CALCIUM‐DEPENDENT RELEASE OF EXOGENOUSLY LOADED γ‐AMINO‐[U‐¹⁴C]BUTYRATE FROM SYNAPTOSOMES: TIME COURSE OF STIMULATION BY POTASSIUM, VERATRIDINE, AND THE CALCIUM IONOPHORE, A23187

Abstract

Abstract —Synaptosomes which had taken up [ 14 C]GABA were applied to a filter and rapidly perfused with various solutions in order to study the time course of release of this putative transmitter and the characteristics of its release. Depolarization of the synaptosomes with veratridine or 56mM‐K + or pretreatment with the Ca 2+ ionophore, A23187, increased the calcium‐dependent efflux of [ 14 C]GABA. Release of [ 14 C]GABA was increased by Ca 2+ within 0.3 s of exposure, and the maximal release rate was not maintained for longer than 0.6 s. The reduction in the rate of release was not attributable to a decrease in calcium influx, but rather appeared to reflect fatigue at some subsequent stage in release. Stimulation by 56mM‐K + also elicited a calcium‐independent increase in the efflux of radioactive GABA, which appeared to arise in part from subcellular particles other than synaptosomes.