Relaxation of guinea-pig fundic strip by adenosine, adenosine triphosphate and electrical stimulation: lack of antagonism by theophylline or ATP treatment

Mechanisms involved in the muscle relaxing effects of STW 5 in guinea pig stomach

INTRODUCTION

The herbal preparation STW 5 ameliorates functional dyspepsia partly by relaxing smooth muscle of the proximal stomach, thus improving gastric accommodation. We explored the unknown pathways responsible for this effect by testing targets known to modulate gastric smooth muscle relaxation.

METHODS

STW 5-induced relaxation of smooth muscle strips from guinea pig gastric corpus before and after pharmacological interventions were recorded with force transducers in an organ bath. ORAI1 mRNA expression was tested in the proximal stomach.

KEY RESULTS

Blockade of Ca2+ -activated K+ and Cl- channels, voltage-gated L- or T-type Ca2+ channels, TRPA1-, TRPV1-, adenosine or 5-HT4 receptors, antagonizing ryanodine receptors, inhibiting cyclooxygenase or sarcoplasmic reticulum calcium ATPase did not affect STW 5-evoked relaxation. Likewise, protein-kinase A or G were not involved. However, the relaxation evoked by STW 5 was significantly reduced by phorbol-12-myristat-13-acetat, an activator of protein-kinase C, by 2- aminoethyldiphenylborinate, an inhibitor of the IP3 receptor-mediated Ca2+ release from the sarcoplasmic reticulum or by SKF-96365, a nonselective store-operated calcium entry (SOCE) blocker. Furthermore, the mixed TRPC3/SOCE inhibitor Pyr3, but not the selective TRPC3 blocker Pyr10, reduced the effect of STW 5. Finally, BTP2, a potent blocker of ORAI-coupled SOCE, almost abolished STW 5-evoked relaxation. Expression of ORAI1 could be demonstrated in the corpus/fundus.

CONCLUSIONS & INFERENCES

STW 5 inhibited SOCE, most likely ORAI channels, which are modulated by IP3- and PKC-dependent mechanisms. Our findings impact on the design of drugs to induce muscle relaxation and help identify phytochemicals with similar modes of actions to treat gastrointestinal disturbances.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Gastric motor responses elicited by vagal stimulation and purine compounds in the atropine-treated rabbit

1. The effects of vagal inhibitory stimulation and of purine compounds were studied in the rabbit stomach. 2. Gastric motility was assessed by the balloon method. Vagal nerves were electrically stimulated at the neck. Purine compounds were injected intra-arterially. 3. In the atropine-treated rabbit, vagal stimulation caused relaxant motor responses followed by a rebound contraction. 4. Among the purine compounds, only ADP and ATP caused relaxant motor responses similar to the effects of vagal inhibitory stimulation. However, the relaxation produced by ATP was more powerful than that due to ADP, especially at lower infusion rates. 5. Vagal inhibitory responses were recorded during and after infusion of ATP. When relaxation by ATP was fully developed, vagal inhibitory stimulation was ineffective. At the highest infusion rates of ATP, a depression of the vagal inhibitory motility was also observed after cessation of the infusion. 6. Relaxant responses to ATP and vagal inhibitory stimulation were not influenced by theophylline, scarcely affected by alpha,beta-methylene ATP, but were reduced or blocked by reactive blue 2. 7. The results are consistent with ATP being an inhibitory neurotransmitter in the stomach of the rabbit.

The relaxing response of the isolated rat duodenum to nicotine

1. The relaxing responses of the atropine-treated isolated rat duodenum to 12 microM nicotine (N) and transmural electrical stimulation (S) have been compared. 2. The effects of both N and S were unaffected by 3-30 microM guanethidine or previous reserpinization (1 mg/kg, i.p. during 5 days). 3. The effects of N and S were completely blocked by 0.5-2.0 mM procaine, 0.1 mM butacaine or 0.3-1.2 microM tetrodotoxin. 4. The relaxing response to N but not that to S was blocked by 0.3 mM hexamethonium or 3 microM mecamylamine and absent in the nicotine-desensitized preparation. 5. The effects of both N and S were unaffected in the adenosine-5'-triphosphate (ATP)-desensitized preparation. 6. It is suggested that the nonadrenergic and noncholinergic (NANC) relaxation of the rat duodenum in response to N and S is also nonpurinergic.

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)

Neural inhibition of gastrointestinal smooth muscle motility in guinea-pigs

The neural inhibition of the gastrointestinal (GI) smooth muscle motility was studied by means of electrical transmural nerve stimulation (ETNS) in guinea-pigs. In untreated muscle strips, ETNS induced four types of response, consisting of three basic components, i.e., contraction and relaxation during ETNS, and after-contraction. Following atropinization, all the responses changed to only one type, i.e., relaxation during ETNS followed by after-contraction. The relaxation of the muscle strips induced by ETNS after atropinization was not significantly reduced under superimposing guanethidine treatment in any site of the GI tract. The maximal relaxation of the muscle strips induced by ETNS after atropine and guanethidine treatments was not uniform throughout the GI tract. The relaxation of the gastric body and colon was greater than that of the jejunum and ileum. The extent of the relaxation was significantly different even in the colon. Theophylline and phentolamine did not reduce the ETNS-induced relaxation following atropine and guanethidine treatments in the distal colon and taenia coli. The findings suggest the following: the non-adrenergic inhibitory (NAI) nerve is the main postganglionic nerve to inhibit the GI smooth muscle motility; and the role of adrenergic nerve is of little importance in respect to postganglionic inhibition. The innervation of the NAI nerve seems not to be uniform throughout the GI tract. Theophylline and phentolamine are not the specific antagonists of the NAI nerve.

Dual action of cholecystokinin-octapeptide on the guinea pig antrum

We examined the mechanism of action of cholecystokinin-octapeptide (CCK-OP) on longitudinal strips of the lesser and greater curvatures of the guinea pig antrum. In the strips of the lesser curvature, CCK-OP produced a concentration-dependent tonic contraction, which was significantly reduced by atropine, but not by tetrodotoxin. In contrast, CCK-OP caused a relaxation of the preparation of the greater curvature in a concentration-dependent manner. The relaxation induced by the peptide was enhanced by atropine, whereas it was blocked by tetrodotoxin. Propranolol, phentolamine, desensitization to adenosine-5'-triphosphate, and desensitization to vasoactive intestinal polypeptide had no effect on CCK-OP-induced relaxation. Cholecystokinin-octapeptide evoked the release of acetylcholine from strips of both sides, and it was not blocked by tetrodotoxin. These findings indicate that the mechanism of action of CCK-OP on the lesser curvature differs from that on the greater curvature. The peptide seems to act directly on smooth muscle cells and to stimulate cholinergic neural activity by sodium channel-independent mechanisms. Additionally, nonadrenergic inhibitory neurons appear to be activated by CCK-OP in the greater curvature.

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

The cholinergic and noncholinergic-nonadrenergic (NCNA) excitatory and inhibitory responses of the guinea-pig ileum to transmural nerve stimulation (TNS) were studied. Unlike the contraction induced by histamine and acetylcholine the responses to TNS, ATP, substance P, bradykinin, 5-HT and GABA were not sustained. The contraction and its fading during TNS involved the activation of cholinergic, adrenergic and NCNA neurons. Substance P, 5-HT and ATP desensitization resulted in reduction of the excitatory NCNA response whereas that due to bradykinin attenuated both the excitatory and inhibitory NCNA responses. The desensitization against TNS and the potential transmitters studies was selective except in the case of ATP. The present results suggest that it is unlikely that ATP, bradykinin or GABA would be the NCNA transmitters in the guinea-pig ileum. The cross-desensitization between the excitatory NCNA transmitter on the one hand, and substance P (markedly expressed) and 5-HT (slightly expressed) on the other hand, give further evidence in favor of the possible transmitter role of substance P-like peptide in excitatory NCNA transmission and of the role of 5-HT in the activation of NCNA neurons.

Adenosine and ATP effects on isolated guinea pig gallbladder

Effects of adenosine, ATP and several derivatives of adenosine were measured in isolated strips of guinea pig gallbladder. Adenosine caused relaxations which were antagonized by theophylline and potentiated by an inhibitor of adenosine uptake, 6-(1-hydroxy-5-nitrobenzylthio)-guanosine (HNBTG). Among several adenosine derivatives, 2-chloroadenosine and 5'-N-ethylcarboxy-midoadenosine were similarly effective while 1-N6-phenylisopropyladenosine was only a weak relaxant. None of the derivatives caused maximal relaxations at 100 microM, and thus absolute potencies could not be determined. ATP caused predominantly contractile effects, with relaxations sometimes being evident at high concentrations. Indomethacin abolished contractile effects of ATP, suggesting prostaglandin involvement, and only relaxations were evident in its presence. Adenosine deaminase abolished the effects of adenosine and partly reduced the relaxant effects of ATP in the presence of indomethacin. In view of the low potency of adenosine and ATP, physiological roles for these compounds in gallbladder motility are not readily evident.

Neither a purine nor VIP is the mediator of inhibitory nerves of opossum oesophageal smooth muscle

Effects of stimulation of intramural nerves in the circular smooth muscle layer of the body of the oesophagus of the opossum (Didelphis marsupialis) were studied, simultaneously measuring the membrane potential of muscle cells using the sucrose-gap technique and contractions of the muscle. Electrical field stimulation of the preparation, superfused with Krebs solution at 27 degrees C, induced a transient hyperpolarization of the smooth muscle cell membrane (i.j.p.) followed by a transient depolarization on which muscle action potentials were often superimposed. The muscle did not develop active tension spontaneously; it therefore did not relax during the i.j.p., but often contracted during the 'off' depolarization. The i.j.p. and the responses following it were characterized as mediated by intramural, non-adrenergic, non-cholinergic (n.a.n.c.) nerves. The i.j.p. amplitude was reduced by raising the external K concentration. When Cl was replaced by isethionate, a small hyperpolarization of the smooth muscle cell membrane ensued along with a comparable small reduction of the i.j.p. amplitude. The 'off' activity following the i.j.p. disappeared completely in Cl-free medium. Apamin (10(-7)-10(-5) M) did not influence this preparation nor the i.j.p. Adenosine and its related adenine nucleotides in concentrations up to 10(-3) M hardly affected the preparation. Prolonged superfusion with adenosine and 6-chloroadenosine revealed a gradually increasing attenuation of the i.j.p. Exogenously applied vasoactive intestinal polypeptide (VIP) induced rhythmic depolarizations of the smooth muscle cell membrane and spontaneous contractions, which were insensitive to neurotoxins but absent in Cl-free media. Field stimulation in the presence of VIP caused an i.j.p. which transiently interrupted the VIP-induced contractile responses. It is concluded that the inhibitory mediator of the intramural n.a.n.c. nerves present in the circular smooth muscle layer of the opossum oesophagus is neither a purine nor VIP. The i.j.p. may result from a selective increase in K permeability of the smooth muscle cell membrane, the 'off' depolarization may involve an increase in the Cl ion conductance. The suggestion is made that the release of neurotransmitter from intramural n.a.n.c. nerves is modulated presynaptically via P1-purinoceptors and that VIP is a likely candidate for an excitatory transmitter, released simultaneously with the inhibitory transmitter.

Contribution of prostaglandins and thromboxanes to the adenosine and ATP-induced contraction of guinea-pig isolated trachea

1 In in vitro experiments adenosine 5'-triphosphate (ATP) and adenosine were found to exert different effects on the guinea-pig isolated trachea depending on whether the trachea had previously been contracted with acetylcholine (ACh) (6.6 x 10(-6) M) or was at resting tone. 2 ATP and adenosine (10(-5) and 10(-3) M) were equipotent in relaxing the precontracted guinea-pig trachea, since concentrations of 1.09 +/- 0.35 and 0.39 +/- 0.16 mM respectively reduced by 25% the ACh-induced contraction. 3 ATP and adenosine (10(-5) and 10(-4) M) caused a moderate contraction of the guinea-pi trachea under resting tone. This effect was antagonized by inhibitors of cyclo-oxygenase (indomethacin 10(-6) M, aspirin 0.3 x 10(-3) M and 3 x 10(-3) M) and of thromboxane synthetase (nictindole 10(-7) M, imidazole 5 x 10(-5) M), which suggests an indirect mechanism of action with release of arachidonic acid derivatives.

Evidence against ATP being the nonadrenergic, noncholinergic inhibitory transmitter in guinea pig stomach

THe adenosine-sensitive P1 purinoceptor antagonists 8-phenyl theophylline and 8-(p-bromophenyl)theophylline (4 microM) antagonized ATP-induced relaxation of spontaneous tone in guinea pig stomach, but caused no significant modification of relaxation to electrical field stimulation (0.2 - 10 Hz, 1 msec for 30 sec, atropine 1.5 microM, and guanethidine 4 microM treated). These results suggest that in fundic muscle ATP acts via hydrolysis to adenosine, with subsequent activation of P1 purinoceptors, and that ATP is not the nonadrenergic, noncholinergic inhibitory transmitter in stomach fundus.

Non-cholinergic, non-adrenergic responses to nerve stimulation of different regions of the guinea-pig small intestine

Non-cholinergic, non-adrenergic responses to nerve stimulation recorded from smooth muscles of the guinea-pig duodenum, jejunum, proximal and terminal ileum were investigated in an attempt to characterize these responses. In the presence of atropine (1-2 mumol X l-1) and guanethidine (10 mumol X l-1) coaxial stimulation induced in all regions of the guinea-pig small intestine an initial relaxation (primary relaxation) upon which contraction (primary contraction) appeared, followed by rebound contraction. Noradrenaline decreased the cholinergic smooth muscle twitches, predominantly at low stimulation frequencies, and had a similar effect on the non-cholinergic, non-adrenergic primary relaxation, primary and rebound contractions. ATP decreased the smooth muscle twitches; however, this agent had only a transient influence on the non-cholinergic, non-adrenergic responses of muscle (tension and membrane potential) to single stimuli. With higher stimulus frequencies ATP increased the primary relaxation and decreased the contraction phases. ATP also inhibited the post-tetanic inhibition induced by non-cholinergic, non-adrenergic nerve stimulation. In most of the muscle cells of the guinea-pig proximal and terminal ileum the non-cholinergic, non-adrenergic nerve stimulation generated i.j.p.s., while about 15-20% of the cells responded with e.j.p.s. During long-lasting stimulation (10s) the i.j.p.s were sometimes "interrupted" by action potentials or by a gradual depolarization of the membrane. The i.j.p.s were followed by a marked rebound depolarization accompanying the action potentials. Those cells which generated i.j.p.s in response to field stimulation, were depolarized by ATP, while those cells, which generated e.j.p.s, were hyperpolarized by ATP. A reduction in the concentration of extracellular sodium chloride decreased both the primary and rebound contractions; the primary contraction was, however, more sensitive than was the rebound contraction. Theophylline increased the primary and rebound contractions with no marked influence on the primary relaxation, lowered the action potential threshold, increased the rebound depolarization and did not markedly influence the i.j.p.s. Quinidine enhanced the primary relaxation and inhibited the primary contraction in a concentration-dependent manner. Inhibition of the rebound contraction by quinidine was slight (less than 50%). The present results demonstrate that primary relaxation, primary and rebound contractions are associated with i.j.p.s and e.j.p.s, and rebound depolarization with action potentials, respectively; they are typical responses of various regions of the guinea-pig small intestine to activation of inhibitory and excitatory non-cholinergic, non-adrenergic nerves. The P1 and P2 receptors, proposed by Burnstock (1975), probably do not mediate the non-cholinergic, nonadrenergic postsynaptic responses of the guinea-pig small intestine...

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.

Effect of arylazido aminopropionyl ATP (ANAPP3), on ATP responses of isolated guinea pig smooth muscle

Arylazido aminopropionyl ATP (ANAPP3), a photoaffinity analogue of adenosine 5'-triphosphate, photoactivated with visible light (+hv), specifically and irreversibly antagonized ATP contractions of the guinea pig vas deferens. ANAPP3 (30 microM) antagonized responses to exogenously added ATP in untreated, and in tissues pretreated with indomethacin (2.9 microM) and 6-(2-hydroxy-5-nitrobenzyl)-thio guanosine (10 microM). It was of interest to see if this pharmacological antagonist of ATP could be used to assess the validity of the purinergic nerve hypothesis by allowing a differentiation between, or proof of the identity of, responses to ATP and the non-adrenergic inhibitory transmitter in guinea pig stomach fundus. After photoactivation (+hv) in the organ bath and subsequent washout, ANAPP3 (30 and 100 microM) failed to antagonize relaxant responses to ATP (1.0 - 1000 microM) in fundic strips. In addition ANAPP3 failed to antagonize ATP-induced inhibition of the twitch response in electrically stimulated guinea pig ileum longitudinal muscle strips. We conclude that ANAPP3 does not antagonize all actions of ATP, which may limit its usefulness in assessing the above hypothesis. Results with this compound suggest that ATP excitatory receptors may differ from those mediating relaxation and other ATP actions.

Attempt to antagonized the stimulatory effect or ATP on insulin secretion

An attempt was made to antagonize the stimulatory effect of ATP on insulin secretion from the isolated perfused rat pancreas. The insulin secretory effect of ATP does not seem to be mediated by cholinergic or beta-adrenergic receptors since neither atropine 0.3 micro mol/1 nor propranolol 1 micro mol/1 had any antagonistic action on insulin secretion induced by ATP (16.5 micro mol/1). Theophylline (50 micro mol/1) did not antagonize the insulin secretion evoked by ATP (16.5 micro mol/1). Apamin (10 nmol/1) was also without antagonistic action. 2-2'Pyridylisatogen tosylate (5 micro mol/1) had not effect on insulin secretion induced by glucose )8.33 mmol/1) or acetylcholine (0.5 micro mol/1 and 0.1 micro mol/1) but inhibited the insulin secretory effect of ATP (16.5 micro mol/1). Thus, the antagonism of 2-2'pyridylisatogen for ATP seems selective. We conclude that a purinoceptor of the P2-type is likely to be present on the B-cell of the rat pancreas.

Influence of adenosine on responses to vagal nerve stimulation in the anesthetized rabbit

The influence of local adenosine infusion into the celiac artery on the gastric contractile responses to centrifugal vagal nerve stimulation was studied in anesthetized rabbits, and was compared with the effects of systemic administration of equivalent amounts of adenosine. Close arterial infusion of adenosine caused a marked reduction of gastric contractions induced by nerve stimulation, whereas corresponding responses to close arterial infusions of acetylcholine were enhanced during adenosine. The comparison with systemic adenosine administration revealed that the influence on gastric neurotransmission was not related to the hypotensive effect of the compound. No effects of adenosine were seen on bronchial activity as measured by insufflation pressure. Variable effects were obtained on cardiac responses to vagal stimulation. Gastric smooth muscle contractions elicited in vitro by transmural nerve stimulation were affected by adenosine in a biphasic manner, initial inhibition followed by potentiation of the apparently cholinergic responses. It is suggested that adenosine may modulate cholinergic neurotransmission in vivo by a dual effect, prejunctional inhibition and postjunctional enhancement.

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.