Evidence against presynaptic α-adrenoreceptor modulation of cardiac sympathetic transmission

Abstract

In addition to its well known actions on postsynaptic adrenoreceptors, noradrenaline released at the sympathetic nerve endings is now believed to modulate subsequent transmitter release through its actions on presynaptic receptors1-7. Much evidence in support of this hypothesis has been obtained from overflow studies in which labelled noradrenaline and metabolites released into the bathing solution are used as an assay of transmitter release. According to Langer4 and Rand, McCulloch and Story6, evidence to favour the existence of such a presynaptic mechanism includes the demonstration that α-adrenoreceptor antagonists produce enhanced transmitter overflow in steady-state conditions of sympat..