Functional consequences of lidocaine binding to slow-inactivated sodium channels.

Abstract

Na channels open upon depolarization but then enter inactivated states from which they cannot readily reopen. After brief depolarizations, native channels enter a fastinactivated state from which recovery at hyperpolarized potentials is rapid (<20 ms). Prolonged depolarization induces a slow-inactivated state that requires much longer periods for recovery (> 1 s). The slow-inactivated state therefore assumes particular importance in pathological conditions, such as ischemia, in which tissues are depolarized for prolonged periods. While use-dependent block of Na channels by local anesthetics has been explained on the basis of delayed recovery of fast-inactivated Na channels, the potential contribution of slow-inactivated channels has been ignored. The principal (tx) subunits from skeletal muscle or brain Na channels display anomalous gating behavior when expressed in Xen0pus oocytes, with a high percentage entering slow-inactivated states after brief depolarizations. This enhanced slow inactivation is eliminated by coexpressing the r subunit with the subsidiary [31 subunit. We compared the lidocaine sensitivity ofct subunits expressed in the presence and absence of the [~1 subunit to determine the relative contributions of fast-inactivated and slow-inactivated channel block. Coexpression of ~31 inhibited the use-dependent accumulation of lidocaine block during repetitive (1-Hz) depolarizations from -100 to -20 mV. Therefore, the time required for recovery from inactivated channel block was measured at -100 mV. Fast-inactivated (ct + [31) channels were mostly unblocked within 1 s of repolarization; however, slowinactivated (et alone) channels remained blocked for much longer repriming intervals (>5 s). The affinity of the slow-inactivated state for lidocaine was estimated to be 15-25 izM, versus 24 izM for the fast-inactivated state. We conclude that slow-inactivated Na channels are blocked by lidocaine with an affinity comparable to that of fast-inactivated channels. A prominent functional consequence is potentiation of use-dependent block through a delay in repriming of lidocaine-bound slow-inactivated channels. Key words: sodium channel 9 slow inactivation 9 lidocaine 9 Xenopus oocytes 9 [31 subunit INTRODUCTION

Structural requirements for voltage‐dependent block of muscle sodium channels by phenol derivatives

“…Prolonged depolarization induces a slow-inactivated state that requires much longer periods for recovery (41 s), and thus, assumes particular importance in pathological conditions, such as ischaemia, in which tissues are depolarized for longer periods (Balser et al, 1996b). To examine drug eects in the presence of a de®nite fraction of slow-inactivated channels, we stepped up the holding potential from 7100 mV to 735 mV for 2.5 s. The membrane potential was then returned to 7100 mV for 10 ms, allowing recovery from fast inactivation.…”

“…In analogy to local anaesthetics and antidysrhythmic drugs such as lidocaine (Balser et al, 1996b;Pugsley & Goldin, 1998;Scheuer, 1999), the blocking potency of all phenol derivatives strongly depends on the kinetic state of the channel, reducing the IC 50 values more than 3 fold when membrane depolarization before the test pulse induces either fast or slow channel Figure 5 Interference of phenol derivatives with fast inactivated channels assessed by shifts in the steady-state availability curve. (a ± d) Steadystate availability curves assessed by a two-pulse protocol in control conditions (circles) and in the presence of representative concentrations of each compound (triangles and squares).…”

“…Although there seems to be a common underlying mechanism for this eect, the acceleration of the current decay in normally gating sodium channels induced by all phenol derivatives was only discernible in the case of lidocaine in mutated channels, in which fast inactivation was disabled and channel closure occurred on a time-scale of about 100 ms (Balser et al, 1996b). These results suggest that, in contrast to lidocaine, the kinetic of drug binding in the case of all phenol derivatives is fast enough to interfere with normal channel inactivation, which occurs on a timescale of 1 ± 2 ms.…”

Interaction of phenol derivatives with ion channels

“…Lidocaineinduced sodium channel blockade is characterized by a higher affinity of the drug for inactivated channels compared with the resting state, and by prolonged recovery from inactivation, introducing a second, slow component representing drug dissociation from inactivated channels [18,[54][55][56][57]. This prolonged recovery from inactivation, once lidocaine is bound, accounts for the consecutive decrease in sodium current relative to the first pulse during repetitive stimulation when the interpulse interval becomes too short to allow recovery from inactivated channel block.…”

“…Voltage-dependent block by all phenolic compounds retains a characteristic set of features that describes local anaesthetic block [18,54,55,57,58], but differs in the kinetics of drug binding and unbinding.…”