Functional consequences of lidocaine binding to slow-inactivated sodium channels.
Na channels open upon depolarization but then enter inactivated states from which they cannot readily reopen. After brief depolarizations, native channels enter a fast-inactivated 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 (alpha) subunits from skeletal muscle or brain Na channels display anomalous gating behavior when expressed in Xenopus oocytes, with a high percentage entering slow-inactivated states after brief depolarizations. This enhanced slow inactivation is eliminated by coexpressing the alpha subunit with the subsidiary beta 1 subunit. We compared the lidocaine sensitivity of alpha subunits expressed in the presence and absence of the beta 1 subunit to determine the relative contributions of fast-inactivated and slow-inactivated channel block. Coexpression of beta 1 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 (alpha + beta 1) channels were mostly unblocked within 1 s of repolarization; however, slow-inactivated (alpha 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 microM, versus 24 microM 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.
Balser
J R
JR
Department of Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland 21205, USA. jrbalser@welchlink.welch.jhu.edu
Nuss
H B
HB
Romashko
D N
DN
Marban
E
E
Tomaselli
G F
GF
D000779:Anesthetics, Local / Q000378:metabolism* / Q000494:pharmacology*; D000818:Animals; D001921:Brain / Q000166:cytology / Q000187:drug effects; D001923:Brain Chemistry / Q000187:drug effects / Q000502:physiology; D005260:Female; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008012:Lidocaine / Q000378:metabolism* / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D051381:Rats; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism*; D014982:Xenopus laevis
D000779:Anesthetics, Local; D015222:Sodium Channels; D008012:Lidocaine