Effect of desensitization induced by adenosine 5'-triphosphate, substance P, bradykinin, serotonin, gamma-aminobutyric acid and endogenous noncholinergic-nonadrenergic transmitter in the guinea-pig ileum

Effects of hyperhomocysteinemia on non-adrenergic non-cholinergic relaxation in isolated rat duodenum

The effect of hyperhomocysteinemia induced by pretreatment with methionine 12 weeks prior to the study on the responses induced by gamma-aminobutyric acid (GABA), electrical field stimulation (EFS), and ATP have been evaluated in isolated rat duodenum. In the presence of adrenergic and cholinergic blockade, EFS (60 V, 1 ms, 1-3 Hz) induced frequency-dependent relaxations of the preparation. GABA and ATP also caused submaximal relaxation of the rat duodenum. The relaxations induced by GABA, EFS, and ATP were not significantly changed in duodenal tissues from hyperhomocysteinemic rats compared with control rats. GABA- and EFS-induced relaxations were inhibited by N-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-4) M) in both hyperhomocysteinemic and control rats. On the other hand, L-NAME incubation did not affect ATP-induced relaxation. These results suggest that hyperhomocysteinemia does not cause an important impairment on non-adrenergic non-cholinergic innervation of the rat duodenum.

Functional antagonism between nitric oxide and ATP in the motor responses of guinea-pig ileum

1. The interaction of nitric oxide and ATP in the non-adrenergic, non-cholinergic (NANC) motor responses and the presence of NADPH-diaphorase and quinacrine-positive myenteric neurones were studied on guinea-pig ileum using mechanographic, histochemical and quinacrine-fluorescence techniques. In the presence of phentolamine, propranolol and atropine, the non-precontracted longitudinally oriented organ bath preparations responded to sodium nitroprusside (1-100 microM) or ATP (5-50 microM) with tetrodotoxin (0.1 microM)-resistant relaxation or contraction, respectively. The effects of ATP were suramin (50 microM)- and apamin (5 microM)-sensitive. 2. The NANC motor responses elicited by electrical stimulation (0.8 ms, 1-20 Hz, 20 s) consisted of tetrodotoxin-sensitive relaxation phase followed by a phase of twitch-like and tonic contractions. 3. NG-nitro-L-arginine (L-NNA, 0.1-0.5 mM) inhibited or abolished the relaxation phase. L-arginine (0.5 mM), but not D-arginine (0.5 mM), restored the relaxation phase in L-NNA-pretreated preparations. The relaxation phase increased after ATP-induced desensitization of purinoceptors and in the presence of suramin (50 mciroM) but was abolished by apamin (5 microM). 4. The phase of contractions was enhanced by L-NNA (0.1-0.5 mM) and restored by L-arginine (0.5 mM). The twitch-like and tonic contractions were decreased during ATP-induced purinoceptor desensitization and in the presence of suramin (50 microLM). Apamin (5 microM) inhibited the tonic contractions. 5. The desensitization of purinoceptors by ATP did not change the L-NNA-induced inhibition of the relaxation phase but decreased the L-NNA-increased phase of contractions. L-NNA reduced the relaxation phase and increased the phase of contractions during purinoceptor desensitization. 6. We conclude that in the longitudinal muscle layer of the guinea-pig ileum, nitric oxide mediates the relaxation phase while ATP contributes via smooth muscle P2 purinoceptors to the phase of contractions suggesting a postjunctional functional antagonism between nitric oxide and ATP. The presence of NADPH-diaphorase- and quinacrine-positive neuronal cells and processes running to the muscle cells confirms a physiological role of nitric oxide and ATP in the ileal neurotransmission.

Rat gastroduodenal motility in vivo: involvement of NO and ATP in spontaneous motor activity

Our studies of fasted anesthetized rats have shown that all spontaneous relaxations of the antrum are nitric oxide (NO) dependent. Duodenal motility is patterned into propagating "grouped" motor activity interposed with "intergroup" periods of nonpropagating motor activity; in the duodenum, only intergroup relaxations are NO dependent. We examined the involvement of NO and ATP in spontaneous motor activities of the gastroduodenum in vivo: contractions and relaxations were recorded and analyzed simultaneously from the antrum (S1) and proximal duodenum (D1) of anesthetized Sprague-Dawley rats (n = 10/group), using extraluminal foil strain gauges. Treatment with the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 mg/kg iv) attenuated (P < 0.05) antral and intergroup relaxations, whereas grouped relaxations were enhanced (P < 0.05). These effects were reversed with L-arginine (300 mg/kg iv). L-NAME also increased (P < 0.05) the amplitude of duodenal contractions. ATP (8 mg. kg-1. min-1 iv) stimulated relaxations at S1 and D1 that were blocked by the P2-purinoceptor antagonist suramin (60 mg/kg iv). This treatment did not affect spontaneous antral relaxations; however, duodenal grouped relaxations were attenuated. Desensitization to the P2x-purinoceptor agonist alpha,beta-methylene ATP (300 micrograms/kg iv) gave results similar to suramin. In contrast, the P2y-purinoceptor agonist 2-methylthio-ATP (2-MeS-ATP; 360 micrograms/kg iv) evoked duodenal relaxations that were attenuated by L-NAME, and desensitization to 2-MeS-ATP attenuated intergroup relaxations. Spontaneous relaxations of the rat antrum and duodenal intergroup relaxations are NO dependent. Both gut regions relax in response to systemically administered ATP; this response is sensitive to suramin. Grouped duodenal relaxations display functional sensitivity to suramin and P2x- purinoceptor desensitization, indicative of the involvement of ATP and P2x purinoceptors. P2y purinoceptors must also be present; however, these occur on elements releasing NO. Although NO does not mediate grouped relaxations or duodenal contractions, the sensitivity of these responses to L-NAME indicates that the pathway(s) controlling these responses is modulated by NO.

Pattern of nonadrenergic, noncholinergic responses during short- or long-lasting electrical stimulation in guinea-pig ileum

1. The pattern of responses of longitudinally oriented guinea pig ileum organ bath preparations was studied during short- (1-5 sec) or long-lasting (20 sec) electrical field stimulation (EFS, 0.8 msec, 40 V, 1-20 Hz). 2. In the presence of phentolamine (5 microM), propranolol (5 microM), and atropine (3 microM), the EFS elicited nonadrenergic, noncholinergic (NANC), tetrodotoxin (0.3 microM)-sensitive responses. 3. The 1-sec EFS evoked relaxation. The response to 5-sec EFS consisted of relaxation followed by twitch, whereas relaxation, twitch and tonic contraction characterized the NANC response to 20-sec EFS. The maximum relaxation was observed at 10-Hz short- or long-lasting EFS. 4. Both N-G-nitro-L-arginine (L-NNA, 0.1-0.5 mM) and apamin (1-5 microM) concentration dependently inhibited the relaxation of the NANC response to 10-Hz 20-sec EFS. During L-NNA treatment, the twitch and the tonic contractions were increased. The inhibitory effect of L-NNA was reversed by L-arginine (0.1-0.5 mM) but not by D-arginine. Sodium nitroprusside (1-10 microM) was without effect. 5. AP 13.2 ACOH (0.1 microM), a blocker of Substance P receptors, inhibited the twitch and the tonic contractions. The contractions were decreased after desensitization of purinoceptors by ATP and in the presence of the GABA(A) receptor antagonist bicuculline (30 microM). 6. Depending on the EFS duration, a subsequent occurrence of relaxation and contractions characterized the NANC responses. It seems that relaxation is mediated by nitric oxide whereas Substance P and ATP are involved in the maintenance of the twitch and the tonic contractions. Nitric oxide appears to exert an inhibitory effect on the excitatory transmitters, whereas purinergic mechanism(s) could modulate the nitric oxide-dependent relaxation.

Tachykinins in the gut. Part I. Expression, release and motor function

The preprotachykinin-A gene-derived peptides substance P and neurokinin (NK) A are expressed in distinct neural pathways of the mammalian gut. When released from intrinsic enteric or extrinsic primary afferent neurons, tachykinins have the potential to influence both nerve and muscle by way of interaction with three different types of tachykinin receptor, termed NK1, NK2 and NK3 receptors. Most prominent among the effects of tachykinins is their excitatory action on gastrointestinal motor activity, which is seen in virtually all regions and layers of the mammalian gut. This action depends not only on a direct activation of the muscle through NK1 and/or NK2 receptors, but also on stimulation of excitatory enteric motor pathways through NK3 and/or NK1 receptors. In addition, tachykinins can inhibit motor activity by stimulating either inhibitory neuronal pathways or interrupting excitatory relays. A synopsis of the available data indicates that endogenous substance P and NKA interact with other enteric transmitters in the physiological control of gastrointestinal motor activity. Derangement of the regulatory roles of tachykinins may be a factor in the gastrointestinal dysmotility associated with infection, inflammation, stress and pain. In a therapeutic perspective, it would seem conceivable, therefore, that tachykinin agonists and antagonists are adjuncts to the treatment of motor disorders that involve pathological disturbances of the gastrointestinal tachykinin system.

Histochemical localization of nitric oxide-synthesizing neurons and vascular sites in the guinea-pig intestine

Laminar preparations of fixed segments of the guinea-pig intestine were examined for nitric oxide synthase activity using reduced nicotinamide adenine dinucleotide phosphate and nitroblue tetrazolium salt as substrates. Under conditions specific for detecting nitric oxide synthase-related diaphorase activity, a subpopulation of neural elements in the myenteric plexus, deep muscular plexus and submucosa were intensely stained. Intensely stained nerve fibres were distributed throughout the meshworks of the myenteric plexus and its innervation of the circular muscle, and in the submucosa within Henle's plexus. Intensely stained nerve cells and their processes were evident in most myenteric ganglia but were rare in ganglia of Henle's plexus. Stained ganglion cells comprised types I, II and VI of the morphologically defined enteric nerve cells. Stained neural elements were increasingly prevalent within successively more caudal segments of the intestine. In addition to neuronal staining, arterioles of the submucosal vascular network displayed distinct, punctate patches of staining distributed over their surface. Perivascular nerve fibre staining was absent. These results show nitric oxide synthase activity to be present within neurons and fibres of the major enteric nerve layers and within submucosal blood vessels throughout the guinea-pig small and large intestine.

5-Hydroxytryptamine is a possible neurotransmitter of the non-cholinergic excitatory nerves in the longitudinal muscle of rainbow trout stomach (Salmo gairdneri)

1. The neurotransmitter of the non-cholinergic excitatory nerves in the rainbow trout stomach was identified on the basis of the pharmacological properties of the contractile responses to transmural stimulation (TMS) and nicotine. 2. TMS caused tetrodotoxin-sensitive contractions of rainbow trout stomach strips in a frequency-dependent manner (0.5-50 Hz). Atropine (1 microM) significantly decreased the contractile response to low-frequency stimulation (0.5-2 Hz), but did not affect that to high-frequency stimulation (3-20 Hz). 3. The atropine-resistant contractile response to TMS (20 Hz) was unaffected by hexamethonium (100 microM), phentolamine (5.4 microM), pyrilamine (1 microM), naloxone (1 microM) or substance P-induced desensitization. 4. 5-Hydroxytryptamine (5-HT, 3 nM-3 microM) caused atropine-resistant contractions in a concentration-dependent manner. In the presence of atropine, methysergide (1 microM) decreased the contractile responses to TMS and 5-HT. 5. Nicotine (3 microM-500 microM) induced atropine-resistant contractions that were completely abolished by tetrodotoxin or hexamethonium. Also methysergide inhibited the contractile responses to nicotine. 6. An acid extract of rainbow trout stomach exhibited atropine-resistant contractions that were decreased by methysergide, in both rainbow trout stomach and guinea-pig ileum longitudinal smooth muscle preparations. 7. The present results indicate that, in longitudinal muscle strips of the rainbow trout stomach, 5-HT is one of the mediators (neurotransmitters) of the non-cholinergic excitatory contractions induced by TMS and nicotine.

Inhibitory action of atrial natriuretic factor on cholinergic and nonadrenergic, noncholinergic neurotransmission in guinea pig small intestine

We studied the effect of synthetic rat atrial natriuretic factor (ANF) (Ser 99-Tyr 126) on the isolated guinea pig proximal ileum. This preparation contained about one-third of the endogenous tissue ANF content which, for the most part, comes from the blood. ANF inhibited, in a dose-dependent manner, cholinergic twitch contractions (EC50 = 4.2 nM), nonadrenergic, noncholinergic (NANC) primary and rebound contractions and histamine-induced sustained tonic contraction (but not carbachol induced contraction) of the longitudinal muscle. Ascending enteric reflex (AER) contractions of the circular muscle were inhibited though not dose-dependently. We suggest pre- and post-synaptic actions of sustained intestinal tissue and blood ANF levels which may play a role in regulating motor activity and muscle tone of the small intestine.

Presence of a substance P-like peptide in an acid extract of the intestinal bulb of the carp (Cyprinus carpio)

1. The effect of an acid extract of the carp intestinal bulb (ECI) on guinea-pig ileum longitudinal smooth muscle (GPLM) and carp intestinal bulb longitudinal smooth muscle (CIBLM) was examined. 2. ECI caused a concentration-dependent contraction of GPLM and CIBLM. This ECI-induced response was reduced by atropine to 30-40% of the control, indicating that part of the contracting activity of ECI is attributable to acetylcholine. The atropine-resistant contracting activity of ECI was not mediated by histamine, 5-hydroxytryptamine, ATP, ADP, angiotensin II, neurotensin, vasoactive intestinal peptide or an opioid peptide. 3. The active material mediating the atropine-resistant contracting activity is probably a peptide, because the contraction in response to ECI was abolished on incubation with pepsin or alpha-chymotrypsin. 4. [D-Pro2, D-Trp7,9]-substance P, [D-Pro4, D-Trp7,9]-substance P (4-11) decreased the atropine-resistant contracting activity of ECI as did desensitization induced by substance P. 5. On a Sephadex G 25 column, the active material was eluted as one peak. The active fractions were pooled and then applied to another Sephadex G25 column to compare the Ve/Vo value for the active material with those for peptides of known molecular weights. The molecular weight of the active material was estimated to be 1200-1700 (1410 +/- 70, n = 6). 6. The results indicate the presence of a substance P-like peptide in the carp intestinal bulb.

Action of histamine on nerve mediated responses of the guinea-pig ileum

The effect of histamine on the responses of the guinea-pig ileum to stimulation of intramural nerves and to some potential nonadrenergic, noncholinergic (NANC) neurotransmitters was analysed. During sustained tonic histamine contraction, electrical stimulation of all intramural nerves elicited a biphasic response (contraction followed by after-relaxation) and application of ATP and bradykinin caused relaxation of the ileum in contrast to their contractile effect on basal tension. Histamine reduced contractile and augmented relaxatory NANC responses, and prevented capsaicin from producing any contractile effect and from significantly influencing the NANC contractions. The present results suggest that, besides its direct effect, histamine activates intramural nerve fibres, mainly the sensory ones, and unmasks NANC relaxation thus modifying the mechanical activity.

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)