Intracellular Mg2+ is a voltage-dependent pore blocker of HCN channels

Sriharsha Vemana, Shilpi Pandey, H. Peter Larsson

Research output: Contribution to journalArticlepeer-review

17 Scopus citations

Abstract

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are activated by membrane hyperpolarization that creates time-dependent, inward rectifying currents, gated by the movement of the intrinsic voltage sensor S4. However, inward rectification of the HCN currents is not only observed in the time-dependent HCN currents, but also in the instantaneous HCN tail currents. Inward rectification can also be seen in mutant HCN channels that have mainly time-independent currents (5). In the present study, we show that intracellular Mg2+ functions as a voltage-dependent blocker of HCN channels, acting to reduce the outward currents. The affinity of HCN channels for Mg 2+ is in the physiological range, with Mg2+ binding with an IC50 of 0.53 mM in HCN2 channels and 0.82 mM in HCN1 channels at +50 mV. The effective electrical distance for the Mg2+ binding site was found to be 0.19 for HCN1 channels, suggesting that the binding site is in the pore. Removing a cysteine in the selectivity filter of HCN1 channels reduced the affinity for Mg2+, suggesting that this residue forms part of the binding site deep within the pore. Our results suggest that Mg2+ acts as a voltage-dependent pore blocker and, therefore, reduces outward currents through HCN channels. The pore-blocking action of Mg2+ may play an important physiological role, especially for the slowly gating HCN2 and HCN4 channels. Mg2+ could potentially block outward hyperpolarizing HCN currents at the plateau of action potentials, thus preventing a premature termination of the action potential.

Original languageEnglish (US)
Pages (from-to)C557-C565
JournalAmerican Journal of Physiology - Cell Physiology
Volume295
Issue number2
DOIs
StatePublished - Aug 2008

Keywords

  • Cysteine
  • Divalent block
  • Hyperpolarization-activated current
  • Hyperpolarization-activated cyclic nucleotide
  • Instantaneous currents
  • Inward rectifying

ASJC Scopus subject areas

  • Cell Biology
  • Physiology

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