ca2+]i.
Purpose of the Study:
- To investigate the relationship between membrane voltage, internal sodium concentration ([Na+]i), and calcium influx in squid giant axons.
- To elucidate the stoichiometry and voltage sensitivity of the Na+/Ca2+ exchanger.
Main Methods:
- Squid giant axons were voltage-clamped and injected with aequorin to measure [Ca2+]i.
- Ionic currents were blocked, and axons were bathed in solutions with varying external calcium ([Ca2+]o) and internal sodium concentrations.
- Aequorin light emission was used to quantify changes in [Ca2+]i in response to voltage pulses.
Main Results:
- Calcium entry was found to be voltage-dependent and sensitive to internal sodium concentration ([Na+]i).
- A steep relationship between [Ca2+]i gain and [Na+]i suggests a stoichiometry of 4 Na+ ions per 1 Ca2+ ion.
- At high external calcium concentrations (50mM), an alternative calcium entry pathway was observed that was not blocked by Cs+.
Conclusions:
- The Na+/Ca2+ exchange in squid giant axons is sensitive to membrane voltage.
- The findings support a model where voltage directly influences the Na+/Ca2+ exchanger's activity.
- An additional, voltage-insensitive calcium entry pathway may exist at high external calcium levels.