Sympathetic transmission in small mesenteric arteries from the rat: influence of impulse pattern.

We have previously studied the transmitters involved in, and the calcium dependency of, the neurogenic response to continuous, regular nerve stimulation in small mesenteric arteries from the rat. We have now studied responses to irregular nerve stimulation in these respects. Small mesenteric arteries from rat were mounted into a myograph and the intramural sympathetic nerves were activated by field stimulation. Irregular nerve stimulation was patterned after recorded activity in human cutaneous nerves. The response to irregular stimulation was reduced by only approximately 50% in the presence of the alpha 1-adrenoceptor antagonist prazosin. In particular, responses to low-frequency stimulation were resistant to prazosin. When the extracellular calcium concentration was reduced from 2.5 to 1.0 mM, approximately 50% of the response to irregular nerve stimulation remained. Responses to low-frequency stimulation were particularly reduced. Thus in these arteries the neurogenic response is caused by noradrenaline and a, perhaps purinergic, co-transmitter. The co-transmitter is important for the response to low-frequency stimulation and for the initial part of the contraction caused by high-frequency stimulation. Reducing the calcium concentration affects more strongly the response to low-frequency nerve stimulation.