Mediation of excitatory neurotransmission by the release of ATP and noradrenaline in sheep mesenteric lymphatic vessels.

1. Spontaneous isometric contractions were measured in rings of sheep mesenteric lymphatics. Field stimulation at short pulse widths increased the frequency of spontaneous contractions and this response was blocked by 3 x 10(-7) M omega-conotoxin and by 10(-6) M guanethidine. 2. Rings that had been incubated with [3H]noradrenaline release 3H in response to field stimulation in a frequency-dependent manner. 3. Exogenous ATP mimicked the response to field stimulation and this was blocked by 10(-4) M suramin but not by prior desensitization with 10(-6) M alpha, beta-methylene ATP. Exogenous noradrenaline was not blocked by 10(-4) M suramin. 4. The excitatory response to field stimulation was not blocked by 10(-4) M suramin but a combination of 10(-4) M suramin and 3 x 10(-6) M phentolamine did block the response. 5. In rings taken from sheep that had been pretreated with reserpine, 10(-4) M suramin alone blocked the response to field stimulation. 6. The results of this study suggest that the excitatory response to stimulation of intramural nerves in sheep mesenteric lymphatics is mediated by the release of both ATP and noradrenaline.