Fig 1. High [K⁺]_e-induced intracellular Ca²⁺ transients in the presence and absence of extracellular Ca²⁺.

A and B: x-y fluorescence images recorded in 24 h cultured sympathetic ganglion neurons loaded with fluo-4/AM before, during and following 30-s depolarizations with 80 mM [K⁺]_e in 2 mM [Ca²⁺]_e (A) and in the absence of extracellular Ca²⁺ with 200 μM EGTA added to the extracellular solution (B). Exposure to elevated [K⁺]_e evoked transient increases in both cyto- and nucleoplasmic Ca²⁺ concentrations. Images in panels 2 and 3 were acquired at 10 and 30 s after initiating the [K⁺]_e challenge. C: High [K⁺]_e depolarizes the membrane potential both in the presence and absence of external Ca²⁺: 80 mM [K⁺]_e rapidly depolarized V_m from a resting membrane potential of -44 mV to 1 mV in 2 mM [Ca²⁺]_e, and from -41 mV to 0 mV in the absence of extracellular Ca²⁺. The level of depolarization was sustained for the duration of the [K⁺]_e challenge, both in the presence and absence of external Ca²⁺. D: Time courses of fractional fluorescence (ΔF/F₀), i.e., [Ca²⁺]_i, in the cytoplasm during exposure to 80 mM [K⁺]_e in the presence and absence of external Ca²⁺. ΔF/F₀ was calculated from the fluorescence intensity measured in the whole cytoplasm for each image taken every second.