Characterization of the membrane conductance changes underlying the apamin-resistant NANC inhibitory junction potential in the guinea-pig proximal and distal colon

Gastric nNOS reduction accompanied by natriuretic peptides signaling pathway upregulation in diabetic mice

AIM

To investigate the relationship between neuronal nitric oxide synthase (nNOS) expression and the natriuretic peptide signaling pathway in the gastric fundus of streptozotocin (STZ)-induced diabetic mice.

METHODS

Diabetic mice were induced by injection of STZ solution. Immunofluorescence labeling of HuC/D, nNOS and natriuretic peptide receptor-A, B, C (NPRs) in the gastric fundus (GF) was used to observe nNOS expression and whether NPRs exist on enteric neurons. The expression levels of nNOS and NPRs in the diabetic GF were examined by western blotting. An isometric force transducer recorded the electric field stimulation (EFS)-induced relaxation and contraction in the diabetic GF. An intracellular recording method assessed EFS-induced inhibitory junction potentials (IJP) on the GF. GF smooth muscles acquired from normal mice were incubated with different concentrations of the NPRs agonist C-type natriuretic peptide (CNP) for 24 h, after which their nNOS expressions were detected by western blotting.

RESULTS

Eight weeks after injection, 43 diabetic mice were obtained from mouse models injected with STZ. Immunofluorescence indicated that the number of NOS neurons was significantly decreased and that nNOS expression was significantly downregulated in the diabetic GF. The results of physiological and electrophysiological assays showed that the EFS-induced relaxation that mainly caused by NO was significantly reduced, while the contraction was enhanced in the diabetic GF. EFS-induced IJP showed that L-NAME sensitive IJP in the diabetic GF was significantly reduced compared with control mice. However, both NPR-A and NPR-B were detected on enteric neurons, and their expression levels were upregulated in the diabetic GF. The nNOS expression level was downregulated dose-dependently in GF smooth muscle tissues exposed to CNP.

CONCLUSION

These findings suggested that upregulation of the NPs signaling pathway may be involved in GF neuropathy caused by diabetes by decreasing nNOS expression.

Nitrergic inhibition of tachykininergic neuro-muscular transmission via cyclic GMP in the hamster ileum

The present study was designed to explore the inhibitory mechanism by nitric oxide (NO) of the tachykininergic neuro-muscular transmissions in the hamster ileum. In the presence of guanethidine (1 µM), atropine (0.5 µM), nifedipine (0.1 µM) and apamin (100 nM), electrical field stimuli (EFS; 0.5 ms duration, 15 V) evoked non-adrenergic, non-cholinergic excitatory junction potentials (EJPs) in circular smooth muscle cells. The EJPs were markedly inhibited by the tachykinin NK1 receptor antagonists [D-Pro(4), D-Trp(7,9)]-SP(4-11) (3 µM). Both the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 200 µM) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 µM), did not affect on the resting membrane potentials, but enhanced the tachykininergic EJPs. In the presence of L-NAME (200 µM), exogenously applied NO (10 µM) and the membrane permeable analogue of guanosine 3',5'-cyclic monophosphate (cGMP), 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP, 3 mM), significantly inhibited the tachykininergic EJPs. Application of EFS (0.5 msec duration, 15 V) with trains of 20 pulses at 20 Hz increased amount of released substance P (SP). The release of SP was further increased by the treatment of L-NAME or ODQ, but markedly reduced by exogenously applied NO and 8-Br-cGMP. These results suggest that the endogenous NO may inhibit the tachykininergic neuro-muscular transmissions by the decrease of SP release from the tachykininergic neurons, possibly through a guanylate cyclase-cGMP-dependent mechanism in the hamster ileum.

Block of inhibitory junction potentials and TREK-1 channels in murine colon by Ca2+ store-active drugs

Post-junctional enteric inhibitory responses are composed of at least two components attributed to the release of a purine and nitric oxide (NO). The nitrergic component is characterized by membrane potential hyperpolarization; however, the conductances involved and the role of Ca(2+) stores in regulating these conductances are controversial. Conventional microelectrode recordings were performed in intact muscle strips and whole-cell voltage clamp experiments were performed on freshly dispersed cells and COS7 cells stably transfected with TREK-1 channels. Here we show that several Ca(2+) store-active compounds, including caffeine, ryanodine, and cyclopiazonic acid, reduce inhibitory junction potentials and responses to sodium nitroprusside in murine colonic muscles. We previously proposed that two-pore K(+) channels of the TREK family mediate a portion of the hyperpolarization response to NO in colonic muscles. We tested the effects of Ca(2+) store-active drugs in COS cells expressing murine TREK-1 channels and found these compounds block TREK-1 currents. These effects were greatly attenuated by dialysing cells with protein kinase A inhibitory peptide (PKAI). Caffeine also blocked stretch-dependent K(+) (SDK) channels, thought to be due to expression of TREK channels, in colonic myocytes, but these effects were not apparent in excised patches. Taken together our data show that Ca(2+) store-active compounds inhibit TREK-1 channels, native SDK channels, and nitrergic inhibitory junction potentials. These effects appear to be due, in part, to the cAMP/PKA stimulatory actions of these drugs and inhibitory effects of TREK channels.

Involvement of purinergic nerves in the NANC inhibitory junction potentials in pigeon oesophageal smooth muscle

1. Electrical field stimulation (EFS) (0.5 ms in train of 2-32 Hz for 300 ms) in smooth muscle of pigeon oesophagus, in the presence of atropine (1 microm) and guanethidine (1 microm), elicited an inhibitory response consisting of a transient hyperpolarization (inhibitory junction potential, IJP) associated with muscle relaxation. 2. Sodium nitroprusside (SNP, 100 microm) induced hyperpolarization correlated to mechanical relaxation. 3. The nitric oxide (NO) synthase inhibitor N(omega)-nitro-l-arginine (from 0.1 to 100 microm) caused a concentration-dependent reduction of electromechanical response to EFS indicating a role for NO in this response. 4. Apamin (1 microm) reduced both IJP and relaxation to EFS but was without effect on the response to SNP indicating a role for purines, which are also blocked by apamin. 5. Adenosine, AMP, ADP and ATP (all from 1 microm to 1 mm) application caused transient hyperpolarization and muscular relaxation with the following order of potency: adenosine > AMP > ADP > ATP. 6. Inhibitory responses evoked by purines are TTX (1 microm) insensitive but they were inhibited by apamin. This indicates that a purine component for the non-adrenergic non-cholinergic (NANC) response exists but the purine receptor site is not located on the neurone. 7. Overall these results suggest that NANC inhibitory response elicited by EFS presents two different components apamin-sensitive, probably purines-mediated and apamin-insensitive probably NO-mediated as apamin only partially block the response to EFS.

An analysis of inhibitory junction potentials in the guinea-pig proximal colon

Intracellular recordings were made from either sheets or isolated bundles of the circular muscle layer of guinea-pig proximal colon and the responses evoked by stimulating inhibitory nerve fibres were analysed. Inhibitory junction potentials (IJPs), evoked by single stimuli, had two components which could be separated on their pharmacological and temporal characteristics and their voltage sensitivities. The initial component, which was abolished by apamin and reduced in amplitude by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), had a brief time course: its amplitude was changed when the external concentration of potassium ions ([K+](o)) was changed. The second component of the IJP had a slower onset than the first component, was abolished by l-nitroarginine (NOLA) and oxadiazolo quinoxalin-1-one (ODQ), an inhibitor of soluble guanylate cyclase: its amplitude was little affected by changing [K+](o) and was increased when the membrane potential of the circular layer was hyperpolarized. The observations suggest that the initial component of the IJP results from the release of ATP which triggers an increase in membrane conductance to K+ and that the second component results from the release of nitric oxide which suppresses a background inward current.

Mechanism of active repolarization of inhibitory junction potential in murine colon

Enteric inhibitory responses in gastrointestinal (GI) smooth muscles involve membrane hyperpolarization that transiently reduce the excitability of GI muscles. We examined the possibility that an active repolarization mechanism participates in the restoration of resting membrane potential after fast inhibitory junction potentials (IJPs) in the murine colon. Previously, we showed these cells express a voltage-dependent nonselective cation conductance (NSCC) that might participate in active repolarization of IJPs. Colonic smooth muscle cells were impaled with micro-electrodes and voltage responses to nerve-evoked IJPs, and locally applied ATP were recorded. Ba2+ (500 muM), a blocker of the NSCC, slowed the rate of repolarization of IJPs. We also tested the effects of Ba2+, Ni2+, and mibefradil, all blockers of the NSCC, on responses to locally applied ATP. Spritzes of ATP caused transient hyperpolarization, and the durations of these responses were significantly increased by the blockers of the NSCC. We considered whether NSCC blockers might affect ATP metabolism and found that Ni2+ decreased ATP breakdown in colonic muscles. Mibefradil had no effect on ATP metabolism. Because both Ni2+ and mibefradil had similar effects on prolonging responses to ATP, it appears that restoration of resting membrane potential after ATP spritzes is not primarily due to ATP metabolism. Neurally released enteric inhibitory transmitter and locally applied ATP resulted in transient hyperpolarizations of murine colonic muscles. Recovery of membrane potential after these responses appears to involve an active repolarization mechanism due to activation of the voltage-dependent NSCC expressed by these cells.

Nitrergic prejunctional inhibition of purinergic neuromuscular transmission in the hamster proximal colon

Neurogenic ATP and nitric oxide (NO) may play important roles in the physiological control of gastrointestinal motility. However, the interplay between purinergic and nitrergic neurons in mediating the inhibitory neurotransmission remains uncertain. This study investigated whether neurogenic NO modulates the purinergic transmission to circular smooth muscles of the hamster proximal colon. Electrical activity was recorded from circular muscle cells of the hamster proximal colon by using the microelectrode technique. Intramural nerve stimulation with a single pulse evoked a fast purinergic inhibitory junction potential (IJP) followed by a slow nitrergic IJP. The purinergic component of the second IJP evoked by paired stimulus pulses at pulse intervals between 1 and 3 s became smaller than that of the first IJP. This purinergic IJP depression could be observed at pulse intervals <3 s, but not at longer ones, and failed to occur in the presence of NO synthase inhibitor. Exogenous NO (0.3-1 microM), at which no hyperpolarization is produced, inhibited purinergic IJPs, without altering the nitrergic IJP and exogenously applied ATP-induced hyperpolarization. In the presence of both purinoceptor antagonist and nitric oxide synthase (NOS) inhibitor, intramural nerve stimulation with 5 pulses at 20 Hz evoked vasoactive intestinal peptide (VIP)-associated IJPs, suggesting that VIP component may be masked in the IJPs of the hamster proximal colon. Our results suggest that neurogenic NO may modulate the purinergic transmission to circular smooth muscles of the hamster proximal colon via a prejunctional mechanism. In addition, VIP may be involved in the neurotransmitter in the hamster proximal colon.

Stretch-activated neuronal pathways to longitudinal and circular muscle in guinea pig distal colon

The role of the longitudinal muscle (LM) layer during the peristaltic reflex in the small and large intestine is unclear. In this study, we have made double and quadruple simultaneous intracellular recordings from LM and circular muscle (CM) cells of guinea pig distal colon to correlate the electrical activities in the two different muscle layers during circumferential stretch. Simultaneous recordings from LM and CM cells (<200 microm apart) at the oral region of the colon showed that excitatory junction potentials (EJPs) discharged synchronously in both muscle layers for periods of up to 6 h. Similarly, at the anal region of the colon, inhibitory junction potentials (IJPs) discharged synchronously in the two muscle layers. Quadruple recordings from LM and CM orally at the same time as from the LM and CM anally revealed that IJPs occurred synchronously in the LM and CM anally at the same time as EJPs in LM and CM located 20 mm orally. Oral EJPs and anal IJPs were linearly related in amplitude between the two muscle layers. Spatiotemporal maps generated from simultaneous video imaging of the movements of the colon, combined with intracellular recordings, revealed that some LM contractions orally could be correlated in time with IJPs in CM cells anally. N(omega)-nitro-L-arginine (L-NA; 100 microM) abolished the IJP in LM, whereas a prominent L-NA-resistant "fast" IJP was always observed in CM. In summary, in stretched preparations, synchronized EJPs in both LM and CM orally are generated by synchronized firing of many ascending interneurons, which simultaneously activate excitatory motor neurons to both muscle layers. Similarly, synchronized IJPs in both LM and CM anally are generated by synchronized firing of many descending interneurons, which simultaneously activate inhibitory motor neurons to both muscle layers. This synchronized motor activity ensures that both muscles around the entire circumference are excited orally at the same time as inhibited anally, thus producing net aboral propulsion.

A possible role of neurotensin in NANC relaxation of longitudinal muscle of the jejunum and ileum of Wistar rats

The mediators of nonadrenergic, noncholinergic (NANC) relaxation in longitudinal muscle of the jejunum and ileum of Wistar rats were examined in vitro. Treatment of the jejunal and ileal segments with alpha-chymotrypsin resulted in decreases in the NANC relaxations induced by electrical field stimulation (EFS) by about one half. The NANC relaxations were also decreased by about one half after the segments had been desensitized to neurotensin. A neurotensin receptor antagonist, SR48692 (10 microM) inhibited the NANC relaxation by 56 and 34% in the jejunal and ileal segments, respectively. An inhibitor of small conductance Ca2+ -activated K+ channel (SK channel), apamin (100 nM) also inhibited the NANC relaxation by 83 and 63%, respectively. Exogenous neurotensin-induced relaxations of the two segments were abolished by apamin. In the ileal segments, N(G)-nitro-L-arginine (L-NOARG, 100 micro M), inhibited the NANC relaxation by 43%. L-NOARG, but not apamin, further inhibited the relaxation which persisted after the desensitization to neurotensin. Apamin with SR48692 inhibited the relaxation only to the same extent as apamin alone. EFS induced inhibitory junction potentials (i.j.ps) in the longitudinal muscle cells of the ileum. I.j.ps consisted of a rapid and a delayed phase. L-NOARG significantly inhibited only the delayed phase. EFS induced only a rapid i.j.ps in the jejunum. SR48692 and apamin inhibited the i.j.ps. These findings suggest that neurotensin and unknown substance(s) mediate NANC relaxation via SK channels in the jejunum of Wistar rats, and that neurotensin via SK channels and nitric oxide not via SK channels separately mediate the relaxation in the ileum.

Molecular properties of small-conductance Ca2+-activated K+ channels expressed in murine colonic smooth muscle

Small-conductance Ca2+-activated K+ (SK) channels are important participants in inhibitory neurotransmission in gastrointestinal smooth muscles. Three isoforms of an SK channel family were cloned from murine proximal colon smooth muscle. The transcripts encoding these subunits (SK1, SK2, and SK3) were detected in murine proximal colon and other peripheral tissues. The mRNA of each subunit was expressed at different levels in murine and canine colonic smooth muscles. The mRNA quantitative ratio of SK transcriptional expression in murine proximal colon is SK2 > SK3 > SK1; transcriptional expression of SK isoforms in canine proximal colon is minimal. SK3 immunohistochemical localization in murine small intestine (jejunum) and proximal colon showed immunoreactivity in circular and longitudinal muscularis. In transversely sectioned muscularis, staining was localized at the cell membrane in smooth muscle cells. Immunoreactivity was more intense in myenteric ganglia between longitudinal and circular muscularis and neuronal processes in circular and longitudinal muscularis. Transient expression of mSK1, mSK2, and mSK3 in COS cells resulted in Ca2+-activated voltage-independent channels. mSK1 is less sensitive to apamin compared with SK2 and showed intracellular Ca2+ sensitivity (10(-8) to 10(-6) M) in asymmetrical K+ (5/140 mM K+) gradients. Our results suggest that SK channel expression varies in colonic myocytes from different species and may contribute differentially to inhibitory junction potentials.

Ongoing nicotinic and non-nicotinic inputs to inhibitory neurons in the mouse colon

1. Intracellular microelectrodes were used to record spontaneous and evoked inhibitory junction potentials (IJP) from the circular muscle layer of the mid-distal region of the mouse isolated colon in the presence of nifedipine (1 micromol/L) and hyoscine (1 micromol/L). 2. The length of the tissue preparation (> 1 cm) or the presence of the mucosa had no effect on the frequency of spontaneous IJP. 3. Hexamethonium (500 micromol/L) reduced the frequency of spontaneous IJP to approximately 70% of the control frequency, whereas D-tubocurarine (280 micromol/L) reduced the frequency to approximately 17% of control. Apamin (250 nmol/L) abolished all spontaneous IJP activity. 4. The greater inhibition of spontaneous IJP in the presence of D-tubocurarine compared with hexamethonium is discussed as a possible 'apamin-like' effect. 5. Although electrically evoked IJP (single pulse at 15 V, 0.6 msec) were not significantly affected by hexamethonium, D-tubocurarine and apamin reduced the amplitude of evoked IJP to approximately 65 and 50% of control, respectively. 6. These results suggest that the properties of spontaneous IJP cannot be inferred by a study of evoked IJP alone.

Simultaneous intracellular recordings from longitudinal and circular muscle during the peristaltic reflex in guinea-pig distal colon

1. Simultaneous intracellular recordings were made from longitudinal muscle (LM) and circular muscle (CM) cells of guinea-pig distal colon during the peristaltic reflex. 2. Spontaneous rhythmical depolarizations with superimposed action potentials (mean amplitude: 19 +/- 2 mV) were regularly recorded from the LM (mean interval: 7 +/- 1 s). In contrast, in the CM layer, spontaneous action potentials occurred with an irregular frequency. Although spontaneous action potentials in LM were rarely correlated in time with those in CM, spontaneous inhibitory junction potentials (sIJPs) were found to occur synchronously in both muscles (5 out of 27 animals; 19 %). 3. Graded inflation of an intra-luminal balloon or mucosal stimulation oral to the recording electrodes elicited gradeable compound IJPs synchronously in both LM (mean amplitude: 6 +/- 1 mV) and CM (mean amplitude: 9 +/- 1 mV) (descending inhibitory reflex). Evoked IJPs were often followed by action potentials in both muscle layers. 4. Mucosal stimuli applied anal to the recording electrodes elicited compound excitatory junction potentials (EJPs) synchronously in both muscles layers that were often associated with the generation of action potentials. In the LM, evoked EJP amplitudes ranged from 3 mV (subthreshold) to 31 mV (including the action potential) and in the CM from 4 mV (subthreshold) to 44 mV (including the action potential). 5. Apamin (500 nM) reduced the evoked IJP in the CM by 55 % (from 11 +/- 2 to 5 +/- 1 mV), but caused no significant reduction in the LM layer (from 8 +/- 1 to 6 +/- 1 mV). Apamin-resistant IJPs in both muscle layers were likely to be due to nitric oxide, since they were abolished by L-NA (100 microM). 6. Atropine (1 microM) abolished the ascending excitatory reflex in both muscles. 7. Injection of neurobiotin into the LM and CM confirmed that simultaneous intracellular recordings were made from different muscle layers. 8. In conclusion, during the peristaltic reflex, the LM and CM layers receive synchronous inhibitory neuromuscular inputs during descending inhibition and synchronous excitatory neuromuscular inputs during ascending excitation. No evidence was found to support reciprocal innervation.

Nitric oxide and thiol reagent modulation of Ca2+-activated K+ (BKCa) channels in myocytes of the guinea-pig taenia caeci

The modulation of large conductance Ca2+-activated K+ (BKCa) channels by the nitric oxide (NO) donors S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) and agents which oxidize or reduce reactive thiol groups were compared in excised inside-out membrane patches of the guinea-pig taenia caeci. When the cytosolic side of excised patches was bathed in a physiological salt solution (PSS) containing 130 mM K+ and 15 nM Ca2+, few BKCa channel openings were recorded at potentials negative to 0 mV. However, the current amplitude and open probability (NPo) of these BKCa channels increased with patch depolarization. A plot of ln(NPo) against the membrane potential (V) fitted with a straight line revealed a voltage at half-maximal activation (V0.5) of 9.4 mV and a slope (K) indicating an e-fold increase in NPo with 12.9 mV depolarization. As the cytosolic Ca2+ was raised to 150 nM, V0.5 shifted 11.5 mV in the negative direction, with little change in K (13.1 mV). NOCys (10 microM) and SNP (100 microM) transiently increased NPo 16- and 3. 7-fold, respectively, after a delay of 2-5 min. This increase in NPo was associated with an increase in the number of BKCa channel openings evoked at positive potentials by ramped depolarizations (between -60 and +60 mV). Moreover, this NOCys-induced increase in NPo was still evident in the presence of 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), the specific blocker of soluble guanylyl cyclase. The sulfhydryl reducing agents dithiothreitol (DTT; 10 and 100 microM) and reduced glutathione (GSH; 1 mM) also significantly increased NPo (at 0 mV) 7- to 9-fold, as well as increasing the number of BKCa channel openings evoked during ramped depolarizations. Sulfhydryl oxidizing agents thimerosal (10 microM) and 4,4'-dithiodipyridine (4,4DTDP; 10 microM) and the thiol-specific alkylating agent N-ethylmaleimide (NEM; 1 mM) significantly decreased NPo (at 0 mV) to 40-50% of control values after 5-10 min. Ramped depolarizations to +100 mV evoked relatively few BKCa channel openings. The effects of thimerosal on NPo were readily reversed by DTT, while the effects of NOCys were prevented by NEM. It was concluded that both redox modulation and nitrothiosylation of cysteine groups on the cytosolic surface of the alpha subunit of the BKCa channel protein can alter channel gating.

Functional antagonism between nitric oxide and ATP in the motor responses of guinea-pig ileum

1. The interaction of nitric oxide and ATP in the non-adrenergic, non-cholinergic (NANC) motor responses and the presence of NADPH-diaphorase and quinacrine-positive myenteric neurones were studied on guinea-pig ileum using mechanographic, histochemical and quinacrine-fluorescence techniques. In the presence of phentolamine, propranolol and atropine, the non-precontracted longitudinally oriented organ bath preparations responded to sodium nitroprusside (1-100 microM) or ATP (5-50 microM) with tetrodotoxin (0.1 microM)-resistant relaxation or contraction, respectively. The effects of ATP were suramin (50 microM)- and apamin (5 microM)-sensitive. 2. The NANC motor responses elicited by electrical stimulation (0.8 ms, 1-20 Hz, 20 s) consisted of tetrodotoxin-sensitive relaxation phase followed by a phase of twitch-like and tonic contractions. 3. NG-nitro-L-arginine (L-NNA, 0.1-0.5 mM) inhibited or abolished the relaxation phase. L-arginine (0.5 mM), but not D-arginine (0.5 mM), restored the relaxation phase in L-NNA-pretreated preparations. The relaxation phase increased after ATP-induced desensitization of purinoceptors and in the presence of suramin (50 mciroM) but was abolished by apamin (5 microM). 4. The phase of contractions was enhanced by L-NNA (0.1-0.5 mM) and restored by L-arginine (0.5 mM). The twitch-like and tonic contractions were decreased during ATP-induced purinoceptor desensitization and in the presence of suramin (50 microLM). Apamin (5 microM) inhibited the tonic contractions. 5. The desensitization of purinoceptors by ATP did not change the L-NNA-induced inhibition of the relaxation phase but decreased the L-NNA-increased phase of contractions. L-NNA reduced the relaxation phase and increased the phase of contractions during purinoceptor desensitization. 6. We conclude that in the longitudinal muscle layer of the guinea-pig ileum, nitric oxide mediates the relaxation phase while ATP contributes via smooth muscle P2 purinoceptors to the phase of contractions suggesting a postjunctional functional antagonism between nitric oxide and ATP. The presence of NADPH-diaphorase- and quinacrine-positive neuronal cells and processes running to the muscle cells confirms a physiological role of nitric oxide and ATP in the ileal neurotransmission.

Cyclic GMP-associated apamin-sensitive nitrergic slow inhibitory junction potential in the hamster ileum

1. The mediators of non-adrenergic, non-cholinergic (NANC) inhibitory junction potentials (i.j.ps) in the circular smooth muscle cells of the hamster ileum were studied. 2. Electrical field stimulation (EFS; 0.5 ms duration, 15 V) of the intramural nerves with a train of five pulses at 20 Hz evoked a rapidly developing hyperpolarization (fast i.j.p.) followed by a sustained hyperpolarization (slow i.j.p.). 3. NG-nitro-L-arginine methyl ester (L-NAME; 50 - 200 microM) and NG-nitro-L-arginine (L-NNA; 50 - 200 microM), NO synthase inhibitors, inhibited or abolished the EFS-induced fast and slow NANC i.j.ps. The effects of these NO synthase inhibitors were reversed by L-arginine (5 mM) but not by D-arginine (5 mM). 4. Exogenously applied nitric oxide (NO; 1 - 100 microM) induced concentration-dependent hyperpolarizations. 5. Oxyhaemoglobin (5 - 50 microM), NO scavenger, inhibited only the slow i.j.p., and the NO-induced hyperpolarization. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxaline-1-one (ODQ; 10 microM) and cystamine (10 mM), guanylate cyclase inhibitors, inhibited only the slow i.j.p. Zaprinast (100 microM), a phosphodiesterase type V inhibitor, enhanced the amplitude and duration of the slow i.j.p. 7. Apamin (100 nM), a small conductance Ca2+-activated K+ channel blocker, inhibited only the slow i.j.p., and NO-induced hyperpolarization. A high concentration of 8-bromoguanosine 3':5'-cyclic monophosphate (8-bromo-cGMP; 1 mM)-induced membrane hyperpolarization which was blocked by apamin. 8. These results suggest that NO, or a related compound, may be the inhibitory transmitter underlying the apamin-sensitive NANC slow i.j.p. and cyclic GMP mediates the slow i. j.p. in the hamster ileum. It is also likely that NO, without involvement of guanylate cyclase is associated with the fast i.j.p.

Nitric oxide modulates cholinergic reflex pathways to the longitudinal and circular muscle in the isolated guinea-pig distal colon

1. The involvement of nitric oxide (NO) in enteric neural pathways underlying reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers activated by mucosal stimulation was examined in the isolated guinea-pig distal colon. 2. A segment of colon spanned two partitions (10 mm apart), which divided the organ bath into three chambers: a recording chamber where LM and CM tension was measured; a stimulation chamber where mucosal stimulation was applied; and a middle chamber separating them. 3. Brushing the mucosa anal and oral to the recording site evoked simultaneous oral contraction and anal relaxation of both the LM and CM. 4. N omega-nitro-L-argininel-NA; 100 microM) or N omega-nitro-L-arginine methyl ester (L-NAME; 100 microM) applied to the middle chamber or stimulation chamber decreased the oral contractile response of the LM and CM (by about 30-40 %), but increased the anal relaxation (> 600 %) and exposed an anal contraction (> 1000 % increase) of both muscles. The addition of L-NA to the recording chamber reduced the anal relaxation of the LM and CM and the anal contraction of the LM, but slightly increased the anal contraction of the CM. 5. S-Nitroso-N-acetylpenicillamine (SNAP; 10 microM), an NO donor, reversed the effects of L-NA in the middle or stimulation chambers. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), a soluble guanylate cyclase inhibitor, mimicked the effects of L-NAin the middle chamber or stimulation chamber, but these effects were not reversed by SNAP. 7. The oral contractile responses, and the anal relaxation and contractile responses of the LM and CM produced by L-NA in the stimulation or middle chambers, were blocked by hexamethonium (300 microM) in any chamber. Atropine (1 microM) in the recording chamber reduced the contractile responses of the LM and CM. 8. In conclusion, endogenous NO facilitates and depresses release of acetylcholine from interneurons in ascending and descending nervous pathways, respectively. These NO effects are mediated through soluble guanylate cyclase in cholinergic interneurons

Spontaneous and evoked inhibitory junction potentials in the circular muscle layer of mouse colon

Intracellular microelectrodes were used to record spontaneous and evoked inhibitory junction potentials (IJPs) from the circular muscle layer of an in vitro preparation of whole mouse colon. Membrane potential recordings were made from cells of the mid to distal region of colon at 36 +/- 1 degrees C in a modified Krebs' solution that contained atropine (1 microM) and nifedipine (1-2 microM). Spontaneously occurring hyperpolarisations of irregular amplitude and frequency (range: up to 20 mV and 2 Hz) were recorded that were resistant to N(G)-nitro-L-arginine (NOLA, 100 microM), but were abolished by tetrodotoxin (TTX, 1.6 microM) or apamin (250 nM). These were considered to be spontaneous IJPs as a consequence of activity in inhibitory motor neurons. Single electrical stimuli (0.6 ms, 15 V), elicited a fast IJP, whose time course could be superimposed on spontaneous IJPs of similar amplitude. The amplitude of evoked IJPs was not depressed by NOLA (100 microM). However, in NOLA (100 microM), further addition of apamin (250 nM) significantly depressed the amplitudes of the evoked IJPs by 44%. NOLA- and apamin-resistant evoked IJPs were abolished by TTX (1.6 microM). It is suggested, that in the circular muscle layer of mouse colon, NO does not mediate the fast hyperpolarisations associated with spontaneous or evoked IJPs. Apamin abolished spontaneous IJPs, but electrical stimuli evoked an IJP with apamin-sensitive and resistant components both of which were non-nitrergic in origin.

Effects of nitric oxide donors, S-nitroso-L-cysteine and sodium nitroprusside, on the whole-cell and single channel currents in single myocytes of the guinea-pig proximal colon

1. The nature of the membrane channels underlying the membrane conductance changes induced by the nitric oxide (NO) donors, S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) were investigated in single myocytes isolated from the circular muscle layer of the guinea-pig proximal colon, by use of standard whole-cell and single channel recording techniques. 2. Under voltage clamp, depolarizing steps from -60 mV elicited a rapidly-developing, little-inactivating outward K+ current (IK) at potentials positive to -40 mV (at 20-25 degrees C). The steady-state level (ISS) of this K current increased in amplitude as the step potential was made to more positive potentials. If the depolarizing steps were made from a holding potential of -80 mV an additional rapidly activating and inactivating outward K+ current was also elicited, superimposed on IK. 3. At 20-25 degrees C, NOCys (2.5 microM), SNP (100 microM) and 8-bromo-cyclic GMP (500 microM) increased the amplitude of ISS of IK elicited from a holding potential of -60 mV. In contrast, NOCys (2-5 microM) had little effect on ISS at 35 degrees C. Higher concentrations (> or = 5 microM at 20-25 degrees C and > or = 10 microM at 35 degrees C) of NOCys decreased the peak amplitude (I[Peak]) and ISS of IK in a concentration-dependent manner. This blockade of IK with NOCys was always associated with an increase of the holding current (IHold), due to the activation of a membrane conductance with a reversal potential between 0 and + 30 mV and which was reduced approximately 50% upon the addition of Cd2+ (1 mM). 4. NOCys (2.5 to 10 microM) or SNP (100 microM) increased the activity of large conductance Ca2+-activated (BK) K' channels in both cell-attached and excised inside-out patches, bathed in either a symmetrical high K+ (130 mM) or an asymmetrically K+ (6 mMout: 130 mMin) physiological saline. Increases in BK channel activity in NOCys (10 microM) or SNP (100 microM) were associated with an increase in the probability of BK channel opening (N.Po), and with a negative shift of the plots of ln(N.Po) against the patch potential, with little change in the slopes of these plots. In cell-attached patches, the increase in N.Po with NOCys was often associated with a decrease in the BK single channel conductance. 5. In both cell-attached and excised patches, NOCys (2.5 to 10 microM) also activated an additional population of channels which allowed inward current flow at potentials positive to EK. In excised inside-out patches bathed in asymmetrical K+ physiological saline, these single channel currents were 2-3 pA in amplitude at -30 mV and reversed in direction near + 10 mV, even if the NaCl (126 mM) concentration in the pipette solution had been replaced with an equimolar concentration of Na gluconate. 6. Under current clamp, NOCys (2.5 microM) and SNP (100 microM) had variable effects on the membrane potential of colonic myocytes, inducing either a small membrane hyperpolarization of <5 mV, or a slowly-developing membrane depolarization of about 5 mV. In contrast, NOCys (5 microM) produced a transient membrane hyperpolarization which was followed by a large depolarization of the membrane potential to positive potentials. The electrotonic potentials elicited in response to an injection of constant hyperpolarizing current (10 pA for 400 ms) were little changed during the NOCys (5 PM)-induced membrane hyperpolarization, but significantly reduced (to 61% of control) during the periods of membrane depolarization. 7. It was concluded that NOCys and SNP, directly increased the number of active BK channels in the membrane of colonic myocytes which leads to a small rapidly oscillating membrane hyperpolarization. The following rebound depolarization in NOCys arises from both the direct opening of a population of cationic channels and the blockade of voltage- and Ca-activated K+ conductances. Finally, the apamin-sensitive K+channels underlying the initial transient hyperpolarization recorded in the intact proximal colon, in response to nerve-released or directly-applied NO, have yet to be identified at the single channel or whole-cell current level.

Effects of nitric oxide (NO) and NO donors on the membrane conductance of circular smooth muscle cells of the guinea-pig proximal colon

1. The membrane conductance changes underlying the membrane hyperpolarizations induced by nitric oxide (NO), S-nitroso-L-cysteine (NC) and sodium nitroprusside (SNP) were investigated in the circular smooth muscle cells of the guinea-pig proximal colon, by use of standard intracellular microelectrode recording techniques. 2. NO (1%), NC (2.5-25 microM) and SNP (1-1000 microM) induced membrane hyperpolarization in a concentration-dependent manner, the hyperpolarizations to NO and NC developing more rapidly than those to SNP. The slower-developing responses to SNP were mimicked by the membrane permeable analogue of guanosine 3':5' cyclic-monophosphate (cyclic GMP), 8-bromo-cyclic GMP (500 microM), and by isoprenaline (10 microM). 3. The hyperpolarizations to NC and SNP were reduced in a low Ca2+ (0.25 mM) saline and upon the addition of haemoglobin (20 microM), but were not effected by NG-nitro-L-arginine (L-NOARG) (100 microM) or omega-conotoxin GVIA (100 nM). the hyperpolarizations to SNP were also significantly reduced by methylene blue (50 microM). 4. Apamin (250 nM) depolarized the membrane potential approximately 10 mV and reduced the initial transient component of the hyperpolarization to NO (1%) and NC (25 microM), but had no effects on the hyperpolarizations to SNP and cyclic GMP. Tetraethylammonium (TEA) (5-15 mM), had little effect on the membrane responses to NO(1%), NC(2.5-25 microM), SNP(100(-1000) microM) or cyclic GMP(500 microM). However, TEA (5-15 mM) reduced the membrane hyperpolarizations to SNP (10 microM) and isoprenaline (10 microM) in a concentration-dependent manner. The hyperpolarization to isoprenaline (10 microM) remaining in the presence of 15 mM TEA was blocked by ouabain (10 microM). 5. The amplitude of electronic potentials (1 s duration) elicited during NO donor hyperpolarizations were little changed or only slightly reduced (5-25%). However, the amplitude of the electrotonic potentials elicited during maintained electrically-induced hyperpolarizations of similar amplitude were significantly increased (30-150%), suggesting that the non-linear membrane properties of the proximal colon partially mask an increase in membrane conductance elicited during the NO donor hyperpolarizations. 6. Membrane hyperpolarization in the presence of an NO donor, 8-bromo-cyclic GMP, isoprenaline, or upon application of a maintained hyperpolarizing electrical current, often evoked oscillations of the membrane potential. These oscillations were prevented by Cs+ (1 mM). 7. These results indicate that NO and NC hyperpolarize the circular muscle of the proximal colon by activating at least two TEA-resistant membrane K+ conductances, one of which is sensitive to apamin blockade. The K+ conductance increases activated by SNP or 8-bromo-cyclic GMP were little effected by apamin, perhaps suggesting a common mechanism. In contrast, the hyperpolarization to isoprenaline appears to involve the activation of TEA-sensitive Ca2(+)-activated K+ ('BK') channels, as well as a Na:K ATPase. Finally, the 'background' membrane conductance of the circular muscle cells of the proximal colon decreased upon membrane hyperpolarization to reveal oscillations of the membrane potential which may well represent 'pacemaker' or 'slow wave' activity.