title: Direct inhibition of cardiac hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by clonidine.
abstract: BACKGROUND
  Inhibition of cardiac sympathetic tone represents an important strategy for treatment of cardiovascular disease, including arrhythmia, coronary heart disease, and chronic heart failure. Activation of presynaptic alpha2-adrenoceptors is the most widely accepted mechanism of action of the antisympathetic drug clonidine; however, other target proteins have been postulated to contribute to the in vivo actions of clonidine.
  
  
  RESULTS
  To test whether clonidine elicits pharmacological effects independent of alpha2-adrenoceptors, we have generated mice with a targeted deletion of all 3 alpha2-adrenoceptor subtypes (alpha2ABC-/-). Alpha2ABC-/- mice were completely unresponsive to the analgesic and hypnotic effects of clonidine; however, clonidine significantly lowered heart rate in alpha2ABC-/- mice by up to 150 bpm. Clonidine-induced bradycardia in conscious alpha2ABC-/- mice was 32.3% (10 microg/kg) and 26.6% (100 microg/kg) of the effect in wild-type mice. A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from alpha2ABC-knockout and wild-type mice. Clonidine inhibited the native pacemaker current (I(f)) in isolated sinoatrial node pacemaker cells and the I(f)-generating hyperpolarization-activated cyclic nucleotide-gated (HCN) 2 and HCN4 channels in transfected HEK293 cells. As a consequence of blocking I(f), clonidine reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild-type and alpha2ABC-knockout mice.
  
  
  CONCLUSIONS
  Direct inhibition of cardiac HCN pacemaker channels contributes to the bradycardic effects of clonidine gene-targeted mice in vivo, and thus, clonidine-like drugs represent novel structures for future HCN channel inhibitors.
authors.affiliation: Department of Pharmacology and Toxicology, University of Würzburg, Würzburg, Germany.
authors.forename: Anne
authors.initials: A
authors.lastname: Knaus
authors.forename: Xiangang
authors.initials: X
authors.lastname: Zong
authors.forename: Nadine
authors.initials: N
authors.lastname: Beetz
authors.forename: Roland
authors.initials: R
authors.lastname: Jahns
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Lohse
authors.forename: Martin
authors.initials: M
authors.lastname: Biel
authors.forename: Lutz
authors.initials: L
authors.lastname: Hein
mesh_terms: D000818:Animals; D001919:Bradycardia / Q000139:chemically induced; D002317:Cardiovascular Agents / Q000494:pharmacology*; D002460:Cell Line; D003000:Clonidine / Q000494:pharmacology*; D054815:Cyclic Nucleotide-Gated Cation Channels; D005260:Female; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels / Q000037:antagonists & inhibitors* / Q000187:drug effects*; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D009124:Muscle Proteins / Q000037:antagonists & inhibitors / Q000187:drug effects*; D015221:Potassium Channels; D013564:Sympathetic Nervous System / Q000187:drug effects*
chemical_list: D002317:Cardiovascular Agents; D054815:Cyclic Nucleotide-Gated Cation Channels; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D003000:Clonidine