Introduction

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The enteric neural pathways subserving intestinal peristalsis involve sensory neurons and interneurons as well as ascending excitatory and descending inhibitory motor neurons (Furness and Costa, 1987; Gershon et al., 1994; Waterman et al., 1994b). It has only recently been demonstrated that inhibitory neural pathways causing relaxation of the longitudinal and circular muscle layers play a crucial role in the coordination and propagation of peristalsis (Waterman et al., 1994a). In terms of their transmitters, enteric inhibitory motor neurons are nonadrenergic noncholinergic neurons, and in the guinea pig small intestine two mechanisms of nonadrenergic noncholinergic inhibitory transmission to the circular muscle have been distinguished (Niel et al., 1983; Costa et al., 1986). One mechanism relies on fast inhibitory junction potentials that are blocked by apamin and are most probably mediated by adenosine triphosphate or a related purine (Niel et al., 1983; Bywater and Taylor, 1986; Costa et al., 1986; Crist et al., 1992). Apamin-insensitive inhibitory transmission involves slow inhibitory junction potentials (Niel et al., 1983; Bywater and Taylor, 1986) that are brought about by vasoactive intestinal polypeptide and nitric oxide (NO) acting in series (He and Goyal, 1993) and that are prevented by NO synthase inhibitors (Lyster et al., 1992).

NO synthase occurs in both enteric inhibitory motor neurons as well as in descending interneurons of the guinea pig small intestine (Costa et al., 1992; Furness et al., 1994; Young et al., 1995), from which NO is released after nerve stimulation (Wiklund et al., 1993b). The most widely reported action of NO in the gut is relaxation of smooth muscle (Sanders and Ward, 1992), which is consistent with the ability of NO to induce slow inhibitory junction potentials in the muscle of the guinea pig small intestine (Lyster et al., 1992; He and Goyal, 1993). However, authentic NO can also cause acetylcholine-mediated contractions of the resting guinea pig ileum (Barthó and Lefebvre, 1994) although the stimulus-evoked release of acetylcholine and tachykinins from enteric neurons is inhibited by NO (Knudsen and Tottrup, 1992; Wiklund et al., 1993a; Kilbinger and Wolf, 1994). These multiple actions of NO suggest that manipulation of the NO system influences enteric motor reflexes and peristalsis of the guinea pig small intestine in a complex manner. The reported data attest to this complexity inasmuch as in one study SNP and other NO donors were found to stimulate peristalsis (Sugisawa et al., 1991) although in other laboratories SNP was shown to inhibit ascending and descending enteric motor reflexes (Yuan et al., 1995) and peristaltic activity (Waterman and Costa, 1994). Furthermore, inhibition of endogenous NO synthesis depresses neuromuscular transmission in the descending inhibitory motor reflex of the guinea pig ileum (Yuan et al., 1995) whereas the peristaltic reflex is facilitated (Ciccocioppo et al., 1994; Suzuki et al., 1994; Waterman and Costa, 1994).

Some of these discrepancies are likely to be the result of differences in experimental conditions and recording protocol. Because in most studies intervals of 10 to 20 min were allowed to elapse between addition of the drugs and recording of their effects (Ciccocioppo et al., 1994; Waterman and Costa, 1994; Yuan et al., 1995), it was the aim of our study 1) to continuously record the immediate and delayed effects of the NO donor SNP and two inhibitors of NO synthase, L-NNA and L-NAME, on peristalsis of the guinea pig isolated ileum, 2) to analyze the dual excitatory/inhibitory action of SNP on peristalsis and, for comparison, on the motor activity of the longitudinal muscle, 3) to examine the time course with which successive exposure of the ileum to L-NAME and apamin, in this or reverse order, abolishes peristalsis and 4) to analyze whether SNP or combined addition of L-NAME and apamin inhibits peristalsis via a similar or a different type of action.

	Methods

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Basic preparation common to all experiments. Adult guinea pigs of either sex and 350 to 450 g body weight were stunned and bled. The ileum was excised, flushed of luminal contents and placed, for up to 4 hr, in Tyrode solution kept at room temperature and oxygenated with a mixture of 95% O₂ and 5% CO₂. The composition of the Tyrode solution was (mM): NaCl 136.9, KCl 2.7, CaCl₂ 1.8, MgCl₂ 1.0, NaHCO₃ 11.9, NaH₂PO₄ 0.4 and glucose 5.6. After dissection, ileal segments were mounted in organ baths that contained oxygenated Tyrode solution maintained at 37°C.

Peristalsis. Peristalsis was studied with a constant intraluminal perfusion system that has been described in detail previously (Costall et al., 1993; Holzer and Maggi, 1994). Briefly, ileal segments (approximately 10 cm in length) were secured horizontally in a silanized glass organ bath containing 30 ml of Tyrode solution. Prewarmed Tyrode solution was continuously infused into the intestinal lumen; the infusion rate was 0.5 ml min¹. The fluid passing the gut lumen was directed into a vertical outlet tubing (Costall et al., 1993) which ended 4 cm above the fluid level in the organ bath. This arrangement required the peristaltic effector system to raise the intraluminal pressure (recorded with a pressure transducer at the aboral end of the segments and displayed on a pen recorder) above 400 Pa to empty the intestinal segments.

View larger version (34K): [in this window] [in a new window]   Fig. 1.   Recording of the effect of sodium nitroprusside (SNP) on peristalsis in the guinea pig isolated ileum. A, Effect of SNP recorded 25 min after addition of vehicle (Tyrode solution) to the bath. B, Effect of SNP recorded 25 min after addition of atropine to the bath. The pressure threshold of peristalsis is marked by arrow heads. The initial stimulation of peristalsis (decrease in pressure threshold) is followed by inhibition of fluid propulsion (elevation of pressure threshold) when atropine is present in the bath (B).

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Longitudinal muscle activity. Segments of 1.5 cm length were suspended vertically in organ baths (capacity 5 ml). The preparations were kept under a resting load of 5 mN, and the mechanical activity of the longitudinal muscle was recorded with isotonic lever displacement measuring systems (HSE, March-Hugstetten, Germany) and displayed on a pen recorder. After a 30-min period of equilibration, the preparations were primed by repeatedly testing their responses to SNP (10 or 100 µM, contact time 6 min) at intervals of 30 min. After reproducible responses had been obtained, the ileal segments were repeatedly exposed to the same concentration of SNP (contact time 20 min) with washout periods of 30 min between the exposures. Atropine (1 µM) was administered to the bath 20 min before the second 20-min challenge of the preparations with SNP, although its vehicle (physiological saline, 1 µl ml¹ bath fluid) was given 20 min before the first 20-min challenge with SNP.

Drugs. The following drugs were used. Apamin, histamine dihydrochloride (both from Serva, Heidelberg, Germany), atropine sulfate and sodium nitroprusside (both from Merck, Darmstadt, Germany) were dissolved in water and diluted in Tyrode solution. Isoproterenol hydrochloride was used in the form of Isuprel injections (0.2 mg ml¹ stabilized aqueous solution, Winthrop, New York, NY). Tyrode solution was used to dissolve L-NNA (10 mM), L-NAME (100 mM), its enantiomer D-NAME (100 mM) and L-arginine (100 mM; all from Bachem, Bubendorf, Switzerland). For completely dissolving L-NNA the solution was sonicated for 2 min followed by vortex stirring.

Statistics. Quantitative data are presented as means ± S.E.M. Statistical evaluation of the results was made with the Mann-Whitney U test, the Wilcoxon test for pair differences or the Quade test (Theodorsson-Norheim, 1987) as appropriate. Probability values P < .05 were regarded as significant.

	Results

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Effect of SNP on peristalsis. Administration of SNP (0.1-100 µM) to the organ bath caused a prompt concentration-dependent stimulation of peristalsis as portrayed by a decrease in the pressure threshold, a response that lasted 10 to 15 min (figs. 1A, 2, A and C and 3A). This facilitatory effect of SNP was accompanied by an increase in the frequency of peristaltic waves, which with 100 µM SNP rose from 0.42 ± 0.06 to 0.69 ± 0.07 min¹ (maximal change, n = 6, P < .01), and an elevation of the residual pressure (fig. 1A). The latter parameter, which is the intraluminal pressure (relative to the zero base-line pressure) measured immediately after the completion of a peristaltic wave, increased from 5 ± 2 to 11 ± 2 Pa (maximal change, n = 6, P < .05) after administration of 100 µM SNP. In addition, SNP reduced the amplitude of the peristaltic waves in all experiments (fig. 1) but this parameter was not quantified in our study. The administration of a 10-fold higher dose of SNP (1 mM, n = 6, data not shown) failed to evoke effects that were larger than those induced by 100 µM SNP.

View larger version (21K): [in this window] [in a new window]   Fig. 3.   Effect of atropine on the concentration-dependent effect of SNP to first decrease (A) and then increase (B) the pressure threshold of peristalsis in the guinea pig isolated ileum. Ordinate: maximal SNP-induced change () in pressure threshold of peristalsis relative to the zero base-line pressure. Abscissa: concentration of SNP. Vehicle (Tyrode solution) or atropine (10 nM) was added to the bath 25 min before SNP. The bars shown are means ± S.E.M. of six to seven experiments. *P < .05, **P < .01 vs the respective values recorded in the presence of the vehicle.

View larger version (36K): [in this window] [in a new window]   Fig. 2.   Time course of the effect of sodium nitroprusside (SNP, 1 and 100 µM) on the pressure threshold of peristalsis in the guinea pig isolated ileum recorded in the presence of vehicle (A and C) or atropine (B and D). Vehicle (Tyrode solution) or atropine (10 nM) was added to the bath 25 min before SNP. Abscissa: time relative to the addition of SNP at time 0. Ordinate: pressure threshold of peristalsis relative to the zero base-line pressure. SNP first decreased the pressure threshold, an effect that in the presence of atropine was followed by a marked increase in the pressure threshold. The figures recorded in the presence of SNP denote the minimal and maximal pressure thresholds measured in the presence of SNP, and the time values given underneath the post-SNP bars refer to the time points when on average the SNP-induced decrease and delayed increase in pressure threshold was maximal. In D peristalsis was invariably abolished by SNP, i.e., even at the intraluminal pressure of 400 Pa no peristalsis was elicited. The bars shown are means ± S.E.M. of six experiments. *P < .05, **P < .01 vs time 5 min.

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Effect of SNP on longitudinal muscle activity. To shed more light on the ability of atropine to unmask SNP-induced inhibition of peristalsis, the action of SNP on the mechanical activity of the quiescent longitudinal muscle of the guinea pig isolated ileum was examined. Administration of SNP (10 and 100 µM) to the organ bath had a biphasic effect on the activity of the longitudinal muscle (fig. 4A). Although the base-line tone of the preparations was low, SNP initially relaxed the muscle, a response that soon was followed by a longer-lasting but transient contraction of the muscle (fig. 4A). Although the relaxation caused by 100 µM SNP was not larger than that caused by 10 µM SNP, the magnitude of the contraction was related to the concentration of SNP and amounted to 10 to 20% of the maximally possible contraction (fig. 4B, table 2). Once the contractile response to SNP had faded away, the tone of the preparations was invariably lower than before exposure to SNP (fig. 4A).

View larger version (27K): [in this window] [in a new window]   Fig. 4.   Effect of SNP on the contractile state of the guinea pig ileum longitudinal muscle in the absence and presence of atropine. A, Recording of the effect of SNP in the presence of vehicle. B, Effect of SNP (10 and 100 µM) in the presence of vehicle and atropine (1 µM, added to the bath 20 min before exposure to SNP). The contractile state of the preparations was recorded as a percentage of the contraction in response to 1 µM histamine (100%) relative to the relaxation caused by 1 µM isoproterenol (0%). The first bar in each triplet denotes the contractile state of the preparation before exposure to SNP, the second bar denotes the maximal relaxation and the third bar the peak contraction, in response to SNP. The bars shown are means ± S.E.M. of six to eight experiments. *P < .05, **P < .01 vs respective bar recorded in the presence of vehicle.

Effect of NO synthase inhibitors on peristalsis. Administration of the NO synthase inhibitor L-NAME (100 µM) to the organ bath stimulated peristalsis as shown by a decrease in the pressure threshold from 92 ± 7 to 67 ± 6 Pa (n = 7, P < .01) and an increase in the frequency of the peristaltic waves from 0.48 ± 0.04 to 0.60 ± 0.05 min¹ (n = 7, P < .01). The effect of a 3-fold higher dose of L-NAME (300 µM) to decrease the pressure threshold of peristalsis (fig. 5A) and to increase the frequency of peristaltic waves from 0.57 ± 0.06 to 0.76 ± 0.06 min¹ (n = 9, P < .01) was not different from that caused by 100 µM L-NAME. The inactive enantiomer, D-NAME (300 µM), failed to influence the pressure threshold (fig. 5A) and the frequency of peristaltic waves (0.45 ± 0.07 min¹ before exposure to D-NAME, 0.44 ± 0.06 min¹ after exposure to D-NAME, n = 7). As with L-NAME, the NO synthase inhibitor L-NNA (30 µM) reduced the pressure threshold (fig. 5B) and increased the frequency of peristaltic waves from 0.53 ± 0.08 to 0.71 ± 0.08 min¹ (n = 7, P < .05).

View larger version (37K): [in this window] [in a new window]   Fig. 5.   Effect of NG-nitro-D-arginine methylester (D-NAME), NG-nitro-L-arginine methylester (L-NAME), NG-nitro-L-arginine (L-NNA) and L-arginine, alone or in combination, on the pressure threshold of peristalsis in the guinea pig isolated ileum. Abscissa: time relative to the addition of drugs at time 0 and concentration of the drugs in the bath. The recordings of the pressure threshold were taken at the indicated time points. A, Left side: D-NAME was administered at time 0. A, Right side: L-NAME was given at time 0, L-arginine at 20 min. B, Left side: L-NNA was given at time 0, L-arginine at 10 min. B, Right side: L-arginine was administered at time 0. Ordinate: pressure threshold of peristalsis relative to the zero base-line pressure. The bars shown are means ± S.E.M. of six to seven experiments. **P < .01 vs time 5 (baseline value); P < .05, P < .01 vs time 20 min (A) or time 10 min (B).

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Effect of L-NAME in combination with apamin on peristalsis. In one of two sets of experiments the ileal segments were exposed to L-NAME (300 µM) 10 min before apamin (0.5 µM) was administered into the organ bath. As described above, L-NAME stimulated peristalsis (fig. 6A) by decreasing the pressure threshold from 109 ± 7 to 70 ± 4 Pa (n = 9, P < .01) and increasing the frequency of the peristaltic waves from 0.57 ± 0.06 to 0.76 ± 0.06 min¹ (n = 9, P < .01). Addition of apamin instantly disrupted the regular pattern of fluid propulsion and caused nonpropulsive contractions of the circular muscle. As can be seen from figure 6A, periods of incoordinated nonpropulsive contractions alternated with brief periods of coordinated peristalsis.

View larger version (59K): [in this window] [in a new window]   Fig. 6.   Recording of the effect of NG-nitro-L-arginine methylester (L-NAME) and apamin, in this or reverse order, on peristalsis of the guinea pig isolated ileum. A, Effect of L-NAME given 10 min before apamin. B, Effect of apamin given 10 min before L-NAME. The pressure threshold is marked by arrowheads. L-NAME or apamin given alone stimulate peristalsis (decrease in pressure threshold, increase in the frequency of peristaltic waves) although combined presence of the drugs causes nonpropulsive spasms of the circular muscle.

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	Discussion

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The results of this study show that interference with inhibitory neuroeffector transmission in the guinea pig isolated ileum modifies fluid propulsion in a distinct manner. Peristalsis was facilitated by apamin, a blocker of certain calcium-dependent potassium channels, and by the NO synthase inhibitor L-NAME, whereas combined administration of both drugs stopped regular peristalsis. It was unexpected to see that the NO donor SNP also stimulated peristalsis as shown by a decrease in the pressure threshold of the peristaltic waves. Only when peristalsis was compromised by a low concentration of atropine did SNP cause a delayed inhibition of peristalsis. The pattern of peristaltic shutdown caused by SNP, however, was profoundly different from that caused by combined administration of L-NAME plus apamin. Whereas SNP abolished motor activity by locking the intestinal segment in a state of complete relaxation in which it was unable to contract, exposure of the intestine to L-NAME plus apamin prevented peristalsis by abolishing the coordination of peristaltic waves although the segment was still able to contract and actually appeared hyperactive but failed to produce regular peristaltic waves.

The analysis of drug effects on peristalsis is complicated by the multiplicity of sites at which drugs can interfere with the neural and muscular effector systems of propulsive motility. Theoretically, the facilitatory influence of L-NAME, apamin and SNP may result from enforcement of excitatory enteric pathways or partial blockade of inhibitory pathways. By taking account of their known actions it would appear that both L-NAME and apamin facilitate peristalsis by interfering with inhibitory neuroeffector transmission. This interpretation is based on the concept that, in the guinea pig small intestine, inhibitory transmission to the circular muscle depends on fast inhibitory junction potentials that are blocked by apamin and on slow inhibitory junction potentials that are prevented by inhibitors of NO synthase (Bywater and Taylor, 1986; Lyster et al., 1992; He and Goyal, 1993). It would follow that interruption of one of these inhibitory transmission mechanisms shifts the balance between excitatory and inhibitory pathways such that stimulation of peristalsis prevails, although this imbalance is not severe enough to distort the basic coordination of regular peristaltic waves.

The stimulant effect of L-NAME and L-NNA on peristalsis, as recorded here by a decrease in the pressure threshold of peristaltic waves, was reduced by L-arginine, although L-arginine alone enhanced the pressure threshold. These observations are in line with those of Waterman and Costa (1994) who saw analogous effects of L-NAME and L-arginine on the volume threshold of peristalsis. These characteristics and the inability of the inactive enantiomer D-NAME to influence peristaltic motility indicate that the actions of L-NAME and L-NNA were due to inhibition of NO synthesis (Kerwin and Heller, 1994). The action of L-NAME, which was only partially counteracted by L-arginine, has previously been found to be relatively resistant to inhibition by L-arginine (Rees et al., 1990).

It is very likely that there are multiple sites of action by which blockade of NO synthase facilitates peristalsis in the guinea pig small intestine as seen here and in other studies (Ciccocioppo et al., 1994; Suzuki et al., 1994; Waterman and Costa, 1994). The facilitatory influence of L-NAME and L-NNA on peristalsis may be closely related to the action of NO synthase inhibitors to enhance the descending inhibitory reflex by facilitating the transmission between sensory neurons and interneurons (Yuan et al., 1995). However, NO synthase inhibitors also block neuromuscular transmission within the descending inhibitory reflex while the ascending excitatory motor reflex remains unaltered (Yuan et al., 1995). How these effects and the ability of NO synthase blockers to facilitate the release of acetylcholine and substance P from myenteric neurons (Knudsen and Tottrup, 1992; Wiklund et al., 1993a; Kilbinger and Wolf, 1994) relates to their facilitatory influence on peristalsis remains to be elucidated. This multiplicity of actions is consistent with the presence of NO synthase in descending interneurons and motor neurons and with the proposed role of NO as neuroneuronal and neuromuscular transmitter substance (Costa et al., 1992; Furness et al., 1994; Waterman and Costa, 1994; Yuan et al., 1995; Young et al., 1995).

When both mechanisms of inhibitory neuroeffector transmission in the guinea pig ileum are blocked by combined administration of L-NAME and apamin, the balance between excitation and inhibition is grossly distorted in favor of excitation, and the intestine becomes hyperactive and may even show episodes of sustained spasm. Hyperexcitability and maintained contraction of the muscle is one factor in the loss of coordinated peristalsis (Waterman et al., 1994b). Another factor is the blockade of inhibitory neurotransmission itself, which has only recently been recognized as being crucial to the alternating cycle of contraction and relaxation moving anally in the peristaltically active gut (Ciccocioppo et al., 1994; Waterman and Costa, 1994). Successive exposure of the guinea pig ileum to L-NAME plus apamin, in this or reverse order, was found to result in prompt disruption of coordinated peristalsis and replacement of regular peristaltic waves by multiple nonpropulsive contractions. This observation extends other studies in which gut distension failed to elicit propulsive peristalsis in intestinal segments that had been incubated with apamin and L-NAME or L-NNA for a period of 10 to 20 min (Ciccocioppo et al., 1994; Waterman and Costa, 1994) and emphasizes the importance of inhibitory motor neurons for the coordination of peristalsis.

From the facilitatory effect of NO synthase inhibition on peristalsis it would seem predictable that exogenous NO, administered by way of the NO donor SNP, inhibits peristaltic motility. However, the reverse was true, and SNP caused a prompt facilitation of peristalsis, an effect that was also noted by Sugisawa et al. (1991) but that is in contradiction with the reported ability of SNP to inhibit peristalsis (Waterman and Costa, 1994) and to depress both ascending excitatory and descending inbitory motor reflexes (Yuan et al., 1995). However, these discrepancies are very likely due to the intervals of 15 to 20 min that were allowed to elapse between drug addition and recording of its effect (Waterman and Costa, 1994; Yuan et al., 1995) whereas in our study the drug-induced changes of motility were continuously recorded. Closer analysis revealed that the action of SNP on intestinal motility is composed of two distinct phases, an initial period of excitation followed by inhibition of motility. The contractile effect of SNP on the longitudinal muscle of the guinea pig ileum is consistent with the ability of exogenous NO to contract the ileum of the guinea pig (Barthó and Lefebvre, 1994) and other species (Barthó and Lefebvre, 1995). The SNP-evoked contraction involves cholinergic neurons, which is in keeping with the involvement of acetylcholine and tachykinins in the contractile response to exogenous NO (Barthó and Lefebvre, 1994). In contrast, the delayed relaxation caused by SNP is likely to mirror the direct inhibitory action of NO on intestinal muscle (Shuttleworth et al., 1991; Lyster et al., 1992; He and Goyal, 1993; Wiklund et al., 1993b; Barthó and Lefebvre, 1994) but may in addition be related to the effect of SNP to depress slow synaptic excitation within the myenteric plexus (Tamura et al., 1993).

In view of the excitatory action of SNP on enteric neurons it would appear that the SNP-induced facilitation of peristalsis results from stimulation of excitatory motor pathways or from a permissive action of tonically released excitatory neurotransmitters. A delayed inhibitory effect of SNP on propulsive motility was seen, in a consistent manner, only when peristaltic activity was compromised by a threshold concentration of atropine. This observation is at variance with the finding of Waterman and Costa (1994) who noted a SNP-evoked increase in the volume threshold of peristalsis in the absence of atropine. It remains to be elucidated whether this discrepancy arises from differences in animal strain, recording conditions or other factors. We assume that in our study atropine caused a subtle shift in the balance between excitatory and inhibitory pathways of peristalsis such that the depressant effect of SNP on peristaltic motility overrode the drug's stimulant effect. The pattern of peristaltic shutdown caused by SNP in the presence of atropine indicates direct relaxation of the muscle to an extent that the muscle is no longer able to contract in response to the excitatory input from enteric neurons, an action that would expectedly be attributed to a transmitter of inhibitory enteric motor neurons. The observation that the effect of SNP on peristalsis was changed by atropine although that of L-NAME remained unaltered further attests to profound differences in the actions of SNP and L-NAME on peristalsis.

The multiple roles of NO in intestinal motor control highlight the difficulties that are encountered in the pharmacological analysis of drugs that may act at several sites, and in an opposing manner, within the enteric pathways subserving peristalsis. Our results illustrate that knowledge of specific cellular effects of drugs such as SNP and L-NAME does in no way allow the final outcome for the physiological process of peristalsis to be readily predicted. As a consequence, complementary studies at the organ level are required to recognize the full impact that a drug may have on propulsive motility. Analysis of the complex motor effects of SNP and L-NAME under this perspective has revealed that NO is an important messenger molecule involved in the coordination of propulsive motility and that both overactivity of the NO system (as reflected by the effects of SNP seen in the presence of atropine) as well as dysfunction of nitrergic inhibitory motor neurons (as reflected by the effects of apamin plus L-NAME) have deleterious effects on peristalsis. In the case of SNP plus atropine it is loss of excitability, and in the case of L-NAME plus apamin loss of the ability to relax, which prevents the physiological process of peristalsis.