Facilitation and depression of ATP and noradrenaline release from sympathetic nerves of rat tail artery

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Abstract
1 Excitatory junction currents (EJCs) were used to measure ATP release; noradrenaline (NA) oxidation currents and fractional overflow of labelled NA, [3H]NA, were used to monitor the release of endogenous and exogenous NA, respectively, from post-ganglionic sympathetic nerves of rat tail artery. 2 During nerve stimulation with 100 pulses at 5-20 Hz the EJCs initially grew in size (maximally by 23%, at 2–10 Hz), and then depressed, maximally by 68% at 20 Hz. 3 The peak amplitude of NA oxidation currents in response to nerve stimulation with 100 pulses at 2–20 Hz grew in size with frequency, while the area was independent of frequency and roughly constant. 4 The size of the NA oxidation currents evoked by nerve stimulation with 4–100 pulses at 20 Hz grew linearly with train length between pulses 4–16. Between pulses 20–100 there was a train length-dependent depression of the signal. 5 Fractional overflow of [3H]NA in response to nerve stimulation with 5–100 pulses at 20 Hz behaved similarly to the EJCs. It initially grew roughly linearly between pulses 5–25, and then showed a dramatic depression similar to that of the EJCs. 6 The α2-adrenoceptor antagonists rauwolscine and yohimbine increased the overflow of [3H]NA and the amplitude of NA oxidation currents, but not that of the EJCs. 7 It is concluded that during high-frequency stimulation (i) the release of ATP and NA is first briefly facilitated then markedly depressed, (ii) facilitation and depression of the two transmitters are similar in magnitude and time course, and (iii) α2-adrenoceptor antagonists differentially modify EJCs and the NA signals. The results obtained in the absence of drugs are compatible with the hypothesis that ATP and NA are released in parallel, while the effects of α2-adrenoceptor antagonists seem to suggest dissociated release.