Effect of different calcium channel blockers on inhibitory junction potentials and slow waves in porcine ileum

Mechanisms of butylidenephthalide for twitch facilitation in electrically stimulated mouse vas deferens

CONTEXT

The rhizome of Ligusticum chuaxiong Hort. (Umbelliferae) has been used by Chinese for several thousand years. Its main constituent, butylidenephthalide (Bdph), was proved to be active in inhibiting rat uterine contractions induced by prostaglandin F_2α and was reported to be a nonspecific antispamodic and a blocker of voltage-dependent Ca²⁺ channels (VDCCs).

OBJECTIVES

The present study investigates the mechanisms of Bdph for twitch facilitation in ICR mouse vas deferens (MVD).

MATERIALS AND METHODS

Electrical field stimulation (EFS, supramaximal voltage ranging from 60-90 V, 1 ms, 0.2 Hz) was applied to the isolated MVD in Krebs solution. Interactions between Bdph (50 µM) and calcium antagonist (verapamil, diltiazem or aspaminol) on the EFS-evoked twitch responses were determined. The number of experiments was 3-18.

RESULTS

Bdph (50 µM)-induced twitch facilitations from 100 to 391.9% were unrelated to activation of postjunctional cholinergic or adrenergic receptors. Verapamil and Bdph unabolished the twitch facilitation each other. Diltiazem unabolished the Bdph-induced twitch facilitation. In contrast, Bdph abolished those induced by diltiazem. Aspaminol at 20 μM abolished the Bdph-induced twitch facilitation. In contrast, Bdph abolished those induced by aspaminol. Tetraethylammonium and 4-aminopyridine, the K⁺ channel blockers, significantly augmented the Bdph-induced twitch facilitation.

DISCUSSION AND CONCLUSIONS

Bdph may bind to the different, more and same subtypes of VDCCs from verapamil, than diltiazem, and as aspaminol does on prejunctional membrane, respectively. Besides a blocker of VDCCs, Bdph may be a blocker of K⁺ channels on prejunctional membrane. Thus, Bdph depolarized the membrane and facilitated the cumulative Ca²⁺-induced twitch responses.

R-Type Ca2+ channels couple to inhibitory neurotransmission to the longitudinal muscle in the guinea-pig ileum

NEW FINDINGS

What is the central question of this study? Subtypes of enteric neurons are coded by the neurotransmitters they synthesize, but it is not known whether enteric neuron subtypes might also be coded by other proteins, including calcium channel subtypes controlling neurotransmitter release. What is the main finding and its importance? Our data indicate that guinea-pig ileum myenteric neuron subtypes may be coded by calcium channel subtypes. We found that R-type calcium channels are expressed by inhibitory but not excitatory longitudinal muscle motoneurons. R-Type calcium channels are also not expressed by circular muscle inhibitory motoneurons. Calcium channel subtype-selective antagonists could be used to target subtypes of neurons to treat gastrointestinal motility disorders. There is evidence that R-type Ca²⁺ channels contribute to synaptic transmission in the myenteric plexus. It is unknown whether R-type Ca²⁺ channels contribute to neuromuscular transmission. We measured the effects of the nitric oxide synthase inhibitor nitro-l-arginine (NLA), Ca²⁺ channel blockers and apamin (SK channel blocker) on neurogenic relaxations and contractions of the guinea-pig ileum longitudinal muscle-myenteric plexus (LMMP) in vitro. We used intracellular recordings to measure inhibitory junction potentials. Immunohistochemical techniques localized R-type Ca²⁺ channel protein in the LMMP and circular muscle. Cadmium chloride (pan-Ca²⁺ channel blocker) blocked and NLA and NiCl₂ (R-type Ca²⁺ channel blocker) reduced neurogenic relaxations in a non-additive manner. Nickel chloride did not alter neurogenic cholinergic contractions, but it potentiated neurogenic non-cholinergic contractions. Relaxations were inhibited by apamin, NiCl₂ and NLA and were blocked by combined application of these drugs. Relaxations were reduced by NiCl₂ or ω-conotoxin (N-type Ca²⁺ channel blocker) and were blocked by combined application of these drugs. Longitudinal muscle inhibitory junction potentials were inhibited by NiCl₂ but not MRS 2179 (P2Y₁ receptor antagonist). Circular muscle inhibitory junction potentials were blocked by apamin, MRS 2179, ω-conotoxin and CdCl₂ but not NiCl₂ . We conclude that neuronal R-type Ca²⁺ channels contribute to inhibitory neurotransmission to longitudinal muscle but less so or not all in the circular muscle of the guinea-pig ileum.

Lack of effect of Z-butylidenephthalide on presynaptic N-type Ca²⁺ channels in isolated guinea-pig ileum

Z-Butylidenephthalide (Bdph) was reported to more potently inhibit electrically induced twitch responses than acetylcholine-induced tonic contraction in isolated guinea-pig ileum (GPI). The aim of the present study was to investigate the inhibitory effects of Z-Bdph on Ca2+ and K+ channels on GPI. In Locke-Ringer’s solution, both responses were isometrically recorded on a polygraph. Incubation of ω-conotoxin MVIIC, but not Z-Bdph, in the electrically stimulated GPI prior to adding ω-conotoxin GVIA, an irreversible blocker of N-type voltage-dependent Ca2+ channels (VDCCs), protected the binding sites and resulted in the twitch responses reversible by washing, suggesting that Z-Bdph did not bind to the N-type VDCCs. Interestingly, we found Z-Bdph concentration dependently delayed the onsets of K+-induced twitch responses, suggesting that Z-Bdph may be a blocker of K+ channels to interfere extracellular K+ across through the pre-junctional membrane of nerve ending in K+-free medium. Z-Bdph similar to nifedipine non-competitively inhibited cumulative ACh-induced phasic contractions, suggesting that Z-Bdph may bind to L-type of inositol-1,4,5-trisphosphate (IP3)-sensitive Ca2+ channels on the endoplasmic reticulum (ER) membrane. In the presence of verapamil, a L-type Ca2+ channel blocker or Z-Bdph, the twitch inhibitions by either were effectively reversed by exogenous Ca2+, suggesting that they may freely pass through pre-junctional N-type, but not L-type which was blocked at least a part by either, of VDDCs open when each electrical coaxial stimulation (ECS) into intracellular space of cholinergic nerve terminal and trigger release of transmitters. In conclusion, results confirm that Z-Bdph more potently inhibits ECS-induced twitch responses than ACh-induced PCs in GPI and suggest that this effect is not mediated by interaction with presynaptic N-type VDCCs.

Calcium channel subtypes for cholinergic and nonadrenergic noncholinergic neurotransmission in isolated guinea pig trachea

The Ca(2+) channel subtypes in the neurotransmission of isolated guinea pig trachea were elucidated by monitoring the effects of specific Ca(2+) channel blockers on cholinergic contractions and nonadrenergic noncholinergic (NANC) relaxation elicited by electrical field stimulation (EFS). In isolated guinea pig trachea, cholinergic contractile responses to low- and high-frequency EFS were inhibited by the selective N-type calcium channel blocker, ω-conotoxin MVIIA. ω-Agatoxin IVA (a selective P-type blocker), ω-conotoxin MVIIC (a nonselective N-, Q-, and P-type blocker), and nifedipine (a selective L-type blocker) were ineffective, whereas Ni(2+) (a T- and R-type blocker) facilitated cholinergic contractions and produced a late contracture when its concentration exceeded 30 μM. The more the concentration of Ni(2+) increased, the greater the number of incidences and the late contracture areas which occurred. Late contracture did not seem to be due to the effects of acetylcholine, tachykinins, or other polypeptides, but disappeared in the absence of indomethacin. The NANC relaxant responses elicited by the low- and high-frequency EFS were inhibited by ω-conotoxin MVIIA or Ni(2+), but unaffected by ω-Agatoxin IVA, ω-conotoxin MVIIC, and nifedipine. In the absence of indomethacin, Ni(2+) did not alter the ω-conotoxin MVIIA (100 nM)-resistant component of cholinergic contraction, but significantly further inhibited that of NANC relaxation. These results suggest that in isolated guinea pig trachea, cholinergic contraction is regulated by N-type calcium channels which may mask T- and R-type calcium channels and may be co-modulated by both, while NANC relaxation is mainly and independently controlled by N-, T-, and R-type calcium channels.

P2Y1 receptors mediate inhibitory neuromuscular transmission and enteric neuronal activation in small intestine

There is increasing evidence that adenosine 5'-triphosphate or a related purine plays a crucial role in smooth muscle relaxation and enteric synaptic neurotransmission. Accordingly, the aim of the present work is to investigate the role P2Y(1) receptors in purinergic inhibitory neurotransmission (pig ileum) and enteric neuronal activation in the small intestine (guinea-pig ileum). Using contractility measurements, micro-electrode recordings and Ca(2+) imaging we found that (i) adenosine 5'-Omicron-2-thiodiphosphate (ADPbetaS) (10 micromol L(-1)) caused smooth muscle relaxation and hyperpolarization that was antagonized by MRS2179 (10 micromol L(-1)) a P2Y(1) receptor antagonist and apamin (1 micromol L(-1)); (ii) electrical field stimulation (EFS) caused a non-nitrergic inhibitory junction potential (IJP) and relaxation that was antagonized by MRS2179 (10 micromol L(-1)); (iii) P2Y(1) receptors were immunolocalized in smooth muscle cells and enteric neurons; (i.v.) superfusion of ADPbetaS (1 micromol L(-1)) induced Ca(2+) transients in myenteric neurons that were inhibited by MRS2179 (1 micromol L(-1)), but not by tetrodotoxin (1 micromol L(-1)); and (v) EFS induced calcium transients were partially inhibited by MRS2179 (1 micromol L(-1)). We conclude that in the small intestine purinergic neuromuscular transmission responsible for the IJP and non-nitrergic relaxation is mediated by P2Y(1) receptors located in smooth muscle cells. Functional P2Y(1) receptors are also present in guinea-pig myenteric neurons. Therefore, P2Y(1) receptors might be an important pharmacological target to modulate gastrointestinal functions.

Roles of interstitial cells of Cajal in intestinal transit and exogenous electrical pacing

The aims of this study were to investigate the role of interstitial cells of Cajal (ICCs) on small intestinal transit and its responses to exogenous pacing in W/W(v) mice. Eleven W/W(v) mice and their controls implanted with four pairs of gastrointestinal electrodes were used for testing the entrainment of slow waves. Another 20 W/W(v) mice and their controls equipped with a duodenal catheter and one pair of intestinal electrodes were used to test small intestinal transit represented by the geometric center (GC). Results were as follows. (1) The effect of pacing on slow wave frequency was sustained only in controls, and not in W/W(v) mice. (2) Both gastric and intestinal slow waves were completely entrained in controls and W/W(v) mice. Higher energy was required for pacing the stomach than the small intestine. (3) There was no significant difference in small intestinal transit between the controls and the W/W(v) mice (GC: 5.4 vs. 5.5). (4) Pacing showed no effects on small intestinal transit in either wild-type (GC: 5.4 vs. 5.6) or W/W(v) mice (GC: 5.5 vs. 5.7). We conclude that myenteric ICCs may not play an important role in the regulation of small intestinal transit in conscious mice. Gastric and intestinal pacing can be achieved without ICCs.

Interstitial cells of Cajal and electrical activity of smooth muscle in porcine ileum

AIM

To identify the interstitial cells of Cajal in the porcine ileum for the first time immunohistochemically and to examine the electrical properties of intestinal smooth muscle in the same region.

METHODS

In vitro intracellular microelectrode recordings were made from smooth muscle cells in cross-sectional preparations from abattoir-derived healthy porcine ileum. Immunohistochemical labelling of interstitial cells of Cajal was performed using an anti-Kit antibody.

RESULTS

Slow waves were recorded in the circular muscle layer of all ileal preparations. The mean resting membrane potential of smooth muscle cells was -61.0 +/- 1.3 mV. Slow waves had a mean amplitude of 8.5 +/- 0.5 mV, a frequency of 9.9 +/- 0.1 cycles per minute and a duration of 5.6 +/- 0.1 s. A waxing and waning pattern of slow wave activity was occasionally observed. In addition, higher frequency spiking activity associated with contractions was observed in some recordings. The L-type calcium channel blocker nifedipine abolished both the spiking activity and the contractions, but had no significant effect on slow wave characteristics. Current-injection manipulation of the resting membrane potential had no effect on slow wave amplitude, frequency or duration. Kit-immunoreactive interstitial cells of Cajal were identified in the ileal samples and were present in the region of the myenteric plexus and in the circular and longitudinal muscle layers.

CONCLUSION

This study recorded slow waves in vitro and demonstrated immunohistochemically the presence of interstitial cells of Cajal in the normal porcine ileum. This study forms a basis for future physiological and pathophysiological comparative studies of intestinal motility.

Role of l-Ca(2+) channels in intestinal pacing in wild-type and W/W(V) mice

Rhythmic contractions generating transit in the digestive tract are paced by a network of cells called interstitial cells of Cajal (ICC) found in the myenteric plexus (MP). ICC generate cyclic depolarizations termed "slow waves" that are passively transmitted to the smooth muscle to initiate contractions. The opening of l-Ca(2+) channels are believed to be primarily responsible for the influx of calcium generating a contraction in smooth muscle. However, l-Ca(2+) channels are not thought to be important in generating the pacing current found in ICC. Using intact segments of circular (CM) and longitudinal (LM) muscle from wild-type mice and mice lacking c-kit kinase (W/W(V)), we found that l-Ca(2+) channel currents are required for pacing at normal frequencies to occur. Application of 1 muM nicardipine caused a significant decrease in contraction amplitude and frequency in LM and CM that was successfully blocked with BAY K 8644. Nicardipine also abolished the pacing gradient found throughout the intestines, resulting in a uniform contraction frequency of 30-40/minute. Stimulating l-Ca(2+) channels with BAY K 8644 neither removed nor recovered the pacing gradient. W/W(V) mice, which lack ICC-MP, also exhibited a pacing gradient in LM. Application of nicardipine to LM segments of W/W(V) mouse intestine did not reduce pacing frequency, and in jejunum, resulted in a slight increase. BAY K 8644 did not affect pacing frequency in W/W(V) tissue. In conclusion, we found that l-Ca(2+) channel activity was required for normal pacing frequencies and to maintain the pacing frequency gradient found throughout the intestines in wild-type but not in W/W(V) mouse intestine.

Otilonium bromide inhibits muscle contractions via L-type calcium channels in the rat colon

The aim of this study is to evaluate in vitro the effect of otilonium bromide (OB) on the mechanical and electrical activities of the rat colonic smooth muscle using muscle bath, microelectrodes and patch-clamp techniques. Otilonium bromide dose dependently inhibited the spontaneous activity (logIC(50) +/- SE: -5.31 +/- 0.05). This effect was not modified by TTX (10(-6) mol L(-1)). Cyclic depolarizations were abolished by OB (10(-4) mol L(-1)). Electrical field stimulation induced inhibitory junction potentials (IJPs) followed by a depolarization with superimposed spikes causing a contraction. In the presence of OB (10(-4) mol L(-1)) IJPs were recorded, but spikes and contractions were abolished. Otilonium bromide (3 x 10(-6) mol L(-1)) inhibited inward current obtained in isolated cells (amphotericin perforated patch technique). The otilonium-sensitive current amplitude was maximal (75pA) around 0 mV. The effect of different doses of OB was tested by depolarizing cells from -70 mV to 0 mV. OB dose dependently inhibited the inward current with an EC(50) of 885 nmol L(-1). Abolishment of the otilonium-sensitive current by 3 x 10(-6) mol L(-1) nifedipine confirmed that it was an L-type Ca(2+) current. Our results show that OB inhibits the spontaneous and triggered muscular contractions. This effect is produced by the inhibition of muscular action potentials carried by L-type calcium current, confirming the spasmolytic properties of OB.

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.

Characterizing voltage-dependent Ca(2+) channels coupled to VIP release and NO synthesis in enteric synaptosomes

In enteric synaptosomes of the rat, the role of voltage-dependent Ca(2+) channels in K(+)-induced VIP release and nitric oxide (NO) synthesis was investigated. Basal VIP release was 39 +/- 4 pg/mg, and cofactor-substituted NO synthase activity was 7.0 +/- 0.8 fmol. mg(-1). min(-1). K(+) depolarization (65 mM) stimulated VIP release Ca(2+) dependently (basal, 100%; K(+), 172.2 +/- 16.2%; P < 0.05, n = 5). K(+)-stimulated VIP release was reduced by blockers of the P-type (omega-agatoxin-IVA, 3 x 10(-8) M) and N-type (omega-conotoxin-GVIA, 10(-6) M) Ca(2+) channels by ~50 and 25%, respectively, but not by blockers of the L-type (isradipine, 10(-8) M), Q-type (omega-conotoxin-MVIIC, 10(-6) M), or T-type (Ni(2+), 10(-6) M) Ca(2+) channels. In contrast, NO synthesis was suppressed by omega-agatoxin-IVA, omega-conotoxin-GVIA, and isradipine by ~79, 70, and 70%, respectively, whereas Ni(2+) and omega-conotoxin-MVIIC had no effect. These findings are suggestive of a coupling of depolarization-induced VIP release primarily to the P- and N-type Ca(2+) channels, whereas NO synthesis is presumably dependent on Ca(2+) influx not only via the P- and N- but also via the L-type Ca(2+) channel. In contrast, none of the Ca(2+) channel blockers affected VIP release evoked by exogenous NO, suggesting that NO induces VIP secretion by a different mechanism, presumably involving intracellular Ca(2+) stores.

In search of selective P2 receptor ligands: interaction of dihydropyridine derivatives at recombinant rat P2X(2) receptors

1,4-Dihydropyridines are regarded as privileged structures for drug design, i.e. they tend to bind to a wide variety of receptor sites. We have shown that upon appropriate manipulation of the substituent groups on a 1,4-dihydropyridine template, high affinity and selectivity for the A(3) subtype of adenosine receptors ('P1 receptors') may be attained. In the present study we have begun to extend this approach to P2 receptors which are activated by ATP and other nucleotides. Nicardipine, a representative dihydropyridine, used otherwise as an L-type calcium channel blocker, was shown to be an antagonist at recombinant rat P2X(2) (IC(50)=25 microM) and P2X(4) (IC(50) approximately 220 microM) receptors expressed in Xenopus oocytes. Thus, this class of compounds represents a suitable lead for enhancement of affinity through chemical synthesis. In an attempt to modify the 1,4-dihydropyridine structure with a predicted P2 receptor recognition moiety, we have replaced one of the ester groups with a negatively charged phosphonate group. Several 4-phenyl-5-phosphonato-1,4-dihydropyridine derivatives, MRS 2154 (2, 6-dimethyl), MRS 2155 (6-methyl-2-phenyl), and MRS 2156 (2-methyl-6-phenyl), were synthesized through three component condensation reactions. These derivatives were not pure antagonists of the effects of ATP at P2X(2) receptors, rather were either inactive (MRS 2156) or potentiated the effects of ATP in a concentration-dependent manner (MRS 2154 in the 0.3-10 microM range and MRS 2155 at >1 microM). Antagonism of the effects of ATP at P2X(2) receptor superimposed on the potentiation was also observed at >10 microM (MRS 2154) or 0.3-1 microM (MRS 2155). Thus, while a conventional dihydropyridine, nicardipine, was found to antagonize rat P2X(2) receptors ninefold more potently than P2X(4) receptors, the effects of novel, anionic 5-phosphonate analogues at the receptor were more complex.

Neural modulation of the cyclic electrical and mechanical activity in the rat colonic circular muscle: putative role of ATP and NO

1. The rat colonic circular muscle displays cyclic episodes of myenteric potential oscillations (MPOs), each of them associated with a spontaneous contraction. Nifedipine 1 microM abolished both MPOs and their associated contractions. TTX (1 microM) increased the amplitude and frequency of spontaneous contractions. 2. Electrical field stimulation (EFS) induced a non-adrenergic non-cholinergic (NANC) inhibitory junction potential (IJP), with two phases: an initial fast hyperpolarization (characterized by IJP amplitude) and a sustained hyperpolarization (characterized by IJP duration). 3. Sodium nitroprusside (10 microM) hyperpolarized and abolished spontaneous contractions even in presence of TTX or 1 microM apamin. ATP (100 microM) also hyperpolarized and abolished spontaneous contractions but its effects were decreased by TTX and abolished by apamin. 4. Suramin (100 microM) or apamin reduced the amplitude of the IJPs, but did not affect their duration. Incubation with L-NOARG (1 mM) reduced the duration but not the amplitude of the IJPs. In presence of L-NOARG plus suramin or L-NOARG plus apamin, both duration and amplitude of the IJPs were reduced but a residual IJP could still be recorded. 5. We conclude that the mechanical and electrical cyclic activity of the rat colonic circular muscle is modulated but not originated by the enteric nervous system and involves L-type calcium channel activity. EFS induces release of NANC inhibitory neurotransmitters which hyperpolarize and relax smooth muscle cells. Both ATP and NO are involved in IJP generation: ATP is responsible for the first phase of the IJPs involving activation of apamin-sensitive potassium channels, whereas NO initiates the second phase which is independent of the activation of such channels.

Slow waves in circular muscle of porcine ileum: structural and electrophysiological studies

The structural and functional bases of pacemaking (slow waves) in porcine ileal circular muscle were studied. The myenteric plexus contained two, structurally distinct types of interstitial cells of Cajal (ICC) interconnected by gap junctions and connected by close contacts to muscle layers. At the deep muscular plexus, ICC were present, not regularly close to nerve axons or in gap junction contact with one another or outer circular muscle, which had many gap junctions. Slow waves (5.2 +/- 2 mV amplitude and 4.6 +/- 0.7 s duration) occurred at 9.9 +/- 1.1 counts/min. Tissue length and time constants were 2.00 +/- 0.3 mm and 111 +/- 37 ms, respectively. Large electrical field-induced hyperpolarizations or depolarizations reduced amplitudes but not frequencies or durations of slow waves; hyperpolarizations progressively reduced inhibitory junction potentials as if the K+ channel opening mediated them. In conclusion, the myenteric plexus ICC of pig ileum, which appears to pace the muscle layers, appears insensitive to voltages applied to the syncytium of circular muscle cells. Limited coupling between ICC and circular muscle or voltage-insensitive pacemaking activity may explain these findings.

Evidence supporting a role for ATP as non-adrenergic non-cholinergic inhibitory transmitter in the porcine ileum

The aim of this study was to investigate the nature of the non-adrenergic non-cholinergic (NANC) inhibitory transmitter of the circular muscle of the porcine ileum. For this purpose, the effects of putative NANC mediators i.e. NO, vasoactive intestinal polypeptide (VIP) and ATP were measured in isolated organ bath experiments (in basal conditions and after incubation with neostigmine 3 x 10[-5] M) and using the microelectrode technique. The NO donor sodium nitroprusside (NaNP) up to 10(-4) M, VIP up to 10(-7) M and ATP up to 10(-4) M failed to cause significant relaxation in the basal state. However, all of them induced marked relaxations when the tissue had been preincubated with neostigmine (3 x 10[-5] M) which was added to increase basal mechanical activity. The resting membrane potential (RMP) was unaffected by NaNP(up to 10(-4) M and VIP up to 10(-7) M whereas ATP (up to 10[-4] M) induced a transient hyperpolarization. The inhibitory junction potentials (IJPs) induced by electrical field stimulation (EFS) were not affected by N omega-nitro-L-arginine (L-NNA) (10[-4] M) whereas suramin, a purinoceptor antagonist, decreased (10[-4] M) or abolished (10[-3] M) the IJPs. Relaxations induced by ATP in neostigmine preincubated tissue were resistant to 10(-6) M tetrodotoxin, an axonal blocker, and inhibited by suramin. Apamin (10[-6] M, a small conductance calcium activated potassium channel blocker, completely abolished the IJP (n=5) and significantly decreased the relaxation induced by ATP (n=5). The present data provide support to the hypothesis that ATP is the NANC inhibitory transmitter in the porcine ileum acting on P2 muscular receptors. Nevertheless, VIP and NaNP do also cause relaxation of preparations preincubated with neostigmine.