Group I secreted PLA2 and arachidonic acid metabolites in the maintenance of cat LES tone

The multi-modality cardiac imaging approach to the Athlete's heart: an expert consensus of the European Association of Cardiovascular Imaging

The term 'athlete's heart' refers to a clinical picture characterized by a slow heart rate and enlargement of the heart. A multi-modality imaging approach to the athlete's heart aims to differentiate physiological changes due to intensive training in the athlete's heart from serious cardiac diseases with similar morphological features. Imaging assessment of the athlete's heart should begin with a thorough echocardiographic examination.Left ventricular (LV) wall thickness by echocardiography can contribute to the distinction between athlete's LV hypertrophy and hypertrophic cardiomyopathy (HCM). LV end-diastolic diameter becomes larger (>55 mm) than the normal limits only in end-stage HCM patients when the LV ejection fraction is <50%. Patients with HCM also show early impairment of LV diastolic function, whereas athletes have normal diastolic function.When echocardiography cannot provide a clear differential diagnosis, cardiac magnetic resonance (CMR) imaging should be performed.With CMR, accurate morphological and functional assessment can be made. Tissue characterization by late gadolinium enhancement may show a distinctive, non-ischaemic pattern in HCM and a variety of other myocardial conditions such as idiopathic dilated cardiomyopathy or myocarditis. The work-up of athletes with suspected coronary artery disease should start with an exercise ECG. In athletes with inconclusive exercise ECG results, exercise stress echocardiography should be considered. Nuclear cardiology techniques, coronary cardiac tomography (CCT) and/or CMR may be performed in selected cases. Owing to radiation exposure and the young age of most athletes, the use of CCT and nuclear cardiology techniques should be restricted to athletes with unclear stress echocardiography or CMR.

Role of rho kinase in the functional and dysfunctional tonic smooth muscles

Tonic smooth muscles play pivotal roles in the pathophysiology of debilitating diseases of the gastrointestinal and cardiovascular systems. Tonic smooth muscles differ from phasic smooth muscles in the ability to spontaneously develop myogenic tone. This ability has been primarily attributed to the local production of specific neurohumoral substances that can work in conjunction with calcium sensitization via signal transduction events associated with the Ras homolog gene family, member A (RhoA)/Rho-associated, coiled-coil containing protein kinase 2 (ROCK II) pathways. In this article, we discuss the molecular pathways involved in the myogenic properties of tonic smooth muscles, particularly the contribution of protein kinase C vs the RhoA/ROCK II pathway in the genesis of basal tone, pathophysiology and novel therapeutic approaches for certain gastrointestinal and cardiovascular diseases. Emerging evidence suggests that manipulation of RhoA/ROCK II activity through inhibitors or silencing of RNA interface techniques could represent a new therapeutic approach for various gastrointestinal and cardiovascular diseases.

HCl-activated neural and epithelial vanilloid receptors (TRPV1) in cat esophageal mucosa

To test whether transient receptor potential channel vanilloid subfamily member-1 (TRPV1) mediates acid-induced inflammation in the esophagus, a tubular segment of esophageal mucosa was tied at both ends, forming a sac. The sac was filled with 0.01 N HCl (or Krebs buffer for control) and kept in oxygenated Krebs buffer at 37 degrees C. The medium around the sac (supernatant) was collected after 3 h. Supernatant of the HCl-filled sac abolished contraction of esophageal circular muscle strips in response to electric field stimulation. Contraction was similarly abolished by supernatant of mucosal sac filled with the TRPV1 agonist capsaicin (10(-6) M). These effects were reversed by the selective TRPV1 antagonist 5'-iodoresiniferatoxin (IRTX) and by the platelet-activating factor (PAF) receptor antagonist CV9388. Substance P and CGRP levels in mucosa and in supernatant increased in response to HCl, and these increases were abolished by IRTX and by tetrodotoxin (TTX) but not affected by CV9388, indicating that substance P and CGRP are neurally released and PAF independent. In contrast, the increase in PAF was blocked by IRTX but not by TTX. Presence of TRPV1 receptor was confirmed by RT-PCR and by Western blot analysis in whole mucosa and in esophageal epithelial cells enzymatically isolated and sorted by flow cytometry or immunoprecipitated with cytokeratin antibodies. In epithelial cells PAF increased in response to HCl, and the increase was abolished by IRTX. We conclude that HCl-induced activation of TRPV1 receptors in esophageal mucosa causes release of substance P and CGRP from neurons and release of PAF from epithelial cells.

Arachidonic acid metabolites follow the preferential course of cyclooxygenase pathway for the basal tone in the internal anal sphincter

Present studies determined the roles of the cyclooxygenase (COX) versus the lipoxygenase (LO) pathways in the metabolic pathway of arachidonic acid (AA) in the internal anal sphincter (IAS) tone. Studies were performed in the rat IAS versus the nontonic rectal smooth muscle (RSM). Indomethacin, the dual COX inhibitor, but not nordihydroguaiaretic acid (NDGA), the LO inhibitor, produced a precipitous decrease in the IAS tone. However, when added in the background of indomethacin, NDGA caused significant reversal of the IAS tone. These inhibitors had no significant effect on the RSM. To follow the significance of COX versus LO pathways, we examined the effects of AA and its metabolites. In the IAS, AA caused an increase in the IAS tone (Emax=38.8+/-3.0% and pEC50=3.4+/-0.1). In the RSM, AA was significantly less efficacious and potent (Emax=11.3+/-3.5% and pEC50=2.2+/-0.3). The AA metabolites (via COXs) PGF2alpha and U-46619 (a stable analog of thromboxane A2) produced increases in the IAS tone and force in the RSM. Conversely, AA metabolites (via 5-LO) lipoxin A4, 5-HETE, and leukotriene C4 decreased the IAS tone. Finally, the contractile effects of AA in the IAS were selectively attenuated by the COX-1 but not the COX-2 inhibitor. Collectively, the specific effects of AA and the COX inhibitor, the Western blot and RT-PCR analyses showing specifically higher levels of COX-1, suggest a preferential role of the COX (specifically COX-1) pathway versus the LO in the regulation of the IAS tone.

Role of phospholipase A2 (group I secreted) in the genesis of basal tone in the internal anal sphincter smooth muscle

The role of phospholipase A(2) (PLA(2)) in the genesis of basal tone in the internal anal sphincter (IAS) is not known. We determined the effects of PLA(2) and inhibitors on the basal tone and intraluminal pressures (IASP) in the rat IAS vs. rectal smooth muscles (RSM). In addition, we determined the correlations between the IAS tone, PLA(2) levels, and the actual enzymatic activity. Inhibition of PLA(2) by 4-bromophenacyl bromide (universal inhibitor of PLA(2)) and MJ33 [selective inhibitor of secreted isoform of PLA(2) (sPLA(2))] caused concentration-dependent decrease in the IAS tone and in the IASP. Maximal decreases in the IAS tone and IASP by 4-bromophenacyl bromide and MJ33 were 58.8 +/- 6.9 and 51.5 +/- 6.3%, and 66.7 +/- 5.1 and 79.8 +/- 8.2%, respectively. The sPLA(2) inhibitors were approximately 100 times more potent in decreasing the IASP than the mean blood pressure. Conversely, the selective inhibitors of the cytosolic and calcium-independent PLA(2) arachidonyl trifluoromethyl ketone and bromoenol lactone, respectively, produced no significant effect. The IAS had characteristically higher levels of sPLA(2) activity (26.5 +/- 4.9 micromol.min(-1).ml(-1)) vs. the RSM (3.2 +/- 0.4 micromol.min(-1).ml(-1)), and higher levels of sPLA(2) as shown by Western blot and RT-PCR. Interestingly, administration of sPLA(2) transformed RSM into the tonic smooth muscle like that of the IAS: it developed basal tone and relaxed in response to the electrical field stimulation. From the present data, we conclude that sPLA(2) plays a critical role in the genesis of tone in the IAS. PLA(2) inhibitors may provide potential therapeutic target for treating anorectal motility disorders.

Role of cADPR in sodium nitroprusside-induced opossum esophageal longitudinal smooth muscle contraction

Nitric oxide (NO) relaxes most smooth muscle, including the circular smooth muscle (CSM) of the esophagus, whereas in the adjacent longitudinal smooth muscle (LSM), it causes contraction. The second messenger pathways responsible for this NO-induced LSM contraction are unclear, given that these opposing effects of NO are both cGMP dependent. In intestinal LSM, but not CSM, cADP ribose (cADPR)-dependent pathways participate in Ca(2+) mobilization and muscle contraction; whether similar differences exist in the esophagus is unknown. The purpose of this study was to determine whether cADPR plays a role in the NO-mediated contraction of opossum esophageal LSM. Standard isometric tension recordings were performed using both LSM and CSM strips from opossum distal esophagus that were hung in 10-ml tissue baths perfused with oxygenated Krebs solution. cADPR produced concentration-dependent contraction of LSM strips with an EC(50) of 1 nM and peak contraction of 57 +/- 18% of the 60 mM KCl-induced contraction. cADPR had no effect on CSM strips at concentrations up to 10(-6) M. The EC(50) of cADPR caused contraction (18 +/- 2% from initial resting length) of isolated LSM cells. Sodium nitroprusside (SNP; 300 muM) induced contraction of LSM strips that averaged 67 +/- 5% of the KCl response. cADPR antagonists 8-bromo-cADPR and 8-amino-cADPR, as well as ryanodine receptor antagonists ryanodine and tetracaine, significantly inhibited the SNP-induced contraction. In conclusion, in the opossum esophagus, 1) cADPR induces contraction of LSM, but not CSM, and 2) NO-induced contraction of LSM appears to involve a cADPR-dependent pathway.

IL-1beta signaling in cat lower esophageal sphincter circular muscle

In a cat model of acute experimental esophagitis, resting in vivo lower esophageal sphincter (LES) pressure and in vitro tone are lower than in normal LES, and the LES circular smooth muscle layer contains elevated levels of IL-1beta that decrease the LES tone of normal cats. We now examined the mechanisms of IL-1beta-induced reduction in LES tone. IL-1beta significantly reduced acetylcholine-induced Ca(2+) release in Ca(2+)-free medium, and this effect was partially reversed by catalase, demonstrating a role of H(2)O(2) in these changes. IL-1beta significantly increased the production of H(2)O(2), and the increase was blocked by the p38 MAPK inhibitor SB-203580, by the cytosolic phospholipase A(2) (cPLA(2)) inhibitor AACOCF3, and by the NADPH oxidase inhibitor apocynin, but not by the MEK1 inhibitor PD-98059. IL-1beta significantly increased the phosphorylation of p38 MAPK and cPLA(2). IL-1beta-induced cPLA(2) phosphorylation was blocked by SB-203580 but not by AACOCF3, suggesting sequential activation of p38 MAPK-phosphorylating cPLA(2). The IL-1beta-induced reduction in LES tone was partially reversed by AACOCF3 and by the Ca(2+)-insensitive PLA(2) inhibitor bromoenol lactone (BEL). IL-1beta significantly increased cyclooxygenase (COX)-2 and PGE(2) levels. The increase in PGE(2) was blocked by SB-203580, AACOCF3, BEL, and the COX-2 inhibitor NS-398 but not by PD-98059 or the COX-1 inhibitor valeryl salicylate. The data suggested that IL-1beta reduces LES tone by producing H(2)O(2), which may affect Ca(2+)-release mechanisms and increase the synthesis of COX-2 and PGE(2). Both H(2)O(2) and PGE(2) production depend on sequential activation of p38 MAPK and cPLA(2). cPLA(2) activates NADPH oxidases, producing H(2)O(2), and may produce arachidonic acid, converted to PGE(2) via COX-2.

HCl-induced inflammatory mediators in cat esophageal mucosa and inflammatory mediators in esophageal circular muscle in an in vitro model of esophagitis

Platelet-activating factor (PAF) and interleukin-6 (IL-6) are produced in the esophagus in response to HCl and affect ACh release, causing changes in esophageal motor function similar to esophagitis (Cheng L, Cao W, Fiocchi C, Behar J, Biancani P, and Harnett KM. Am J Physiol Gastrointest Liver Physiol 289: G418-G428, 2005). We therefore examined HCl-activated mechanisms for production of PAF and IL-6 in cat esophageal mucosa and circular muscle. A segment of normal mucosa was tied at both ends, forming a mucosal sac (Cheng L, Cao W, Fiocchi C, Behar J, Biancani P, and Harnett KM. Am J Physiol Gastrointest Liver Physiol 289: G860-G869, 2005) that was filled with acidic Krebs buffer (pH 5.8) or normal Krebs buffer (pH 7.0) as control and kept in oxygenated Krebs buffer for 3 h. The supernatant of the acidic sac (MS-HCl) abolished contraction of normal muscle strips in response to electric field stimulation. The inhibition was reversed by the PAF antagonist CV3988 and by IL-6 antibodies. PAF and IL-6 levels in MS-HCl and mucosa were significantly elevated over control. IL-6 levels in mucosa and supernatant were reduced by CV3988, suggesting that formation of IL-6 depends on PAF. PAF-receptor mRNA levels were not detected by RT-PCR in normal mucosa, but were significantly elevated after exposure to HCl, indicating that HCl causes production of PAF and expression of PAF receptors in esophageal mucosa and that PAF causes production of IL-6. PAF and IL-6, produced in the mucosa, are released to affect the circular muscle layer. In the circular muscle, PAF causes production of additional IL-6 that activates NADPH oxidase to induce production of H(2)O(2). H(2)O(2) causes formation of IL-1beta that may induce production of PAF in the muscle, possibly closing a self-sustaining cycle of production of inflammatory mediators.

In vitro model of acute esophagitis in the cat

We have shown that IL-1beta and IL-6, possibly originating from the mucosa in response to injury, inhibit neurally mediated contraction of esophageal circular muscle but do not affect ACh-induced contraction, reproducing the effect of experimental esophagitis on esophageal contraction. To examine the interaction of mucosa and circular muscle in inflammation, we examined the effect of HCl on in vitro esophageal mucosa and circular muscle. Circular muscle strips, when directly exposed to HCl, contracted normally. However, when circular muscle strips were exposed to supernatants of mucosa incubated in HCl (2-3 h, pH 5.8), contraction decreased, and the inhibition was partially reversed by an IL-6 antibody. Supernatants from the mucosa of animals with in vivo-induced acute esophagitis (AE) similarly reduced contraction. IL-6 levels were higher in mucosal tissue from AE animals than in control mucosa and in AE mucosa supernatants than in normal mucosa supernatants. IL-6 levels increased significantly in normal mucosa and supernatants in response to HCl, suggesting increased production and release of IL-6 by the mucosa. IL-6 increased H2O2 levels in the circular muscle layer but not in mucosa. Exposure of the mucosa to HCl caused IL-1beta to increase only in the mucosa and not in the supernatant. These data suggest that HCl-induced damage occurs first in the mucosa, leading to the production of IL-1beta and IL-6 but not H2O2. IL-1beta appears to remain in the mucosa. In contrast, IL-6 is produced and released by the mucosa, eventually resulting in the production of H2O2 by the circular muscle, with this affecting circular muscle contraction.

Platelet-activating factor and prostaglandin E2 impair esophageal ACh release in experimental esophagitis

ACh is a neurotransmitter in cat esophageal circular muscle, as atropine nearly abolishes contraction of in vitro circular muscle strips in response to electric field stimulation (EFS) (5, 12). Experimental esophagitis reduced EFS- but not ACh-induced contraction of esophageal circular muscle, suggesting that esophagitis impairs neurotransmitter release. Because IL-1beta and IL-6 are produced in esophagitis and reproduce these changes in normal esophageal muscle (12), we examined the role of IL-1beta and IL-6 in this motor dysfunction. IL-1beta, IL-6 (12), H2O2, PGE2, and platelet-activating factor (PAF) were elevated in esophagitis specimens. Normal muscle incubated (2 h) in IL-1beta and IL-6 had increases in H2O2, PGE2, and PAF levels. H2O2 contributed to increased PGE2 and PAF, as the increase was partially (60-80%) reversed by the H2O2 scavenger catalase. EFS-induced [3H]ACh release from muscle strips significantly (42%) decreased in esophagitis and after 2 h incubation in PGE2 and in PAF C-16. Similarly, EFS-induced but not ACh-induced muscle contraction decreased in esophagitis and after incubation in PGE2 and PAF C-16. Finally, in normal muscle strips treated with IL-1beta electrical field stimulation (EFS)-induced contraction was partially restored by indomethacin or by the PAF antagonist CV3988 and was completely restored by the combination of CV3988 and indomethacin, whereas in strips treated with IL-6, EFS-induced contraction was partially restored by the PAF antagonist CV3988 and not affected by indomethacin. We conclude that IL-1beta-induced production of H2O2 causes formation of PGE2 and PAF that inhibit ACh release from esophageal cholinergic neurons without affecting ACh-induced contraction of esophageal circular muscle. IL-6 causes production of H2O2, PAF, and other unidentified inflammatory mediators.

Inflammation induced changes in arachidonic acid metabolism in cat LES circular muscle

Myogenic lower esophageal sphincter (LES) tone is maintained by arachidonic acid metabolites, such as PGF(2alpha) and thromboxane A(2)/B(2). Experimental esophagitis in cat reduces LES in vivo pressure and in vitro tone. Because IL-1beta may mediate esophagitis-associated reduction in ACh release in esophagus, we examined whether IL-1beta may also play a role in esophagitis-induced reduction of LES tone. A cat model of experimental esophagitis was obtained by repeated esophageal perfusion with HCl (Biancani P, Barwick K, Selling J, and McCallum R. Gastreonterology 87: 8-16, 1984 and Sohn UD, Harnett KM, Cao W, Rich H, Kim N, Behar J, and Biancani P. J Pharmacol Exp Ther 283: 1293-1304, 1997.). LES circular muscle strips were examined in muscle chambers as previously described (Biancani P, Billett G, Hillemeier C, Nissenshon M, Rhim BY, Sweczack S, and Behar J. Gastroenterology 103: 1199-1206, 1992). Levels of inflammatory mediators were measured. IL-1beta levels were higher in esophagitis than in normal LES. IL-1beta reduced normal LES tone, and the reduction was reversed by catalase, suggesting a role of H(2)O(2). This was confirmed by IL-1beta-induced production of H(2)O(2) in normal LES and elevated H(2)O(2) levels in esophagitis. H(2)O(2) by itself is sufficient to explain the changes that occur in the muscle, reducing its ability to contract. H(2)O(2) increased PGE(2) in normal LES, and PGE(2) levels were elevated in esophagitis LES, whereas PGF(2alpha) levels were unchanged. H(2)O(2) also increased levels of 8-isoprostanes, stable prostaglandin-like compounds formed by free radical-induced peroxidation of arachidonic acid, and 8-isoprostane levels were elevated in esophagitis. The PGF(2alpha) analog 8-iso-PGF(2alpha) caused little contraction of LES strips but reduced PGF(2alpha) binding and contraction of normal LES. In esophagitis, PGF(2alpha) binding and contraction were reduced in LES, suggesting that isoprostanes may contribute to reduction in tone in esophagitis. The data suggest that, in esophagitis, IL-1beta causes production of H(2)O(2). H(2)O(2) increases PGE(2), which relaxes the LES, and 8-iso-F(2alpha), which blocks PGF(2alpha)-mediated contraction.

Signal-transduction pathways that regulate smooth muscle function I. Signal transduction in phasic (esophageal) and tonic (gastroesophageal sphincter) smooth muscles

Contraction of esophageal (Eso) and lower esophageal sphincter (LES) circular muscle depends on distinct signal-transduction pathways. ACh-induced contraction of Eso muscle is linked to phosphatidylcholine metabolism, production of diacylglycerol and arachidonic acid (AA), and activation of the Ca(2+)-insensitive PKCepsilon. Although PKCepsilon does not require Ca(2+) for activation, either influx of extracellular Ca(2+) or release of Ca(2+) from stores is needed to activate the phospholipases responsible for hydrolysis of membrane phospholipids and production of second messengers, which activate PKCepsilon. In contrast, the LES uses two distinct intracellular pathways: 1) a PKC-dependent pathway activated by low doses of agonists or during maintenance of spontaneous tone, and 2) a Ca(2+)-calmodulin-myosin light chain kinase (MLCK)-dependent pathway activated in response to maximally effective doses of agonists during the initial phase of contraction. The Ca(2+) levels, released by agonist-induced activity of phospholipase C, determine which contractile pathway is activated in the LES. The Ca(2+)-calmodulin-MLCK-dependent contractile pathway has been well characterized in a variety of smooth muscles. The steps linking activation of PKC to myosin light chain (MLC20) phosphorylation and contraction, however, have not been clearly defined for LES, Eso, or other smooth muscles. In addition, in LES circular muscle, a low-molecular weight pancreatic-like phospholipase A2 (group I PLA2) causes production of AA, which is metabolized to prostaglandins and thromboxanes. These AA metabolites act on receptors linked to heterotrimeric G proteins to induce activation of phospholipases and production of second messengers to maintain contraction of LES circular muscle. We have examined the signal-transduction pathways activated by PGF(2alpha) and by thromboxane analogs during the initial contractile phase and found that these pathways are the same as those activated by other agonists. In response to low doses of agonists or during maintenance of tone, presumably due to low levels of calcium release, a PKC-dependent pathway is activated, whereas at high doses of PGF(2alpha) and thromboxane analogs, in the initial phase of contraction, calmodulin is activated, PKC activity is reduced, and contraction is mediated, in part, through a Ca(2+)-calmodulin-MLCK-dependent pathway. The PKC-dependent signaling pathways activated by PGF(2alpha) and by thromboxanes during sustained LES contraction, however, remain to be examined, but preliminary data indicate that a distinct PKC-dependent pathway may be activated during maintenance of tonic contraction, which is different from the one activated during the initial contractile response. The initial contractile response to low levels of agonists depends on activation of G(q). Sustained contraction in response to PGF(2alpha) may involve activation of the monomeric G protein RhoA, because the contraction is inhibited by the RhoA-kinase antagonist Y27632. This shift in signal-transduction pathways between initial and sustained contraction has been recently reported in intestinal smooth muscle.

Proinflammatory cytokines alter/reduce esophageal circular muscle contraction in experimental cat esophagitis

Cholinergic mechanisms are largely responsible for esophageal contraction in response to swallowing or to in vitro electrical field stimulation (EFS). After induction of experimental esophagitis by repeated acid perfusion, the responses to swallowing and to EFS were significantly reduced but contraction in response to ACh was not affected, suggesting that cholinergic mechanisms are damaged by acid perfusion but that myogenic mechanisms are not. Measurements of ACh release in response to EFS confirmed that release of ACh was reduced in esophagitis compared with normal controls. To examine factors contributing to this neuropathy, normal esophageal strips were incubated for 1-2 h with the proinflammatory cytokines IL-1beta (100 U/ml), IL-6 (1 ng/ml), or TNF-alpha (1 ng/ml). IL-1beta and IL-6 levels, measured by Western blot analysis, increased in esophagitis compared with normal circular muscle. IL-1beta and IL-6 reduced contraction in response to EFS (2-10 Hz, 0.2 ms) but did not affect ACh-induced contraction, suggesting that these cytokines inhibit ACh release without affecting myogenic contractile mechanisms. EFS-induced ACh release was significantly reduced in normal esophageal strips by incubation in IL-1beta or IL-6, suggesting that they may contribute to the contractility changes. TNF-alpha at 1 ng/ml, however, did not affect the response to ACh or to electrical stimulation but inhibited both at higher concentrations. TNF-alpha levels were low in normal muscle and did not increase with esophagitis. The data suggest that the proinflammatory cytokines IL-1beta and IL-6 contribute to reduced esophageal contraction by inhibiting release of ACh from myenteric neurons.

PGF(2alpha)-induced contraction of cat esophageal and lower esophageal sphincter circular smooth muscle

Lower esophageal sphincter (LES) tone depends on PGF(2alpha) and thromboxane A(2) acting on receptors linked to G(i3) and G(q) to activate phospholipases and produce second messengers resulting in muscle contraction. We therefore examined PGF(2alpha) signal transduction in circular smooth muscle cells isolated by enzymatic digestion from cat esophagus (Eso) and LES. In Eso, PGF(2alpha)-induced contraction was inhibited by antibodies against the alpha-subunit of G(13) and the monomeric G proteins RhoA and ADP-ribosylation factor (ARF)1 and by the C3 exoenzyme of Clostridium botulinum. A [(35)S]GTPgammaS-binding assay confirmed that G(13), RhoA, and ARF1 were activated by PGF(2alpha). Contraction of Eso was reduced by propranolol, a phospholipase D (PLD) pathway inhibitor and by chelerythrine, a PKC inhibitor. In LES, PGF(2alpha)-induced contraction was inhibited by antibodies against the alpha-subunit of G(q) and G(i3), and a [(35)S]GTPgammaS-binding assay confirmed that G(q) and G(i3) were activated by PGF(2alpha). PGF(2alpha)-induced contraction of LES was reduced by U-73122 and D609 and unaffected by propranolol. At low PGF(2alpha) concentration, contraction was blocked by chelerythrine, whereas at high concentration, contraction was blocked by chelerythrine and CGS9343B. Thus, in Eso, PGF(2alpha) activates a PLD- and protein kinase C (PKC)-dependent pathway through G(13), RhoA, and ARF1. In LES, PGF(2alpha) receptors are coupled to G(q) and G(i3), activating phosphatidylinositol- and phosphatidylcholine-specific phospholipase C. At low concentrations, PGF(2alpha) activates PKC. At high concentration, it activates both a PKC- and a calmodulin-dependent pathway.

Opposing roles of K(+) and Cl(-) channels in maintenance of opossum lower esophageal sphincter tone

The ionic basis underlying the maintenance of myogenic tone of lower esophageal sphincter circular muscle (LES) was investigated in opossum with the use of standard isometric tension and conventional intracellular microelectrode recordings in vitro. In tension recording studies, nifedipine (1 microM) reduced basal tone to 27.7 +/- 3.8% of control. The K(+) channel blockers tetraethylammonium (TEA, 2 mM), charybdotoxin (100 nM), and 4-aminopyridine (4-AP, 2 mM) enhanced resting tone, whereas apamin and glibenclamide were without affect. Cl(-) channel blockers DIDS (500 microM) and 5-nitro-2-(3-phenylpropylamino)-benzoic acid (500 microM), as well as niflumic acid (0.1-300 microM), decreased basal tone, but tamoxifen was without effect. Intracellular microelectrode recordings revealed ongoing, spontaneous, spike-like action potentials (APs). Nifedipine abolished APs and depolarized resting membrane potential (RMP). Both TEA and 4-AP significantly depolarized RMP and augmented APs, whereas niflumic acid dose-dependently hyperpolarized RMP and abolished APs. These data suggest that, in the opossum, basal tone is associated with continuous APs and that K(+) and Ca(2+)-activated Cl(-) channels have important opposing roles in the genesis of LES tone.