Figure 6. Effects of extracellular Ca²⁺ removal.

A: Near-simultaneous FluoZin-3 (top) and Fura-6F (bottom) records from co-loaded neurons. Extracellular Ca²⁺ was washed out for 10 min before exposure to NMDA in the nominally Ca²⁺-free solution. Subsequently 2mM Ca²⁺ was introduced 15 min into the NMDA exposure. In the nominal absence of extracellular Ca²⁺, initial FluoZin-3 increases were reduced but not abolished (compare with Figure 4). Reintroduction of Ca²⁺ produced prompt and sustained FluoZin-3 fluorescence increases. The lower panel of A shows that Ca²⁺ removal virtually abolished Fura-6F responses, until sustained increases were observed after reintroduction of Ca²⁺. B: As in control conditions, late FluoZin-3 loss was coincident with arrival of sustained Ca²⁺ increases in somata. The relationship between the time at which 50% FluoZin-3 fluorescence intensity (compared with plateau levels prior to arrival of Ca²⁺ overload) is lost is plotted against the time of half-maximal Ca²⁺ increase as the sustained Ca²⁺ response invaded somata for the 6 neurons shown in A.