Effect of papaverine on synaptic transmission in the guinea-pig ileum

1. The effect of papaverine, a well known smooth muscle relaxant, was investigated on neural transmission within the enteric nervous system. Segments of guinea-pig ileum were placed in a partitioned bath to enable drugs, including papaverine, to be applied to enteric nerve pathways without interfering with the recording of the smooth muscle contraction. Ascending excitatory enteric nerve pathways were activated by electrical field stimulation in the anal compartment (10 Hz for 2 s, 45 mA, 0.5 ms pulse duration) and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded isotonically. 2. Tetrodotoxin (0.6 microM) and hexamethonium (100 microM) both abolished, or greatly reduced, the contractions when applied to either compartment indicating that nicotinic synapses are involved in this pathway. 3. Papaverine (0.3-30 microM) applied independently to each compartment depressed in a concentration-dependent manner, the nerve-mediated contractions. The IC50 of this inhibitory effect was 3.53 microM for the oral and 4.76 microM for the anal compartments, respectively. Two other phosphodiesterase (PDE) inhibitors, 3-isobutyl-1-methylxanthine (IBMX 10-300 microM) and theophylline (30-1000 microM) added to the anal compartment also inhibited the nerve mediated contractions. Papaverine applied to the anal bath, after IBMX 100 microM (or theophylline 300 microM) further inhibited the nerve-mediated contractions, but was less effective than when applied alone. 4. Phentolamine (1 microM), an alpha-adrenoceptor antagonist, reduced the inhibitory effect of papaverine, but not that of IBMX (100 microM) or theophylline (300 microM). A combination of phentolamine and IBMX (or theophylline) prevented the inhibitory effect of papaverine. 5. Tetrodotoxin, but not papaverine or hexamethonium, inhibited the contraction elicited by electrical stimulation just anal to the partition indicating that papaverine did not affect the generation or conduction of nerve action potentials. 6. Verapamil (1 microM) and nifedipine (1 microM), two smooth muscle relaxants which act by blocking L-type calcium channels, only inhibited the contractions when applied directly to the recording (oral) compartment. This indicates that L-type Ca2+ channels are probably not involved in synaptic transmission in these ascending pathways and thus that the PDE inhibitors do not inhibit synaptic transmission by acting on these channels. omega-Conotoxin GVIA (10 nM), a potent inhibitor of the N-type Ca2+ channels, blocked the nerve-mediated contractions applied to either compartment. Whether the PDE inhibitors exert their inhibitory actions via these channels remains to be established. 7. The results indicate that the PDE inhibitors, papaverine, IBMX and theophylline inhibit excitatory enteric neural pathways by depressing synaptic transmission. The inhibitory effect of papaverine (but not IMBX or theophylline) involves, at least in part, the release of noradrenaline from sympathetic nerves acting on alpha-adrenoceptors on enteric neurones.

High-Power Infrared (8-Micrometer Wavelength) Superlattice Lasers

A quantum-cascade long-wavelength infrared laser based on superlattice active regions has been demonstrated. In this source, electrons injected by tunneling emit photons corresponding to the energy gap (minigap) between two superlattice conduction bands (minibands). A distinctive design feature is the high oscillator strength of the optical transition. Pulsed operation at a wavelength of about 8 micrometers with peak powers ranging from approximately 0.80 watt at 80 kelvin to 0.2 watt at 200 kelvin has been demonstrated in a superlattice with 1-nanometer-thick AlInAs barriers and 4.3-nanometer-thick GaInAs quantum wells grown by molecular beam epitaxy. These results demonstrate the potential of strongly coupled superlattices as infrared laser materials for high-power sources in which the wavelength can be tailored by design.

Synthesis and pharmacological activity of 2-(substituted phenyl)-3-[2 or 3-[(4-substituted phenyl-4-hydroxy)piperidino]ethyl or propyl]-1,3-thiazolidin-4-ones

A series of aryl-hydroxy-piperidinoalkyl-thiazolidinones was synthesized and evaluated to inhibit castor oil-induced diarrhea in mice. The dose dependent antidiarrheal activity of the most active compound 2-(p-nitrophenyl)-3-¿2-[(4-(p-chlorophenyl)-4-hydroxy)piperidino]ethyl]- 1, 3-thiazolidin-4-one (6) was counteracted by naloxone, resulting comparable with that of loperamide, a mu opiate agonist.

The role of constitutive and inducible nitric oxide synthase in senna- and cascara-induced diarrhoea in the rat

The role of constitutive and inducible nitric oxide (NO) synthase in rats treated with senna and cascara was studied. Senna (60 mg/kg p.o.) and cascara (800 mg/kg p.o.) ex vivo significantly increased Ca(2+)-dependent constitutive NO synthase activity in the rat colon. Induction of NO synthase (12% of the total NO synthase) was associated with cascara, but not senna, administration. Dexamethasone (0.03-0.3 mg/kg i.p.), which inhibits the expression of inducible NO synthase, significantly and dose-dependently reduced cascara-(but not senna-) induced diarrhoea and colonic fluid secretion. These findings suggest that senna probably exerts its laxative effect through stimulation of the constitutive isoform of NO synthase, while the inducible isoform of NO synthase also seems to be involved in the laxative effect of cascara.

Nitric oxide-donating compounds and cyclic GMP depress the spontaneous contractile activity of the isolated rabbit jejunum

The effects of sodium nitroprusside (SNP) and acidified sodium nitrite (ASN) solutions, nitric oxide (NO)-donating compounds, and NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthesis, were studied on the spontaneous contractile activity of the isolated rabbit jejunum. The addition of SNP (10(-5) to 10(-3) mol/l or ASN (10(-5) to 10(-3) mol/l) the organ bath inhibited the amplitude of the spontaneous contractions in rabbit isolated jejunum in a concentration dependent fashion, while L-NAME (3 x 10(-5) to 3 x 10 mol/l) was without effect. Methylene blue (3 x 10(-7) to 3 10(-6) mol/l), which inhibits soluble guanylate cyclase, and oxyhemoglobin (10(-5) mol/l), an NO scavenger, counteracted the effects of both SNP (3 x 10(-4) mol/l) and ASN (10(-4) mol/l). The spontaneous motility of rabbit jejunum was also inhibited in a concentration-dependent manner by 8-Br-cyclic GM (10(-5) to 10(-3) mol/l), a permeable analogue of cyclic GM. These results provide evidence that exogenous NO may inhibit spontaneous contractility and that this effect might be mediated, in part, by cyclic GMP, whereas endogenous NO does not seem to play a role in the regulation of the spontaneous motility of rabbit jejunum in vitro.

Tunable Fano interference in intersubband absorption

New modulation-doped quantum semiconductor structures that exhibit strong Fano interference effects have been designed and demonstrated. Intersubband absorption experiments clearly demonstrate the ability to engineer Fano resonances and their evolution toward bound-to-bound transitions as the continuum is progressively modified under the action of an electric field normal to the layers.

Antidiarrhoeal activity of new thiazolidinones related to loperamide

A series of thiazolidinones related to loperamide was synthesized and evaluated for antidiarrhoeal activity in mice, using the castor oil test. Of five compounds tested, antidiarrhoeal activity was found only for 2-(p-nitrophenyl)-3-¿3-[(4-(p-chlorophenyl)-4-hydroxy)piperidino]ethyl¿- 1,3-thiazolidin-4-one. The compound was less active than loperamide (ED50 values = 48.7 (24.8-95.6) and 0.91 (0.24-3.40) mg kg-1 respectively), but was also less toxic (LD50 values = 745.9 (545.2-929.8) and 108.9 (85.5-138.7) mg kg-1, respectively). Its antidiarrhoeal activity was counteracted by naloxone. Our results support the hypothesis that this compound, like loperamide, is an opiate-receptor agonist.

The osmotic and intrinsic mechanisms of the pharmacological laxative action of oral high doses of magnesium sulphate. Importance of the release of digestive polypeptides and nitric oxide

A common use for high doses of oral magnesium salts is to produce a laxative effect to treat constipation. In the intestinal lumen the poorly absorbable magnesium ions (and other ions such as sulphate) exert an osmotic effect and cause water to be retained in the intestinal lumen. This increases the fluidity of the intraluminal contents and results in a laxative action. Although the laxative action of magnesium is thought to be due to a local effect in the intestinal tract, it is also possible that released hormones such as cholecystokinin or activation of constitutive nitric oxide synthase might contribute to this pharmacological effect. Under normal circumstances the pharmacological administration of high doses of oral magnesium salts is safe and some salts--such as magnesium hydroxide--also have an antacid effect to neutralize stomach acid. However, high doses of magnesium or prolonged use may allow sufficient absorption into the systemic circulation to cause renal or other organ toxicity.

Relationship between nitric oxide and platelet-activating factor in castor-oil induced mucosal injury in the rat duodenum

The modulation of platelet activating factor (PAF) formation in duodenal tissue by nitric oxide (NO) released in response to castor oil was studied in rats pretreated with NG-nitro-L-arginine methyl ester (L-NAME, 6.25-25 mg/kg, i.p.), an inhibitor of NO synthase, NG-nitro-D-arginine methyl ester (D-NAME, 25 mg/kg, i.p.), the inactive enantiomer of L-NAME or isosorbide-5-mononitrate (IMN, 30-90 mg/kg, p.o.), a NO donating compound. Castor oil (2 ml/rat orally) increased PAF production in the rat duodenum 3 h after challenge. L-NAME, but not D-NAME, enhanced the amount of PAF formed by duodenal tissue, while IMN (30-90 mg/kg) counteracted the effects of L-NAME (12.5 mg/kg) and also reduced PAF release in the tissue of rats treated with castor oil. L-NAME 12.5 mg/kg, but not D-NAME, enhanced both macroscopic damage and acid phosphatase release induced by castor oil. These effects were reduced by a PAF antagonist BN 52021 (3-t-Butyl-hexahydro-4, 7b, 11-trihydroxy-8-methyl-9H-1, 7a-epoxymethano-1H, 6aH-cyclopenta [c] furo [2, 3b] furo [3'2':3,4] cyclopenta [1.2-d]furan-5,9,12(4H)trione) 10 and 20 mg/kg i.p. Such findings suggest that endogenous nitric oxide could reduce PAF biosynthesis in castor oil-treated rats.

NG-nitro-L-arginine methyl ester reduces senna- and cascara-induced diarrhoea and fluid secretion in the rat

Senna (60 mg/kg orally) and cascara (800 mg/kg orally)-induced diarrhoea and net fluid secretion were studied in rats for a time period of 1-8 h. NG-Nitro-L-arginine methyl ester (L-NAME) (2.5-25 mg/kg i.p. twice, 15 min before and 4 h after laxative administration), an inhibitor of nitric oxide synthase, reduced the diarrhoeal response. This effect was counteracted by L-arginine (600 and 1500 mg/kg i.p. 15 min before laxative administration), the precursor of nitric oxide (NO). The senna- and cascara-stimulated fluid secretion was reduced by NG-nitro-L-arginine methyl ester 25 mg/kg i.p. (twice, 15 min before and 4 h after laxative administration), while the stereoisomer NG-nitro-D-arginine methyl ester (D-NAME) 25 mg/kg i.p. was without effect. These results suggest a possible involvement of NO in senna- and cascara-induced diarrhoea and fluid secretion.

Acetic acid-induced colitis in normal and essential fatty acid deficient rats

Eicosanoids and platelet-activating factor (PAF) production increases in experimental colitis. Both eicosanoids and PAF seem to arise from similar membrane phospholipids. To support both these suggestions we have investigated whether a fat-free diet, which should alter production of eicosanoids and PAF, affects experimental colitis. Essential fatty acid deficient (EFAD) rats were obtained by putting 4-week-old animals on a fat-free diet for 3 months. Experimental colitis was induced by a single intracolonic administration of 2 ml of 4% acetic acid. One to seven days later the animals were sacrificed and the colon removed to assess macroscopically and histologically intestinal damage. Eicosanoids and PAF levels were also measured in the mucosa scrapings by specific radioimmunoassay. The injury to the colon was more evident in control rats compared with EFAD rats. Besides colonic tissue of control rats showed a highly significant increase of PGE2, LTB4 and PAF, compared with levels in EFAD rats. Our results indicate that fat-free diet reduces tissue damage, and at the same time PGE2, LTB4 and PAF colonic content.

NG-nitro-L-arginine methyl ester modulates intestinal secretion and motility produced by carbachol

The effects of the nitric oxide (NO) synthesis inhibitor, NG-nitro-L-arginine methyl ester, on carbachol-induced diarrhoea, fluid accumulation and motility changes were studied. Pretreatment of mice with NG-nitro-L-arginine methyl ester (1-25 mg/kg i.p.) and NG-nitro-L-arginine (2.5-50 mg/kg i.p.) but not NG-nitro-D-arginine methyl ester (25 mg/kg i.p.) prevented in a dose-related manner the carbachol (0.5 mg/kg i.p.)-induced diarrhoea in mice. L-Arginine (150-1500 mg/kg i.p.) administered to mice pretreated with NG-nitro-L-arginine methyl ester counteracted the antidiarrhoeal activity of NG-nitro-L-arginine methyl ester in a dose-related manner. Pretreatment of rats with NG-nitro-L-arginine methyl ester (2.5-25 mg/kg i.p.) decreased the intestinal fluid accumulation induced by carbachol in rats. NG-Nitro-D-arginine methyl ester was without effect. Intraperitoneal pretreatment of rats with NG-nitro-L-arginine methyl ester (2.5-25 mg/kg) reduced the increase in small intestinal transit induced by carbachol. NG-nitro-L-arginine methyl ester had no effect. These results provide evidence that nitric oxide may play a role in diarrhoea, intraluminal fluid accumulation and motility changes induced by carbachol.

Dissociation of castor oil-induced diarrhoea and intestinal mucosal injury in rat: effect of NG-nitro-L-arginine methyl ester

1. Castor oil (2 ml orally) produced diarrhoea in rats 1-7 h after challenge, which was associated with gross damage to the duodenal and jejunal mucosa. 2. The injury was accompanied by release of acid phosphatase into the gut lumen, indicating cellular injury. 3. Intraperitoneal injection of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME, 2.5-50 mg kg-1 twice), prevented the diarrhoea. The dose of L-NAME (50 mg kg-1) completely blocked the diarrhoea but increased the release of acid phosphatase and worsened the gross damage. 4. The NO donating compound, isosorbide-5-mononitrate (IMN, 150 mg kg-1 twice) reversed the effects of L-NAME (50 mg kg-1) on castor oil-induced diarrhoea, gross damage and acid phosphatase release. 5. The apparent dissociation of the diarrhoeal and intestinal mucosal damaging effects of castor oil suggest that NO has a protective effect on the rat duodenal and jejunal mucosa, but that NO mediates, in part, the diarrhoea effect of this laxative.

Nitric oxide involvement in sodium choleate-induced fluid secretion and diarrhoea in rats

Bile salt-induced diarrhoea, net water and electrolyte secretion, gastrointestinal transit and nitric oxide (NO) synthase activity were studied in rats. NG-Nitro-L-arginine methyl ester (2.5-25 mg/kg i.p.), an inhibitor of NO synthase, and dexamethasone (0.03-0.3 mg/kg i.p.), an inhibitor of the inducible isoform of NO synthase, antagonized the diarrhoeal response. The NO precursor, L-arginine and isosorbide-5-mononitrate (an NO donor), reversed the inhibitory effect of NG-nitro-L-arginine methyl ester. The bile salt-stimulated fluid secretion, transit through the gut and NO synthase all were inhibited by NG-nitro-L-arginine methyl ester (but not NG-nitro-D-arginine methyl ester). NO synthase activity also was inhibited by dexamethasone. The results are consistent with bile salt induction of epithelial cell injury and concomitant synthesis of NO, mainly through activation of the inducible form of the enzyme. We believe that in this study NO is a mediator of intestinal secretion and motility changes that enhance transit of luminal contents through the gut, resulting in diarrhoea.

Nitric oxide as a mediator of the laxative action of magnesium sulphate

1. Magnesium sulphate was studied for its effects on diarrhoea, fluid secretion, gastrointestinal transit and nitric oxide (NO) synthase activity in rats. 2. At a dose of 2 g kg-1 orally magnesium sulphate produced diarrhoea that was delayed in onset and intensity in a dose-related manner by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). This was prevented by the NO precursor, L-arginine and the NO donating compound, isosorbide-5-mononitrate (IMN). 3. Nitric oxide synthase activity was stimulated in gut tissue from rats given magnesium sulphate and this was inhibited by L-NAME. Dexamethasone (1 mg kg-1, i.p.), an inhibitor of inducible NO synthase, had no effect on magnesium sulphate-induced diarrhoea. 4. Magnesium sulphate stimulated fluid and electrolyte accumulation in the intestinal lumen; these effects were prevented by L-NAME but not D-NAME. 5. Gastrointestinal transit of a non-absorbable marker (charcoal suspension) was increased by oral magnesium sulphate from a mean value of 54.1% to 72.9% (P < 0.01), and this was prevented by pretreatment with L-NAME. 6. The results demonstrate that oral magnesium sulphate produces diarrhoea in rats by increasing the accumulation of fluid in the intestinal lumen and enhancing flow from the proximal to distal intestine. The mechanism involves release of NO, probably through stimulation of the constitutive form of NO synthase. Whether or not the effects of magnesium sulphate are due to an osmotic action or an intrinsic effect of the magnesium or sulphate ions cannot be determined from these experiments.

Nitric oxide as a mediator of bisacodyl and phenolphthalein laxative action: induction of nitric oxide synthase

Bisacodyl and phenolphthalein are diphenylmethane laxatives that have effects on intestinal water and electrolyte transport and smooth muscle contractility. Nitric oxide (NO) is produced in the intestine, where it stimulates electrolyte secretion and relaxes smooth muscle. Therefore, we studied in rats the effect of these laxatives on diarrhea, fluid transport in vivo, gastrointestinal transit and NO synthase activity in the absence and presence of inhibitors of NO synthesis. Both laxatives (50 mg/kg p.o.) produced diarrhea, which was delayed in onset by 25 mg/kg (i.p.) of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). The L-NAME effect was reversed by the NO donor isosorbide-5-mononitrate (30-120 mg/kg i.p.). L-Arginine (600 and 1500 mg/kg i.p.) prevented the inhibitory effect of L-NAME on diarrhea. The laxatives evoked water and electrolyte secretion and enhanced the transit of a suspension of charcoal through the gastrointestinal tract. This was inhibited by L-NAME but not D-NAME. The inhibitor of inducible NO synthase, dexamethasone (0.03-0.3 mg/kg i.p.), prevented the effects of both laxatives on electrolyte and water transport. Stimulation by these drugs of NO synthase was also inhibited by dexamethasone. The results demonstrate that bisacodyl and phenolphthalein stimulate water and electrolyte secretion, promote transit of intraluminal contents and produce diarrhea in association with enhanced production of NO. Furthermore, it appears that the NO is derived principally from activation of an inducible form of NO synthase.

Inhibition of intestinal motility and secretion by flavonoids in mice and rats: structure-activity relationships

Intraperitoneal administration of some flavonoids (apigenin, flavone, kaempferol, morin, myricetin, naringin and rutin; 12.5-50 mg kg-1) significantly (P < 0.05-0.01) reduced small (28-69%) and large (83-134%) intestinal transit in mice. Other flavonoids (naringenin, silibinin, silymarin and taxifolin, 100-200 mg kg-1) reduced (23-41%; P < 0.05-0.01) intestinal transit at doses of 100-200 mg kg-1 while hesperitin, catechin and phloridzin (up to 200 mg kg-1) had no effect. This effect was antagonized by yohimbine (87-96%) and phentolamine (87-91%) but not by prazosin, propranolol, atropine, hexamethonium, mepyramine, cyproheptadine and naloxone. Yohimbine (92-96%) also antagonized the inhibitory effect of flavonols (12.5-50 mg kg-1) (P < 0.05-0.01) on intraluminal accumulation of fluid and diarrhoea induced by castor oil. By contrast, verapamil potentiated the flavonol effect. It is suggested that these effects, influenced by the structure of the molecules, are mediated by alpha 2-adrenergic receptors and calcium.

Platelet activating factor potentiates rat paw oedema induced by different phlogogen agents

Carrageenin oedema is enhanced by the simultaneous injection in the rat paw of platelet activating factor (PAF). The enhancement of carrageenin oedema is observed throughout the time course of the experiments. This enhancement is also present when the oedema-producing agent is dextran, cellulose sulphate, histamine, 5-hydroxytryptamine, bradykinin or prostaglandin E2. Both verapamil and BN 52021 abolished the enhancement induced by PAF without modifying significantly carrageenin oedema. In essential fatty acid deficient (EFAD) rats depleted of kininogen and amines, carrageenin oedema is not modified by PAF. These findings suggest that PAF interacts with other inflammatory mediators regulating the formation of oedema induced by irritants injected locally.