Extracellular hemin is a reverse use-dependent gating modifier of cardiac voltage-gated Na channels.

Heme (Fe-protoporphyrin IX) is a well-known protein prosthetic group; however, heme and hemin (Fe-protoporphyrin IX) are also increasingly viewed as signaling molecules. Among the signaling targets are numerous ion channels, with intracellular-facing heme-binding sites modulated by heme and hemin in the sub-µM range. Much less is known about extracellular hemin, which is expected to be more abundant, in particular after hemolytic insults. Here we show that the human cardiac voltage-gated sodium channel hNa1.5 is potently inhibited by extracellular hemin (  ≈ 80 nM), while heme, dimethylhemin, and protoporphyrin IX are ineffective. Hemin is selective for hNa1.5 channels: hNa1.2, hNa1.4, hNa1.7, and hNa1.8 are insensitive to 1 µM hemin. Using domain chimeras of hNa1.5 and rat rNa1.2, domain II was identified as the critical determinant. Mutation N803G in the domain II S3/S4 linker largely diminished the impact of hemin on the cardiac channel. This profile is reminiscent of the interaction of some peptide voltage-sensor toxins with Na channels. In line with a mechanism of select gating modifiers, the impact of hemin on Na1.5 channels is reversely use dependent, compatible with an interaction of hemin and the voltage sensor of domain II. Extracellular hemin thus has potential to modulate the cardiac function.