Figure 4.

ABA activates Ca²⁺ channels. (A) Records from two patches, cell-attached (Left) and excised inside-out (Right) before (Top) and after (Bottom) adding 20 μM ABA to the bath. Traces in ABA at 55 s (cell-attached) and 20 s (excised patch) after additions. Membrane voltage clamped to −100 mV and with 30 mM Ba²⁺ in bath and pipette (cell-attached) or with 30 mM Ba²⁺ inside and 10 mM Ba²⁺ outside (excised patch). Scale: vertical, 2 pA; horizontal, 1 s. Point-amplitude histograms of channel openings for excised patch data plotted on a log scale (far right): events −ABA taken over 100 s; events +ABA taken over 5 s and scaled by a factor of 20 for comparison. (B) Current–voltage curves obtained by summing currents recorded from one excised, inside-out patch during 10-s voltage ramps between 0 and −200 mV. Baseline (leakage) current corrected by subtracting corresponding data (−ABA) with no opening events. Data are the sum of 40 ramps (−ABA) and 10 ramps (+ABA) taken immediately before and after adding ABA. Sum before ABA addition scaled by 0.25 for comparison, and both current-voltage curves fitted to a Boltzmann function of the form

1

where g_max is the relative conductance maximum, V the clamp voltage, E_Ba the Ba²⁺ equilibrium voltage (= −14 mV), δ the voltage sensitivity factor, V_1/2 the voltage giving half-maximal current activation, and z, F, R, and T have their usual meanings. Fitted parameters (+ABA): δ, 0.93 ± 0.05; V_1/2, −83 ± 2 mV. Fittings of currents summed before adding ABA gave V_1/2, −142 ± 48 mV if δ was constrained to a value of unity. Variation in P_o (see Fig. 5) accounts for the lower activation by ABA when summed over 50 s.