Autoregulation of neuromuscular transmission in the guinea-pig saphenous artery.

The participation of presynaptic adrenoceptors and receptors mediating the postsynaptic excitatory junction potential (e.j.p.) responses in feedback regulation of neurotransmission in the guinea-pig saphenous artery was studied by electrophysiological recording of the synaptic potentials. Stimulation of perivascular nerves elicited e.j.p. and slow depolarization in the artery. With repetitive stimulation, the successive e.j.p.s decreased in amplitude. Prazosin abolished the slow depolarization without any effect on the e.j.p. The depression of e.j.p. with repetitive stimulation was converted to facilitation in the presence of yohimbine. Yohimbine inhibited the first e.j.p. in a train but did not have a significant effect on the slow depolarization and contraction. A high concentration of ANAPP3, a specific P2-antagonist, depressed the e.j.p. The slow depolarization and contraction, however, were significantly potentiated by ANAPP3, even at low concentrations. Propranolol had no effect on the neural responses. The results suggest that there are functional presynaptic alpha 2-adrenoceptors and P2-receptors that could play a physiological role in the autoregulation of neuromuscular transmission in the guinea-pig saphenous artery.