Quantitative measurements of the cytosolic Ca2+ activity within isolated guinea pig nerve-endings using entrapped arsenazo III and quin2.

The absorbance changes of intrasynaptosomally entrapped arsenazo III have been converted into values of free Ca2+ concentration by correcting for the nonlinear response of arsenazo III at different concentrations of the dye as well as for changes in internal pH. An average resting value for free Ca2+ concentration around 0.4 microM is obtained. Depolarization with veratridine or gramicidin increases this value to around 3 microM. Measurements of cytosolic free Ca2+ with the quin2 method gives much lower values in similar conditions. The release of prelabelled [14C]noradrenaline from the nerve-endings is maximally activated when the internal free Ca2+ concentration rises as measured with arsenazo III to about 4 microM when titrated with increasing concentrations of ionophore A23187.