J. G. De Man

Non-adrenergic non-cholinergic relaxation mediated by nitric oxide in the canine ileocolonic junction

The nature of the inhibitory non-adrenergic non-cholinergic (NANC) neurotransmitter was studied in circular muscle strips of the canine terminal ileum and ileocolonic junction. Nitric oxide (NO) induced tetrodotoxin-resistant NANC relaxation, similar to that induced by electrical stimulation or acetylcholine (ACh). Incubation with the stereospecific inhibitors of NO biosynthesis, NG-monomethyl-L-arginine (L-NMMA) and NG-nitro-L-arginine (L-NNA), resulted in an increase of basal tension in the ileocolonic junction which was partly reversed by L-arginine but not by D-arginine. Moreover, L-NMMA and L-NNA, but not D-NMMA, concentration dependently inhibited the NANC relaxation in response to electrical stimulation and ACh, but not that in response to NO or nitroglycerin. This inhibitory effect was reversed by L-arginine but not by D-arginine. Hemoglobin reduced the NANC relaxation in response to electrical stimulation, ACh and nitroglycerin, and abolished the responses to NO. Our results suggest that NO or a NO releasing substance mediates the NANC relaxation in the canine terminal ileum and ileocolonic junction.

Ca2+ dependency of the release of nitric oxide from non‐adrenergic non‐cholinergic nerves

1 The role of Ca2+ in nitrergic neurotransmission was studied in the canine ileocolonic junction. 2 The specific N‐type voltage‐sensitive Ca2+ channel blocker ω‐conotoxin GVIA (CTX, 10–100 nm) significantly reduced the electrically‐evoked (2–16 Hz, 1–2 ms pulse width) non‐adrenergic non‐cholinergic (NANC) relaxations, preferentially affecting those to low frequency stimulation, in circular muscle strips of the ileocolonic junction. In contrast, the nerve‐mediated NANC‐relaxations in response to acetylcholine (30 μm), γ‐aminobutyric acid (100 μm) and adenosine 5′‐triphosphate (100 μm), as well as the relaxations to nitric oxide (NO) (3–10 μm) and nitroglycerin (1 μm), remained unaffected. 3 A NO‐related substance (NO‐R), released from the ileocolonic junction in response to NANC nerve stimulation (4 and 16 Hz, 2 ms pulse width), was assayed with a superfusion bioassay cascade. CTX (50 nm) reduced the release of NO‐R induced by electrical impulses (4 Hz: from 18 ± 4% to 6 ± 4%; 16 Hz: from 33 ± 2% to 14 ± 4%, n = 5), but not that in response to the nicotinic receptor agonist, 1,1‐dimethyl‐4‐phenylpiperazinium (DMPP, 0.3 mm). In Ca2+‐free medium, the release of NO‐R evoked by electrical impulses or DMPP was inhibited. The L‐type Ca2+ channel blockers verapamil (1–3 μm) and nifedipine (1 μm) had no effect. 4 From these results we conclude that the release of NO‐R in response to NANC nerve stimulation is Ca2+‐dependent. The electrically‐evoked release of NO‐R results from Ca2+ entry through CTX‐sensitive N‐type voltage‐sensitive Ca2+ channels, whereas that induced by nicotinic receptor activation involves CTX‐insensitive Ca2+ channels, different from the L‐ or N‐type.

The role of nitric oxide in serotonin-induced relaxations in the canine terminal ileum and ileocolonic junction.

SummaryThe role of nitric oxide (NO) in 5-HT-induced non-adrenergic non-cholinergic (NANC) relaxations was studied on circular muscle strips of the canine ileocolonic junction (ICJ) and terminal ileum. During an acetylcholine-induced contraction, NO (10−5 M) evoked a transient relaxation, whereas 5-HT (10−4 M) caused an initial NANC relaxation followed by a contraction. This initial relaxation to 5-HT, but not the relaxation to NO, was significantly inhibited by the stereospecific inhibitors of the NO biosynthesis NG-monomethyl-Larginine (L-NMMA) and NG-nitro-Irarginine (L-NNA). Larginine, but not D-arginine, prevented the inhibitory effect of L-NMMA and LNNA. The enantiomer of L-NMMA, D-NMMA, had no effect. Hemoglobin abolished the NO-induced relaxations and significantly inhibited the relaxation to 5-HT.From these experiments it is concluded that the 5-HT-induced NANC relaxation is mediated by NO or a NO releasing substance.

α2-Adrenoceptor-mediated modulation of the nitrergic innervation of the canine isolated ileocolonic junction

1 The effects of specific α‐adrenoceptor agonists and antagonists on electrically‐evoked non‐adrenergic non‐cholinergic (NANC) relaxations, previously demonstrated as nitrergic, were investigated in isolated circular muscle strips of the canine ileocolonic junction. 2 During a substance P‐induced contraction and in the presence of atropine and guanethidine, the specific α1‐adrenoceptor agonist, phenylephrine and antagonist, prazosin, as well as the specific α2‐adrenoceptor antagonist, yohimbine, had no effect on the NANC relaxations evoked by electrical field stimulation. In contrast, clonidine and the more specific α2‐adrenoceptor agonist, UK‐14,304, significantly reduced the electrically‐induced relaxations, preferentially those in response to low frequency stimulation. The inhibitory effect of UK‐14,304 on these relaxations was antagonized by yohimbine. 3 During a noradrenaline‐induced contraction, clonidine, but not UK‐14,304 significantly augmented the relaxations to electrical stimulation. 4 The adrenoceptor agonists and antagonists used had no effect on concentration‐response curves to NO or on the relaxation induced by nitroglycerin. 5 These results indicate that stimulation of prejunctional α2‐adrenoceptors inhibits the nitrergic NANC relaxations induced by field stimulation and thus suggest prejunctional regulation of nitric oxide release via α2‐adrenoceptors in the canine ileocolonic junction.

Nitric oxide release in response to stimulation of nonadrenergic, noncholinergic nerves

The nature of the inhibitory nonadrenergic, noncholinergic (NANC) neurotransmitter was studied in the canine ileocolonic junction. In a first series of experiments, circular muscle strips were mounted in organ baths filled with aerated Krebs-Ringer solution for isometric tension recording. During a norepinephrine-induced contraction and in the presence of atropine, nitric oxide (NO) induced tetrodoxin-resistant relaxations that were similar to the NANC relaxations obtained by electrical stimulation and acetylcholine (ACh).