The effect of hypoxia on neuroeffector transmission in the bovine retractor penis and rat anococcygeus muscles.

The effects of reducing the PO2 of the bathing fluid were studied on non-adrenergic non-cholinergic (NANC) transmission in isolated preparations of the bovine retractor penis muscle, the rat anococcygeus muscle, the guinea-pig taenia caeci and the guinea-pig urinary bladder. Hypoxia rapidly and reversibly impaired NANC transmission in the bovine retractor penis and rat anococcygeus muscles but did not affect transmission in the guinea-pig taenia caeci or bladder, suggesting that different NANC mechanisms are involved. Although neurally-evoked relaxation of the bovine retractor penis was impaired by hypoxia, relaxations produced by vasoactive intestinal peptide, prostaglandin E1, sodium nitroprusside or an inhibitory factor isolated from the bovine retractor penis were unaffected. Since the inhibitory factor is similar to, and may actually be the NANC transmitter, the results suggest that the site of action of hypoxia in impairing transmission is prejunctional at the inhibitory nerve endings.     

Suramin antagonizes responses to P2-purinoceptor agonists and purinergic nerve stimulation in the guinea-pig urinary bladder and taenia coli.

1. Suramin, an inhibitor of several types of ATPase, was investigated for its ability to antagonize responses mediated via P2X-purinoceptors in the guinea-pig urinary bladder and P2Y-purinoceptors in the guinea-pig taenia coli. 2. In isolated strips of bladder detrusor muscle, suramin (100 microM-1 mM) caused a non-competitive antagonism of responses to alpha, beta-methylene ATP with an estimated pA2 of approximately 4.7, and inhibited responses to stimulation of the intramural purinergic nerves, with a similar pA2 value. At a concentration of 10 microM, suramin had little effect, but at a concentration of 1 microM, suramin potentiated responses to alpha,beta-methylene ATP, and potentiated responses to electrical stimulation of intramural purinergic nerves. 3. In isolated strips of taenia coli, in which a standard tone had been induced by carbachol (100 nM), suramin at 100 microM and 1 mM significantly antagonized relaxant responses to ATP (at an EC50 concentration) with an estimated pA2 of 5.0 +/- 0.82 and relaxant responses to electrical stimulation of the intramural non-adrenergic, non-cholinergic inhibitory nerves, either single pulses or trains of 8 Hz for 10 s, with estimated pA2 values of 4.9 +/- 0.93 and 4.6 +/- 1.01, respectively. Suramin had no significant effect at 1 or 10 microM. 4. Suramin, at any of the concentrations tested, did not affect contractile responses to histamine (10 microM) or carbachol (10 microM) in the bladder detrusor preparations. In the taenia coli, suramin did not affect either the relaxant responses to noradrenaline (at an EC50 concentration) or the contractile responses to carbachol (100 nM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that part of the NANC relaxant response of guinea-pig trachea to electrical field stimulation is mediated by nitric oxide.

1. The nitric oxide (NO) synthesis inhibitors NG-monomethyl L-arginine (L-NMMA) and L-nitroarginine methyl ester (L-NAME) reduced relaxations of guinea-pig tracheal smooth muscle elicited by stimulation of intramural non-adrenergic, non-cholinergic (NANC) nerves, but D-NMMA had no effect. L-NAME was 10-30 times more potent than L-NMMA. Relaxations produced by sodium nitroprusside and vasoactive intestinal polypeptide (VIP) were not affected by L-NMMA or L-NAME. 2. The inhibitory effect of L-NMMA on NANC-mediated relaxations was partially reversed by L-arginine but was not affected by D-arginine. 3. VIP antibody and alpha-chymotrypsin abolished or greatly reduced the relaxant action of VIP and reduced relaxations elicited by stimulation of NANC nerves; the residual NANC relaxation was further reduced by L-NAME. 4. The results suggest that NO and VIP are mediators of NANC-induced relaxations of guinea-pig tracheal smooth muscle. We propose the term 'nitrergic' to describe transmission processes which are mediated by NO.     

Studies on the stereoselectivity of the P2-purinoceptor.

ATP, 2-chloro-ATP, 2-methylthio-ATP, and their unnatural L-enantiomers, were synthesized and their effects tested on the guinea-pig taenia coli and urinary bladder, and the stimulated frog ventricle. The potent P2-purinoceptor agonists, 2-chloro-ATP and 2-methylthio-ATP were, respectively, 30 and 200 times more effective than ATP in relaxing the guinea-pig taenia, but approximately as effective as ATP in contracting the guinea-pig bladder and augmenting the force of contraction of the frog ventricle. A high degree of stereoselectivity was observed for relaxations of the guinea-pig taenia coli produced by the P2-purinoceptoragonists, and 2-methylthio-ATP was over 700 times more effective than its L-enantiomer. In contrast, stereoselectivity for contraction of the guinea-pig bladder was observed only at low concentrations with each pair of enantiomers, and a similar low stereoselectivity was displayed by the frog ventricle. These results show that P2-purinoceptors mediating inhibitory responses in the guinea-pig taenia coli can show a high degree of stereoselectivity, while P2-purinoceptors mediating excitatory responses in the guinea-pig bladder and in the frog ventricle show little stereoselectivity. The partial stereoselectivity of the P2-purinoceptor in smooth muscle contrasts with the absolute stereospecificity of P1-purinoceptors for adenosine on smooth muscle and autonomic nerve terminals and the absolute stereospecificity of the receptor for ADP on the human platelet.     

Suramin and the inhibitory junction potential in taenia caeci of the guinea-pig

The effect of surinam on the inhibitory junction potential evoked in smooth muscle cells of guinea-pig taenia caeci by stimulation of intramural nerves at 22 degrees C was investigated. The amplitude of the inhibitory junction potential was reduced concentration dependently by suramin. The suppression of this response by suramin became less pronounced when the number of stimuli increased (pulse rate: 20/s). These results indicate that suramin reduces the inhibitory junction response by interacting with P2-purinoceptors.     

Direct evidence for ATP release from non-adrenergic, non-cholinergic ("purinergic") nerves in the guinea-pig taenia coli and bladder

Demonstration of release of ATP from smooth muscle preparations during stimulation of purinergic nerves is complicated by the difficulty in showing whether it comes from nerve or muscle. ATP released during relaxation of the guinea-pig taenia coli and contraction of bladder strips in response to purinergic nerve stimulation was measured in the superfusate using the luciferin-luciferase ATP assay method. The amount of ATP increased 2-6 fold during isometric responses to purinergic nerve stimulation. This release was blocked by tetrodotoxin but not by adrenergic nerve destruction with 6-hydroxydopamine. No significant release of ATP was detected during comparable responses elicited by direct muscle stimulation. These results provide further support for the purinergic nerve hypothesis.     

Comparison of the effects of ultraviolet light and purinergic nerve stimulation on the guinea-pig taenia coli

1 The responses of the guinea-pig taenia coli, urinary bladder, and the rabbit portal vein to ultraviolet (u.v.) light were compared to those elicited by purinergic nerve stimulation and exogenous adenosine triphosphate (ATP).2 In the presence of sodium nitrite, u.v. light between 340-380 nm produced a maximum relaxation of the taenia coli. The relaxation was reversible and fast in onset. It was unaffected by atropine, guanethidine or low concentrations of phentolamine or propranolol. When the tone was low, the relaxation was usually followed by a ;rebound contraction' upon cessation of stimulation. Thus, the response to u.v. light closely resembles the responses to both purinergic nerve stimulation and exogenously applied ATP.3 U.v. light did not initiate impulses in purinergic nerves since its action was unaffected by tetrodotoxin; nor did it release ATP from nerve terminals (in contrast to its release during purinergic nerve stimulation). The adenosine-uptake inhibitor, dipyridamole, which potentiates the responses to purinergic nerve stimulation and ATP, did not affect the response to u.v. light.4 Agents known to alter postjunctional responses to purinergic nerve stimulation and ATP also altered the response to u.v. light. High concentrations of the 2-substituted imidazoline compounds, antazoline and phentolamine, which antagonize the responses to purinergic nerve stimulation and ATP, reduced the responses to u.v. irradiation. The prostaglandin synthesis inhibitor, indomethacin, which abolishes the ;rebound contraction' following stimulation of purinergic nerves, also blocks the ;rebound ;contraction' following u.v. irradiation. Increases in the K(+) concentration produced parallel changes in the inhibitory responses to u.v. light and purinergic nerve stimulation.5 U.v. light produced relaxation and inhibition of spontaneous activity of the rabbit portal vein (relaxed by ATP), but had no effect on the guinea-pig urinary bladder (contracted by ATP) and ureter (unaffected by ATP).6 It is suggested that u.v. light is acting on some part of the purinergic receptor complex which is involved in the mediation of inhibitory responses to ATP and purinergic nerve stimulation, and may therefore provide a way of investigating the chemistry of inhibitory purinergic receptors.     

Effects of omega-conotoxin GVIA on autonomic neuroeffector transmission in various tissues.

1. The effects of omega-conotoxin GVIA (conotoxin), a potent inhibitor of neuronal N-type Ca2+ channels, have been examined on responses to stimulation of noradrenergic, cholinergic and non-adrenergic, non-cholinergic (NANC) nerves in a range of isolated tissues to investigate the role of conotoxin-sensitive Ca2+ channels in neurotransmission. 2. Contractions elicited by field stimulation of noradrenergic nerves in rat and mouse anococcygeus muscles, rabbit ear artery and rat vas deferens (epididymal portion) were inhibited by conotoxin. Responses to noradrenaline, and to adenosine triphosphate in the vas deferens, were not affected. 3. Positive chronotropic responses to field stimulation of noradrenergic nerves were inhibited by conotoxin in rat and mouse atria, but responses to noradrenaline and tyramine were not affected. 4. The stimulation-induced release of noradrenaline was inhibited by conotoxin in the rabbit ear artery and in rat and mouse atria. 5. Relaxations in response to stimulation of the noradrenergic perivascular mesenteric nerves were reduced or abolished by conotoxin in rat and rabbit jejunum. The response to noradrenaline in rat jejunum was not affected. 6. Contractions elicited by stimulation of cholinergic nerves were inhibited by conotoxin in rat jejunum and mouse ileum (perivascular mesenteric nerves), and in guinea-pig taenia caeci (field stimulation). Responses to acetylcholine in rat jejunum and mouse ileum were not affected. 7. Contractions elicited by stimulation of the cholinergic plus NANC pelvic nerves were inhibited by conotoxin in rabbit colon, and to a lesser extent in guinea-pig colon. The stimulation-induced contraction of the guinea-pig colon was inhibited by conotoxin by a greater proportion in the presence than in the absence of atropine. Responses to acetylcholine were not affected in the rabbit colon but were slightly reduced in the guinea-pig colon. 8. Relaxations in response to field stimulation of NANC nerves were inhibited by conotoxin in guinea-pig taenia caeci and rat gastric fundus strips, and in rat anococcygeus muscle when the tone was raised by guanethidine but not when it was raised by carbachol. The relaxations produced by sodium nitroprusside in the rat gastric fundus and anococcygeus were not affected. 9. Contractions of the rat bladder elicited by stimulation of the peri-urethral nerves, which are NANC- and cholinergically mediated, were relatively insensitive to inhibition by conotoxin. The response were almost completely abolished by tetrodotoxin. 10. The conotoxin-induced inhibitions of responses to nerve stimulation developed slowly and persisted after removal of conotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)     

Theophylline antagonizes some effects of purines in the intestine but not those of intramural inhibitory nerve stimulation.

1 Hyoscine- and guanethidine-treated preparations of longitudinal muscle of rabbit duodenum, guinea-pig taenia caeci and fundic strip relaxed when exposed to noradrenaline, adenosine, adenosine triphosphate (ATP) or to field stimulation of their intramural nerves. 2 In guinea-pig taenia caeci and fundus, theophylline 100 mumol/l had no effect on responses to noradrenaline, adenosine, ATP and intramural nerve stimulation. 3 In rabbit duodenum, theophylline 100 mumol/l antagonized some responses to adenosine but had no effect on responses to noradrenaline, ATP and intramural nerve stimulation. 4 Theophylline 1 mmol/l itself relaxed the intestinal tissues and in the fundic strip and taenia caeci, these relaxant effects were associated with abolition of spike activity and cellular hyperpolarization. In the taenia caeci, the amplitude of inhibitory post-junctional potentials was reduced. 5 Theophylline 1 mmol/l antagonized the twitch suppression produced by adenosine and ATP in the transmurally-stimulated guinea-pig ileum but not that evoked by noradrenaline. 6 It is concluded that theophylline can selectively antagonize some actions of purines in the intestine but that it does not specifically antagonize the effect of intramural inhibitory nerve stimulation.     

Adenosine 5'-(2-fluorodiphosphate) is a selective agonist at P2-purinoceptors mediating relaxation of smooth muscle

The pharmacological effects of ATP and of an adenine nucleotide analogue, adenosine 5'-(2-fluorodiphosphate) (ADP-beta-F) on various guinea-pig isolated smooth muscle preparations were investigated. ATP relaxed the taenia coli and the aorta, and contracted the urinary bladder and vas deferens. ADP-beta-F mimicked the effects of ATP on the taenia coli and aorta, but did not contract the bladder or vas deferens. The lack of potency of ADP-beta-F on the bladder was not due to its rapid degradation by ectonucleotidases, as it was degraded more slowly than ATP by both the bladder and the taenia coli. These results are consistent with the suggestion that the P2-purinoceptors mediating contraction of smooth muscle (P2X) are different from those mediating inhibitory effects (P2Y), and suggest that ADP-beta-F is a specific agonist at the P2Y-purinoceptor.     

Evidence against vasoactive intestinal polypeptide being the non-adrenergic, non-cholinergic inhibitory transmitter released from nerves supplying the smooth muscle of the guinea-pig taenia coli

Vasoactive intestinal polypeptide (VIP), located within intrinsic neurones of the guinea-pig taenia coli, has been proposed as a neurotransmitter which may mediate certain non-adrenergic, non-cholinergic effects within the gastrointestinal tract. The relaxation of the taenia coli produced by exogenous VIP has a longer latency and time to maximum than the relaxation which is obtained during electrical stimulation of the non-adrenergic, non-cholinergic nerves. The responses to exogenous VIP (0.03-1 microM) were abolished by the proteolytic enzyme alpha-chymotrypsin (1 U/ml), whereas there were no statistically significant changes in the inhibitory responses to intramural nerve stimulation (0.1-5 Hz) or exogenous ATP (0.03-100 microM), which closely mimics the nerve-mediated response. Apamin is a neurotoxin which prevents the increase in K+ conductance associated with the inhibitory junction potential produced by stimulation of the non-adrenergic, non-cholinergic inhibitory nerves. In the presence of apamin (0.005-1 microM) the responses to intramural nerve stimulation and exogenous ATP were significantly antagonised, and often converted to contractile responses. The VIP-induced relaxations during apamin treatment were not significantly decreased. These results suggest that VIP is unlikely to be the transmitter released from the non-adrenergic, non-cholinergic nerves of the guinea-pig taenia coli, and are consistent with the view that these nerves are purinergic in nature.     

The effect of catecholamines and ATP on the smooth muscle cell membrane of the guinea-pig taenia coli

A comparative study was made of the effects of some catecholamines and purine-like compounds on the smooth muscle cell membrane of the guinea-pig taenia coli, using the double sucrose-gap method. The inhibitory junction potential (IJP) might be mediated by purinergic or adrenergic nerves. Therefore, the effects of these compounds on the membrane resistance and the membrane potential and also on the IJP were measured in the presence of guanethidine and atropine. The potency of the catecholamines tested to mimic the IJP with regard to membrane hyperpolarization and decrease of membrane resistance suggested an action on α-adrenoceptors: adrenaline > noradrenaline ? isoprenaline. Equipotent concentrations of adrenaline and adenosine triphosphate (ATP) were found to exert the same hyperpolarization of the smooth muscle membrane in both the presence and absence of chloride. From the effects observed with phentolamine and tolazoline it is concluded that the IJP is not mediated by adrenergic nerves, for the adrenaline response was abolished, whereas the IJP was unaffected. Furthermore, the ATP-blocking potencies of phentolamine and imidazole could not be demonstrated. On the other hand, the palallelism observed between the responses obtained with ATP and with transmural stimulation is in favor of the hypothesis that the IJP is mediated by purinergic nerves.     

The lack of involvement of cyclic nucleotides in the smooth muscle relaxant action of BRL 34915.

1. The relaxant effect of BRL 34915 has been examined on four isolated preparations, the bovine retractor penis (BRP), guinea-pig taenia coli, guinea-pig trachea and rabbit aortic strip contracted by either histamine, carbachol, noradrenaline or 10 mM KCl. Even though the probability of the involvement of external calcium entering through voltage-operated channels in these tissues varied, there was little corresponding variation in sensitivity to BRL 34915. 2. The relaxant effect of BRL 34915 on the BRP and guinea-pig taenia coli was unaffected by haemoglobin 3.3 microM or Apamin 0.5 microM, concentrations which blocked completely the relaxant effect of non-adrenergic, non-cholinergic (NANC) nerve stimulation in these tissues. 3. BRL 34915 in doses causing maximum relaxation did not increase the levels of either cyclic AMP or cyclic GMP in the BRP, although the appropriate enzymes were present and could be stimulated by forskolin or sodium nitroprusside. 4. In the BRP isoprenaline 30 microM acting through beta-receptors caused maximum relaxation but did not raise the levels of cyclic AMP, even though lower doses of 2 microM did raise the levels of this nucleotide in the rabbit uterus. 5. These results provide some indirect evidence that membrane hyperpolarization may not be the only cause of the smooth muscle relaxation induced by BRL 34915. However, neither a rise in cyclic AMP nor cyclic GMP are satisfactory alternative mechanisms.     

Mode of action of cholecystokinin octapeptide on smooth muscles of stomach, ileum and gall bladder

Mechanical activity of preparations isolated from canine and guinea-pig stomach, ileum and gall bladder was recorded. At least 2 strips were cut out from each organ and investigated simultaneously in thermostatically controlled organ baths. Responses to acetylcholine (ACH) were used for comparison. Octapeptide of cholecystokinin (CCK-OP) at concentrations of 5 X 10(-11) M to 10(-8) M produced dose-dependent tonic concentrations in all muscle strips and showed a higher affinity but lower efficacy as compared to ACH. Atropine (10(-7) M 10(-5) M) had no effect on the CCK-OP responses in stomach and gall bladder muscle strips but it significantly decreased the CCK-OP responses in ileum muscle strips. Dibutyryl cyclic GMP (dbc GMP) at concentrations of 10(-5) M-5 X 10(-4) M did not change the ACH dose-response curves but shifted to the right in parallel to the control the dose-response curves for CCK-OP in all muscle preparations with pA2 values 5, 5.3, 7.2, and 6 for canine stomach and guinea-pig stomach, ileum and gall bladder, respectively. Michaelis-Menten's analysis suggested a competitive type of interaction of dbc GMP on CCK-OP contractile responses of guinea-pig ileum and gall bladder; at a higher concentration (5 X 10(-4) M) the antagonistic effect of dbc GMP in canine and guinea-pig stomach appeared to be a mixed or uncompetitive type. The data suggest that the contractile effects of CCK-OP in the iliac smooth muscle are caused by cholinergic and direct mechanisms whereas on the gall bladder and gastric smooth muscles by direct myogenic mechanisms only.     

A modulating role of prostaglandins in contractions of the guinea-pig ileum

1 The role of prostaglandins in contractions of the guinea-pig ileum evoked either directly by acetylcholine or indirectly by angiotensin and by coaxial stimulation has been investigated.2 Prostaglandin E(2) in low concentration (6 nM) slightly augmented both types of contraction. Indomethacin, an inhibitor of prostaglandin synthesis, markedly reduced the indirectly evoked contractions but did not affect contractions in response to acetylcholine. The addition of prostaglandin E(2) to the preparation treated previously with indomethacin restored the effect of indirect stimulation.3 The pretreatment of the preparation with guanethidine or alpha-methyl-p-tyrosine prevented the inhibitory effect of indomethacin on indirectly evoked contractions. Prostaglandin E(2) addition to such preparations considerably augmented both types of contraction.4 The stimulation of non-cholinergic, non-adrenergic inhibitory nerves in the taenia coli and ileum preparations evoked hyperpolarization and relaxation of the preparations followed by action potentials and contraction. These responses were not changed by indomethacin pretreatment and prostaglandin E(2), but rebound contraction was sometimes augmented by the prostaglandin.5 Two mechanisms for the effects of prostaglandin E(2) are suggested: a direct effect on smooth muscle, and an indirect action through the sympathetic nerves which by release of noradrenaline affect the acetylcholine release from parasympathetic nerve endings.     

Effects of phosphorothioate analogues of ATP, ADP and AMP on guinea-pig taenia coli and urinary bladder.

Phosphorothioate analogues of ATP, ADP and AMP were tested on the guinea-pig taenia coli and urinary bladder. The Rp diastereoisomers of the phosphorothioate analogues, ATP-alpha-S and ADP-alpha-S were respectively 7 and 3 times more effective than the Sp diastereoisomers in causing relaxation of the taenia coli. No stereoselectivity was observed for the diastereoisomers of ATP-beta-S. In guinea-pig bladder, no stereoselectivity was observed for any of the phosphorothioate analogues. These results show that P2-purinoceptors mediating inhibitory responses in the guinea-pig taenia coli show marked stereoselectivity, while P2-purinoceptors mediating excitatory responses in the guinea-pig bladder show little stereoselectivity.     

The effect of cold storage on the adrenergic mechanisms of intestinal smooth muscle

1. In the guinea-pig taenia caecum, fluorescent adrenergic fibres terminate in both muscle layers. The density of these fibres is greater in the taenia than in the underlying circular muscle layer. The myenteric plexus and individual ganglion cells are also densely innervated by intensely fluorescent adrenergic nerve fibres.

2. After three days of cold storage, the specific fluorescence disappeared from all tissue layers of the taenia caecum and smooth muscle fibres. In contrast, cholinesterase active substances were still demonstrable in all tissue layers even after seven days of cold storage but the density of these substances was decreased.

3. Cold storage (3-7 days) decreased the tissue noradrenaline content and did not modify the cholinesterase enzyme activity (4 days).

4. In cold stored strips, the inhibitory response to nicotine, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) or electrical transmural stimulation was abolished and enhancement of the contractile response occurred. Cold storage also inhibited the inhibitory action of tyramine. Similar results were observed after reserpine treatment.

5. In fresh taenia, the relaxation produced by nicotine, DMPP and electrical transmural stimulation was inhibited by adrenoceptor blocking agents and bretylium. In cold storage preparations, contraction produced by these stimuli was blocked by parasympathetic blocking agents and potentiated by anti-cholinesterase. These results indicate that the inhibitory response to these stimulants is mediated by stimulation of the adrenergic nerve system more than by non-adrenergic nerves; the excitatory effect is probably due to stimulation of cholinergic nerves.

6. These results suggest that the adrenergic mechanisms of the taenia caecum are more labile in cold storage than the cholinergic mechanisms. Thus, the inhibitory action of cold storage on the relaxation produced by nicotine, DMPP, and transmural stimulation is probably explained by selective physical degeneration of the adrenergic nerve terminal. Also, enhancement of the contractile response to these stimulants in cold stored preparations is explained by the lack of adrenergic inhibitory mechanisms.

     

Evidence that adenosine triphosphate or a related nucleotide is the transmitter substance released by non-adrenergic inhibitory nerves in the gut

1. Stimulation of the vagal non-adrenergic inhibitory innervation caused the release of adenosine and inosine into vascular perfusates from the stomachs of guinea-pigs and toads.2. Stimulation of portions of Auerbach's plexus isolated from turkey gizzard caused the release of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP).3. ATP, added to solutions perfused through the toad stomach vasculature, was broken down to adenosine, inosine and adenine.4. Of a series of purine and pyrimidine derivatives tested for inhibitory activity on the guinea-pig isolated taenia coli, ATP and ADP were the most potent.5. ATP caused inhibition of twelve other gut preparations previously shown to contain non-adrenergic inhibitory nerves. The inhibitory action of ATP was not prevented by tetrodotoxin.6. Quinidine antagonized relaxations of the guinea-pig taenia coli caused by catecholamines or adrenergic nerve stimulation. Higher concentrations of quinidine antagonized relaxations caused by ATP or non-adrenergic inhibitory nerve stimulation.7. When tachyphylaxis to ATP had been produced in the rabbit ileum, there was a consistent depression of the responses to non-adrenergic inhibitory nerve stimulation but not of responses to adrenergic nerve stimulation.8. It is suggested that ATP or a related nucleotide is the transmitter substance released by the non-adrenergic inhibitory innervation of the gut.     

Physiological role of prostaglandins in adrenergic and non-adrenergic inhibitory neurotransmission in guinea pig taenia coli]

Prostaglandin (PG) E1, PGE2 and PGF2alpha and their biosynthesis inhibitor, indomethacin, were tested for their effects on the inhibitory responses of taenia induced by electrical stimulation of adrenergic and/or non-adrenergic inhibitory nerves in the perivascular nerve-taenia preparation isolated from guinea-pig caecum. The response to adrenergic nerve stimulation was considerably reduced by PGE1 and PGE2, while it was little affected by PGF2alpha. Although PGE1 and PGE2 produced contraction of taenia, it may be possible to dismiss from consideration their action of contraction of taenia in accounting for their inhibitory effect on the response to nerve stimulation since the following observations were made, 1) when the taenia were contracted by histamine, the response to adrenergic nerve stimulation was not reduced and 2) in the presence of polyphloretin phosphate, PGE1 and PGE2 did not contract taenia but reduced the response to adrenergic nerve stimulation. In the presence of indomethacin, the response to adrenergic nerve stimulation was increased greatly. On the contrary, the inhibitory response to non-adrenergic inhibitory nerve stimulation was not affected by application of PGE1, PGE2, PGF2alpha and indomethacin. These results suggest that endogenous PG of E series in guinea-pig taenia may play a role in modulating adrenergic neurotransmission. Attempts to demonstrate that PG could operate on a non-adrenergic inhibitory neurotransmission by a negative feedback mechanism were without success.     

Effects of loperamide on the guinea pig taenia coli.

The effects of an antidiarrhoeal agent, loperamide, were studied using isolated guinea pig taenia coli. Loperamide in concentrations ranging from 10(-7) M inhibited the cholinergic, contractile responses induced by electrical transmural stimulation, nicotine and serotonin, and the nonadrenergic, relaxing responses induced by electrical transmural stimulation and nicotine. However, the adrenergic response to perivascular nerve stimulation was not affected by these concentrations. The inhibitory effects of loperamide were not reversed by washing with a drug-free solution. Morphine (10(-6 M to 10(-5) M) also inhibited both cholinergic and nonadrenergic responses, but the effect was reversible. Naloxone (1o(-6) M) attenuated the inhibitory effects of both drugs. Unlike morphine, loperamide in concentrations higher than 5 x 10(-6) M relaxed the strips and reduced the contractile response to acetylcholine noncompetitively, and these effects were not blocked by naloxone. These results suggest that loperamide at low concentrations acts selectively on the opiate receptors located in both cholinergic and nonadrenergic nerves and at higher concentrations also acts directly on smooth muscle thus producing relaxation of the intestinal tone.