Sympathetic neurons possess the ability to release Ca²⁺ from intracellular stores in response to prolonged membrane depolarization in the absence of extracellular Ca²⁺.

<p><b>A</b>: Time course of cytosolic fluo-4 ΔF/F₀ in a sympathetic neuron in response to a 30-s exposure to 80 mM [K⁺]_e in 2 mM [Ca²⁺]_e and a 30-s exposure to 80 mM [K⁺]_e in Ca²⁺-free solution, followed by a second 30-s application of 80 mM [K⁺]_e in 2 mM [Ca²⁺]_e applied 70 seconds after the Ca²⁺ transient was elicited in Ca²⁺- free solution. <b>B</b>: Bar graphs of peak magnitude of the cytosolic ΔF/F₀ transients (left panel) and their rise and decay times during exposure to 80 mM [K⁺]_e in the presence and absence of extracellular Ca²⁺ (with 200 μM EGTA added). Rise times and decay times were measured as the intervals from 10% to 90% and from 90% to 10%, respectively, of peak ΔF/F₀. Values are mean ± SEM from 48 to 87 cells. *<i>P</i> < 0.0001 versus 2 mM [Ca²⁺]_e, paired <i>t</i>-test. <b>C</b>: Plots of sympathetic neuron membrane potential as a function of [K⁺]_e. Values are mean ± SEM (n = 5–8 experiments for each [K⁺]_e studied). Lines are linear fits with slopes of 67.5 mV (2 mM [Ca²⁺]_e) and 62.2 mV (0 mM [Ca²⁺]_e) per 10-fold change in [K⁺]_e. <b>D</b> and <b>E</b>: Peak magnitude of cytosolic ΔF/F₀ transients as a function of membrane potential. Shown are the relationships between membrane potential and peak ΔF/F₀ amplitude in response to 30-s exposures to 40, 60, 80 or 100 mM [K⁺]_e in 2 mM [Ca²⁺]_e (<b>D</b>) or in the absence of extracellular Ca²⁺ (<b>E</b>). Values are mean ± SEM (n = 18–87 cells for each [K⁺]_e). Solid lines represent best fits of the data to a linear function (D) and a polynomial function (E). Dashed lines denote membrane potentials generated by each [K⁺]_e. <b>B</b>: * <i>P</i> < 0.001 versus 40 mM [K⁺]_e; ** <i>P</i> < 0.03 versus 60 mM [K⁺]_e. <b>C</b>: * <i>P</i> < 0.05 versus 40 mM [K⁺]_e; repeated measures ANOVA and Dunn’s method for multiple comparisons.