Figure 2. Inward currents in a single cultured HISM cell.

A, superimposed current traces elicited by depolarizing pulses to 0 mV from −100 mV in control (135 mm Na⁺, 2.5 mm Ca²⁺) and after replacement of Na⁺ by Cs⁺ (125 mm) with 10 mm CaCl₂ or BaCl₂ added to the external solution. B, I-V relationships for the peak inward current in control (bull) and high-Cs⁺ external solution with 10 mm Ca²⁺ (•) or 10 mm Ba²⁺ (▵) in the same cell. C, peak inward Na⁺ current was measured at +10 mV by applying voltage steps from −100 mV every 10 s upon cumulative application of ascending concentrations of TTX as shown by the horizontal lines on the logarithmic concentration scale. External solutions contained Na⁺ and Ca²⁺ but not Mg²⁺. D, examples of I_Na from the same experiment. The concentrations of TTX applied are indicated. E, concentration-effect curve for the experiment illustrated in C and D. Data points represent relative current (I_TTX/I_Control) fitted by the logistic function with an IC₅₀ value of 56 nm. I_TTX is the current in TTX.