Effects of met-enkephalin on the mechanical activity and distribution of met-enkephalin-like immunoreactivity in the cat small intestine

Naloxone-dependent effects of Met-enkephalin (10(-8) M) on the spontaneous and electrically induced mechanical activities were studied in longitudinal and circular preparations isolated from the cat duodenum, jejunum and ileum. Met-Enkephalin changed the spontaneous activity of all preparations tested with the exception of the circular preparations from the ileum. Met-Enkephalin-induced responses of the longitudinal preparations from the ileum were abolished by treatment with tetrodotoxin (10(-7) M), while the responses of both longitudinal and circular preparations from the duodenum and jejunum were only partially depressed, being resistant to tetrodotoxin components. The latter were most pronounced in the duodenum. The neurogenic electrically induced (0.5 msec, 5 Hz, 150 pulses) responses of all the preparations consisted mainly of contractile components which were significantly and naloxone-dependently reduced by Met-enkephalin (10(-8) M). The contractile components of the responses, which were reduced by Met-enkephalin, were entirely abolished by atropine (3 x 10(-6) M). Both Met-enkephalin and atropine inhibitory effects on the neurogenic responses were more pronounced in the ileum. Met-Enkephalin was found in nerve fibers of the myenteric plexus distributed mainly among the circular muscle. Single immunoreactive nerve fibers were observed in the longitudinal muscle layer of the duodenum but not in the jejunum and ileum. The distribution of Met-enkephalin-like immunoreactivity along the small intestine did not show significant differences among the three intestinal regions tested. The results obtained suggest that Met-enkephalin can modulate the mechanical activity of the cat small intestine, inhibiting cholinergic transmission and/or activating smooth muscle opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies of the innervation of rabbit myometrium and cervix

We characterized the innervation of isolated circular and longitudinal-oriented muscle strips from the nulliparous rabbit uterus and cervix by field stimulation (FS). FS with increasing frequency (2.5-50 pps) and voltage (2.5-70 V) caused graded increases in isometric contraction with no relaxation or inhibition of spontaneous activity. Tetrodotoxin (TTX, 3.1 X 10(-6) M) significantly reduced the FS response by 75% in all strips at higher stimulus frequencies. Contractile responses to FS were also significantly inhibited by atropine (3.5 X 10(-6) M) in circular uterus and in longitudinal cervix. Guanethidine (5 X 10(-6) M) reduced the response in all strips, as did phentolamine (3.6 X 10(-6) M) in longitudinal uterus and circular cervix. Propranolol (3.9 X 10(-6) M) did not significantly change the response in longitudinal uterus or circular cervix. In longitudinal uterus, combined guanethidine and atropine produced significant inhibition, but not statistically different from either drug alone. Similar results were seen in circular uterus. Electron microscopy and glyoxylic acid histofluorescence indicate that both blood vessels and smooth muscle in rabbit uterus are supplied with adrenergic nerves. The results suggest the presence of TTX-sensitive adrenergic and cholinergic excitatory innervation of rabbit uterus and cervix.     

Long-term hypoxia changes myometrial responsiveness and oxytocin receptors in the pregnant ewe: differential effects on longitudinal versus circular smooth muscle

Previous studies from our laboratory demonstrated that long-term hypoxia (LTH) altered in vitro contractile responses to oxytocin in full-thickness myometrial strips from pregnant sheep. The present study was designed to determine, first, if the reduced contractile response to oxytocin following LTH is the result of combined effects on longitudinal and circular smooth muscle or if the effect is specific to a single muscle layer and, second, if the reduced contractile response to oxytocin following LTH is caused by changes in oxytocin-receptor protein. Pregnant ewes were maintained at high altitude (3820 m) from Day 30 to Days 137-142 of gestation, when the ewes were killed for collection of myometrial tissue. Tissue was also collected from age-matched, normoxic controls. Longitudinal and circular layers were separated, length-tension curves generated to determine optimal resting tension, and all strips exposed to increasing half-log doses of oxytocin ranging from 10-12 to 10-6.5 M. The expression of oxytocin-receptor protein was measured using Western blot analysis. We found that LTH did not affect KCl-induced contraction of either smooth muscle layer, whereas the sensitivity of both myometrial layers to oxytocin was altered. A decreased maximum contractile response of the circular layer to oxytocin was also observed. Additionally, LTH decreased expression of oxytocin-receptor protein in the circular layer and increased levels in the longitudinal layer. Results from the present study indicate that LTH alters contractile responses and oxytocin-receptor protein expression in a layer-specific manner in the pregnant sheep myometrium.     

Effect of prostaglandin F2 alpha on propulsive activity of the isolated segmental colon of the guinea-pig.

The effects of prostaglandin F2alpha (PGF 2alpha) on propulsive activity in segments of isolated colon and on isolated strips of guinea-pig colon were investigated. Using experimental conditions under which spontaneous propulsive activity was negligible, PGF2alpha (5X10(-8)X1X10(-6)M), added to the bathing medium increased propulsive activity in a concentration dependent manner. This increase of propulsive activity was abolished in the presence of atropine or tetrodotoxin (1X10(-7)g/ml). The contractions produced by PGF2alpha (5X10(-7) -1X10(-5)M) in isolated longitudinal and circular smooth muscle strips of guinea-pig colon were unaffected in the presence of atropine or tetrodotoxin (1X10(-7) g/ml). From these results it is concluded that under the conditions employed in this study propulsive activity stimulated by PGF2alpha may depend on the contractions of both muscle layers and stimulation of the peristalic reflex.     

Prostaglandin F and 13,14-dihydro-15-keto prostaglandin F in the endometrium and uterine flushings of sheep before implantation

From 22 women undergoing hysterectomy at various stages of the menstrual cycle, strip preparations were dissected from the outer, longitudinal and the inner, circular smooth muscle layers of the ampullary-isthmic junction (AIJ). The strips were mounted in organ baths, and isometric tension was recorded. Spontaneous contractions were recorded mainly in circular muscle strips. Contractions were elicited by 127 mM-K+, 10(-6) M-noradrenaline and 10(-6) M-PGF-2 alpha. Potassium induced biphasic responses that were slightly different in the two tissues. In circular muscle strips, noradrenaline and PGF-2 alpha induced phasic contractions superimposed on a rise in tone. In longitudinal muscle specimens, the two compounds produced tonic responses. All types of mechanical activity were inhibited by removal of extracellular calcium. K+-induced responses and phasic contractions produced by noradrenaline and PGF-2 alpha could be abolished by 10(-6) M-nifedipine whereas the tonic contractions in the circular and longitudinal muscle were more resistant to the calcium antagonist. The results suggest that K+-induced responses in circular and longitudinal muscle of the human AIJ, and the phasic contractions in circular muscle, depend on calcium influx via potential-sensitive membrane channels. Receptor-operated calcium channels seem to be involved in the tonic contractions observed mainly in the longitudinal smooth muscle.     

The effect of leukotriene D4 on the isolated stomach and colon of the rat

The effect of synthetic leukotriene D4 (LTD4) was evaluated on isolated gastric longitudinal or circular smooth muscle and distal colon of the rat. The concentrations of LTD4, 2.5 X 10(-10)M to 5 X 10(-7)M, evoked minimal to maximal contractile responses. In addition, selected prostaglandins were used to identify the mediator of LTD4-induced contraction of gastric smooth muscle. FPL 55712 inhibited LTD4-induced contractions of gastric longitudinal or circular muscle. Indomethacin inhibited only LTD4-induced contractions of the longitudinal muscle. A combination of FPL 55712 and indomethacin produced greater inhibition of LTD4-induced contractions of longitudinal muscle than either agent alone. However, the same combination of inhibitors showed no greater effect than FPL 55712 alone on LTD4-induced contractions of circular smooth muscle. Unlike PGI2, PGF2, PGA2, or PGD2, PGE2 evoked contraction of the longitudinal muscle and relaxation of the circular muscle of the stomach. The dissimilar effect of PGE2 in the two smooth muscle layers of the rat stomach may signify that PGE2 is the prostaglandin released by LTD4 from the longitudinal and circular gastric muscle. However, the opposing pharmacologic effects following LTD4-induced release of prostaglandins in the circular muscle of the stomach would preclude the appearance of an inhibitory effect of indomethacin in this tissue. In contrast, PGE2 and other prostaglandins contract gastric longitudinal muscle in response to LTD4. Thus, these studies clearly suggest that LTD4 has both a direct and indirect effect on gastric smooth muscle of the rat. Unlike the stomach, LTD4-induced contraction of the distal colon was not inhibited by indomethacin while FPL 55712 antagonized contractions. Thus, these findings indicate a differential mechanism of stimulation of rat gastrointestinal tissue by LTD4.     

Reciprocal activity of longitudinal and circular muscle during intestinal peristaltic reflex

A two-compartment, flat-sheet preparation of rat colon was devised, which enabled exclusive measurement of longitudinal muscle activity during the ascending and descending phases of the peristaltic reflex. A previous study using longitudinal muscle strips revealed the operation of an integrated neuronal circuit consisting of somatostatin, opioid, and VIP/pituitary adenylate cyclase-activating peptide (PACAP)/nitric oxide synthase (NOS) interneurons coupled to cholinergic/tachykinin motor neurons innervating longitudinal muscle strips that could lead to descending contraction and ascending relaxation of this muscle layer. Previous studies in peristaltic preparations have also shown that an increase in somatostatin release during the descending phase causes a decrease in Met-enkephalin release and suppression of the inhibitory effect of Met-enkephalin on VIP/PACAP/NOS motor neurons innervating circular muscle and a distinct set of VIP/PACAP/NOS interneurons. The present study showed that in contrast to circular muscle, longitudinal muscle contracted during the descending phase and relaxed during the ascending phase. Somatostatin antiserum inhibited descending contraction and augmented ascending relaxation of longitudinal muscle, whereas naloxone had the opposite effect. VIP and PACAP antagonists inhibited descending contraction of longitudinal muscle and augmented ascending relaxation. Atropine and tachykinin antagonists inhibited descending contraction of longitudinal muscle. As shown in earlier studies, the same antagonists and antisera produced opposite effects on circular muscle. We conclude that longitudinal muscle contracts and relaxes in reverse fashion to circular muscle during the peristaltic reflex. Longitudinal muscle activity is regulated by excitatory VIP/PACAP/NOS interneurons coupled to cholinergic/tachykinin motor neurons innervating longitudinal muscle.     

Motility regulating effects of galanin on smooth muscle of porcine ileum.

We studied the effect of synthetic porcine galanin on circular and longitudinal oriented strips of pig ileal muscle. Galanin 10(-11)-10(-6) M had no effect on resting tension in the two layers. In circular muscle precontracted with carbachol 10(-6) M, galanin dose-dependently inhibited the amplitude of contractions to a maximum of 33 +/- 8% at 10(-6) M. In longitudinal muscle the amplitude of contractions induced by carbachol 10(-7) M or transmural field stimulation increased after addition of galanin 10(-9)-10(-7) M to a maximum of 21 +/- 6%, while at higher concentrations inhibition occurred. Maximal inhibition was 36 +/- 14% at galanin 10(-6) M. Tetrodotoxin did not influence the effects of galanin in the preparations. The results indicate that in the homologous species galanin inhibits the circular muscle layer, possibly by a direct action on the smooth muscle. In the longitudinal muscle the effect of galanin is apparently excitatory. The inhibition observed with high concentration of galanin could be due to tachyphylaxis and desensitization. Alternatively, an additional population of low affinity, inhibitory receptors may exist.     

Arecoline excites the colonic smooth muscle motility via M3 receptor in rabbits

Arecoline is an effective component of areca (betel nuts, a Chinese medicine named pinang or binglang). The purpose of this study was to investigate the effect of arecoline on the motility of distal colon in rabbits and its mechanisms involved. Strips of colonic smooth muscle were suspended in organ baths containing Krebs solution, and their isometric contractions were examined. The response of smooth muscle to arecoline in colonic strips was recorded. The effects of atropine, gallamine and 1,1-dimethyl-4-diphenylacetoxypiperidiniumiodide (4-DAMP) on arecoline-induced contraction were also observed. Arecoline (1 nM - 1 microM) produced a concentration-dependent contraction in both the longitudinal and the circular smooth muscle of rabbit colon. Atropine (10 microM) abolished the arecoline (80 nM)--induced contraction. M3 receptor antagonist, 4 - DAMP (0.4 microM), abolished the arecoline (80 nM)--related response, whereas M2 receptor antagonist, gallamine (0.4 microM), did not affect the effect of arecoline. These results suggest that arecoline excites the colonic motility via M3 receptor in rabbits.     

Motility effects of opioid peptides in dog intestine

Six opioid peptides, like morphine, were found to produce dose-dependent contractions of dog isolated intestine when administered as intraarterial boluses. The increases in incidence and amplitude of intestinal contractions were antagonized by naloxone. The rank order of potency of the opioid agonists tested was D-Ala2-met-enkephalinamide greater than D-Ala2-leu-enkephalinamide greater than met-enkephalin greater than beta-endorphin 1-31 greater than morphine greater than morphiceptin greater than dynorphin 1-13. The contractions induced by two opioid agonists displayed differential sensitivity to blockade by tetrodotoxin (TTX). Met-enkephalin was barely affected by concentrations of TTX that markedly reduced responses to morphiceptin. Some portion of the motility effect of metenkephalin may be exerted directly on intestinal smooth muscle.     

Identification of bombesin receptors on isolated muscle cells from human intestine

Smooth muscle cells were isolated separately from the longitudinal and circular muscle layers of human jejunum obtained at surgery and used to determine whether amphibian bombesin-14 and 3 mammalian homologues, GRP-(1-27), GRP-(18-27) and neuromedin B, can cause contraction by acting directly on muscle cells. Circular and longitudinal muscle cells contracted identically in response to bombesin-14 (C50 2 x 10(-12) M). The contractile response was not affected by selective muscarinic, opioid, CCK or serotonin antagonists but was inhibited by the substance P (SP) derivative, [D-Arg1, D-Pro2, D-Trp7,9, Leu11]SP. All 3 mammalian bombesins were less potent than bombesin-14. GRP-(1-27) and GRP-(18-27) were equipotent (C50 4 x 10(-11) M) but 20 times less potent than bombesin-14. Neuromedin B (C50 6 x 10(-12) M) was 3 times less potent than bombesin-14. All bombesins, however, were more potent than other enteric neuropeptides (e.g., tachykinins, opioid peptides). The study demonstrates conclusively the ability of bombesins to cause direct contraction of intestinal smooth muscle cells.     

Contractile effects of 16-methyl analogues of PGE2 on the circular and longitudinal muscles of the guinea-pig isolated colon.

The mechanical effects of 16-methyl analogues of PGE2, mainly 16,16-dimethyl PGE2, on circular and longitudinal muscles of the guinea-pig isolated proximal colon were investigated. In circular muscle strips, PGE2 100 nM produced an initial contraction followed by relaxation, while 16(R)-methyl PGE2 and 16,16-dimethyl PGE2 (1 nM - 1 microM) produced sustained contractions. In longitudinal muscle strips, PGE2 and 16-methyl analogues of PGE2 produced only contractions. The contractile responses of both muscle strips to 16,16-dimethyl PGE2 were not influenced by atropine or tetrodotoxin, indicating that these analogues act directly on the muscles, but were eliminated by the omission of extracellular Ca ions or in the presence of 1 mM lanthanum ions. However, verapamil, a Ca channel blocker, did not block the contractile response to the methyl analogues in circular muscle strips, although it completely inhibited the contractile response of longitudinal muscle strips. These results suggest that the contractile effect of 16-methyl analogues of PGE2 on the circular muscle may be due to an increased influx of Ca ions mainly via receptor-sensitive and partly voltage-sensitive Ca channels, while the contractile effect of the analogues on the longitudinal muscle may be due to an increase in influx of Ca ions via voltage-sensitive Ca channels.     

Functional and immunocytochemical evidence that galanin is a physiological regulator of human jejunal motility.

The neuropeptide galanin has species-dependent effects on intestinal motility. It has a contractile effect on rat jejunal muscle while it relaxes guinea-pig ileum by inhibiting cholinergic transmission. Its effect on human gut motility has been unknown. Extensive work led to the discovery of selective galanin analogues such as M15 [galanin(1-12)-Pro-substance-P(5-11)], M35 [galanin(1-12)-Pro-bradykinin(2-9)-amide] that competitively inhibit various actions of galanin in the central nervous system. The present study was designed to examine the effect of galanin, M15 and M35 on longitudinal jejunal smooth muscle strips isolated from humans and rats, and to localize galanin-immunoreactivity in human jejunum. Galanin and ACh were equally effective in stimulating contractions of the isolated jejunal muscle: sigmoid curve fitting showed that maximal contractile response to galanin and ACh were 25.7+/-11.1 mN and 23.7+/-9.7 in humans, while 8.0+/-0.6 and 8.1+/-0.3 mN in rats, respectively. These effects of galanin were not inhibited by either atropine (5 x 10(-6) M) or tetrodotoxin (3 x 10(-6) M). The potency of galanin inducing the contractile actions were similar in humans and rats. Interestingly, neither M15 nor M35 (up to 10(-7) M) were able to inhibit the responses of the smooth muscle to galanin. However, both putative galanin receptor antagonists showed agonist effects in our experimental models. In accordance with the functional studies, both the longitudinal and the circular muscle layers were abundant in nerve fibers and varicosities showing galanin immunoreactivity. Our data suggest that galanin is a potent physiological regulator of jejunal contractions in humans. Its action on the jejunum, however, is mediated by galanin receptors that are different from those located in the central nervous system.     

Modulation by GABA(B) and delta opioid receptors of neurally induced responses in isolated guinea-pig taenia coli and human colonic circular muscle.

The GABA-ergic and opioid modulation of neurally induced muscle responses was studied in isolated guinea-pig taenia coli and human colonic circular muscle, using identical field stimulation parameters (rectangular pulses of 0.5 ms duration, 9 V x cm(-1) intensity, trains of 3 pulses at 0.5 Hz, repeated every 1/3/5 min). The stimulation-induced contractions were inhibited in both preparations by GABA and baclofen; the IC50 values in human colonic circular muscle were approximately 100 and 31.0 microM, respectively. In guinea-pig taenia coli, the inhibition by 10(-4) M GABA was dose-dependently reversed by 10(-4)-10(-3) M of GABA(B) receptor antagonist CGP 35348; antagonism by phaclofen was less effective in the same concentration range. In human colonic circular muscle, inhibition by 3 x 10(-5) M baclofen was fully reversed by 10(-3) M CGP 35348. With the exception of caecum, the delta 2 opioid receptor agonist deltorphin II was a potent inhibitor in human colonic circular muscle. 10(-8) M Deltorphin caused a 74.4 +/- 9.6% (n = 4) inhibition which was reversed by 10(-6) M of delta receptor selective peptide antagonist BOC-Tyr-Pro-Gly-Phe-Leu-Thr(OtBu). Deltorphin II was ineffective in guinea-pig taenia coli even at 10(-6) M; the same concentration caused an 84.3 +/- 7.9 (n = 4) inhibition in human preparations. It is concluded that: 1) GABA-ergic modulatory mechanisms are present both in human colonic circular muscle and guinea-pig taenia coli; 2) the GABA receptors involved are of type B; and 3) delta opioid receptor-mediated modulation functions only in human colonic circular muscle in regions other than the caecum.     

Action of FMRFamide-like Peptides on Porcine Gastrointestinal Motility In Vitro

Decker, B., B. Vadokas, U. Kutschenreuter, K. Golenhofen, K. Voigt, G. P. Mcgregor and K. Mandrek. Action of FMRFamide-like peptides on porcine gastrointestinal motility in vitro. Peptides 18(10) 1531–1537, 1997.—Mechanical activity was recorded in circular and longitudinal smooth muscle preparations isolated from extensive regions of the porcine gastrointestinal tract in response to the FMRFamide-like neuropeptides F8Famide and A18Famide. In all preparations, the peptides were about equipotent in producing phasic contractions or enhancing spontaneous activity. The most prominent responses were observed in jejunal longitudinal strips which were on the average 91% (±4% SEM, n = 15; 10⁻⁶ M) of the histamine (10⁻⁵ M) responses. The peptide-induced phasic activity was completely abolished by nifedipine but was unaffected by tetrodotoxin, atropine, phentolamine, yohimbine, phenoxybenzamine, propranolol, methysergide, cimetidine, indomethacin, levallorphane or naloxone. Both peptides enhanced acetylcholine-induced contractions. However, bovine ileum and guinea-pig taenia coli was not affected by these peptides. The results indicate that F8F- and A18F-amide contract porcine gastrointestinal smooth muscle by acting directly via non-opioid receptors on L-type calcium channels. In addition an increase of the sensitivity to cholinergic stimulation occurs.     

Effects of stimulation of nervous fibres upon contractile activity of the tracheobronchial lymphatic node capsules' smooth muscles

The contractile activity of smooth muscle of tracheobronchial lymph nodes' capsules was recorded in vitro. The field electric stimulation (0.5 ms pulses, 55 V nominal, 4 min trains at frequencies 0.5, 1, 2, 4, 8, 16 and 32 Hz) of strips from lymph node produced a frequency-dependent increase in baseline tension and frequency of phase contractions. Evoked contractions were significantly (about 80%) reduced by tetrodotoxin (1 x 10(-6) M). The blockage of M-cholinoreceptors with atropine (1 x 10(-6) M) did not affect the field-evoked responses. Contractile field-evoked effects were significantly reduced by the phentolamine (1 x 10(-7)-1 x 10(-6) M) but not completely. Field-evoked contractions were slightly affected by the propranolol (1 x 10(-7)-1 x 10(-6) M). We conclude that the contractile activity of bovine tracheobronchial lymph node capsular smooth muscle is modulated by excitatory adrenergic nerves.     

Inhibition of peristaltic activity by cannabinoids in the isolated distal colon of mouse.

The effects of the cannabinoid receptor agonist Win 55,212-2 and of the competitive cannabinoid receptor antagonist SR 141716A on the electrically-evoked peristalsis of isolated distal colon of mouse were studied. Intraluminal pressure, longitudinal displacement, ejected fluid volume and changes in morphology of external intestinal wall were simultaneously recorded in the pre-drug period and in presence of Win 55,212-2 alone or in combination with SR 141716A. In the pre-drug period (control), peristaltic activity was characterised by regular, monophasic waves and the intraluminal content propelled towards anterograde (oro-aboral) direction with a propulsion velocity of 1.25 +/- 0.1 mm x s(-1). Pressure and shortening waves showed a peak amplitude of 2.44 +/- 0.32 kPa and 1.8 +/- 0.72 mm, respectively. The mean amount of fluid volume ejected during each contraction was 80 +/- 12.6 microl. The addition of Win 55,212-2 [10(-7)-10(-4) M] to the organ bath determined a dose-related attenuation of peristaltic activity consequent to the decrease of circular and longitudinal muscle strength. The decrease of contractile activity was followed by dose-dependent decrease of the amount of fluid ejected during peristalsis. The effects of Win 55,212-2 [10(-7)-10(-5) M] were prevented by SR 141716A, indicating the presence of cannabinoid CB1 receptors in the mouse distal colon. SR 141716A alone enhanced both tonic and phasic motor activities in the colonic longitudinal smooth muscle, suggesting that CB1 receptor antagonists could act either through antagonising the effect of endogenous CB1 receptor agonist or by an agonist effect on these receptors. The present results further support the hypothesis that cannabinoids perform a neuromodulatory role in various tracts of gastrointestinal system and first demonstrate their action also in the distal colon of rodents.     

Effect of methionine-enkephalin on the spontaneous electrical and mechanical activity of the smooth muscle of the rat portal vein

In the longitudinal smooth muscle of the isolated rat portal vein, methionine-enkephalin (Met-enkephalin) increased the spontaneous contraction with a concentration as low as 10(-8)M. When the membrane activity was recorded using a microelectrode, Met-enkephalin enhanced the spike burst activity but without any effect on the resting membrane potential. Naloxone, phentolamine, atropine and reserpine pre-treatment did not inhibit the excitatory effect of Met-enkephalin on the spontaneous contraction. These results suggest that the excitatory effect of Met-enkephalin on the mechanisms involved in the automaticity may be a direct action on smooth muscle or relate to presynaptic action on a non-adrenergic non-cholinergic system.     

Spontaneous electromechanical activity in the rat duodenum in vitro.

Isolated rat duodenum shows spontaneous mechanical and electrical activities. Mechanical activity consists in changes both in endoluminal pressure and in isometric tension. Electrical activity is characterized by slow waves with superimposed bursts. This spontaneous activity is tetrodotoxin (TTX) resistant and therefore it is myogenic in origin. Indeed, TTX pretreatment, even in the presence of atropine and guanethidine, caused an increase in amplitude and in frequency of the electrical and mechanical activities. This finding indicates the presence of tonically active inhibitory intramural non adrenergic, non cholinergic (NANC) nerves. Duodenal longitudinal strips showed a spontaneous mechanical activity resembling that one recorded from isolated segment. Instead, circular strips are quiescent under resting condition and a contractile activity can be detected only after TTX pretreatment suggesting that: i) the circular smooth muscle layer is tonically inhibited by intramural NANC nerves and, ii) the contractions observed in the rat duodenum are due to the activity of the longitudinal one.