HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) All Versions of this Article: 295/2/C557    most recent 00154.2008v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Vemana, S. Articles by Larsson, H. P. Search for Related Content PubMed PubMed Citation Articles by Vemana, S. Articles by Larsson, H. P.

MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

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

Neurological Sciences Institute, Oregon Health and Science University, Beaverton, Oregon

Submitted 15 March 2008 ; accepted in final form 22 June 2008

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 Mg²⁺ 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.

inward rectifying; hyperpolarization-activated current; instantaneous currents; divalent block; cysteine; hyperpolarization-activated cyclic nucleotide