Release of Noradrenaline from the Dog Heart In Situ after Intravenous and Intracoronary Administration of 5-Hydroxytryptamine

The following experiments have been carried out in order to establish the validity of the hypothesis that the cardiac stimulatory effects of 5-hydroxytryptamine (5-HT) are mediated by catecholamines. Release of cardiac noradrenaline (NA) has been observed after administration of 5-HT either intravenously or intracoronarily. After labelling the heart with 3H-5-HT, it was possible to release this amine by cardioaccelerator nerve stimulation even after treatment with an adrenergic β blocker. It is concluded that the cardiac stimulatory effects of 5-HT are mediated by NA; this NA is mainly derived from sympathetic nerve endings by intra-neuronal exchange for 5-HT.