Novel dual ‘small’ vesicle model of ATP- and noradrenaline-mediated sympathetic neuromuscular transmission

The main question asked was if sympathetic nerves in guinea-pig vas deferens release the co-transmitters ATP and noradrenaline from the same, or different vesicles, i.e. in fixed combinations or independently. The extracellularly recorded excitatory junction current (EJC) and the fractional increase in overflow of tritium (ΔT) were used to monitor the per pulse secretion of ATP and [3H]NA, respectively, during electrical stimulation with 1–3000 pulses at 0.1–40 Hz. The frequency- and train length-dependence and α2-adrenoceptor-mediated autoinhibition of these parameters, and of the ATP-mediated twitch contraction, were compared first in the presence of cocaine (to block noradrenaline reuptake), then after brief exposure to phenoxybenzamine (PBA, to irreversibly ‘destroy’ α2-autoreceptors). Parallel variations of EJC/p(ulse) and ΔT/p(ulse) under all conditions would support, non-parallel variations argue against exocytosis of ATP and noradrenaline from the same vesicles. The main findings were that facilitation and α2-autoinhibition of EJC/p and ΔT/p were remarkably similar during stimulation at 2 Hz but increasingly dissimilar at higher frequencies. ΔT/p remained strongly facilitated and tightly controlled by activation of α2-autoreceptors at 10–40 Hz, but both the facilitation and the sensitivity to α2-autoinhibition of EJC/p were inversely related to frequency. At 40 Hz EJCs were ‘small’, minimally facilitated and totally unaffected by cocaine or PBA, i.e. insensitive to α2-autoinhibition. Nevertheless, activation of α2-receptors during the 40 Hz train strongly restricted the ‘post-tetanic augmentation’ (PTA) of the first EJC 10 s after the tetanus. Comparison between the frequency dependence of EJCs and the twitch contraction in the presence of cocaine or after PBA treatment indicates that it is the ‘summed EJC per second’, i.e. the ATP-driven current injection per unit time into smooth muscle, which triggers the twitch. The working hypothesis is proposed that these nerves use two classes of ‘small vesicles’ (SVs) to store and release either ‘big’ or ‘small’ ATP and noradrenaline ‘quanta’, and that differences in properties (Ca2+ affinity, capacity) of Ca2+ receptors in the SV membranes enable the nerves to selectively secrete ‘big quanta’ at low frequency and ‘small quanta’ during trains at high frequency.