Figure 2.

Inactivation of current through I_K,in is correlated with [Ca²⁺]_i elevation. Voltage steps of 20 s duration from −50 to −200 mV leading to limited (A) and profound (B) increases in [Ca²⁺]_i (bottom trace, note different scales) in two broad bean guard cells. The clamp current (top trace, note different scales) shows the characteristic time course for I_K,in activation (A) in the absence of an extensive rise in [Ca²⁺]_i, and activation followed by a decay in current amplitude (B) with the more pronounced rise in [Ca²⁺]_i. C, Summary of relative I_K,in inactivation as a function of the change in [Ca²⁺]_i (Δ[Ca²⁺]_i) evoked by 20-s voltage steps from −50 to −200 mV recorded in 52 independent experiments (solid points). Histograms show the means ± se of measurements binned in successive pools of 10 or 11 experiments. Inactivation of I_K,in was calculated from the ratio (I_max − I_final)/I_max, with I_max determined at maximum I_K,in amplitude and I_final taken as the final current amplitude without correction for instantaneous current. Δ[Ca²⁺]_i values were determined from the mean [Ca²⁺]_i recorded over periods of 1 s immediately before and at the end of the voltage steps. Note that the analysis does not account for measurements in which [Ca²⁺]_i was initially high, nor does it account for measurements in which clamp steps yielded little inward current. The distribution is therefore probably skewed to the right along the x axis, but nonetheless shows that current inactivation was associated with the rise in [Ca²⁺]_i. D, [Ca²⁺]_i elevation (Δ[Ca²⁺]_i) after 20-s steps to −200 mV is dependent on the resting [Ca²⁺]_i level. Data from C plotted as a function of [Ca²⁺]_i before voltage steps to −200 mV. Histograms show the means ± se of measurements binned in successive pools with [Ca²⁺]_i ≤ 80 nm, 80 nm < [Ca²⁺]_I ≤ 300 nm, and [Ca²⁺]_I > 300 nm at rest. Note the logarithmic abscissa. The decline in the mean Δ[Ca²⁺]_i from high starting [Ca²⁺]_i values is not consistent with saturation of the Fura-2 signal.