Fate of [3H]amezinium in sympathetically innervated rabbit tissues

Perfused rabbit hearts accumulated intra-aortically infused [³H]amezinium. At concentrations of 1 or 10 nM, the accumulation proceeded at a constant rate for at least 60 min. After 60 min, tissue medium ratios were between 6 (1 μM) and approx. 100 (1 or 10 nM [³H]amezinium). Cocaine or pretreatment with 6-hydroxydopamine abolished, and pretreatment with reserpine reduced the accumulation of [³H]amezinium (1nM). Kinetic analysis yielded a K_m value of 0.9 μM and a V_max of 1.2 nmole g^?1 min^?1. When hearts had been labelled with 1 or 10 nM [³H]amezinium, the fractional rate of the subsequent efflux was very low (0.001 min^?1). It was greatly increased when the animals had been pretreated with reserpine. Electrical stimulation of the sympathetic nerves released [³H] amezinium from pre-labelled rabbit pulmonary artery strips. The electrically evoked overflow was abolished by tetrodotoxin or omission of Ca²⁺; it was enhanced by cocaine, desipramine and yohimbine and decreased by clonidine. The results show that amezinium, at least at low concentrations, is selectively taken up into postganglionic sympathetic neurones, that it is partly sequestered in the vesicles, and that it is released by action potentials. Amezinium is a structurally novel substrate of both the noradrenaline transport mechanism of the axolemma and the transport mechanism of the noradrenaline-storing synaptic vesicles.