Transmitter characteristics of small mesenteric arteries from the rat

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Abstract
We have studied the neurogenic response of small mesenteric arteries from the rat to evaluate the involvement of possible co‐transmitters under various modes of stimulation.Segments of small branches of the mesenteric artery were mounted in a myograph and the intramural nerves were activated with transmural electrical stimulation.A single stimulation of the nerves caused a contraction that was reduced by only 20% in the presence of adrenergic blocking agents (prazosin or phenoxybenzamine), whereas the steady‐state response to continuous nerve stimulation of high frequency was reduced by 90–95 %. In contrast, all responses to applied noradrenaline in doses up to at least 1 mM were eliminated by phenoxybenzamine treatment. The stable ATP analogue, α,β‐methylene ATP, reduced the response to a single nerve stimulation by 70%, but reduced the contraction caused by continuous high‐frequency nerve stimulation by only 10%. None of these agents affected the response to applied neuropeptide Y (NPY). The response of relaxed vessels to nerve stimulation was totally blocked by the combination of an adrenoceptor‐blocking agent and α,β‐methylene ATP, although even in this situation a further neurogenic response could be revealed in vessels precontracted with vasopressin.Responses to either single stimuli or brief burst stimulations were potentiated after high‐frequency stimulation. Both the adrenergic and non‐adrenergic components were enhanced to roughly the same extent. Also the potentiated response was eliminated by the combined application of prazosin and α,β‐methylene ATP.The non‐adrenergic transmitter in the sympathetic nerves of small arteries thus appears to be the dominant transmitter during low‐frequency nerve stimulation, causing rapid but phasic activation. Noradrenaline is the most important transmitter for higher frequencies, exerting slower but sustained contractions. The post‐stimulatory potentiation affects both the adrenergic and the non‐adrenergic part of the neurogenic response.

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