Cytosolic Ca²⁺ changes induced by long-lasting depolarizations of increasing amplitudes. (A) Cytosolic Ca²⁺ changes monitored with Fura-2 (upper traces) in response to 50-s depolarizing pulses of increasing amplitudes from a holding potential of −80 mV (lower traces) in the same fiber. Voltage pulses were given every 2 min, and the fluorescence images were collected at a frequency of 1 Hz. (B) Relationships between the mean maximal changes in Fura-2 fluorescence and membrane potential (○) and between mean values of pulse end decays of Fura-2 signal (measured between the right arrowhead and the dotted line in the example trace) normalized to maximal Fura-2 fluorescence change (measured between the dotted line and the left arrowhead) and membrane potential (n = 8) (•). The solid line relationship was fitted with a Boltzmann equation with V_1/2 of −15 mV and a slope factor of 12 mV, excluding −40- and −30-mV data points. The dotted line curve linking −50-, −40-, and −30-mV data points was drawn by eye.