Absence of direct chronotropic action of angiotensin infused into the sinus node artery

Angiotensin was shown to have no significant direct chronotropic action when perfused into the canine sinus node through its artery in situ. A slight cardiac acceleration which occurs during the systemic effect (aortic hyper-tension) from angiotensin is not due to sensitization of the sinus node to norepinephrine. Since it can be reversed and blocked with naphthylisoproterenol, this sinus tachycardia is presumably mediated by catecholamines delivered to the heart from an extracardiac source. Slowing of the sinus node during hypertension from angiotensin in the nonatropinized dog is due to a vagal reflex and is not caused by a direct negative chronotropic effect on the sinus node.