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

Summary

• 1. Absorbance changes associated with divalent cation binding to arsenazo III were used to measure changes in Ca²⁺, Sr²⁺, and Ba²⁺ concentrations under a variety of experimental conditions.

• 2. The rate of the falling phase of an absorbance change signal, measured in nerve cell bodies injected with arsenazo III and under membrane potential control, was taken as an index of divalent cation buffering. With influx of ions through the membrane or with ionophoretic injection, we found the buffering, i.e., the dye-absorbance signal's falling rate, to be greatest for Ca²⁺ ions: the sequence was Ca²⁺ > Sr²⁺ ≫ Ba²⁺.

• 3. Injecting Ca²⁺ or Sr²⁺ into the center of a nerve cell produced a significantly greater amplitude of arsenazo III signal than the same injection near the cell membrane. We did not find this to be the case for Ba²⁺ or Mg²⁺ injections.

• 4. We conclude that the Ca²⁺ regulatory system binds Ca²⁺ most strongly compared to the other ions tested, and there is a variable distribution of buffering machinery within the nerve soma, with increased buffer capacity near the plasma membrane of the cell. A preliminary report of some of the results presented in this paper has appeared previously (Tillotson and Gorman, 1980).