Guanylate cyclase inhibitors: effect on tone, relaxation, and cGMP content of lower esophageal sphincter

Comparative Relaxant Effects of Ataciguat and Zaprinast on Sheep Sphincter of Oddi

BACKGROUND

Relaxing the sphincter of Oddi (SO) is an important process during endoscopic retrograde cholangiopancreatography (ERCP) procedures. This issue suggests that the easier the sphincterotomy and cannulation, the more post-ERCP complications decrease.

AIMS

To compare the relaxant effects of ataciguat (a novel soluble guanylyl cyclase activator) and zaprinast (an inhibitor of phosphodiesterase 5) on sheep SO in vitro, thus testing whether they can be used during ERCP.

STUDY DESIGN

Animal experimentation.

METHODS

Sheep SO rings were placed in tissue baths and their isometric tension to ataciguat and zaprinast were tested. We also tested their isometric tension against ataciguat in the presence of 1H-(1,2,4) oxadiazole (4,3-a) quinoxalin-1-one (ODQ) which is a soluble guanylyl cyclase inhibitor.

RESULTS

Ataciguat and zaprinast both triggered concentration addicted relaxation on sheep SO rings (p=0.0018, p=0.0025 respectively) but the relaxation of the ataciguat was significantly greater than that of zaprinast at all concentrations (p=0.0024). It was observed that decreased relaxation responses were initiated by ataciguat in the presence of ODQ (p=0.0012).

CONCLUSION

Ataciguat and zaprinast both have relaxing effects on sphincter of Oddi, although that of zaprinast is lower. We believe that ataciguat and zaprinast can be used in ERCP procedures in order to relax the sphincter of Oddi and thus can be used locally in order to decrease complications.

Nitric oxide activation of a potassium channel (BKCa) in feline lower esophageal sphincter

AIM: To assess the effect of nitric oxide (NO) on the large conductance potassium channel (BK_Ca) in isolated circular (CM) and sling (SM) muscle cells and muscle strips from the cat lower esophageal sphincter (LES) to determine its regulation of resting tone and relaxation.

METHODS: Freshly enzymatically-digested and isolated circular smooth muscle cells were prepared from each LES region. To study outward K⁺ currents, the perforated patch clamp technique was employed. To assess LES resting tone and relaxation, muscle strips were mounted in perfused organ baths.

RESULTS: (1) Electrophysiological recordings from isolated cells: (a) CM was more depolarized than SM (-39.7 ± 0.8mV vs -48.1 ± 1.6 mV, P < 0.001), and maximal outward current was similar (27.1 ± 1.5 pA/pF vs 25.7 ± 2.0 pA/pF, P > 0.05); (b) The NO donor sodium nitroprusside (SNP) increased outward currents only in CM (25.9 ± 1.9 to 46.7 ± 4.2 pA/pF, P < 0.001) but not SM (23.2 ± 3.1 to 27.0 ± 3.4 pA/pF, P > 0.05); (c) SNP added in the presence of the BK_Ca antagonist iberiotoxin (IbTX) produced no increase in the outward current in CM (17.0 ± 2.8 vs 13.7 ± 2.2, P > 0.05); and (d) L-NNA caused a small insignificant inhibition of outward K⁺ currents in both muscles; and (2) Muscle strip studies: (a) Blockade of the nerves with tetrodotoxin (TTX), or BK_Ca with IbTX had no significant effect on resting tone of either muscle; and (b) SNP reduced tone in both muscles, and was unaffected by the presence of TTX or IbTX.

CONCLUSION: Exogenous NO activates BK_Ca only in CM of the cat. However, as opposed to other species, exogenous NO-induced relaxation is predominantly by a non-BK_Ca mechanism, and endogenous NO has minimal effect on resting tone.

Study on the cyclic GMP-dependency of relaxations to endogenous and exogenous nitric oxide in the mouse gastrointestinal tract

BACKGROUND AND PURPOSE

cGMP mediates nitrergic relaxations of intestinal smooth muscle, but several studies have indicated that cGMP-independent mechanisms may also be involved. We addressed this contention by studying the effect of ODQ and ns2028, specific inhibitors of soluble guanylate cyclase, on nitrergic relaxations of the mouse gut.

EXPERIMENTAL APPROACH

Mouse gastric fundus and small intestinal muscle preparations were mounted in organ baths to study relaxations to exogenous NO, NO donors and electrical field stimulation (EFS) of enteric nerves.

KEY RESULTS

In gastric fundus longitudinal muscle strips, ODQ and NS2028 abolished the L-nitroarginine-sensitive relaxations to EFS and the relaxations to NO and NO donors, glyceryl trinitrate (GTN), SIN-1 and sodium nitroprusside (SNP). EFS of intestinal segments and muscle strips showed L-nitroarginine-resistant relaxations, which were abolished by the purinoceptor blocker suramin. In the presence of suramin, ODQ and NS2028 abolished all relaxations to EFS in intestinal segments and strips. ODQ and NS2028 abolished the relaxations to exogenous NO and to the NO donors GTN, SIN-1 and SNP in circular and longitudinal intestinal muscle strips. Intestinal segments showed residual relaxations to NO and GTN.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that relaxations to endogenous NO in the mouse gastric fundus and small intestine are completely dependent on cGMP. ODQ and NS2028 incompletely blocked nitrergic relaxations to exogenous NO in intact intestinal segments. However, it is unlikely that this is due to the involvement of cGMP-independent pathways because ODQ and NS2028 abolished all relaxations to endogenous and exogenous NO in intestinal muscle strips.

Generation of nitric oxide in the opossum lower esophageal sphincter during physiological experimentation

Lipopolysaccharide (LPS), given in vivo, modulates opossum esophageal motor functions by inducing the inducible nitric oxide synthase (iNOS), which increases nitric oxide (NO) production. Superoxide, a NO scavenger, is generated during this endotoxemia. Superoxide is cleared by superoxide dismutase (SOD) and catalase (CAT) to protect the physiological function of NO. This study examined whether lower esophageal sphincter (LES) motility, NO release, and iNOS and nitrotyrosine accumulation in the LES are affected by LPS in vitro. Muscle strips from the opossum LES were placed in tissue baths containing oxygenated Krebs buffer. NO release was measured with a chemiluminescence NOx analyzer, and Western blots were performed to analyze iNOS and nitrotyrosine production. The percent change in resting LES tone after a 6-hour exposure to LPS was significantly increased compared to pretreatment values. The percent LES relaxation upon electrical stimulation was significantly decreased in the control group at 6 hours, indicating that the LPS treatment had an effect. The NO concentration in the tissue bath of LPS- treated muscle without nerve stimulation was significantly less than that of LPS treatment combined with SOD/CAT or SOD/CAT alone. iNOS and nitrotyrosine were detectable and increased over time in the LES muscle of both the control and LPS-treated groups. Antioxidant enzymes may play a role in regulating NO-mediated neuromuscular functions in the LES.

Role of cGMP as a mediator of nerve-induced motor functions of the opossum esophagus

Stimulation of esophageal nerves produces biphasic relaxation of the lower esophageal sphincter (LES) and an off response of circular esophageal muscle. Previously, we proposed that cGMP mediates nerve-induced hyperpolarization of circular LES muscle but not LES relaxation. These experiments explore whether cGMP mediates LES relaxation or the off response. Strips of muscle from the opossum esophagus and LES were connected to force-displacement transducers, placed in tissue baths containing oxygenated Krebs solution at 37 degrees C, and stimulated by an electrical field. 1H-[1,2, 4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of guanylyl cyclase, antagonized the off response, shortened its latency, and blocked the first phase of LES relaxation. ODQ also antagonized LES relaxation by exogenous nitric oxide (NO) but not relaxations by vasoactive intestinal polypeptide (VIP). Part of the nerve-induced LES relaxation and the off response appear to be mediated by the second messenger cGMP. These studies indicate that VIP-induced LES relaxation is not mediated by cGMP and therefore do not support the hypothesis that VIP produces LES relaxation by causing the generation of NO.

Nitric oxide in the contractile action of bradykinin, oxytocin, and prostaglandin F2 alpha in the estrogenized rat uterus

Experiments were performed on uteri from estrogen-primed female rats. Bradykinin (BK) (10(-8) M) significantly augmented biosynthesis of prostaglandin F2 alpha (PGF2alpha) and prostaglandin E2 (PGE2), and this synthesis was completely blocked by NG-monomethyl L-arginine (NMMA) (300 microM), a competitive inhibitor of nitric oxide synthase (NOS). Blockade of prostaglandin synthesis by indomethacin caused rapid dissipation of isometric developed tension (IDT) induced by BK. Blockade of NOS with NMMA had similar but less marked effects. Combining the two inhibitors produced an even more rapid decay in IDT, suggesting that BK-induced NO release maintains IDT by release of prostanoids. The decline of frequency of contraction (FC) was not significantly altered by either indomethacin or NMMA but was markedly accelerated by combination of the inhibitors, which suggests that PGs maintain FC and therefore FC decline is accelerated only when PG production is blocked completely by combination of the two inhibitors of PG synthesis. The increase in IDT induced by oxytocin was unaltered by indomethacin, NMMA or their combination indicating that neither NO nor PGs are involved in the contractions induced by oxytocin. However, the decline in FC with time was significantly reduced by the inhibitor of NOS, NMMA, suggesting that FC decay following oxytocin is caused by NO released by the contractile process. In the case of PGF2alpha, NMMA resulted in increased initial IDT and FC. The decline in FC was rapid and dramatically inhibited by NMMA. Receptor-mediated contraction by BK, oxytocin, and PGF2alpha is modulated by NO that maintains IDT by releasing PGs but reduces IDT and FC via cyclic GMP.

Modulation of baroreceptor activity by nitric oxide and S-nitrosocysteine

The goal of this study was to determine whether nitric oxide (NO) and the NO donor, S-nitrosocysteine (cysNO), modulate the activity of carotid sinus baroreceptors. Baroreceptor activity was recorded from the vascularly isolated carotid sinus in anesthetized rabbits. Baroreceptor activity decreased in a dose-dependent manner after injection of either NO or cysNO as constant pressure was maintained, and activity recovered spontaneously over time, within seconds to minutes. The baroreceptor pressure-activity relation was shifted significantly to the right by cysNO, with a profound suppression of activity at high pressure. Baroreceptor activity at 160 mm Hg averaged 76 +/- 8%, 60 +/- 6%, and 36 +/- 5% of the control maximum during exposure to 10(-4), 2 to 3 x 10(-4), and 10(-3) mol/L cysNO, respectively. The inhibition of activity by the L and D isomers of cysNO was equivalent and was blocked by reduced hemoglobin, suggesting that the effect was mediated by NO. The suppression of baroreceptor activity by cysNO was not related to vascular relaxation as measured by videomicrometer. Inhibition of soluble guanylate cyclase with methylene blue or 6-anilinoquinoline-5,8-quinone (LY83583, 10(-5) mol/L) did not attenuate and dibutyryl cGMP (10(-3) mol/L) did not mimic the suppression of baroreceptor activity by cysNO, suggesting a cGMP-independent mechanism. Activation of endogenous NO formation with thimerosal (10(-5) to 10(-4) mol/L) reduced maximum baroreceptor activity in five of eight experiments to 59 +/- 7% of the control maximum.(ABSTRACT TRUNCATED AT 250 WORDS)