Neurotransmission in isolated sheep mesenteric lymphatics

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 10(-7) M tetrodotoxin. The alpha-antagonists phentolamine, prazosin, and yohimbine failed to block the excitatory response in a dose of 10(-6) M. Exogenous noradrenaline (10(-6) M) increased the frequency and force of spontaneous contractions and this effect was blocked by a 10(-6) M phentolamine. Atropine 10(-6) M failed to block the excitatory response to field stimulation. alpha beta-methylene ATP caused an intense transient excitatory effect followed by recovery to a frequency level just above that of control but the excitatory effect of field stimulation was not blocked in these desensitized vessels. When vessels were exposed to a mixture of 10(-4) M noradrenaline and 10(-5) M phentolamine field stimulation did not further increase the frequency of spontaneous contractions. These results demonstrate that the innervation of sheep mesenteric lymphatics is different from that of bovine mesenteric lymphatics. The identity of the transmitter is as yet unknown but it does not appear to be ATP nor is it noradrenaline acting on postsynaptic alpha-receptors.