Insights into sinus arrhythmia of the dog: Acetylcholine perfusion of canine right atrium results in beat-to-beat patterns that mimic sinus arrhythmia supporting exit block in the sinoatrial conduction pathways.

Sinus arrhythmia of the dog is unique because of the pronounced alternating beat-to-beat intervals. The clustering of these short (faster rates) and long (slower rates) intervals is not just influenced by autonomic input from breathing; sinus arrhythmia can persist in the panting or apneic dog. The multiplicity of central and peripheral influences on the sinus node complicates the unraveling of the mechanisms of sinus arrhythmia. Studies of the sinus node suggest that acetylcholine can slow cellular depolarization and block sinoatrial conduction. Electrocardiographic monitoring of the dog supports this notion in that abrupt bifurcation into short and long intervals develop at lower heart rates. We sought to determine whether this phenomenon could be recapitulated in canine atrial preparations perfused with acetylcholine and whether selective pharmacologic blockade of the voltage and calcium clocks could provide insight into its mechanism. Spontaneous beat to beat (A-A) intervals were obtained from monophasic action potential recordings of perfused canine right atrial preparations before and during perfusion with acetylcholine (2-5 μM). The calcium clock was blocked with ryanodine (2-3 μM). The membrane clock was blocked with diltiazem hydrochloride (I blocker; 0.25 μM) and ZD7288 (I blocker; 3 μM). Hyperpolarization was hindered by blockade of I/I with tertiapin Q (100 nM) before and during acetylcholine perfusion. Acetylcholine resulted in beat clusters similar to those seen in sinus arrhythmia of the dog. Beat clusters were consistent with intermittent 2:1 and 3:1 sinoatrial conduction block. Tertiapin Q abolished this patterning suggesting a role of I/I in the mechanism of these acetylcholine-induced beat-to-beat patterns.