Fig. 4.

Rates of slow (A) and fast modes (B) of flecainide block associated with the illustrated reaction schemes. Block was induced by 50 μM flecainide in the presence of various cytoplasmic concentrations of caffeine, Ca²⁺, and Mg²⁺. For the fast-mode, on rate, R_Fon = 1/τ_o and R_Foff = 1/τ_F. For the slow-mode, on rate, R_Son equals the inverse of the time between long closures (defined here as closures longer than 200 ms) and R_Soff equals the inverse of τ_S. Note that slow-mode inhibition was not resolved in 0.1 μM Ca²⁺ alone. Asterisks indicate significant differences from 0.1 μM Ca²⁺ plus caffeine in (A) and 0.1 μM Ca²⁺ alone in (B) (*P < 0.05; **P < 0.01).