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 sympathe tic co-transmitter adenosine 5'-triphosphate (ATP) from individual rel ease sites is monoquantal and intermittent; the average release probab ility may be as low as 0.01.(1-3) Indirect evidence from biochemical s tudies of noradrenaline overflow is compatible with a similar monoquan tal, low probability release of noradrenaline as well.(10) In the pres ent 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 sympatheti c nerve varicosities. The results seem to provide the first direct evi dence that noradrenaline, similarly to ATP, may be released intermitte ntly 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 pu lses) at high frequencies. The results show that during stimulation at 20 Hz the peak amplitude of the noradrenaline oxidation current respo nse grew linearly with the train length, but at 50 Hz the curve descri bing this growth was sigmoid in shape.