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Lidocaine inhibition of cloned hyperpolarization-activated cucline nucleotide-modulated (HCN) channels Yip, Raymond Hon Leung

Abstract

Hyperpolarization-activated Cyclic Nucleotide-modulated (HCN) channels underlie the hyperpolarization-activated or "funny" current (If or Ih), which is important in regulating excitability in the neurons of the central and peripheral nervous Systems and in the conduction tissue of the heart. There are four mammalian isoforms of HCN channels (HCN1-4), each exhibiting different activation and deactivation kinetics. Lidocaine, a local anesthetic and antiarrhythmic drug, has been shown to inhibit HCN-mediated currents in the rabbit sinoatrial (SA) node, which expresses various HCN isoforms. It also inhibits Ih in rat small DRG neurons. Lidocaine has been shown to block Na⁺ channels by interacting with the aromatic side chain of a phenylalanine residue in the region lining the channel pore. With the exception of a recent study on rabbit HCN4, this is the first study examining lidocaine inhibition on cloned HCN channels. I hypothesize that lidocaine, in micromolar concentrations, inhibits HCN1 channels from the intracellular side by interacting with amino acid residues lining the channel pore. Lidocaine, at concentrations ranging from 50 to 3200 μM, inhibited mouse HCN1 channels expressed in Chinese hamster ovary (CHO) cells with an IC₅₀ of 669 ± 320 μM. The onset of lidocaine action is fast (~6 seconds) and concentration-dependent with maximum inhibition occurring at approximately 20 seconds, and inhibition is partially or completely reversible. The fact that currents does not return completely may be attributed to a phenomenon called current rundown, or to incomplete washout of lidocaine. Extracellular application of the permanently charged derivative of lidocaine, QX-314, did not inhibit the channel, which suggests that lidocaine inhibition, in its charged form, occurs from the intracellular side. A point mutation at the phenylalanine residue does not remove lidocaine inhibition, indicating that this residue is not responsible for interaction with lidocaine. These results support the hypothesis that lidocaine inhibits HCNl from the intracellular side, with molecular mechanisms of inhibition yet to be determined.

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