Direct inhibition of cardiac hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by clonidine.
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.
Knaus
Anne
A
Department of Pharmacology and Toxicology, University of Würzburg, Würzburg, Germany.
Zong
Xiangang
X
Beetz
Nadine
N
Jahns
Roland
R
Lohse
Martin J
MJ
Biel
Martin
M
Hein
Lutz
L
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*
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