Intermittent release of noradrenaline by single pulses and release during short trains at high frequencies from sympathetic nerves in rat tail artery.

As shown by electrophysiological analysis, the release of the sympathetic co-transmitter adenosine 5'-triphosphate (ATP) from individual release sites is monoquantal and intermittent; the average release probability may be as low as 0.01. Indirect evidence from biochemical studies of noradrenaline overflow is compatible with a similar monoquantal, low probability release of noradrenaline as well. In the present study our first aim was to address this issue more directly in rat tail artery, using continuous amperometry to monitor in real time the release of noradrenaline from a relatively small number of sympathetic nerve varicosities. The results seem to provide the first direct evidence that noradrenaline, similarly to ATP, may be released intermittently during nerve stimulation at low frequency. Our second aim was to use the same technique to study the release of noradrenaline caused by nerve stimulation with single pulses or short trains (two to eight pulses) at high frequencies. The results show that during stimulation at 20 Hz the peak amplitude of the noradrenaline oxidation current response grew linearly with the train length, but at 50 Hz the curve describing this growth was sigmoid in shape.