Kinetics of hyperpolarization-induced CNGA1 block by extracellular PhTx. (A) Current transient elicited by stepping membrane voltage from 100 to −150 mV in the continued presence of 0.1 µM of extracellular PhTx. The superimposed curve is a single-exponential fit. (B) Reciprocal of the time constant (mean ± SEM; n = 4–6) for channel block (1/τon), obtained from fits as shown in A, is plotted against the extracellular concentration of PhTx for eight voltages. The (unresolved) lines through the data are linear fits whose slope represents the apparent second-order rate constant (kon) for blocker binding. (C) Natural logarithm of kon from B is plotted against membrane voltage. The plot is fitted with the equation ln kon = ln kon (0 mV) − zonVF/RT, yielding kon (0 mV) = 1.04 ± 0.06 × 109 M−1s−1 (open circle) and zon = 0.01 ± 0.01.
