Differential adrenergic response to extrinsic denervation in canine longitudinal jejunal and ileal smooth muscle

Inhibition by nitric oxide and nonadrenergic, noncholinergic (NANC) nerves is preserved in a canine model of extrinsic denervation: Implications for small bowel transplantation

BACKGROUND

Small bowel transplantation (SBT) is complicated by changes in graft motility, especially in the early postoperative period. This dysmotility may be related in part to the extrinsic denervation necessitated by the procedure, but specific neurotransmitter response to SBT is incompletely understood. The aim of this study was to evaluate the role of nitric oxide and nonadrenergic, noncholinergic (NANC) enteric neural input in the nonimmunologic etiology of the dysmotility seen after SBT.

METHODS

A technique of jejunoileal extrinsic denervation (without disruption of mesenteric vascular supply) was used as a model of canine jejunoileal autotransplantation to avoid potential confounding factors such as ischemia-reperfusion and postallotransplant immunologic effects. Longitudinal smooth muscle strips from ileum and jejunum were studied with in vitro tissue chamber methodology at 0, 2, and 8 weeks after this experimental model to explore early and late effects of denervation. Effects of exogenous nitric oxide (NO) and electric field stimulation (EFS), which releases native, endogenous enteric neurotransmitters) were evaluated in neurally intact control dogs and those undergoing extrinsic denervation.

RESULTS

Exogenous NO caused a dose-dependent inhibition of spontaneous contractile activity and in some muscle strips a decrease in basal tone in both groups of dogs. These effects were unchanged by neural blockade with tetrodotoxin and preserved after extrinsic denervation. EFS produced inhibition of spontaneous contractile activity in ileum and a complex, inconsistent response in jejunum. The response to EFS in both ileum and jejunum was unchanged after extrinsic denervation.

CONCLUSIONS

Nitric oxide inhibits contractile activity in canine longitudinal muscle of small bowel. Motility changes seen after this large animal model of extrinsic denervation are not caused by changes in NO or NANC neural function. The variability observed between different segments of intestine is important to consider in the context of SBT.

Nitrergic mechanisms mediating inhibitory control of longitudinal smooth muscle contraction in mouse small intestine

Studies using genetic manipulation to investigate mechanisms of control of physiologic function often necessitate mouse models. However, baseline functional analysis of murine small intestinal motility has not been well defined. Our aim was to define nitrergic mechanisms regulating mouse small intestinal longitudinal muscle. Endogenous nitric oxide (NO) is an important neuroregulatory substance mediating inhibition of contractile activity in murine small bowel. Full-thickness muscle strips of jejunum and ileum from C57BL/6 mice (n > or =6 mice) cut in the direction of longitudinal muscle were studied. Numerous conditions of electrical field stimulation (EFS) and effects of exogenous NO and NO donors were studied in the absence or presence of inhibitors of nitric oxide synthase (NOS) and 1H-[1,2,4]-oxadiazaolo-[4,3-a]-quinoxalin-1-one (ODQ), a downstream inhibitor of guanylyl cyclase. EFS induced a frequency-dependent inhibition of contractile activity in both jejunum and ileum (P < 0.05). As the voltage of EFS was increased, inhibition turned to excitation in the jejunum; in contrast, the ileum demonstrated a voltage-dependent increasing inhibition (P < 0.05 each). EFS-induced inhibition was blocked by NOS inhibitors and ODQ. NO donors inhibited spontaneous contractile activity abolished by ODQ. NO appears to be an endogenous inhibitory neurotransmitter in murine longitudinal small bowel muscle. Nitrergic mechanisms mediate inhibitory control of murine longitudinal small intestinal muscle. Differences exist in neuroregulatory control between jejunum and ileum that may be related to their known difference in motor patterns.

Contractile activity of mouse small intestinal longitudinal smooth muscle

BACKGROUND

Interest in genomic modulation experimentally often necessitates use of mouse models.

MATERIALS AND METHODS

AIM

To characterize and quantitate smooth muscle contractile activity of the mouse small intestine using in vitro techniques. Full-thickness jejunal and ileal muscle strips from mice were cut in the direction of longitudinal muscle, suspended in tissue baths (37 degrees C), and connected to force transducers. Spontaneous contractility and two dose-response curves to the cholinergic agonist bethanechol and adrenergic agonist norepinephrine were quantitated for 6 h.

RESULTS

Total contractile activity increased over 4 to 5 h in jejunum (P < 0.01) but not in ileum. Frequency of contractions (counts/min) in jejunum increased from 16 to 33 (P < 0.01) in the first 4 h, then remained stable; ileal frequency did not change. One hour of cold preservation had no major effect on contractile activity and frequency. Bethanechol increased and norepinephrine decreased contractile activity in dose-dependent fashion. The dose of bethanechol producing 50% increase in maximal response did not differ between the first and second dose-response; in contrast, the concentration of norepinephrine producing 50% decrease in activity for the second dose-response in jejunum was decreased compared to the first dose-response (P < 0.01). Cold preservation had no substantive effect on agonist responses.

CONCLUSION

Experiments in murine jejunal but not ileal longitudinal muscle in vitro must consider early changes in contractile activity after tissue harvest. These experiments serve as a baseline for comparison of stimuli or genetic modifications on murine contractile activity of longitudinal muscle in vivo.

Chronic extrinsic denervation of the small bowel: effect on adrenergic and cholinergic contractile mechanisms in canine ileal circular muscle

BACKGROUND

Small bowel transplantation necessitates chronic extrinsic denervation and is often followed by enteric dysfunction. Our aim was to study canine ileal contractile activity after extrinsic denervation.

METHODS

In vitro dose responses to cholinergic and adrenergic agonists were evaluated in canine ileal muscle strips in control subjects and after jejunoileal extrinsic denervation (EX DEN) at 0, 2, and 8 weeks after operative preparation.

RESULTS

Spontaneous activity and the increased activity after tetrodotoxin (enteric nerve blockade) did not differ between groups. Response to acetylcholine and bethanechol did not differ at any time in the control or EX DEN group. In contrast, the EX DEN group demonstrated a procontractile hypersensitivity to norepinephrine at 2 and 8 weeks that was not seen in the control group. This adrenergic hypersensitivity in the EX DEN group was insensitive to intramural neural blockade with tetrodotoxin.

CONCLUSIONS

Extrinsic denervation does not affect basal contractile activity, augmented contractile activity to intramural neural blockade, nor response to cholinergic agonists. However, it induces a procontractile adrenergic hypersensitivity in canine ileal circular muscle mediated at the level of the smooth muscle and not at the enteric nervous system.

Neuronal adrenergic and muscular cholinergic contractile hypersensitivity in canine jejunum after extrinsic denervation

Extrinsic denervation may be responsible for motor dysfunction after small bowel transplantation. The aim of this study was to examine the role of extrinsic innervation of canine jejunum on contractile activity. An in vitro dose response of cholinergic and adrenergic agonists was evaluated in canine jejunal strips of circular muscle at 0, 2, and 8 weeks in a control group and after jejunoileal extrinsic denervation (EX DEN). Neurons in circular muscle were quantitated by means of immunohistochemical techniques. Adrenergic and cholinergic responses did not differ at any time in the control group. However, at 2 and 8 weeks, extrinsic denervation caused an increased sensitivity to the procontractile effects of the cholinergic agonist bethanechol at the level of the smooth muscle cells, and increased sensitivity to the inhibitory effects of the adrenergic agent norepinephrine mediated at the level of the enteric nervous system. Immunohistochemical analysis showed a reduction in all neurons and a complete lack of adrenergic fibers in the EX DEN group after 2 and 8 weeks. Extrinsic denervation induces enteric neuronal cholinergic and adrenergic smooth muscle hypersensitivity in canine jejunal circular muscle.

Effect of extrinsic denervation on muscarinic neurotransmission in the canine ileocolonic region

To explore the hypothesis that denervation hypersensitivity increases ileocolonic motor activity after extrinsic denervation, we compared muscarinic neurotransmission in canine ileocolonic loops that were isolated and either extrinsically innervated or extrinsically denervated. We recorded ileal, ileocolonic sphincter (ICS) and colonic pressures, and colonic tone, compliance and relaxation during ileal distention. Muscarinic effects were probed by neostigmine, and minimally effective doses of muscarinic receptor antagonists. Denervation augmented ileal, ICS and colonic contractile activity; colonic high-amplitude propagating contractions (HAPCs) were also augmented; colonic relaxation during ileal distention was abolished. Neostigmine induced HAPCs in both loop preparations. Pirenzipine (M1 antagonist) reduced ileal contractile activity in all loops and reduced colonic relaxation during ileal distention in innervated loops. Pirenzipine also reduced colonic tone and colonic HAPCs, more in denervated loops. Darifenacin (M3 antagonist) reduced ileocolonic contractile activity and tone more than did AF-DX 116 (M2 antagonist) in all loops. Cholinergic receptor subtypes modulate different facets of ileocolonic motor activity in the canine ileocolonic region. Increased sensitivity at M1 muscarinic receptors may partly account for the effects of extrinsic denervation.