An inactivation stabilizer of the Na⁺ channel acts as an opportunistic pore blocker modulated by external Na⁺

The Na⁺ channel is the primary target of anticonvulsants carbamazepine, phenytoin, and lamotrigine. These drugs modify Na⁺ channel gating as they have much higher binding affinity to the inactivated state than to the resting state of the channel. It has been proposed that these drugs bind to the Na⁺ channel pore with a common diphenyl structural motif. Diclofenac is a widely prescribed anti-inflammatory agent that has a similar diphenyl motif in its structure. In this study, we found that diclofenac modifies Na⁺ channel gating in a way similar to the foregoing anticonvulsants. The dissociation constants of diclofenac binding to the resting, activated, and inactivated Na⁺ channels are ∼880 μM, ∼88 μM, and ∼7 μM, respectively. The changing affinity well depicts the gradual shaping of a use-dependent receptor along the gating process. Most interestingly, diclofenac does not show the pore-blocking effect of carbamazepine on the Na⁺ channel when the external solution contains 150 mM Na⁺, but is turned into an effective Na⁺ channel pore blocker if the extracellular solution contains no Na⁺. In contrast, internal Na⁺ has only negligible effect on the functional consequences of diclofenac binding. Diclofenac thus acts as an "opportunistic" pore blocker modulated by external but not internal Na⁺, indicating that the diclofenac binding site is located at the junction of a widened part and an acutely narrowed part of the ion conduction pathway, and faces the extracellular rather than the intracellular solution. The diclofenac binding site thus is most likely located at the external pore mouth, and undergoes delicate conformational changes modulated by external Na ⁺ along the gating process of the Na⁺ channel.