Effect of perfusion rate of cholinergic agonist on sinus node automaticity

Recent evidence supports a complex relationship between pressure in the sinus node artery and heart rate. In addition, it has been suggested that acetylcholine effects vary depending upon the pressure at which the drug is injected. We examined the cardiac chronotropic responses to acetylcholine, delivered via the sinus node artery using a constant flow perfusion technique. Dogs were anesthetized with chloralose and prepared to record ECG, arterial pressure and bipolar electrograms from the sinus node, sulcus terminalis, right atrium, right ventricle and His bundle. The sinus node artery was catheterized, distribution verified and autologously perfused via the femoral artery. Both vagi and both stellate ganglia were transected. Analog data were processed by computer for each cycle length during perfusion with normal Tyrode solution or Tyrode solution containing acetylcholine. Perfusion of normal Tyrode solution (1-4ml/min) resulted in prolongation in cycle length which was greater at higher flow rates but rapidly dissipated at all flow rates. Beyond mechanically-induced bradycardia, acetylcholine initially prolonged cycle length but cycle length prolongation faded with time. Delivery of acetylcholine at higher flow rates resulted in significantly greater prolongation of cycle length. Cycle length always returned back toward control although perfusion of acetylcholine continued. Thus, responses to acetylcholine are influenced not only by drug concentration but also by the flow rate at which the drug is delivered. This suggests a coupling of mechanical and pharmacologic components of chronotropic influences at the sinus node.