Verapamil block of the delayed rectifier K current in chick embryo dorsal root ganglion neurons

We have used the patch-clamp method in the whole-cell configuration to investigate the mechanism of block of the delayed rectifier K current (I-DRK) by verapamil in embryonic chick dorsal root ganglion (DRG) neurons. Verapamil induced a dose-dependent decay of the current, without altering its activation kinetics. This observation, together with the good description of I-DRK time course at various blocker concentrations with the computer simulation of a three-state chain model (closed left right arrow open left right arrow open-blocked), indicates that verapamil acts as a state-dependent, open-channel blocker. To account for the double-exponential time course of recovery from block, this minimal kinetics scheme was expanded to include a closed-blocked state resulting from channel closure (at hyperpolarized voltages) with verapamil still bound to it. The apparent block and unblock rate constants assessed from verapamil-induced current decay in the presence of external Na were 0.95 plusminus 0.05 ms(-1)mM(-1) and 0.0037 plusminus 0.0016 ms(-1), respectively. When external Na was replaced by K, only the unblock rate constant changed, to 0.02 plusminus 0.009 ms(-1). Under these ionic conditions it was also observed that the recovery from block was modified from the double-exponential time course in the presence of external Na (tau(1) similar to or equal to 160 ms; tau(2) similar to or equal to 1600 ms), to a faster single-exponential recovery (tau similar to or equal to 100 ms). We tested the voltage dependence of block by applying stimulation protocols aimed at eliminating bias easily introduced by the shift of the gating equilibrium and by the coupling of channel activation and block. Under these experimental conditions the resulting block rate constant was not measurably voltage dependent.