Figure 4:
Reverse use dependence of NaV1.5 current inhibition.
(A) Top, pulse protocol for measuring the use dependence of hemin action; holding potential was −120 mV. Repetitive depolarizations, 5 ms each in duration, from −120 to −10 mV were applied. The gaps indicate waiting intervals at −120 mV to yield various pulse frequencies, in this case 20 Hz. Center and bottom, whole-cell current responses of NaV1.5 channels before (black) and after application of 1 µM hemin (red). (B) Mean relative change in peak current amplitude with hemin with respect to the corresponding pulse under control conditions as a function of the pulse number for the indicated pulse frequencies. Straight lines connect the data points for clarity. (C) Pulse protocol and superimposed current traces in the presence of hemin. Between the two test pulses to −10 mV to measure current, 50 ms depolarizing pulses of variable voltage followed by 40 ms at −140 mV were inserted (blue). (D) Mean relative peak current of pulse 2 over pulse 1 as a function of the prepulse voltage. Superimposed fit: Boltzmann function with the indicated parameters. Open symbols denote control values in the absence of hemin, connected by straight dashed lines. (E) Pulse protocol and superimposed current trace in the presence of hemin. Between the two test pulses, a depolarization to −10 mV of variable duration was inserted, followed by 40 ms at −140 mV (blue). (F) Mean relative peak current of pulse 2 over pulse 1 as a function of the prepulse duration. Superimposed fit: Exponential, yielding the indicated time constant. (G) Pulse protocol and superimposed current trace in the presence of hemin. Between the first and the second pulse a 50 ms depolarization to 40 mV was inserted (blue), between test pulses 2 and 3 the duration at −140 mV was varied (blue mark). (H) Mean relative peak current of pulse 3 over pulse 2 as a function of the recovery duration. Superimposed fit: Exponential, yielding the indicated time constant. Data in (B), (D), (F), (H) are mean ± sem with n in parentheses.