Selective effects of clonidine on the canine sinus node

Area of Science:

• Cardiovascular Pharmacology
• Autonomic Nervous System Physiology

Background:

• Clonidine is known to affect blood pressure and heart rate.
• Its precise mechanism within the heart's sinus node requires further elucidation.

Purpose of the Study:

• To investigate the direct and indirect chronotropic effects of clonidine on the canine sinus node.
• To determine if clonidine's effects are mediated by altering sympathetic or vagal nerve activity.

Main Methods:

• Selective perfusion of the sinus node artery in pentobarbital-anesthetized dogs.
• Administration of varying doses of clonidine (0.001–10 µg/ml).
• Assessment of sinus rate changes and responses to sympathetic (stellate ganglion stimulation) and pharmacological (norepinephrine, atropine, vagal stimulation) challenges.

Main Results:

• Clonidine caused dose-related reductions in sinus rate.
• Intranodal clonidine significantly inhibited heart rate acceleration from sympathetic nerve stimulation.
• Clonidine did not affect the heart rate response to norepinephrine infusion or atropine.
• Effects on vagal stimulation response were inconsistent.

Conclusions:

• Clonidine directly slows sinus node rate and inhibits sympathetic-mediated heart rate increases.
• These effects suggest clonidine may inhibit norepinephrine release within the sinus node.
• The findings support a local mechanism of action for clonidine within the heart's primary pacemaker.