Figure 5. Modification of I_Kx by External Ba²⁺.

Recordings were made from cells bathed in Ringer solution containing 5 mM Cs⁺ and 0.1 mM Cd²⁺.

(A) Whole-cell current in response to voltage steps from –30 mV before (left) and after (right) 5 mM Ba²⁺ was added to the Cs⁺- and Cd²⁺-containing Ringer solution (note that more depolarizing steps are shown after Ba²⁺ application).

(B) Time-dependent current in 5 mM Ba²⁺ had kinetics similar to those of the control I_Kx. The points are mean time constants, τ (±SD, n = 3–6), of single exponentials fitted to the time-dependent current component elicited by test steps from –30 mV in 5 mM Ba²⁺. The continuous smooth curve describes 1/(α + β) (derived as in Figure 3C) and was estimated for data from six experiments in 5 mM BaCl₂. The broken curve is the control repeated from Figure 3C.

(C) Activation curves, from tail currents measured at –30 mV, for I_Kx in 0 (closed circles), 1 (open circles), 5 (closed squares), and 25 (open squares) mM added Ba²⁺. The smooth curves are the Boltzmann equation fitted using a least-squares method. The midpoint of the activation curve in 5 mM Ba²⁺ was –6.9 ± 1.7 mV, with a slope factor of 11.2 ± 0.9 mV (means ± SD, n = 7).

(D) Concentration dependence of the Ba²⁺-induced shifts in the midpoint of the activation curve for I_Kx. The points are means (±SD) for cells in 0 (n = 7), 1 (n = 4), 5 (n = 7), or 25 (n = 3) mM added Ba²⁺.