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 α2-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.Methods and Results—
To test whether clonidine elicits pharmacological effects independent of α2-adrenoceptors, we have generated mice with a targeted deletion of all 3 α2-adrenoceptor subtypes (α2ABC−/−). α2ABC−/− mice were completely unresponsive to the analgesic and hypnotic effects of clonidine; however, clonidine significantly lowered heart rate in α2ABC−/− mice by up to 150 bpm. Clonidine-induced bradycardia in conscious α2ABC−/− mice was 32.3% (10 μg/kg) and 26.6% (100 μg/kg) of the effect in wild-type mice. A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from α2ABC-knockout and wild-type mice. Clonidine inhibited the native pacemaker current (If) in isolated sinoatrial node pacemaker cells and the If-generating hyperpolarization-activated cyclic nucleotide–gated (HCN) 2 and HCN4 channels in transfected HEK293 cells. As a consequence of blocking If, clonidine reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild-type and α2ABC-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.