Evaluation of P2-purinoceptor antagonists at two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli

Effects of excitatory and inhibitory neurotransmission on motor patterns of human sigmoid colon in vitro

BACKGROUND AND PURPOSE

To characterize the in vitro motor patterns and the neurotransmitters released by enteric motor neurons (EMNs) in the human sigmoid colon.

EXPERIMENTAL APPROACH

Sigmoid circular strips were studied in organ baths. EMNs were stimulated by electrical field stimulation (EFS) and through nicotinic ACh receptors.

KEY RESULTS

Strips developed weak spontaneous rhythmic contractions (3.67+/-0.49 g, 2.54+/-0.15 min) unaffected by the neurotoxin tetrodotoxin (TTX; 1 microM). EFS induced strong contractions during (on, 56%) or after electrical stimulus (off, 44%), both abolished by TTX. Nicotine (1-100 microM) inhibited spontaneous contractions. Latency of off-contractions and nicotine responses were reduced by N(G)-nitro-L-arginine (1 mM) and blocked after further addition of apamin (1 microM) or the P2Y(1) receptor antagonist MRS 2179 (10 microM) and were unaffected by the P2X antagonist NF279 (10 microM) or alpha-chymotrypsin (10 U mL(-1)). Amplitude of on- and off-contractions was reduced by atropine (1 microM) and the selective NK(2) receptor antagonist Bz-Ala-Ala-D-Trp-Phe-D-Pro-Pro-Nle-NH(2) (1 microM). MRS 2179 reduced the amplitude of EFS on- and off-contractions without altering direct muscular contractions induced by ACh (1 nM-1 mM) or substance P (1 nM-10 microM).

CONCLUSIONS AND IMPLICATIONS

Latency of EFS-induced off-contractions and inhibition of spontaneous motility by nicotine are caused by stimulation of inhibitory EMNs coreleasing NO and a purine acting at muscular P2Y(1) receptors through apamin-sensitive K(+) channels. EFS-induced on- and off-contractions are caused by stimulation of excitatory EMNs coreleasing ACh and tachykinins acting on muscular muscarinic and NK(2) receptors. Prejunctional P2Y(1) receptors might modulate the activity of excitatory EMNs. P2Y(1) and NK(2) receptors might be therapeutic targets for colonic motor disorders.

Functional evidence that ATP or a related purine is an inhibitory NANC neurotransmitter in the mouse jejunum: study on the identity of P2X and P2Y purinoceptors involved

1. Conflicting views exist on whether ATP is a neurotransmitter in the enteric nervous system. We investigated the role of ATP in enteric transmission in circular muscle strips of the mouse jejunum. 2. On PGF2alpha-precontracted muscle strips and in the presence of atropine and guanethidine, electrical field stimulation (EFS, 1-8 Hz) of nonadrenergic noncholinergic (NANC) nerves induced transient relaxations that were abolished by the nerve-conductance blocker tetrodotoxin. The NO synthase blocker l-nitroarginine (l-NOARG) partially inhibited the NANC relaxations to EFS, but fast-twitch relaxations to EFS were still observed in the presence of l-NOARG. 3. In the presence of l-NOARG, ATP, the P2X receptor agonist alphabetaMeATP and the P2Y receptor agonist ADPbetaS relaxed jejunal muscle strips. Tetrodotoxin did not affect the relaxation to ATP and ADPbetaS, but inhibited that to alphabetaMeATP. 4. The l-NOARG-resistant NANC relaxations to EFS were almost abolished by apamin, a blocker of small-conductance Ca2+ activated K+ channels, and by suramin and PPADS, blockers of P2 purinoceptors. Relaxations to ATP were almost abolished by apamin and suramin but not affected by PPADS. 5. Desensitisation of alphabetaMeATP-sensitive P2X receptors, the P2X receptor blocker Evans blue and the P2X1,2,3 receptor blocker NF 279 inhibited the l-NOARG-resistant NANC relaxations to EFS and that to alphabetaMeATP without affecting the relaxation to ADPbetaS. Brilliant blue G, a P2X2,5,7 receptor blocker, did not affect the relaxations to EFS. 6. Desensitisation of P2Y receptors and MRS 2179, a P2Y1 receptor blocker, virtually abolished the l-NOARG-resistant NANC relaxations to EFS and the relaxation to ADPbetaS without affecting the relaxation to alphabetaMeATP. 7. Dipyridamole, an adenosine uptake inhibitor, or theophylline and 8-phenyltheophylline, blockers of P1 and A1 purinoceptors, respectively, did not affect the purinergic NANC relaxations to EFS. 8. Our results suggest that ATP or a related purine acts as an inhibitory NANC neurotransmitter in the mouse jejunum, activating P2 but not P1 purinoceptors. Relaxations to the purinergic NANC neurotransmitter mainly involve P2Y receptors of the P2Y1 subtype that are located postjunctionally. Purinergic NANC neurotransmission also involves P2X receptors, most likely of the P2X1 and P2X3 subtype, located pre- and/or postjunctionally.

Alteration of the purinergic modulation of enteric neurotransmission in the mouse ileum during chronic intestinal inflammation

1. The effect of chronic intestinal inflammation on the purinergic modulation of cholinergic neurotransmission was studied in the mouse ileum. Chronic intestinal inflammation was induced by infection of mice with the parasite Schistosoma mansoni during 16 weeks. 2. S. mansoni infection induced a chronic inflammatory response in the small intestine, which was characterised by intestinal granuloma formation, increased intestinal wall thickness, blunted mucosal villi and an enhanced activity of myeloperoxidase. 3. In control ileum and in chronically inflamed ileum, electrical field stimulation (EFS) of longitudinal muscle strips induced frequency-dependent contractions that were abolished by tetrodotoxin (TTX) and atropine. Carbachol induced dose-dependent contractions that were not affected by TTX but abolished by atropine. 4. In control ileum, adenosine and ATP dose-dependently inhibited the contractions to EFS. Theophylline and 8-phenyltheophylline, P(1) and A(1) receptor antagonists respectively, prevented this inhibitory effect of adenosine and ATP. PPADS, DMPX and MRS 1220, antagonists of P(2), A(2) and A(3) receptors, respectively, did not prevent this inhibitory effect of adenosine and ATP. Adenosine and ATP did not affect the contractions to carbachol. 5. The inhibitory effect of adenosine and ATP on contractions to EFS in control ileum was mimicked by the stable adenosine analogue methyladenosine and by the A(1)-receptor agonist N(6)-cyclohexyladenosine, but not by the A3 receptor agonist 2-Cl IB-MECA or by the ATP analogues alphabeta-methylene-ATP and ADPbetaS. The inhibitory effect of adenosine on contractions to EFS was lost after prolonged (90 min) treatment of control ileum with methyladenosine (100 micro M). 6. In chronically inflamed ileum, adenosine, methyladenosine, N(6)-cyclohexyladenosine and ATP all failed to inhibit the cholinergic nerve-mediated contractions to EFS. Also theophylline, 8-phenyltheophylline, PPADS, DMPX and MRS 1220 had no effect on the contractions to EFS and carbachol. The loss of effect of adenosine and ATP was still evident after 52 weeks of infection. 7. These results indicate that in physiological conditions neuronal adenosine A(1) receptors modulate cholinergic nerve activity in the mouse ileum. However, during chronic intestinal inflammation, this purinergic modulation of cholinergic nerve activity is impaired. This suggests that chronic intestinal inflammation leads to a dysfunction of specific neuronal regulatory mechanisms in the enteric nervous system.

Nitrergic and purinergic regulation of the rat pylorus

The role of nitric oxide (NO) and ATP in the regulation of nonadrenergic, noncholinergic (NANC) inhibitory transmission in the pylorus remains unclear. In the presence of atropine and guanethidine, electric field stimulation induced NANC relaxations in a frequency-dependent manner (1-20 Hz) in the rat pylorus. NANC relaxations were significantly inhibited by N(G)-nitro-L-arginine methyl ester (L-NAME; 10(-4) M). P(2X) purinoceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 3 x 10(-5) M) and P(2Y) purinoceptor antagonist reactive blue 2 (2 x 10(-5) M) had no effect on NANC relaxations. However, the combined administration of L-NAME and PPADS, but not reactive blue 2, evoked greater inhibitory effects on NANC relaxation than that evoked by L-NAME alone. alpha-Chymotrypsin and vasoactive intestinal polypeptide antagonist did not affect NANC relaxations. ATP (10(-5)-10(-3) M) and P(2X) purinoceptor agonist alpha, beta-methyleneadenosine 5'-triphosphate (10(-7)-10(-5) M), but not P(2Y) purinoceptor agonist 2-methylthioadenosine 5'-triphosphate (10(-7)-10(-5) M), induced muscle relaxations in a dose-dependent manner, and relaxations were significantly reduced by PPADS and unaffected by TTX. These studies suggest that NO and ATP act in concert to mediate NANC relaxation of the rat pylorus. ATP-induced relaxation appears to be mediated by P(2X) purinoceptors located on smooth muscle cells.

Intracellular calcium events activated by ATP in murine colonic myocytes

ATP is a candidate enteric inhibitory neurotransmitter in visceral smooth muscles. ATP hyperpolarizes visceral muscles via activation of small-conductance, Ca(2+)-activated K(+) (SK) channels. Coupling between ATP stimulation and SK channels may be mediated by localized Ca(2+) release. Isolated myocytes of the murine colon produced spontaneous, localized Ca(2+) release events. These events corresponded to spontaneous transient outward currents (STOCs) consisting of charybdotoxin (ChTX)-sensitive and -insensitive events. ChTX-insensitive STOCs were inhibited by apamin. Localized Ca(2+) transients were not blocked by ryanodine, but these events were reduced in magnitude and frequency by xestospongin C (Xe-C), a blocker of inositol 1,4,5-trisphosphate receptors. Thus we have termed the localized Ca(2+) events in colonic myocytes "Ca(2+) puffs. " The P(2Y) receptor agonist 2-methylthio-ATP (2-MeS-ATP) increased the intensity and frequency of Ca(2+) puffs. 2-MeS-ATP also increased STOCs in association with the increase in Ca(2+) puffs. Pyridoxal-phospate-6-azophenyl-2',4'-disculfonic acid tetrasodium, a P(2) receptor inhibitor, blocked responses to 2-MeS-ATP. Spontaneous Ca(2+) transients and the effects of 2-MeS-ATP on Ca(2+) puffs and STOCs were blocked by U-73122, an inhibitor of phospholipase C. Xe-C and ryanodine also blocked responses to 2-MeS-ATP, suggesting that, in addition to release from IP(3) receptor-operated stores, ryanodine receptors may be recruited during agonist stimulation to amplify release of Ca(2+). These data suggest that localized Ca(2+) release modulates Ca(2+)-dependent ionic conductances in the plasma membrane. Localized Ca(2+) release may contribute to the electrical responses resulting from purinergic stimulation.

Characterization of the ATPase released during sympathetic nerve stimulation of the guinea-pig isolated vas deferens

The release of ATPase activity evoked by electrical field stimulation (EFS) (8 Hz, 25 s) was investigated in several tissues in which adenosine 5'-triphosphate (ATP) acts as a neurotransmitter. Superfusate collected during EFS of sympathetic nerves of the guinea-pig, rat and mouse isolated vas deferens and parasympathetic nerves of the guinea-pig isolated urinary bladder contained ATPase activity. ATP breakdown was fastest in superfusate collected from the guinea-pig isolated vas deferens. However, EFS of the enteric nerves of the guinea-pig isolated taenia coli did not release any detectable ATPase. The ATPase released from the guinea-pig isolated vas deferens metabolized ATP at similar rates at incubation temperatures of 37 degrees C and 20 degrees C. Lineweaver-Burke analysis of the initial rates of ATP hydrolysis gave a K(M) of 39 microM and a V(max) of 1039 pmol ATP metabolized min(-1) ml(-1) superfusate. 6-N,N-diethyl-D-beta,gamma-dibromomethyleneATP (ARL 67156), pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and pyridoxal-5'-phosphate (P-5-P) all inhibited the ATPase activity in a concentration-dependent manner with a potency order of ARL 67156 = PPADS>P-5-P. In conclusion, EFS of several tissues in which ATP is a neurotransmitter causes the release of an ATPase and activity is greatest in the guinea-pig vas deferens. The enzyme has pharmacological and kinetic characteristics that are similar to ectonucleoside triphosphate diphosphohydrolases.

Purinergic activation of spontaneous transient outward currents in guinea pig taenia colonic myocytes

Spontaneous transient outward currents (STOCs) were recorded from smooth muscle cells of the guinea pig taenia coli using the whole cell patch-clamp technique. STOCs were resolved at potentials positive to -50 mV. Treating cells with caffeine (1 mM) caused a burst of outward currents followed by inhibition of STOCs. Replacing extracellular Ca(2+) with equimolar Mn(2+) caused STOCs to "run down. " Iberiotoxin (200 nM) or charybdotoxin (ChTX; 200 nM) inhibited large-amplitude STOCs, but small-amplitude "mini-STOCs" remained in the presence of these drugs. Mini-STOCs were reduced by apamin (500 nM), an inhibitor of small-conductance Ca(2+)-activated K(+) channels (SK channels). Application of ATP or 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) increased the frequency of STOCs. The effects of 2-MeS-ATP persisted in the presence of charybdotoxin but were blocked by combination of ChTX (200 nM) and apamin (500 nM). 2-MeS-ATP did not increase STOCs in the presence of pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, a P(2) receptor blocker. Similarly, pretreatment of cells with U-73122 (1 microM), an inhibitor of phospholipase C (PLC), abolished the effects of 2-MeS-ATP. Xestospongin C, an inositol 1,4,5-trisphosphate (IP(3)) receptor blocker, attenuated STOCs, but these events were not affected by ryanodine. The data suggest that purinergic activation through P(2Y) receptors results in localized Ca(2+) release via PLC- and IP(3)-dependent mechanisms. Release of Ca(2+) is coupled to STOCs, which are composed of currents mediated by large-conductance Ca(2+)-activated K(+) channels and SK channels. The latter are thought to mediate hyperpolarization and relaxation responses of gastrointestinal muscles to inhibitory purinergic stimulation.

Different receptors mediating the inhibitory action of exogenous ATP and endogenously released purines on guinea-pig intestinal peristalsis

1 Adenosine 5'-triphosphate (ATP) is an enteric neurotransmitter which acts at purine receptors on intestinal nerve and muscle. This study set out to shed light on the receptor mechanisms by which exogenous and endogenous ATP influences intestinal peristalsis. 2 Peristalsis in isolated segments of the guinea-pig small intestine was triggered by a perfusion-induced rise of the intraluminal pressure. Motor changes were quantified by alterations of the peristaltic pressure threshold (PPT) at which propulsive muscle contractions were elicited. 3 ATP (>/= 3 microM) increased PPT and abolished peristalsis at concentrations of 100-300 microM. Adenosine 5'-O-2-thiodiphosphate (ADPbetaS, 3-100 microM) was more potent, whereas alpha,beta-methylene ATP (alpha,beta-meATP, 3-100 microM) was less potent, than ATP in depressing peristalsis. 4 8-Phenyltheophylline (10 microM) attenuated the anti-peristaltic effect of 10 and 30 microM ATP but not that of higher ATP concentrations. Apamin (0.5 microM) counteracted the ability of ATP, ADPbetaS and alpha,beta-meATP to enhance PPT. Suramin (300 microM) and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 150 microM) antagonized the inhibitory effect of alpha,beta-meATP on peristalsis but did not alter the effect of ATP and ADPbetaS. 5 PPADS (50-150 microM) reduced PPT by as much as 50%. This stimulant effect on peristalsis was prevented by suramin (300 microM) but left unaltered by apamin (0.5 microM) and NG-nitro-L-arginine methyl ester (300 microM). 6 These data show that exogenous and endogenous ATP inhibits intestinal peristalsis via different apamin-sensitive purinoceptor mechanisms. Exogenous ATP depresses peristalsis mostly via suramin- and PPADS-insensitive P2 receptors, whereas endogenous purines act via P2 receptors sensitive to both suramin and PPADS.

ATP is a mediator of the fast inhibitory junction potential in human jejunal circular smooth muscle

The neurotransmitter(s) that generates the fast component of the inhibitory junction potential (IJP-F) in human jejunal circular smooth muscle is not known. The aim of this study was to determine the role of ATP and purinergic receptors in the generation of the IJP-F in human jejunal circular smooth muscle strips. The P2-receptor antagonist suramin (100 microM) reduced the IJP-F by 28%. Apamin (1 microM) reduced the IJP-F by 25%. Desensitization of muscle strips with the putative P2x-receptor agonist alpha, beta-methylene ATP (alpha,beta-MeATP, 100 microM) decreased the IJP-F by 44%, and desensitization with the putative P2y-receptor agonist adenosine 5'-O-2-thiodiphosphate (ADPbetaS) completely abolished the IJP-F. Desensitization with the putative P2y-receptor agonist 2-methylthioATP had no effect on the IJP-F. Exogenous ATP evoked a hyperpolarization with a time course that matched the IJP-F. The ATP-evoked hyperpolarization was reduced by apamin and suramin, reduced by desensitization with alpha,beta-MeATP (69% decrease), and abolished by desensitization with ADPbetaS. These data suggest that the IJP-F in human jejunal circular smooth muscle is mediated in part by ATP through an ADPbetaS-sensitive P2 receptor.

P2-purinoceptor antagonists: III. Blockade of P2-purinoceptor subtypes and ecto-nucleotidases by compounds related to suramin

Effects of suramin and five analogs or fragments of suramin were studied on contractions of the rat vas deferens elicited by alpha, beta-methylene ATP (alpha, beta-MeATP; mediated by P2X-purinoceptors), relaxations of the carbachol-precontracted guinea-pig taenia coli elicited by adenosine 5'-O-(2-thiodiphosphate) (ADP beta S; mediated by P2Y-purinoceptors), and the degradation of ATP by rat vas deferens tissue. One compound, NF023, differed from suramin by removal of two p-methylbenzamido groups, whereas another, BSt101, differed from NF023 by additional removal of the three sulphonate residues from one of the terminal naphthalene rings. The compounds all shifted the concentration-response curve of alpha, beta-MeATP in the rat vas deferens to the right and simultaneously increased the maximum of the curve. Where three concentrations were tested, the Arunlakshana-Schild regression was linear, and the slope did not differ from 1. The apparent Kd values were between 1 and 3672 microM. In the guinea-pig taenia coli, the compounds shifted the concentration-response curve of ADP beta S to the right in a parallel manner, but in the one case where three concentrations were tested, the slope of the Arunlakshana-Schild regression was lower than 1. Apparent Kd values were between 10 and 786 microM. The removal of ATP from the medium by vas deferens tissue was decreased only by suramin, NF023 and BSt101, with IC25% values between 170 and 590 microM.

P2-purinoceptor antagonists: II. Blockade of P2-purinoceptor subtypes and ecto-nucleotidases by compounds related to Evans blue and trypan blue

Effects of Evans blue and four derivatives as well as of trypan blue and four derivatives, mostly smaller fragments but two compounds with an additional ethylene bridge in the center of the molecule, were studied on contractions of the rat vas deferens elicited by alpha, beta-methylene ATP (alpha, beta-MeATP; mediated by P2X-purinoceptors), relaxations of the carbachol-precontracted guinea-pig taenia coli elicited by adenosine 5'-O-(2-thiodiphosphate) (ADP beta S; mediated by P2Y-purinoceptors), and the degradation of ATP by rat vas deferens tissue. All compounds shifted the concentration-response curve of alpha, beta-MeATP in the rat vas deferens to the right, and most compounds increased the maximum of the curve. Each member of the Evans blue series was similar in potency to the corresponding member of the trypan blue series. Where three concentrations were tested, the Arunlakshana-Schild regression was linear, and the slope did not differ from 1. The apparent Kd values were between 0.8 and 385 microM. In the guinea-pig taenia coli, only the members of the trypan blue group were relatively potent, shifting the concentration-response curve of ADP beta S to the right in a surmountable manner. In 2 of 3 cases where three concentrations were tested, the slope of the Arunlakshana-Schild regression was lower than 1. Apparent Kd values in the trypan blue group were between 5.2 and 324 microM. The removal of ATP from the medium by vas deferens tissue was decreased mainly by the members of the Evans blue group, with IC25% values between 13 and 158 (in 1 case > 1000) microM. The results indicate that the position of the sulphonate residues at the terminal naphthalene rings of these compounds hardly influences P2X purinoceptor affinity but greatly influences P2Y affinity and ecto-nucleotidase blockade. Among active compounds, apparent purinoceptor affinity and ecto-nucleotidase blockade increase with the size of the molecules up to Evans blue and trypan blue themselves; introduction of a central ethylene bridge does not result in a further gain in potency. NH01, the desmethyl derivative of Evans blue, seems to be interesting because it is the compound with the highest P2X- versus P2Y-selectivity presently available.

Evaluation of P2-purinoceptor antagonists at two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli

The guinea-pig taenia coli possesses two relaxation-mediating receptors for nucleotides: a prototypic P2Y-purinoceptor, which is activated by adenosine 5'-O-(2-thio-diphosphate) (ADP beta S), and a separate receptor for alpha, beta-methylene ATP (alpha,beta-MeATP). Effects of several as yet incompletely characterized P2-purinoceptor antagonists at these receptors were examined. The concentration-relaxation curve of ADP beta S was shifted to the right by reactive blue 2, suramin, 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulp honic acid (XAMR0721; at 1000 microM only), pyridoxalphosphate-6-azophenyl-2',5'-disulphonic acid (iso-PPADS), pyridoxal 5-phosphate, trypan blue and Evans blue (at 320 microM only). Schild plots for the antagonism of reactive blue 2, suramin, iso-PPADS and pyridoxal 5-phosphate against ADP beta S had slopes < 1. The concentration-relaxation curve of alpha,beta-MeATP was shifted to the right by reactive blue 2, suramin, XAMR0721, iso-PPADS, pyridoxal 5-phosphate and trypan blue but not by Evans blue (320 microM). Schild plots for the antagonism of suramin, XAMR0721 and iso-PPADS against alpha,beta-MeATP had slopes > 1. Only XAMR0721 differed clearly in potency against the two nucleotides: it was considerably more potent against alpha,beta-MeATP than against ADP beta S. 2-Methylthio ATP (MeSATP; 1 microM) and ATP (100 microM) were degraded by pieces of taenia coli. All antagonists except trypan blue attenuated the degradation of either or one of the two nucleotides. The selective effect of XAMR0721 against alpha,beta-MeATP confirms the existence of two relaxation-mediating P2-purinoceptors in guinea-pig taenia coli. Comparison of the apparent affinities of the antagonists for the two taenia coli receptors with affinities for the P2X-purinoceptor of the rat vas deferens shows that reactive blue 2, suramin, iso-PPADS, pyridoxal 5-phosphate and trypan blue have little selectivity for any of the three receptors. XAMR0721, which has been shown to possess relatively high affinity for the P2Y-purinoceptor in turkey erythrocytes, was very weak at the P2Y-receptor of the taenia, thus supporting the existence of pharmacologic P2Y-receptor subtypes. Evans blue, with little effect in the taenia coli but a marked effect in the rat vas defrens, is the most selective P2X- (versus P2Y-) purinoceptor antagonists presently known, although its effect on the degradation of nucleotides must be kept in mind.