Effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery.

The effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery were investigated using a microelectrode method. Dopamine did not modify the membrane potential or the membrane resistance of the smooth muscle, but did reduce the amplitude of excitatory junction potentials (e.j.p.) and enhance the facilitation of e.j.p. produced by repetitive stimulation. Phentolamine (10(-7) M) enhanced the amplitude and the facilitation of the e.j.p., and with the addition of dopamine (10(-6) M) there was a reduction in the amplitude of e.j.p. but not in the facilitation. Haloperidol and sulpiride (greater than 10(-6) M) increased the amplitude of e.j.p. without altering the postjunctional membrane properties. Haloperidol and sulpiride did not increase the facilitation of e.j.p. produced by repetitive stimulation. In the presence of haloperidol or sulpiride (10(-5) M), dopamine (10(-6) M) did not suppress the amplitude of the e.j.p. These results indicate that in the guinea-pig mesenteric artery, dopamine inhibits the release of transmitter at the presynaptic membrane.