Effects of Ca2+ concentration and Ca2+ channel blockers on noradrenaline release and purinergic neuroeffector transmission in rat tail artery

1 The effects of Ca2+ concentration and Ca2+ channel blockers on noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves have been investigated in rat tail arteries in vitro. Intracellularly recorded excitatory junction potentials (e.j.ps) were used as a measure of ATP release and continuous amperometry was used to measure NA release. 2 Varying the extracellular Ca2+ concentration similarly affected the amplitudes of e.j.ps and NA-induced oxidation currents evoked by trains of ten stimuli at 1 Hz. 3 The N-type Ca2+ blocker, omega-conotoxin GVIA (omega-CTX GVIA, 0.1 mu M) reduced the amplitudes of both e.j.ps (evoked by trains of ten stimuli at 1 Hz) and NA-induced oxidation currents (evoked by trains of ten stimuli at 1 Hz and 50 stimuli at 10 Hz) by about 90%. 4 The omega-CTX GVIA resistant e.j.ps and NA-induced oxidation currents evoked by trains of 50 stimuli at 10 Hz were abolished by the non-selective Ca2+ channel blocker, Cd2+ (0.1 mM), and were reduced by omega-conotoxin MVIIC (0.5 mu M) and omega-agatoxin IVA (40 nM). 5 Nifedipine (10 mu M) had no inhibitory effect on omega-CTX GVIA resistant e.j.ps and NA-induced oxidation currents. 6 Thus both varying Ca2+ concentration and applying Ca2+ channel blockers results in similar effects on NA and ATP release from postganglionic sympathetic nerves. These findings are consistent with the hypothesis that NA and ATP are co released together from the sympathetic nerve terminals.