The perfusion of the sinus node artery in dogs in situ was performed. The selective administration of nicotine and DMPP into the sinus node artery induced a biphasic response, i.e., a negative chronotropic response followed by a positive one. Both compounds produced similar negative chronotropic responses, but positive responses were more markedly induced by DMPP than by nicotine.
These negative chronotropic responses were blocked by atropine, hexamethonium and tetrodotoxin. The positive response to nicotine was blocked by propranolol and tetrodotoxin. DMPP-induced positive chronotropic effect was blocked by propranolol but never by tetrodotoxin.
It is suggested that the positive chronotropic action of DMPP is different from that of nicotine on the sino-atrial node and is similar to that of tyramine.