Figure 2. Intrasynaptosomal [Calcium²+] ([Ca²+]ⁱ) and glutamate release in response to depolarization of synaptosomes (1.34 mg protein) by increasing Potassium sup + from 5 to 35 mM. (A) The [Ca²+]ⁱchanges as determined by the Fura-2/AM fluorescence ratio method. In the presence of 1.3 mM Calcium²+, a large initial transient was observed on the increase in external [Potassium sup +]. The large initial transient subsequently decreased to a plateau that was typically half that of the initial transient. In the absence of added Calcium²+ (nominally Calcium²+ -free), no Calcium²+ transient is observed. (B) Nicotinamide adenine dinucleotide phosphate (NADPH) fluorescence response to added KCl in the presence of 1.3 mM Calcium²+ and in nominally Calcium²+ -free solution. The initial slope of the fluorescence response in 1.3 mM Calcium²+ gives an estimate of the initial glutamate release of 1.5 nmol, based on the equivalence of its slope to one fourth of the slope observed on addition of the internal standard of 6 nmol (done at 900 s and not shown in the tracing; see Figure 4, for example).