Abstract
The sinus node artery of 11 anesthetized mongrel dogs was perfused at a constant pressure of 100mmHg. A relatively small dose of norepinephrine, 0.01 to 1.0μg, usually induced a positive chronotropic response when injected directly into the sinus node artery. Occasionally, however, a combination of a positive and a negative chronotropic response or only a negative chronotropic response was apparent. A larger dose of norepinephrine, 10 to 30μg, consistently induced a triphasic response: there was a brief positive chronotropic response, followed by a longer negative chronotropic response, and finally a sustained positive chronotropic response. The negative chronotropic response usually began before the systemic blood pressure became elevated as a consequence of the systemic effects of the norepinephrine. The negative chronotropic response was not influenced appreciably by bilateral vagotomy. However, it was completely blocked by a small dose of atropine, 1μg. These results suggest that norepinephrine releases acetylcholine from parasympathetic nerve terminals.
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