Compound-specific Na+ channel pore conformational changes induced by local anaesthetics.
Upon prolonged depolarizations, voltage-dependent Na+ channels open and subsequently inactivate, occupying fast and slow inactivated conformational states. Like C-type inactivation in K+ channels, slow inactivation is thought to be accompanied by rearrangement of the channel pore. Cysteine-labelling studies have shown that lidocaine, a local anaesthetic (LA) that elicits depolarization-dependent ('use-dependent') Na+ channel block, does not slow recovery from fast inactivation, but modulates the kinetics of slow inactivated states. While these observations suggest LA-induced stabilization of slow inactivation could be partly responsible for use dependence, a more stringent test would require that slow inactivation gating track the distinct use-dependent kinetic properties of diverse LA compounds, such as lidocaine and bupivacaine. For this purpose, we assayed the slow inactivation-dependent accessibility of cysteines engineered into domain III, P-segment (mu1: F1236C, K1237C) to sulfhydryl (MTSEA) modification using a high-speed solution exchange system. As expected, we found that bupivacaine, like lidocaine, protected cysteine residues from MTSEA modification in a depolarization-dependent manner. However, under pulse-train conditions where bupivacaine block of Na+ channels was extensive (due to ultra-slow recovery), but lidocaine block of Na+ channels was not, P-segment cysteines were protected from MTSEA modification. Here we show that conformational changes associated with slow inactivation track the vastly different rates of recovery from use-dependent block for bupivacaine and lidocaine. Our findings suggest that LA compounds may produce their kinetically distinct voltage-dependent behaviour by modulating slow inactivation gating to varying degrees.
Fukuda
Koji
K
Department of Anaesthesiology, Vanderbilt University, School of Medicine, Nashville, TN 37232-6602, USA.
Nakajima
Tadashi
T
Viswanathan
Prakash C
PC
Balser
Jeffrey R
JR
D000779:Anesthetics, Local / Q000494:pharmacology*; D000818:Animals; D006801:Humans; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D011487:Protein Conformation; D051381:Rats; D015222:Sodium Channels / Q000737:chemistry* / Q000502:physiology*
D000779:Anesthetics, Local; D015222:Sodium Channels