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 noradrenal ine (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sy mpathetic nerves have been investigated in rat tail arteries in vitro. Intr acellularly recorded excitatory junction potentials (e.j.ps) were used as a measure of ATP release and continuous amperometry was used to measure NA r elease. 2 Varying the extracellular Ca2+ concentration similarly affected the ampli tudes of e.j.ps and NA-induced oxidation currents evoked by trains of ten s timuli 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 evo ked by trains of 50 stimuli at 10 Hz were abolished by the non-selective Ca 2+ channel blocker, Cd2+ (0.1 mM), and were reduced by omega-conotoxin MVII C (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 r esults in similar effects on NA and ATP release from postganglionic sympath etic nerves. These findings are consistent with the hypothesis that NA and ATP are co released together from the sympathetic nerve terminals.