Extra‐vesicular Binding of Noradrenaline and Guanethidine in the Adrenergic Neurones of the Rat Heart: A Proposed Site of Action of Adrenergic Neurone Blocking Agents

1 The binding and efflux characteristics of [14C]-guanethidine and [3H]-noradrenaline were studied in heart slices from rats which were pretreated with reserpine and nialamide. 2 Binding of both compounds occurred at extra-vesicular sites within the adrenergic neurone. After a brief period of rapid washout, the efflux of [14C]-guanethidine and [3H]-noradrenaline proceeded at a steady rate. The efflux of both compounds appeared to occur from a single intraneuronal compartment. 3 (+)-Amphetamine accelerated the efflux of [14C]-noradrenaline; this effect was inhibited by desipramine. 4 Unlabelled guanethidine and amantadine also increased the efflux of labelled compounds. Cocaine in high concentrations increased slightly the efflux of [14C]-guanethidine but not that of [3H]-noradrenaline. 5 Heart slices labelled with [3H]-noradrenaline became refractory to successive exposures to releasing agents although an appreciable amount of labelled compound was still present in in these slices. 6 It is suggested that [14C]-guanethidine and [3H]-noradrenaline are bound at a common extravesicular site within the adrenergic neurone. Binding of guanethidine to the extra-vesicular site may be relevant to its pharmacological action, i.e., the blockade of adrenergic transmission.