Figure 2.

The biophysical and pharmacological properties of the MVIIC-insensitive current. (A) Peak-current-voltage (I–V) relationship before (Ctrl) and after MVIIC (8 μM) application (Left, n = 9). Sample Ca²⁺ current recordings elicited at −30 mV before and after MVIIC application (Right). The holding potential was −80 mV with Ca²⁺ as the charge carrier (also in B and C). (B) Time course of the MVIIC block monitored by Ca²⁺ current evoked by an action potential waveform command. (Inset) Sample Ca²⁺ currents before (a) and after (b) MVIIC application. The dashed line is trace b scaled to trace a. (C) I–V relationship of the MVIIC-insensitive Ca²⁺ current before (ctrl) and after application of NiCl₂ (100 μM, n = 4; 1 mM, n = 3) or CdCl₂ (50 μM, n = 4). (Right) Sample currents elicited at −20 mV before (ctrl) and after application of 100 μM NiCl₂. The experiments were performed in the presence of MVIIC (same for D and E). (D) Time course of inactivation of the MVIIC-insensitive Ba²⁺ current. The current was elicited by a 200-ms pulse to +10 mV with 2 mM Ba²⁺ as the charge carrier (black trace). The inactivation time course was fit with a single exponential curve with a time constant of 43 ms (grayish white trace). (E) Voltage-dependence of inactivation of the MVIIC-insensitive Ba²⁺ (n = 6) and Ca²⁺ (n = 4) currents (Left). The current (I) was elicited from various holding potentials (V_h = −120 to −10 mV) to 0 mV, and was normalized with respect to the peak current elicited from V_h = −120 mV. (Right) An example with Ba²⁺ as the charge carrier. The interval between each test pulse was 30 sec. The grouped data (Left) was fit with a sum of two Boltzmann expressions: I = C₁/{1 + exp[(V_h − V_half1)/k₁]} + (1 − C₁)/{1 + exp[(V_h − V_half2)/k₂]}. When Ba²⁺ was the charge carrier, C₁ = 0.56, V_half1 = −82 mV, k₁ = 7 mV, V_half2 = −46 mV and k₂ =6 mV. For Ca²⁺ as the charge carrier, C₁ = 0.66, V_half1 = −67 mV, k₁ = 9 mV, V_half2 = −31 mV and k₂ = 5 mV.