Role of action potentials and calcium influx in ATP- and UDP-induced noradrenaline release from rat cultured sympathetic neurones

Adenine and uracil nucleotides release noradrenaline from rat postganglioni c sympathetic neurones by activation of P2X-receptors and distinct receptor s for uracil nucleotides, respectively. The present study on cultured neuro nes of rat thoracolumbal paravertebral ganglia has analysed the involvement of action potentials and calcium influx in the nucleotide-induced transmit ter release. ATP and UDP (100 mu M each) caused a marked release of previously incorpora ted [H-3]noradrenaline. The P2-receptor antagonists suramin (300 mu M) and cibacron blue 3GA (3 mu M) decreased the ATP-induced but not the UDP-induce d release. The response to ATP was decreased by the sodium channel blocker tetrodotoxin (0.5 mu M; by 47%), by the N-type calcium channel blocker omeg a-conotoxin GVIA (100 nM; by 35%), and by the alpha(2)-adrenoceptor agonist UK-14,304 (1 mu M by 45%); it was not changed by the potassium channel blo cker tetraethylammonium (10 mM). The response to UDP was abolished by tetro dotoxin, greatly decreased by omega-conotoxin (by 78%), also abolished by U K-14,304, and increased by tetraethylammonium (by 410%). ATP (100 mu M) cau sed a marked increase in intraaxonal free calcium as measured by fura-2 mic rofluorimetry. Withdrawal of extracellular calcium diminished the calcium r esponse to ATP by 85%, and tetrodotoxin and omega-conotoxin diminished it b y about 40%. As studied with the amphotericin B-perforated patch method, AT P (100 mu M) induced a large depolarisation and action potential firing. UD P (100 mu M) induced only a small depolarisation but it also elicited actio n potentials. UDP increased the excitability of the neurones. The results indicate that the ATP-induced release of noradrenaline depends on influx of calcium from the extracellular space. Calcium passes through t wo structures: volt age-gated channels and - probably - the P2X-receptor it self. Only that component of ATP-induced transmitter release which is trigg ered by opening of voltage-gated calcium channels is sensitive to modulatio n by alpha(2)-adrenoceptors. In contrast to ATP, the UDP-induced release of noradrenaline is entirely due to generation of action potentials followed by calcium influx through voltage-gated channels. All of the UDP-induced re lease is therefore sensitive to alpha(2)-adrenoceptor modulation.