1. Arteriolar diameter was measured using an optical method in preparations of guinea‐pig submucosal plexus in vitro. Electrical stimulation of one or more neurones in ganglia of the submucosal plexus causes a cholinergic vasodilatation in normal animals. The vasomotor innervation to the arterioles was studied in guinea‐pigs in which the extrinsic nerves to the intestine had been removed. Tissues were processed for immunohistochemistry after the in vitro experiments. 2. Extrinsic denervation resulted in complete loss of catecholamine fluorescence, NPY (neuropeptide Y) and CGRP (calcitonin gene‐related peptide) immunofluorescence around the blood vessels and no neurogenic vasoconstriction was observed up to 60 days post‐denervation. Vasodilatation in response to ganglionic stimulation was increased; smaller arterioles (outside diameter less than 40 microns) showed a greater enhancement of neurogenic vasodilatation than larger arterioles. 3. Nerve‐evoked vasodilatations were only partially inhibited by muscarinic antagonists at 30‐60 days after extrinsic denervations. 4. The non‐cholinergic neurogenic vasodilatation was abolished by the substance P antagonists, spantide, [D‐Arg1, D‐Pro2, D‐Trp7.9, Leu11]substance P and [D‐Arg1, D‐Phe5, D‐Trp7.9, Leu11]substance P. These antagonists did not alter the cholinergic vasodilatation in normal or extrinsically denervated arterioles. 5. Exogenous substance P dilated all submucosal arterioles; the concentration which produced half‐maximal vasodilatations was 2.5 mM in both normal and extrinsically denervated arterioles. Substance P antagonists inhibited the vasodilatation caused by substance P at concentrations similar to those needed to block nerve‐mediated vasodilatation. 6. There was a strong correlation between the finding of non‐cholinergic vasodilatation in response to ganglionic stimulation, and the presence of substance P‐immunoreactive fibres running from ganglion to arteriole. This correlation did not exist for VIP (vasoactive intestinal peptide). 7. These results suggest that intrinsic intestinal substance P‐containing nerve fibres supply submucosal arterioles after sympathetic efferents and sensory afferents are removed. Stimulation of these nerves releases substance P to produce arteriolar dilatation.
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