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

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Abstract
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).