Figure 3.

Increases in [Ca²⁺]_i are dependent on the extracellular Ca²⁺ concentration and triggered by Ca²⁺ influx. Data from one Vicia guard cell in 5 mM K⁺-Mes, pH 6.1, and KCl (= 10 mM K⁺), with 0.02–20 mM CaCl₂ (A–C) and from a second cell with 2 mM MnCl₂ instead of CaCl₂ (D). (A) [Ca²⁺]_i record during exposures to 0.02, 0.2, and 2 mM CaCl₂ (bars above). The cell was clamped to −50 mV throughout with 4-s steps to −200 mV at times indicated (⊔). Time scale, 2 min. (B) Mean rate of rise in [Ca²⁺]_i (d[Ca²⁺]_i/dt) during voltage steps plotted as a function of extracellular Ca²⁺ concentration shows roughly a 10-fold increase with each decade change in [Ca²⁺]_o. (C) [Ca²⁺]_i rise evoked on stepping to −200 mV continues to increase after returning to −50 mV. Data from the A in 2 mM [Ca²⁺]_o replotted on an expanded time scale. Period of clamp step to −200 mV indicated by vertical hairlines. Time scale, 10 s. (D) Fura2 fluorescence (Upper) recorded on excitation with 340 nm (f₃₄₀, fine line), 360 nm (f₃₆₀, heavy line), and 390 nm (f₃₉₀, dotted line) light, and the corresponding [Ca²⁺]_i signal (Lower). Time scale, 20 s. Note the approximate 20% quench of fluorescence excited at all three wavelengths during the 20-s step to −200 mV (⊔, Upper) and the absence of any change in [Ca²⁺]_i. (Inset) Fura2 fluorescence excited at 340, 360, and 390 nm during the first 20-s voltage step (⊔) in Fig. 1B for comparison.