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 postganglionic sympathetic neurones by activation of P2X-receptors and distinct receptors for uracil nucleotides, respectively. The present study on cultured neurones of rat thoracolumbal paravertebral ganglia has analysed the involvement of action potentials and calcium influx in the nucleotide-induced transmitter release. ATP and UDP (100 µM each) caused a marked release of previously incorporated [3H]noradrenaline. The P2-receptor antagonists suramin (300 µM) and cibacron blue 3GA (3 µM) decreased the ATP-induced but not the UDP-induced release. The response to ATP was decreased by the sodium channel blocker tetrodotoxin (0.5 µM; by 47%), by the N-type calcium channel blocker ω-conotoxin GVIA (100 nM; by 35%), and by the α2-adrenoceptor agonist UK-14,304 (1 µM; by 45%); it was not changed by the potassium channel blocker tetraethylammonium (10 mM). The response to UDP was abolished by tetrodotoxin, greatly decreased by ω-conotoxin (by 78%), also abolished by UK-14,304, and increased by tetraethylammonium (by 410%). ATP (100 µM) caused a marked increase in intraaxonal free calcium as measured by fura-2 microfluorimetry. Withdrawal of extracellular calcium diminished the calcium response to ATP by 85%, and tetrodotoxin and ω-conotoxin diminished it by about 40%. As studied with the amphotericin B-perforated patch method, ATP (100 µM) induced a large depolarisation and action potential firing. UDP (100 µM) induced only a small depolarisation but it also elicited action 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 two structures: voltage-gated channels and – probably – the P2X-receptor itself. Only that component of ATP-induced transmitter release which is triggered by opening of voltage-gated calcium channels is sensitive to modulation by α2-adrenocep-tors. 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 release is therefore sensitive to α2-adrenoceptor modulation.