At first the aim of our study was to observe the simultaneous responses of two hearts after intraarterial (into a. femoralis) adrenaline administration, in the rat with its own heart and a transplanted one – hence non-innervated. After these, some next experiments were performed: in some rats the His bundle of the heterotopically transplanted heart was damaged before transplantation. In all experiments the heart rate was observed on ECG and simultaneously, the arterial blood pressure was recorded from femoral artery in Vetbutal-anaesthetized rats. Results: 1) both the heterotopically transplanted, non-innervated heart and the animal’s own heart reacted to adrenaline administeration by producing bradycardia, 2) the heterotopically transplanted heart with the damaged His bundle – hence with a ventricular block reacted to adrenaline administration by raising the heart rate, whereas at the same time and in the same animal its own heart reacted by producing bradycardia. Conclusions: 1) the cause of bradycardia after adrenaline administration does not lie in the reflex from the arcus aortae, since we observed bradycardia after adrenaline administration also in the transplanted, non-innervated heart; therefore the baroreceptor reflex is not the cause of bradycardia after adrenaline administration; 2) bradycardia after adrenaline occurs in both the proper heart and the transplanted, non-innervated one, as a result of an interaction between two cholinergic centres which must be situated above and below the point of the His bundle interruption. The role of acetylcholine in the heart results from the interaction between these two centres.

Keywords: Bradycardia, Adrenaline Administration, Baroreceptors, Heart Transplant, Intercalated Discs, Long-Evans Rats