Spontaneous release of large packets of noradrenaline from sympathetic nerve terminals in rat mesenteric arteries in vitro

Continuous amperometry was used to monitor noradrenaline (NA) release from sympathetic nerves supplying rat mesenteric arteries in vitro. During electrical stimulation the amplitude of oxidation currents evoked by successive stimuli varied over a small range, with occasional events of larger amplitude. In the absence of stimulation, spontaneous oxidation currents (s.o.cs) were recorded. The frequency of s.o.cs was increased by alpha -latrotoxin (1 nM). This agent also increased the frequency of spontaneous excitatory junction potentials (s.e.j.ps), which monitor the packeted release of adenosine 5' triphosphate (ATP). The frequency of s.o.cs recorded 20-25 min after applying alpha -latrotoxin was about four times the control value, but that of s.e.j.ps was about 30 times the control value. The findings suggest that continuous amperometry can detect the spontaneous packeted release of NA, probably from large dense-cored vesicles. In contrast, s.e.j.ps monitor spontaneous release of neurotransmitter (ATP) from a different store, most likely the small dense-cored vesicles.