4-Aminopyridine injected into the sinus node artery produced a long-lasting decrease in sinus rate preceded by a transient increase. With a large dose, a long-lasting increase in sinus rate was superimposed on the long-lasting decrease phase. The long-lasting decrease and increase in sinus rate were abolished by 1-hyoscyamine and propranolol respectively, and both were reduced by tetrodotoxin. These results can be interpreted as being due to excitation of autonomic nerves by 4-aminopyridine.
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