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 Na1.7 and Na1.5. 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 Na1.7 in HEK and CHO cells with an IC value of 72 and 57 μM and Na1.5 in CHO cells with an IC 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 Na1.5 and Na1.7 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.