α-Adrenoreceptor blocker phentolamine inhibits voltage-gated sodium channels via the local anaesthetic binding site

Background and Purpose

Phentolamine is a non-selective α-adrenoreceptor antagonist used to reverse local anaesthesia, for example, during dental procedures when a vasoconstrictor is co-applied. Phentolamine-mediated vasodilation leads to faster clearance of injected drugs. Previous electrophysiological studies hypothesized that phentolamine acts as a modulator of voltage-gated sodium channels, which could conflict with its indication as local anaesthetic reversal agent.

Experimental Approach

We performed manual and high throughput patch-clamp recordings on HEK and CHO cells expressing Na_V1.7 and Na_V1.5. We investigated the effects of phentolamine on sodium channel biophysics and the additive impact of phentolamine on cells preconditioned with the local anaesthetic mexiletine. We used site-directed mutagenesis, homology modelling and drug docking to identify phentolamine's binding site. We compared the effect on sodium channels with other clinically established α-adrenoreceptor antagonists.

Key Results

Phentolamine inhibits Na_V1.7 in HEK and CHO cells with an IC₅₀ value of 72 and 57 μM and Na_V1.5 in CHO cells with an IC₅₀ of 27 μM. Phentolamine enhances the tonic block induced by the local anaesthetic mexiletine. Phentolamine binds to sodium channels at the local anaesthetic receptor site. The α-adrenoreceptor antagonists alfuzosin, urapidil and phenoxybenzamine show lower potency on Na_V1.5 and Na_V1.7 in patch-clamp recordings.

Conclusions and Implications

Phentolamine blocks voltage-gated sodium channels via the local anaesthetic receptor site. This may conflict with its current indication as an antidote for local anaesthetics. We propose alternative α-adrenoreceptor antagonists as possible candidates for local anaesthetic reversal because these are less potent inhibitors of both cardiac and neuronal voltage-gated sodium channels.