Some implications of the high intrasynaptic norepinephrine concentrations in resistance arteries

Norepinephrine concentrations in excess of 10(-4) M are achieved at the post-junctional membrane of the adrenergic synaptic cleft in resistance arteries at least for short periods of time. These concentrations are greater than the threshold of low affinity non-alpha-adrenoceptors-'extraceptors', that respond to norepinephrine and would be sufficient to overcome the blockade by alpha-adrenoceptor antagonists. The relationship of such activity to the excitatory junction potential and the relevance of this to contraction in vivo remains a matter of conjecture.