ATP is a mediator of the fast inhibitory junction potential in human jejunal circular smooth muscle

Comparison of inhibitory neuromuscular transmission in the Cynomolgus monkey IAS and rectum: special emphasis on differences in purinergic transmission

KEY POINTS

Inhibitory neuromuscular transmission (NMT) was compared in the internal anal sphincter (IAS) and rectum of the Cynomolgus monkey, an animal with high gene sequence identity to humans. Nitrergic NMT was present in both muscles while purinergic NMT was limited to the rectum and VIPergic NMT to the IAS. The profile for monkey IAS more closely resembles humans than rodents. In both muscles, SK3 channels were localized to PDGFRα⁺ cells that were closely associated with nNOS⁺ /VIP⁺ nerves. Gene expression levels of P2RY subtypes were the same in IAS and rectum while KCNN expression levels were very similar. SK3 channel activation and inhibition caused faster/greater changes in contractile activity in rectum than IAS. P2Y1 receptor activation inhibited contraction in rectum while increasing contraction in IAS. The absence of purinergic NMT in the IAS may be due to poor coupling between P2Y1 receptors and SK3 channels on PDGFRα⁺ cells.

ABSTRACT

Inhibitory neuromuscular transmission (NMT) was compared in the internal anal sphincter (IAS) and rectum of the Cynomolgus monkey, an animal with a high gene sequence identity to humans. Electrical field stimulation produced nitric oxide synthase (NOS)-dependent contractile inhibition in both muscles whereas P2Y1-dependent purinergic NMT was restricted to rectum. An additional NOS-independent, α-chymotrypsin-sensitive component was identified in the IAS consistent with vasoactive intestinal peptide-ergic (VIPergic) NMT. Microelectrode recordings revealed slow NOS-dependent inhibitory junction potentials (IJPs) in both muscles and fast P2Y1-dependent IJPs in rectum. The basis for the difference in purinergic NMT was investigated. PDGFRα⁺ /SK3⁺ cells were closely aligned with nNOS⁺ /VIP⁺ neurons in both muscles. Gene expression of P2RY was the same in IAS and rectum (P2RY1>>P2RY2-14) while KCNN3 expression was 32% greater in rectum. The SK channel inhibitor apamin doubled contractile activity in rectum while having minimal effect in the IAS. Contractile inhibition elicited with the SK channel agonist CyPPA was five times faster in rectum than in the IAS. The P2Y1 receptor agonist MRS2365 inhibited contraction in rectum but increased contraction in the IAS. In conclusion, both the IAS and the rectum have nitrergic NMT whereas purinergic NMT is limited to rectum and VIPergic NMT to the IAS. The profile in monkey IAS more closely resembles that of humans than rodents. The lack of purinergic NMT in the IAS cannot be attributed to the absence of PDGFRα⁺ cells, P2Y1 receptors or SK3 channels. Rather, it appears to be due to poor coupling between P2Y1 receptors and SK3 channels on PDGFRα⁺ cells.

Upregulation of L-type calcium channels in colonic inhibitory motoneurons of P/Q-type calcium channel-deficient mice

Enteric inhibitory motoneurons use nitric oxide and a purine neurotransmitter to relax gastrointestinal smooth muscle. Enteric P/Q-type Ca2+ channels contribute to excitatory neuromuscular transmission; their contribution to inhibitory transmission is less clear. We used the colon from tottering mice (tg/tg, loss of function mutation in the α1A pore-forming subunit of P/Q-type Ca2+ channels) to test the hypothesis that P/Q-type Ca2+ channels contribute to inhibitory neuromuscular transmission and colonic propulsive motility. Fecal pellet output in vivo and the colonic migrating motor complex (ex vivo) were measured. Neurogenic circular muscle relaxations and inhibitory junction potentials (IJPs) were also measured ex vivo. Colonic propulsive motility in vivo and ex vivo was impaired in tg/tg mice. IJPs were either unchanged or somewhat larger in tissues from tg/tg compared with wild-type (WT) mice. Nifedipine (L-type Ca2+ channel antagonist) inhibited IJPs by 35 and 14% in tissues from tg/tg and WT mice, respectively. The contribution of N- and R-type channels to neuromuscular transmission was larger in tissues from tg/tg compared with WT mice. The resting membrane potential of circular muscle cells was similar in tissues from tg/tg and WT mice. Neurogenic relaxations of circular muscle from tg/tg and WT mice were similar. These results demonstrate that a functional deficit in P/Q-type channels does not alter propulsive colonic motility. Myenteric neuron L-type Ca2+ channel function increases to compensate for loss of functional P/Q-type Ca2+ channels. This compensation maintains inhibitory neuromuscular transmission and normal colonic motility.

Furthering the debate on the role of interstitial cells of Cajal in enteric inhibitory neuromuscular neurotransmission

The gut, a muscular organ, performs a critical role in transporting ingested contents, yet it is also controlled to periodically stop transport to maximize digestion and toxin detection. The complex intraluminal composition and rheology challenge the mechanistic requirements of inhibitory neuromuscular neurotransmission. The interstitial cells of Cajal (ICCs)-generated slow wave may tune the promiscuous luminal chemical environment, which prepares the smooth muscle membrane potential for a depolarizing or hyperpolarizing response as needed. Slow waves are abolished during stimulation-induced inhibitory junction potentials (IJPs) due to purinergic-nitrergic tandem neurotransmission. Recent data demonstrating intact IJPs in a genomic knockout of ICCs provide rigorous evidence of the noncontribution of ICCs during evoked neurotransmission. This perspective article discusses the priority areas of investigations in enteric musculomotor transmission, for understanding its near-perfect design for chemical space sensing, as well as diseases in which the luminal transport braking process becomes dysfunctional, leading to delayed gastric emptying or intestinal transit.

β-Nicotinamide adenine dinucleotide acts at prejunctional adenosine A1 receptors to suppress inhibitory musculomotor neurotransmission in guinea pig colon and human jejunum

Intracellular microelectrodes were used to record neurogenic inhibitory junction potentials in the intestinal circular muscle coat. Electrical field stimulation was used to stimulate intramural neurons and evoke contraction of the smooth musculature. Exposure to β-nicotinamide adenine dinucleotide (β-NAD) did not alter smooth muscle membrane potential in guinea pig colon or human jejunum. ATP, ADP, β-NAD, and adenosine, as well as the purinergic P2Y1 receptor antagonists MRS 2179 and MRS 2500 and the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine, each suppressed inhibitory junction potentials in guinea pig and human preparations. β-NAD suppressed contractile force of twitch-like contractions evoked by electrical field stimulation in guinea pig and human preparations. P2Y1 receptor antagonists did not reverse this action. Stimulation of adenosine A1 receptors with 2-chloro-N6-cyclopentyladenosine suppressed the force of twitch contractions evoked by electrical field stimulation in like manner to the action of β-NAD. Blockade of adenosine A1 receptors with 8-cyclopentyl-1,3-dipropylxanthine suppressed the inhibitory action of β-NAD on the force of electrically evoked contractions. The results do not support an inhibitory neurotransmitter role for β-NAD at intestinal neuromuscular junctions. The data suggest that β-NAD is a ligand for the adenosine A1 receptor subtype expressed by neurons in the enteric nervous system. The influence of β-NAD on intestinal motility emerges from adenosine A1 receptor-mediated suppression of neurotransmitter release at inhibitory neuromuscular junctions.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Purinergic neuromuscular transmission in the gastrointestinal tract; functional basis for future clinical and pharmacological studies

Nerve-mediated relaxation is necessary for the correct accomplishment of gastrointestinal (GI) motility. In the GI tract, NO and a purine are probably released by the same inhibitory motor neuron as inhibitory co-transmitters. The P2Y1 receptor has been recently identified as the receptor responsible for purinergic smooth muscle hyperpolarization and relaxation in the human gut. This finding has been confirmed in P2Y1 -deficient mice where purinergic neurotransmission is absent and transit time impaired. However, the mechanisms responsible for nerve-mediated relaxation, including the identification of the purinergic neurotransmitter(s) itself, are still debatable. Possibly different mechanisms of nerve-mediated relaxation are present in the GI tract. Functional demonstration of purinergic neuromuscular transmission has not been correlated with structural studies. Labelling of purinergic neurons is still experimental and is not performed in routine pathology studies from human samples, even when possible neuromuscular impairment is suspected. Accordingly, the contribution of purinergic neurotransmission in neuromuscular diseases affecting GI motility is not known. In this review, we have focused on the physiological mechanisms responsible for nerve-mediated purinergic relaxation providing the functional basis for possible future clinical and pharmacological studies on GI motility targeting purine receptors.

Cholinergic neuromuscular transmission mediated by interstitial cells of Cajal in the myenteric layer in mouse ileal longitudinal smooth muscles

To elucidate the roles played by the interstitial cells of Cajal in the myenteric layer (ICC-MY) in cholinergic neuromuscular transmission, we recorded mechanical and electrical activities in response to electrical field stimulation (EFS) of the ileal longitudinal muscle strips from WBB6F1-W/W(V) (W/W(V)) mutant mice, that lacked ICC-MY and compared with those in WBB6F1-+/+ (+/+) control mice. In +/+ muscle strips, EFS induced phasic contractions, which were abolished or strongly attenuated by atropine or tetrodotoxin. In W/W(V) preparations, EFS induced similar phasic contractions, but the cholinergic component was smaller than that in +/+ strips. This was despite of the fact that the contractions because of exogenous applications of carbachol and high K(+) solution in W/W(V) strips were comparable to or rather greater than those in the +/+ preparations. EFS induced atropine-sensitive excitatory junction potentials (EJPs) in the +/+ longitudinal smooth muscle cells but not in W/W(V) cells. In the presence of eserine, EFS induced atropine-sensitive EJPs in W/W(V) cells. These results suggest that ICC-MY mediate the cholinergic neuromuscular transmission in mouse ileal longitudinal smooth muscles. In addition, the other pathway in which ICC-MY are not involved can operate concomitantly.

Differential release of β-NAD(+) and ATP upon activation of enteric motor neurons in primate and murine colons

BACKGROUND

The purinergic component of enteric inhibitory neurotransmission is important for normal motility in the gastrointestinal (GI) tract. Controversies exist about the purine(s) responsible for inhibitory responses in GI muscles: ATP has been assumed to be the purinergic neurotransmitter released from enteric inhibitory motor neurons; however, recent studies demonstrate that β-nicotinamide adenine dinucleotide (β-NAD(+)) and ADP-ribose mimic the inhibitory neurotransmitter better than ATP in primate and murine colons. The study was designed to clarify the sources of purines in colons of Cynomolgus monkeys and C57BL/6 mice.

METHODS

High-performance liquid chromatography with fluorescence detection was used to analyze purines released by stimulation of nicotinic acetylcholine receptors (nAChR) and serotonergic 5-HT(3) receptors (5-HT(3)R), known to be present on cell bodies and dendrites of neurons within the myenteric plexus.

KEY RESULTS

Nicotinic acetylcholine receptor or 5-HT(3)R agonists increased overflow of ATP and β-NAD(+) from tunica muscularis of monkey and murine colon. The agonists did not release purines from circular muscles of monkey colon lacking myenteric ganglia. Agonist-evoked overflow of β-NAD(+), but not ATP, was inhibited by tetrodotoxin (0.5 μmol L(-1)) or ω-conotoxin GVIA (50 nmol L(-1)), suggesting that β-NAD(+) release requires nerve action potentials and junctional mechanisms known to be critical for neurotransmission. ATP was likely released from nerve cell bodies in myenteric ganglia and not from nerve terminals of motor neurons.

CONCLUSIONS & INFERENCES

These results support the conclusion that ATP is not a motor neurotransmitter in the colon and are consistent with the hypothesis that β-NAD(+), or its metabolites, serve as the purinergic inhibitory neurotransmitter.

P2Y1 purinoreceptors are fundamental to inhibitory motor control of murine colonic excitability and transit

Activation of enteric inhibitory motor neurons causes inhibitory junctional potentials (IJPs) and muscle relaxation in mammalian gastrointestinal (GI) muscles, including humans. IJPs in many GI muscles are bi-phasic with a fast initial hyperpolarization (fIJP) due to release of a purine neurotransmitter and a slower hyperpolarization component (sIJP) due to release of nitric oxide. We sought to characterize the nature of the post-junctional receptor(s) involved in transducing purinergic neural inputs in the murine colon using mice with genetically deactivated P2ry1. Wild-type mice had characteristic biphasic IJPs and pharmacological dissection confirmed that the fIJP was purinergic and the sIJP was nitrergic. The fIJP was completely absent in P2ry1(−/−) mice and the P2Y1 receptor antagonist MRS2500 had no effect on electrical activity or responses to electrical field stimulation of intrinsic nerves in these mice. Contractile experiments confirmed that purinergic responses were abolished in P2ry1(−/−) mice. Picospritzing of neurotransmitter candidates (ATP and its primary metabolite, ADP) and β-NAD (and its primary metabolite, ADP-ribose, ADPR) caused transient hyperpolarization responses in wild-type colons, but responses to β-NAD and ADPR were completely abolished in P2ry1(−/−) mice. Hyperpolarization and relaxation responses to ATP and ADP were retained in colons of P2ry1(−/−) mice. Video imaging revealed that transit of fecal pellets was significantly delayed in colons from P2ry1(−/−) mice. These data demonstrate the importance of purinergic neurotransmission in regulating colonic motility and confirm pharmacological experiments suggesting that purinergic neurotransmission is mediated via P2Y1 receptors.

Functional evidence for purinergic inhibitory neuromuscular transmission in the mouse internal anal sphincter

The neurotransmitter(s) underlying nitric oxide synthase (NOS)-independent neural inhibition in the internal anal sphincter (IAS) is still uncertain. The present study investigated the role of purinergic transmission. Contractile and electrical responses to electrical field stimulation of nerves (0.1-5 Hz for 10-60 s) were recorded in strips of mouse IAS. A single stimulus generated a 28-mV fast inhibitory junction potential (IJP) and relaxation. The NOS inhibitor N(omega)-nitro-l-arginine (l-NNA) reduced the fast IJP duration by 20%. Repetitive stimulation at 2.5-5 Hz caused a more sustained IJP and sustained relaxation. l-NNA reduced relaxation at 1 Hz and the sustained IJP at 2.5-5 Hz. All other experiments were carried out in the presence of NOS blockade. IJPs and relaxation were significantly reduced by the P2 receptor antagonists 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS) (100 microM), by desensitization of P2Y receptors with adenosine 5'-[beta-thio]diphosphate (ADP-betaS) (10 microM), and by the selective P2Y1 receptor blocker 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS2179) (10 microM). Relaxation and IJPs were also significantly reduced by the K(+) channel blocker apamin (1 microM). Removal of extracellular potassium (K(o)) increased IJP amplitude to 205% of control, whereas return of K(o) 30 min later hyperpolarized cells by 19 mV and reduced IJP amplitude to 50% of control. Exogenous ATP (3 mM) relaxed muscles in the presence of TTX (1 microM) and hyperpolarized cells by 15 mV. In conclusion, these data suggest that purinergic transmission significantly contributes to NOS-independent neural inhibition in the mouse IAS. P2Y1 receptors, as well as at least one other P2 receptor subtype, contribute to this pathway. Purinergic receptors activate apamin-sensitive K(+) channels as well as other apamin-insensitive conductances leading to hyperpolarization and relaxation.

Beta-nicotinamide adenine dinucleotide is an inhibitory neurotransmitter in visceral smooth muscle

Peripheral inhibitory nerves are physiological regulators of the contractile behavior of visceral smooth muscles. One of the transmitters responsible for inhibitory neurotransmission has been reputed to be a purine, possibly ATP. However, the exact identity of this substance has never been verified. Here we show that beta-nicotinamide adenine dinucleotide (beta-NAD), an inhibitory neurotransmitter candidate, is released by stimulation of enteric nerves in gastrointestinal muscles, and the pharmacological profile of beta-NAD mimics the endogenous neurotransmitter better than ATP. Levels of beta-NAD in superfusates of muscles after nerve stimulation exceed ATP by at least 30-fold; unlike ATP, the release of beta-NAD depends on the frequency of nerve stimulation. beta-NAD is released from enteric neurons, and release was blocked by tetrodotoxin or omega-conotoxin GVIA. beta-NAD is an agonist for P2Y1 receptors, as demonstrated by receptor-mediated responses in HEK293 cells expressing P2Y1 receptors. Exogenous beta-NAD mimics the effects of the enteric inhibitory neurotransmitter. Responses to beta-NAD and inhibitory junction potentials are blocked by the P2Y1-selective antagonist, MRS2179, and the nonselective P2 receptor antagonists, pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid and suramin. Responses to ATP are not blocked by these P2Y receptor inhibitors. The expression of CD38 in gastrointestinal muscles, and specifically in interstitial cells of Cajal, provides a means of transmitter disposal after stimulation. beta-NAD meets the traditional criteria for a neurotransmitter that contributes to enteric inhibitory regulation of visceral smooth muscles.

Inhibitory neuromuscular transmission mediated by the P2Y1 purinergic receptor in guinea pig small intestine

ATP is a putative inhibitory neurotransmitter responsible for inhibitory junction potentials (IJPs) at neuromuscular junctions (IJPs) in the intestine. This study tested the hypothesis that the purinergic P2Y(1) receptor subtype mediates the IJPs. IJPs were evoked by focal electrical stimulation in the myenteric plexus and recorded with "sharp" intracellular microelectrodes in the circular muscle coat. Stimulation evoked three categories of IJPs: 1) purely purinergic IJPs, 2) partially purinergic IJPs, and 3) nonpurinergic IJPs. Purely purinergic IJPs were suppressed by the selective P2Y(1) purinergic receptor antagonist MRS2179. Purely purinergic IJPs comprised 26% of the IJPs. Partially purinergic IJPs (72% of the IJPs) consisted of a component that was abolished by MRS2179 and a second unaffected component. The MRS2179-insensitive component was suppressed or abolished by inhibition of formation of nitric oxide by N(omega)-nitro-l-arginine methyl ester (l-NAME) in some, but not all, IJPs. An unidentified neurotransmitter, different from nitric oxide, mediated the second component in these cases. Nonpurinergic IJPs were a small third category (4%) of IJPs that were abolished by l-NAME and unaffected by MRS2179. Exogenous application of ATP evoked IJP-like hyperpolarizing responses, which were blocked by MRS2179. Application of apamin, which suppresses opening of small-conductance Ca(2+)-operated K(+) channels in the muscle, decreased the amplitude of the purinergic IJPs and the amplitude of IJP-like responses to ATP. The results support ATP as a neurotransmitter for IJPs in the intestine and are consistent with the hypothesis that the P2Y(1) purinergic receptor subtype mediates the action of ATP.

Pacemaker activity and inhibitory neurotransmission in the colon of Ws/Ws mutant rats

The aim of this study was to characterize the pacemaker activity and inhibitory neurotransmission in the colon of Ws/Ws mutant rats, which harbor a mutation in the c-kit gene that affects development of interstitial cells of Cajal (ICC). In Ws/Ws rats, the density of KIT-positive cells was markedly reduced. Wild-type, but not Ws/Ws, rats showed low- and high-frequency cyclic depolarization that were associated with highly regular myogenic motor patterns at the same frequencies. In Ws/Ws rats, irregular patterns of action potentials triggered irregular muscle contractions occurring within a bandwidth of 10-20 cycles/min. Spontaneous activity of nitrergic nerves caused sustained inhibition of muscle activity in both wild-type (+/+) and Ws/Ws rats. Electrical field stimulation of enteric nerves, after blockade of cholinergic and adrenergic activity, elicited inhibition of mechanical activity and biphasic inhibitory junction potentials both in wild-type and Ws/Ws rats. Apamin-sensitive, likely purinergic, inhibitory innervation was not affected by loss of ICC. Variable presence of nitrergic innervation likely reflects the presence of direct nitrergic innervation to smooth muscle cells as well as indirect innervation via ICC. In summary, loss of ICC markedly affects pacemaker and motor activities of the rat colon. Inhibitory innervation is largely maintained but nitrergic innervation is reduced possibly related to the loss of ICC-mediated relaxation.

Evidence that ATP or a related purine is an excitatory neurotransmitter in the longitudinal muscle of mouse distal colon

BACKGROUND AND PURPOSE

This study analysed the contribution of the purinergic system to enteric neurotransmission in the longitudinal muscle of mouse distal colon.

EXPERIMENTAL APPROACH

Motor responses to exogenous ATP and to nerve stimulation in vitro were assessed as changes in isometric tension.

KEY RESULTS

ATP induced a concentration-dependent contraction, reduced by 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzene disulphonic acid (PPADS), suramin, P2Y purinoreceptor desensitisation with adenosine 5'-O-2-thiodiphosphate (ADPbetaS), and atropine, but unaffected by P2X purinoceptor desensitisation with alpha,beta-methylene ATP (alpha,beta-meATP) and by 2,2-dimethyl-propionic acid 3-(2-chloro-6-methylaminopurin-9-yl)-2-(2,2-dimethyl-propionyloxymethyl)-propyl ester (MRS 2395), a P2Y(12) selective antagonist. The response to ATP was increased by 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS 2179), a P2Y(1) selective antagonist, tetrodotoxin (TTX) or N(omega)-nitro-L-arginine methyl ester (L-NAME). ADPbetaS, a P2Y-purinergic agonist, induced muscular contraction, with the same pharmacological profile as the ATP-induced contraction. ADP, a natural ligand for P2Y(1) receptors, induced muscular relaxation, antagonized by MRS 2179 and by TTX or L-NAME. Nerve stimulation elicited a transient nitrergic relaxation, followed by contraction. Contractile responses was reduced by atropine, PPADS, suramin, P2Y purinoceptor desensitisation, but not by P2X purinoceptor desensitisation, MRS 2179 or MRS 2395. None of the purinergic antagonists modified the nerve-evoked relaxation.

CONCLUSIONS AND IMPLICATIONS

In the longitudinal muscle of mouse distal colon, ATP, through ADPbetaS-sensitive P2Y purinoceptors, contributed to the excitatory neurotransmission acting directly on smooth muscle and indirectly via activation of cholinergic neurons. Moreover, P2Y1 purinoceptors appear to be located on nitrergic inhibitory neurons. This study provides new insights into the role of purines in the mechanism inducing intestinal transit in mouse colon.

Study on the cyclic GMP-dependency of relaxations to endogenous and exogenous nitric oxide in the mouse gastrointestinal tract

BACKGROUND AND PURPOSE

cGMP mediates nitrergic relaxations of intestinal smooth muscle, but several studies have indicated that cGMP-independent mechanisms may also be involved. We addressed this contention by studying the effect of ODQ and ns2028, specific inhibitors of soluble guanylate cyclase, on nitrergic relaxations of the mouse gut.

EXPERIMENTAL APPROACH

Mouse gastric fundus and small intestinal muscle preparations were mounted in organ baths to study relaxations to exogenous NO, NO donors and electrical field stimulation (EFS) of enteric nerves.

KEY RESULTS

In gastric fundus longitudinal muscle strips, ODQ and NS2028 abolished the L-nitroarginine-sensitive relaxations to EFS and the relaxations to NO and NO donors, glyceryl trinitrate (GTN), SIN-1 and sodium nitroprusside (SNP). EFS of intestinal segments and muscle strips showed L-nitroarginine-resistant relaxations, which were abolished by the purinoceptor blocker suramin. In the presence of suramin, ODQ and NS2028 abolished all relaxations to EFS in intestinal segments and strips. ODQ and NS2028 abolished the relaxations to exogenous NO and to the NO donors GTN, SIN-1 and SNP in circular and longitudinal intestinal muscle strips. Intestinal segments showed residual relaxations to NO and GTN.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that relaxations to endogenous NO in the mouse gastric fundus and small intestine are completely dependent on cGMP. ODQ and NS2028 incompletely blocked nitrergic relaxations to exogenous NO in intact intestinal segments. However, it is unlikely that this is due to the involvement of cGMP-independent pathways because ODQ and NS2028 abolished all relaxations to endogenous and exogenous NO in intestinal muscle strips.

P2Y1 receptors mediate inhibitory purinergic neuromuscular transmission in the human colon

Indirect evidence suggests that ATP is a neurotransmitter involved in inhibitory pathways in the neuromuscular junction in the gastrointestinal tract. The aim of this study was to characterize purinergic inhibitory neuromuscular transmission in the human colon. Tissue was obtained from colon resections for neoplasm. Muscle bath, microelectrode experiments, and immunohistochemical techniques were performed. 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) was used as a selective inhibitor of P2Y(1) receptors. We found that 1) ATP (1 mM) and adenosine 5'-beta-2-thiodiphosphate (ADPbetaS) (10 microM), a preferential P2Y agonist, inhibited spontaneous motility and caused smooth muscle hyperpolarization (about -12 mV); 2) MRS 2179 (10 microM) and apamin (1 microM) significantly reduced these effects; 3) both the fast component of the inhibitory junction potential (IJP) and the nonnitrergic relaxation induced by electrical field stimulation were dose dependently inhibited (IC(50) approximately 1 microM) by MRS 2179; 4) ADPbetaS reduced the IJP probably by a desensitization mechanism; 5) apamin (1 microM) reduced the fast component of the IJP (by 30-40%) and the inhibitory effect induced by electrical field stimulation; and 6) P2Y(1) receptors were localized in smooth muscle cells as well as in enteric neurons. These results show that ATP or a related purine is released by enteric inhibitory motoneurons, causing a fast hyperpolarization and smooth muscle relaxation. The high sensitivity of MRS 2179 has revealed, for the first time in the human gastrointestinal tract, that a P2Y(1) receptor present in smooth muscle probably mediates this mechanism through a pathway that partially involves apamin-sensitive calcium-activated potassium channels. P2Y(1) receptors can be an important pharmacological target to modulate smooth muscle excitability.

Inhibitory responses to exogenous adenosine in murine proximal and distal colon

The aims of the present study were firstly, to characterize pharmacologically the subtypes of P(1) purinoreceptors involved in the inhibitory effects induced by exogenous adenosine in longitudinal smooth muscle of mouse colon, and secondly, to examine differences in the function and distribution of these receptors between proximal and distal colon. Adenosine (100 microM-3 mM) caused a concentration-dependent reduction of the amplitude of spontaneous contractions in the proximal colon, and muscular relaxation in the distal colon. In the proximal colon, adenosine effects were antagonized by a selective A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM), but were not modified by 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM) or by 9-chloro-2-(2-furanyl)-5-((phenylacetyl)amino)- [1,2,4]triazolo[1,5-c]quinazoline (MRS 1220, 0.1 microM), selective A(2) and A(3) receptor antagonists, respectively. In the distal colon, adenosine effects were antagonized by DPCPX, DMPX, and by a selective A(2B) receptor antagonist, 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(n-propyl) xanthine (MRS 1754, 10 microM), but not by 8-(3-chlorostyryl)-caffeine (CSC, 10 microM), a selective A(2A) receptor antagonist, or by MRS 1220. Tetrodotoxin (TTX 1 microM), the nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 microM), or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM), an inhibitor of soluble guanylyl cyclase, reduced adenosine effects only in distal colon. In addition, L-NAME induced a further reduction of adenosine relaxation in the presence of DPCPX, but not in the presence of MRS 1754. From these results we conclude that, in the murine proximal colon, adenosine induces inhibitory effects via TTX-insensitive activation of A(1) receptor. In the distal colon, adenosine activates both A(1) and A(2B) receptors, the latter located on enteric inhibitory neurons releasing NO.

Nucleotide regulation of the voltage-dependent nonselective cation conductance in murine colonic myocytes

ATP is proposed to be a major inhibitory neurotransmitter in the gastrointestinal (GI) tract, causing hyperpolarization and smooth muscle relaxation. ATP activates small-conductance Ca(2+)-activated K(+) channels that are involved in setting the resting membrane potential and causing inhibitory junction potentials. No reports are available examining the effects of ATP on voltage-dependent inward currents in GI smooth muscle cells. We previously reported two types of voltage-dependent inward currents in murine proximal colonic myocytes: a low-threshold voltage-activated, nonselective cation current (I(VNSCC)) and a relatively high-threshold voltage-activated (L-type) Ca(2+) current (I(L)). Here we have investigated the effects of ATP on these currents. External application of ATP (1 mM) did not affect I(VNSCC) or I(L) in dialyzed cells. ATP (1 mM) increased I(VNSCC) and decreased I(L) in the perforated whole-cell configuration. UTP and UDP (1 mM) were more potent than ATP on I(VNSCC). ADP decreased I(L) but had no effect on I(VNSCC). The order of effectiveness was UTP = UDP > ATP > ADP. These effects were not blocked by pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS), but the phospholipase C inhibitor U-73122 reversed the effects of ATP on I(VNSCC). ATP stimulation of I(VNSCC) was also reversed by protein kinase C (PKC) inhibitors chelerythrine chloride or bisindolylmaleimide I. Phorbol 12,13-dibutyrate mimicked the effects of ATP. RT-PCR showed that P2Y(4) is expressed by murine colonic myocytes, and this receptor is relatively insensitive to PPADS. Our data suggest that ATP activates I(VNSCC) and depresses I(L) via binding of P2Y(4) receptors and stimulation of the phospholipase C/PKC pathway.

Impact of caveolin-1 knockout on NANC relaxation in circular muscles of the mouse small intestine compared with longitudinal muscles

Recently, we showed that caveolin-1 (cav1) knockout mice (Cav1(-/-) mice) have impaired nitric oxide (NO) function in the longitudinal muscle (LM) layer of the small intestine. The defect was a reduced responsiveness of the muscles to NO compensated by an increase in the function of apamin-sensitive, nonadrenergic, noncholinergic (NANC) mediators. In the present study, we examined similarly the effects of cav1 knockout on the relaxation in circular muscle (CM) of the mouse small intestine. CM of Cav1(-/-) mice also showed defective NO function, but less than in LM, as well as more activation of apamin-sensitive NANC mediators. CM of Cav1(-/-) mice, like LM, lacked cav1 but retained small amounts of cav3 and caveolae in the outer CM layer. In addition, we also examined the effects of a soluble guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinazolin-1-one (ODQ), on electric field stimulation (EFS)-mediated relaxation in both LM and CM. ODQ had an effect similar to the block of NO synthesis. Moreover, we compared the actions of two NO donors in the LM and CM of control and Cav1(-/-) mice. Similar to LM, CM of Cav1(-/-) mice showed a reduced responsiveness to the NO donors sodium nitroprusside and S-nitroso-N-acetyl penicillamine. However, both ODQ and apamin blocked the inhibitory effects of the NO donors in LM, whereas apamin had no effect in CM. In conclusion, cav1 knockout affects NO function in both LM and CM, but its effects in CM differ significantly.

Excitatory purinergic neurotransmission in smooth muscle of guinea-pig [corrected] taenia caeci

Non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmission has been an area of intense interest in gut motor physiology, whereas excitatory NANC neurotransmission has received less attention. In order to further explore excitatory NANC neurotransmission, we performed conventional intracellular recordings from guinea-pig taenia caeci smooth muscle. Tissue was perfused with oxygenated Krebs solution at 35 degrees C and nerve responses evoked by either oral or aboral nerve stimulation (NS) (4 square wave pulses, 0.3 ms duration, 20 Hz). Electrical activity was characterized by slow waves upon which one to three action potentials were superimposed. Oral NS evoked an inhibitory junction potential (IJP) at either the valley or peak of the slow wave. Application of nifedipine (1 microM) abolished slow waves and action potentials, but membrane potential flunctuations (1-3 mV) and IJPs remained unaffected. Concomitant application of apamin (300 nM), a small-conductance Ca(2+)-activated K(+) channel blocker, converted the IJP to an EJP that was followed by slow IJP. Further administration of N(G)-nitro-l-arginine methyl ester (l-NAME, 200 microM), a nitric oxide synthase inhibitor, abolished the slow IJP without affecting the EJP, implying that the slow IJP is due to nitrergic innervation. The EJP was abolished by tetrodotoxin (1 microM), but was not significantly affected by atropine (3 microM) and guanethidine (3 microM) or hexamethonium (500 microM). Substance P (SP, 1 microM) desensitization caused slight attenuation of the EJP, but the EJP was abolished by desensitization with alpha,beta-methylene ATP (50 microM), a P2 purinoceptor agonist that is more potent than ATP at the P2X receptor subtype, suramin (100 microM), a non-selective P2 purinoceptor antagonist, and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 100 microM) , a selective P2X purinoceptor antagonist. In contrast, the EJP was unaffected by MRS-2179 (2 microM), a selective P2Y(1) receptor antagonist. Aboral NS evoked an apamin- and l-NAME-sensitive IJP, but virtually no NANC EJP. These data suggest the presence of polarized excitatory purinergic neurotransmission in guinea-pig taenia caeci, which appears to be mediated by P2X purinoceptors, most likely the P2X(1) subtype.

Participation of ATP in nonadrenergic, noncholinergic relaxation of longitudinal muscle of wistar rat jejunum

A role of ATP in nonadrenergic, noncholinergic (NANC) relaxations was examined in the Wistar rat jejunum. Electrical field stimulation (EFS) induced NANC relaxation of longitudinal muscle of the jejunal segments in a frequency-dependent manner. A purinoceptor antagonist, adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS, 100 muM) inhibited the relaxation: relaxations induced by EFS at lower or higher frequencies were either completely or partially inhibited, respectively. After the jejunal segments had been desensitized to ATP, the relaxations were decreased to the same extent as those inhibited by A3P5PS. An inhibitor of small conductance Ca(2+)-activated K(+) channels (SK channels), apamin (100 nM), completely inhibited EFS-induced relaxations. Treatment of the segments with an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin (1 muM), significantly inhibited the relaxations. The exogenous ATP-induced relaxation of longitudinal muscle occurred with a concomitant decrease in intracellular Ca(2+) levels. Apamin and thapsigargin abolished these ATP-induced responses. A3P5PS significantly inhibited the inhibitory junction potentials which were induced in the longitudinal muscle cells. In addition, apamin significantly inhibited the hyperpolarization that was induced by exogenous ATP in the cells. These findings in the Wistar rat jejunum suggest that ATP participates in the NANC relaxation via activation of SK channels induced by Ca(2+) ions that are released from the thapsigargin-sensitive store site.

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.

Inhibitory pathways in the circular muscle of rat jejunum

1. Conflicting data have been reported on the contribution of nitric oxide (NO) to inhibitory neurotransmission in rat jejunum. Therefore, the mechanism of relaxation and contribution to inhibitory neurotransmission of NO, adenosine 5'-triphosphate (ATP), vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) was examined in the circular muscle of Wistar-Han rat jejunum. 2. Mucosa-free circular muscle strips were precontracted with methacholine in the presence of guanethidine and exposed to electrical field stimulation (EFS) and exogenous NO, ATP, VIP and PACAP. All stimuli induced reduction of tone and inhibition of phasic motility. Only electrically induced responses were sensitive to tetrodotoxin (3 x 10(-6) m). 3. NO (10(-6)-10(-4) m)-induced concentration-dependent relaxations that were inhibited by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ; 10(-5) m) and the small conductance Ca(2+)-activated K(+)-channel blocker apamin (APA; 3 x 10(-8) m). 4. Relaxations elicited by exogenous ATP (10(-4)-10(-3) m) were inhibited by the P2Y purinoceptor antagonist reactive blue 2 (RB2; 3 x 10(-4) m), but not by APA and ODQ. 5. The inhibitory responses evoked by 10(-7) m VIP and 3 x 10(-8) m PACAP were decreased by the selective PAC(1) receptor antagonist PACAP(6-38) (3 x 10(-6) m) and APA. The VPAC(2) receptor antagonist PG99-465 (3 x 10(-7) m) reduced relaxations caused by VIP, but not those by PACAP, while the VPAC(1) receptor antagonist PG97-269 (3 x 10(-7) m) had no influence. 6. EFS-induced relaxations were inhibited by the NO-synthase inhibitor N(omega)-nitro-l-arginine methyl ester (3 x 10(-4) m), ODQ and APA, but not by RB2, PG97-269, PG99-465 and PACAP(6-38). 7. These results suggest that NO is the main inhibitory neurotransmitter in the circular muscle of Wistar-Han rat jejunum acting through a rise in cyclic guanosine monophosphate levels and activation of small conductance Ca(2+)-dependent K(+) channels.

Mechanism of active repolarization of inhibitory junction potential in murine colon

Enteric inhibitory responses in gastrointestinal (GI) smooth muscles involve membrane hyperpolarization that transiently reduce the excitability of GI muscles. We examined the possibility that an active repolarization mechanism participates in the restoration of resting membrane potential after fast inhibitory junction potentials (IJPs) in the murine colon. Previously, we showed these cells express a voltage-dependent nonselective cation conductance (NSCC) that might participate in active repolarization of IJPs. Colonic smooth muscle cells were impaled with micro-electrodes and voltage responses to nerve-evoked IJPs, and locally applied ATP were recorded. Ba2+ (500 muM), a blocker of the NSCC, slowed the rate of repolarization of IJPs. We also tested the effects of Ba2+, Ni2+, and mibefradil, all blockers of the NSCC, on responses to locally applied ATP. Spritzes of ATP caused transient hyperpolarization, and the durations of these responses were significantly increased by the blockers of the NSCC. We considered whether NSCC blockers might affect ATP metabolism and found that Ni2+ decreased ATP breakdown in colonic muscles. Mibefradil had no effect on ATP metabolism. Because both Ni2+ and mibefradil had similar effects on prolonging responses to ATP, it appears that restoration of resting membrane potential after ATP spritzes is not primarily due to ATP metabolism. Neurally released enteric inhibitory transmitter and locally applied ATP resulted in transient hyperpolarizations of murine colonic muscles. Recovery of membrane potential after these responses appears to involve an active repolarization mechanism due to activation of the voltage-dependent NSCC expressed by these cells.

Functional evidence that ATP or a related purine is an inhibitory NANC neurotransmitter in the mouse jejunum: study on the identity of P2X and P2Y purinoceptors involved

1. Conflicting views exist on whether ATP is a neurotransmitter in the enteric nervous system. We investigated the role of ATP in enteric transmission in circular muscle strips of the mouse jejunum. 2. On PGF2alpha-precontracted muscle strips and in the presence of atropine and guanethidine, electrical field stimulation (EFS, 1-8 Hz) of nonadrenergic noncholinergic (NANC) nerves induced transient relaxations that were abolished by the nerve-conductance blocker tetrodotoxin. The NO synthase blocker l-nitroarginine (l-NOARG) partially inhibited the NANC relaxations to EFS, but fast-twitch relaxations to EFS were still observed in the presence of l-NOARG. 3. In the presence of l-NOARG, ATP, the P2X receptor agonist alphabetaMeATP and the P2Y receptor agonist ADPbetaS relaxed jejunal muscle strips. Tetrodotoxin did not affect the relaxation to ATP and ADPbetaS, but inhibited that to alphabetaMeATP. 4. The l-NOARG-resistant NANC relaxations to EFS were almost abolished by apamin, a blocker of small-conductance Ca2+ activated K+ channels, and by suramin and PPADS, blockers of P2 purinoceptors. Relaxations to ATP were almost abolished by apamin and suramin but not affected by PPADS. 5. Desensitisation of alphabetaMeATP-sensitive P2X receptors, the P2X receptor blocker Evans blue and the P2X1,2,3 receptor blocker NF 279 inhibited the l-NOARG-resistant NANC relaxations to EFS and that to alphabetaMeATP without affecting the relaxation to ADPbetaS. Brilliant blue G, a P2X2,5,7 receptor blocker, did not affect the relaxations to EFS. 6. Desensitisation of P2Y receptors and MRS 2179, a P2Y1 receptor blocker, virtually abolished the l-NOARG-resistant NANC relaxations to EFS and the relaxation to ADPbetaS without affecting the relaxation to alphabetaMeATP. 7. Dipyridamole, an adenosine uptake inhibitor, or theophylline and 8-phenyltheophylline, blockers of P1 and A1 purinoceptors, respectively, did not affect the purinergic NANC relaxations to EFS. 8. Our results suggest that ATP or a related purine acts as an inhibitory NANC neurotransmitter in the mouse jejunum, activating P2 but not P1 purinoceptors. Relaxations to the purinergic NANC neurotransmitter mainly involve P2Y receptors of the P2Y1 subtype that are located postjunctionally. Purinergic NANC neurotransmission also involves P2X receptors, most likely of the P2X1 and P2X3 subtype, located pre- and/or postjunctionally.

ATP and sensory transduction in the enteric nervous system

ATP is a neurotransmitter in the central and peripheral nervous systems and is also involved in peripheral inflammation and transmission of the sensation of pain. Recently, the regulated release of ATP from non-neuronal sources has been shown to play a role in the activation of sensory nerve terminals. Within the enteric nervous system, which is present in the wall of the gastrointestinal tract, ATP plays three major roles. ATP acts as an inhibitory transmitter from the enteric motor neurons to the smooth muscle via P2Y receptors. ATP is released as an excitatory neurotransmitter between enteric interneurons and from the interneurons to the motor neurons via P2Y and P2X receptors. Finally, ATP may act as a sensory mediator, from epithelial sources to the intrinsic sensory nerve terminals. Thus, ATP participates in the transduction of sensory stimuli from the gut lumen and in the subsequent initiation and propagation of enteric reflexes.

Role of Ca2+-activated Cl- channels and MLCK in slow IJP in opossum esophageal smooth muscle

The possible contribution of Ca2+-activated Cl- channel [I(Cl(Ca))] and myosin light-chain kinase (MLCK) to nonadrenergic, noncholinergic slow inhibitory junction potentials (sIJP) was studied using conventional intracellular microelectrode recordings in circular smooth muscle of opossum esophageal body and guinea pig ileum perfused with Krebs solution containing atropine (3 microM), guanethidine (3 microM), and substance P (1 microM). In opossum esophageal circular smooth muscle, resting membrane potential (MP) was -51.9 +/- 0.7 mV (n = 89) with MP fluctuations of 1-3 mV. A single square-wave nerve stimulation of 0.5 ms duration and 80 V induced a sIJP with amplitude of 6.3 +/- 0.2 mV, half-amplitude duration of 635 +/- 19 ms, and rebound depolarization amplitude of 2.4 +/- 0.1 mV (n = 89). 9-Anthroic acid (A-9-C), niflumic acid (NFA), wortmannin, and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) abolished MP fluctuations, sIJP, and rebound depolarization in a concentration-dependent manner. A-9-C and NFA but not wortmannin and ML-9 hyperpolarized MP. In guinea pig ileal circular smooth muscle, nerve stimulation elicited an IJP composed of both fast (fIJP) and slow (sIJP) components, followed by rebound depolarization. NFA (200 microM) abolished sIJP and rebound depolarization but left the fIJP intact. These data suggest that in the tissues studied, activation of I(Cl(Ca)), which requires MLCK, contributes to resting MP, and that closing of I(Cl(Ca)) is responsible for sIJP.

The P(2)-purinoceptor antagonist suramin is a competitive antagonist at vasoactive intestinal peptide receptors in the rat gastric fundus

The P(2)-purinoceptor antagonist, suramin, was used to investigate the possible involvement of adenosine 5'-triphosphate (ATP) in the inhibitory non-adrenergic non-cholinergic (NANC) innervation of the rat gastric fundus. ATP (1-30 microM) produced biphasic responses consisting of concentration-dependent relaxations followed by concentration-dependent contractions. Suramin (200 microM) significantly reduced relaxations and abolished contractions to ATP. Under NANC conditions, electrical field stimulation (EFS) induced frequency-dependent relaxations. Suramin (200 microM) and the peptidase alpha-chymotrypsin (1 u ml(-1)) had the same effects on EFS-induced relaxations: their duration was reduced, but their magnitude was unaffected. Cumulative relaxations to vasoactive intestinal peptide (VIP; 0.1-100 nM), and to the VIP analogue pituitary adenylate cyclase activating peptide 1-27 (PACAP; 0.2-100 nM), were almost completely abolished by alpha-chymotrypsin (1 u ml(-1)), and were inhibited by suramin (3-200 microM) in an apparently competitive manner. Schild plot analysis indicated that suramin had pA(2) values of 5.1+/-0.2 (Hill slope=0.9+/-0.2) and 5.6+/-0.1 (Hill slope=1.0+/-0.1), against VIP and PACAP, respectively. Concentration-dependent relaxations to nitric oxide (1-30 microM) and cumulative relaxations to isoprenaline (0.1-300 nM) were not affected by suramin (200 microM). No conclusions can be made regarding the possible involvement of ATP in EFS-induced NANC relaxations. The results suggest that suramin acts as a competitive antagonist at VIP receptors in the rat gastric fundus.

Effect of exogenous ATP on canine jejunal smooth muscle

Intracellular recordings were made from the circular smooth muscle cells of the canine jejunum to study the effect of exogenous ATP and to compare the ATP response to the nonadrenergic, noncholinergic (NANC) inhibitory junction potential (IJP) evoked by electrical field stimulation (EFS). Under NANC conditions, exogenous ATP evoked a transient hyperpolarization (6.5 +/- 0.6 mV) and EFS evoked a NANC IJP (17 +/- 0.4 mV). Omega-conotoxin GVIA (100 nM) and a low-Ca(2+), high-Mg(2+) solution abolished the NANC IJP but had no effect on the ATP-evoked hyperpolarization. The ATP-evoked hyperpolarization and the NANC IJP were abolished by apamin (1 microM) and N(G)-nitro-L-arginine (100 microM). Oxyhemoglobin (5 microM) partially (38.8 +/- 5.5%) reduced the amplitude of the NANC IJP but had no effect on the ATP-evoked hyperpolarization. Neither the NANC IJP nor the ATP-evoked hyperpolarization was affected by P2 receptor antagonists or agonists, including suramin, reactive blue 2, 1-(N, O-bis-[5-isoquinolinesulfonyl]-N-methyl-L-tyrosyl)-4-phenylpiperazine , pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, alpha, beta-methylene ATP, 2-methylthioadenosine 5'-triphosphate tetrasodium salt, and adenosine 5'-O-2-thiodiphosphate. The data suggest that ATP evoked an apamin-sensitive hyperpolarization in circular smooth muscle cells of the canine jejunum via local production of NO in a postsynaptic target cell.