Purinergic nerves mediate the non-nitrergic relaxation of the human ileum in response to electrical field stimulation

The Effect of Ischemia and Reperfusion on Enteric Glial Cells and Contractile Activity in the Ileum

BACKGROUND

We investigated the effects of ischemia followed by different periods of reperfusion (I/R) on immunoreactive S100β-positive glial and Hu-immunoreactive neurons co-expressing the P2X2 receptor in the myenteric plexus of the rat ileum.

METHODS

The ileal artery was occluded for 35 min with an atraumatic vascular clamp. The animals were killed 24 h, 72 h, and 1 week after ischemia. Sham animals were not submitted to ileal artery occlusion. The relative density, size, and co-localization of P2X2 receptor-expressing cells in relation to S100β-immunoreactive glial and Hu-immunoreactive neuronal cells were evaluated. Additionally, we analyzed the effects of I/R on gastrointestinal transit and ileum contractile activity.

RESULTS

The cellular density of P2X2 receptor and neuronal Hu immunoreactivity/cm(2) decreased after I/R, whereas glial S100β immunoreactivity/cm(2) increased. No significant differences between sham and I/R groups were observed regarding the perikarya area of Hu-positive neurons. The area of S100β-immunoreactive glial cells increased by 24.1 % 1 week after I/R compared with the 24 h group. Methylene blue progression along the small intestine decreased (P < 0.05) from 24.5 ± 2.3 % in the sham group to 17.2 ± 2.0 % 1 week post-ischemia. We noted a significant (P < 0.05) decrease in the maximal contraction amplitude triggered by electrical field stimulation in the presence of ATP in preparations submitted to 24 h of I/R.

CONCLUSIONS

Changes in the P2X2 receptor density parallel myenteric neuronal loss following I/R of the rat ileum. This, together with the increase in the activated (oversized) glial cells, may contribute to decreased GI motility after I/R.

Pharmacological characterization of purinergic inhibitory neuromuscular transmission in the human colon

BACKGROUND

In the present study, we further characterize the purinergic receptors mediating the inhibitory junction potential (IJP) and smooth muscle relaxation in the human colon using a new, potent and selective agonist (MRS2365), and antagonists (MR2279 and MRS2500) of the P2Y(1) receptor. The P2Y(12) antagonist AR-C66096 was tested as well. Using this pharmacological approach, we tested whether β-nicotinamide adenine dinucleotide (β-NAD) fulfilled the criteria to be considered an inhibitory neurotransmitter in the human colon.

METHODS

We carried out muscle bath and microelectrode experiments on circular strips from the human colon and calcium imaging recordings on HEK293 cells, which constitutively express the human P2Y(1) receptor.

KEY RESULTS

Both the fast component of IJP and non-nitrergic relaxation was concentration-dependently inhibited by MRS2279 and MRS2500. This antagonism was confirmed in HEK293 cells. However, AR-C66096 did not modify either inhibitory response. Adenosine 5'-Ο-2-thiodiphosphate and MRS2365 caused a smooth muscle hyperpolarization and transient inhibition of spontaneous motility that was antagonized by MRS2279 and MRS2500. β-Nicotinamide adenine dinucleotide inhibited the spontaneous motility (IC(50) = 3.3 mmol L(-1) ). Nevertheless, this effect was not antagonized by high concentrations of P2Y(1) antagonists.

CONCLUSIONS & INFERENCES

Inhibitory purinergic neuromuscular transmission in the human colon was pharmacologically assessed by the use of new P2Y(1) receptor antagonists MRS2179, MRS2279, and MRS2500. The rank order of potency of the P2Y(1) antagonists is MRS2500 > MRS2279 > MRS2179. We found that β-NAD partially fulfills the criteria to be considered an inhibitory neurotransmitter in the human colon, but the relative contribution of each purine (ATP/ADP vsβ-NAD) requires further studies.

The NANC relaxation of the human ileal longitudinal and circular muscles is inhibited by MRS 2179, a P2 purinoceptor antagonist

AIMS

Functional innervation of the human small intestine may be different from that of experimental animals. These experiments set out to assess the mediating roles of P(2) purinoceptors in the non-adrenergic, non-cholinergic (NANC) relaxation of the human ileum longitudinal and circular muscles.

MAIN METHODS

In organ bath experiments NANC relaxations were evoked by electrical field stimulation (EFS). The relaxant effects of ATP and sodium nitroprusside were also examined.

KEY FINDINGS

In the longitudinal muscle, relaxation in response to EFS (2 or 10 Hz for 30 s) or the relaxant effect of exogenous ATP were strongly inhibited or abolished, respectively, by the P(2) purinoceptor antagonist MRS 2179 (10 microM). MRS 2179 had a smaller effect at 3 microM. The NANC relaxation was also inhibited by apamin (3 microM), but not by N(G)-nitro-L-arginine (100 microM), an inhibitor of nitric oxide synthesis. Both apamin (3 microM) and MRS 2179 (3 microM, a concentration below the effective range if administered alone) strongly inhibited the NANC response in preparations pretreated with the nitric oxide synthase inhibitor. NANC relaxations of the circular muscle were also inhibited by MRS 2179 (3-10 microM).

SIGNIFICANCE

MRS 2179-sensitive P(2) purinoceptors play a mediating role the NANC relaxation in the ileal longitudinal and circular muscle. There seems to be a supra-additive relationship between the purinergic and nitrergic mechanisms in the longitudinal muscle.

Purinergic and nitrergic junction potential in the human colon

The aim of the present work is to investigate a putative junction transmission [nitric oxide (NO) and ATP] in the human colon and to characterize the electrophysiological and mechanical responses that might explain different functions from both neurotransmitters. Muscle bath and microelectrode techniques were performed on human colonic circular muscle strips. The NO donor sodium nitroprusside (10 microM), but not the P2Y receptor agonist adenosine 5'-O-2-thiodiphosphate (10 microM), was able to cause a sustained relaxation. NG-nitro-L-arginine (L-NNA) (1 mM), a NO synthase inhibitor, but not 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) (10 microM), a P2Y antagonist, increased spontaneous motility. Electrical field stimulation (EFS) at 1 Hz caused fast inhibitory junction potentials (fIJPs) and a relaxation sensitive to MRS 2179 (10 microM). EFS at higher frequencies (5 Hz) showed biphasic IJP with fast hyperpolarization sensitive to MRS 2179 followed by sustained hyperpolarization sensitive to L-NNA; both drugs were needed to fully block the EFS relaxation at 2 and 5 Hz. Two consecutive single pulses induced MRS 2179-sensitive fIJPs that showed a rundown. The rundown mechanism was not dependent on the degree of hyperpolarization and was present after incubation with L-NNA (1 mM), hexamethonium (100 microM), MRS 2179 (1 microM), and NF023 (10 microM). We concluded that single pulses elicit ATP release from enteric motor neurons that cause a fIJP and a transient relaxation that is difficult to maintain over time; also, NO is released at higher frequencies causing a sustained hyperpolarization and relaxation. These differences might be responsible for complementary mechanisms of relaxation being phasic (ATP) and tonic (NO).

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

P2 purinoceptor antagonists inhibit the non-adrenergic, non-cholinergic relaxation of the human colon in vitro

Neurotransmitters released by myenteric neurons regulate movements of intestinal smooth muscles. There has been little pharmacological evidence for a role of purinergic mechanisms in the non-adrenergic, non-cholinergic (NANC) relaxation of the human large intestine. We used P(2) purinoceptor antagonists to assess whether such receptors are involved in the NANC relaxation of the circular muscle of the human sigmoid colon. It was also investigated whether the guanylate cyclase enzyme mediates the NANC response. Human colonic circular strips were tested in organ bath experiments with isotonic recording. NANC, non-nitrergic relaxations induced by electrical field stimulation (1 and 10 Hz, in the presence of atropine, guanethidine, and 100 microM N(G)-nitro-L-arginine [L-NOARG]) were strongly inhibited by a combination of the P(2) purinoceptor antagonists pyridoxal-phosphate-6-azophenyl-2',4'-sulfonic acid (PPADS) (50 microM) and suramin (100 microM). PPADS plus suramin was ineffective in the absence of L-NOARG. L-NOARG alone significantly reduced the NANC relaxation to electrical stimulation. PPADS plus suramin strongly inhibited the relaxation in response to exogenous alpha,beta-methylene ATP. The guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (3 microM) inhibited the NANC relaxation, but did not add to its reduction by L-NOARG. L-NOARG was still slightly effective in the presence of ODQ. Vasoactive intestinal polypeptide tachyphylaxis failed to influence the non-nitrergic NANC relaxation. It is concluded that nitric oxide (NO) and ATP co-mediate, in a non-additive manner, the NANC relaxation. NO probably acts through the guanylate cyclase, though a small fraction of its effect might be mediated by other mechanisms. Activators of the guanylate cyclase other than NO do not seem to participate in the NANC relaxation.