Localization of neuronal Ca2+ buffering near plasma membrane studied with different divalent cations

Area of Science:

• Neuroscience
• Biochemistry
• Cell Biology

Background:

• Divalent cations, particularly calcium (Ca2+), play critical roles in neuronal function.
• Understanding the buffering mechanisms of these ions within nerve cells is essential for elucidating cellular signaling pathways.
• Arsenazo III is a metallochromic indicator sensitive to divalent cations.

Purpose of the Study:

• To quantify and compare the buffering capacities of different divalent cations (Ca2+, Sr2+, Ba2+) in nerve cell bodies.
• To investigate the spatial distribution of buffering mechanisms within the nerve soma.
• To assess the binding affinity of the Ca2+ regulatory system for various divalent cations.

Main Methods:

• Utilized absorbance changes of arsenazo III to measure Ca2+, Sr2+, and Ba2+ concentrations.
• Employed nerve cell bodies injected with arsenazo III under membrane potential control.
• Assessed buffering rates by measuring the falling phase of the absorbance signal after ion influx or injection.
• Compared signal amplitudes following central versus peripheral ion injections.

Main Results:

• The buffering rate followed the sequence: Ca2+ > Sr2+ >> Ba2+.
• Ca2+ and Sr2+ injections produced larger arsenazo III signals when injected centrally compared to peripherally.
• This spatial difference in signal amplitude was not observed for Ba2+ or Mg2+ injections.
• The dye-absorbance signal's falling rate indicated Ca2+ buffering was most effective.

Conclusions:

• The neuronal Ca2+ regulatory system exhibits the strongest binding affinity for Ca2+ compared to other tested divalent cations.
• Buffering machinery is heterogeneously distributed within the nerve soma, with higher capacity near the plasma membrane.
• These findings highlight the specific and localized nature of calcium buffering in nerve cells.