Properties of the inhibitory junction potential in smooth muscle of the guinea-pig gastric fundus

Mechanical and electrical responses modulated by excitation of inhibitory nerves during stimulation with high-potassium solutions in circular smooth muscle of the rabbit rectum

The properties of the mechanical responses produced by solutions containing high concentrations of potassium ion (high-K solution, [K(+)](o) = 9-27 mM) were investigated in circular smooth muscle preparations isolated from the rabbit rectum. Isometric recording of mechanical responses of the muscle revealed spontaneous contractions, which successively decreased and finally disappeared in most preparations. Stimulation of the smooth muscle with high-K solutions elicited an increase in both amplitude and frequency of twitch contractions (sustained component), with about a 2 min delay in the beginning (initial inhibition), and a transient large contraction shortly after the cessation of stimulation (after contraction). Transmural nerve stimulation (TNS) with electrical pulses for 1 min at 1 Hz frequency produced a sustained inhibition, but a transient contraction followed after termination of TNS. In the presence of tetrodotoxin (TTX), the TNS-induced responses were abolished, while a high-K solution elicited increased twitch contractions with a short delay and abolished the after contraction. Suramin produced effects similar to TTX on the responses produced by high-K solutions or TNS, but this was not the case for atropine, guanethidine or N(omega)-nitro-L-arginine (L-NA). Recording membrane potentials with microelectrodes revealed that TNS evoked an inhibitory junction potential (i.j.p.) which was non-adrenergic, non-cholinergic and non-nitrergic in nature. High-K solutions elicited a tri-phasic change in the membrane potential; an initial hyperpolarization, followed by a sustained depolarization and finally a transient depolarization on cessation of high-K stimulation. TTX or suramin inhibited the i.j.p.s and altered the tri-phasic change in the membrane potential produced by a high-K solution to a mono-phasic depolarization. No significant modulation of electrical responses of the membrane induced by TNS or high-K solution was elicited by atropine, guanethidine or L-NA. The results indicated that the circular smooth muscle of the rabbit rectum is innervated by inhibitory nerves, and that stimulation with high-K solutions caused inhibitory neuronal modulation of both electrical and mechanical responses of the smooth muscle, in a suramin-sensitive way.

Regulation of basal tone, relaxation and contraction of the lower oesophageal sphincter. Relevance to drug discovery for oesophageal disorders

The lower oesophageal sphincter (LOS) is a specialized region of the oesophageal circular smooth muscle that allows the passage of a swallowed bolus to the stomach and prevents the reflux of gastric contents into the oesophagus. The anatomical arrangement of the LOS includes semicircular clasp fibres adjacent to the lesser gastric curvature and sling fibres following the greater gastric curvature. Such anatomical arrangement together with an asymmetric intrinsic innervation and distinct proportion of neurotransmitters in both regions produces an asymmetric pressure profile. The LOS tone is myogenic in origin and depends on smooth muscle properties that lead to opening of L-type Ca(2+) channels; however it can be modulated by enteric motor neurons, the parasympathetic and sympathetic extrinsic nervous system and several neurohumoral substances. Nitric oxide synthesized by neuronal NOS is the main inhibitory neurotransmitter involved in LOS relaxation. Different putative neurotransmitters have been proposed to play a role together with NO. So far, only ATP or related purines have shown to be co-transmitters with NO. Acetylcholine and tachykinins are involved in the LOS contraction acting through acetylcholine M(3) and tachykinin NK(2) receptors. Nitric oxide can also be involved in the regulation of LOS contraction. The understanding of the mechanisms that originate and modulate LOS tone, relaxation and contraction and the characterization of neurotransmitters and receptors involved in LOS function are important to develop new pharmacological tools to treat primary oesophageal motor disorders and gastro-oesophageal reflux disease.

Impact of caveolin-1 knockout on NANC relaxation in circular muscles of the mouse small intestine compared with longitudinal muscles

Recently, we showed that caveolin-1 (cav1) knockout mice (Cav1(-/-) mice) have impaired nitric oxide (NO) function in the longitudinal muscle (LM) layer of the small intestine. The defect was a reduced responsiveness of the muscles to NO compensated by an increase in the function of apamin-sensitive, nonadrenergic, noncholinergic (NANC) mediators. In the present study, we examined similarly the effects of cav1 knockout on the relaxation in circular muscle (CM) of the mouse small intestine. CM of Cav1(-/-) mice also showed defective NO function, but less than in LM, as well as more activation of apamin-sensitive NANC mediators. CM of Cav1(-/-) mice, like LM, lacked cav1 but retained small amounts of cav3 and caveolae in the outer CM layer. In addition, we also examined the effects of a soluble guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinazolin-1-one (ODQ), on electric field stimulation (EFS)-mediated relaxation in both LM and CM. ODQ had an effect similar to the block of NO synthesis. Moreover, we compared the actions of two NO donors in the LM and CM of control and Cav1(-/-) mice. Similar to LM, CM of Cav1(-/-) mice showed a reduced responsiveness to the NO donors sodium nitroprusside and S-nitroso-N-acetyl penicillamine. However, both ODQ and apamin blocked the inhibitory effects of the NO donors in LM, whereas apamin had no effect in CM. In conclusion, cav1 knockout affects NO function in both LM and CM, but its effects in CM differ significantly.

Caveolin-1 gene knockout impairs nitrergic function in mouse small intestine

Caveolin-1 is a plasma membrane-associated protein that is responsible for caveolae formation. It plays an important role in the regulation of the function of different signaling molecules, among which are the different isoforms of nitric oxide synthase (NOS). Nitric oxide (NO) is known to be an important inhibitory mediator in the mouse gut. Caveolin-1 knockout mice (Cav1(-/-)) were used to examine the effect of caveolin-1 absence on the NO function in the mouse small intestine (ileum and jejunum) compared to their genetic controls and BALB/c controls. Immunohistochemical staining showed loss of caveolin-1 and NOS in the jejunal smooth muscles and myenteric plexus interstitial cells of Cajal (ICC) of Cav1(-/-) mice; however, nNOS immunoreactive nerves were still present in myenteric ganglia. Under nonadrenergic noncholinergic (NANC) conditions, small intestinal tissues from Cav1(-/-) mice relaxed to electrical field stimulation (EFS), as did tissues from control mice. Relaxation of tissues from control mice was markedly reduced by N-omega-nitro-L-arginine (10(-4) M), but relaxation of Cav1(-/-) animals was affected much less. Also, Cav1(-/-) mice tissues showed reduced relaxation responses to sodium nitroprusside (100 microM) compared to controls; yet there were no significant differences in the relaxation responses to 8-bromoguanosine-3': 5'-cyclic monophosphate (100 microM). Apamin (10(-6) M) significantly reduced relaxations to EFS in NANC conditions in Cav1(-/-) mice, but not in controls. The data from this study suggest that caveolin-1 gene knockout causes alterations in the smooth muscles and the ICC, leading to an impaired NO function in the mouse small intestine that could possibly be compensated by apamin-sensitive inhibitory mediators.

Gastrointestinal Function Regulation by Nitrergic Efferent Nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

Effects of endogenous and exogenous nitric oxide on electrical responses of circular smooth muscle isolated from the guinea-pig stomach antrum

The effects of endogenous and exogenous nitric oxide (NO) on electrical activity were investigated in circular smooth muscle preparations isolated from the guinea-pig stomach antrum. The actions of endogenous NO were evaluated from the effects of inhibition of NO synthesis by N(omega)-nitro-L-arginine (nitroarginine), while those of exogenous NO were assessed from the effects of SIN-1, an NO donor. Antral circular smooth muscle generated slow potentials periodically at a frequency of about 1 cycle per min (cpm), and unitary potentials were also generated in a random fashion in the interval between slow potentials. Application of nitroarginine (10(-5) M) increased the frequency of slow potentials, with no significant alteration of the resting membrane potential and amplitude of slow potentials. Frequency analysis of unitary potentials revealed that nitroarginine also increased the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials evoked by depolarizing pulses was about 10 s, but was decreased to 6 s by nitroarginine. In the presence of nitroarginine, SIN-1 (10(-9)-10(-7) M) reduced the amplitude and frequency of slow potentials: low concentrations (<10(-8) M) reduced only the frequency of slow potentials, while higher concentrations (10(-8)-10(-7) M) reduced both the amplitude and frequency of slow potentials, in a concentration-dependent manner, before abolishing the slow potentials. The power spectrum of the unitary potentials indicated that SIN-1 (>10(-8) M) reduced the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials was increased again to about 10 s by SIN-1. Thus, the excitatory effects of nitroarginine could be antagonized by SIN-1, suggesting that the inhibitory effects of endogenous NO are comparable to those of exogenous NO produced by SIN-1. The results also suggested that the effects of NO on smooth muscle are insignificant and NO selectively inhibits the activity of intramuscular interstitial cells of Cajal (ICC-IM).

Participation of ATP in nonadrenergic, noncholinergic relaxation of longitudinal muscle of wistar rat jejunum

A role of ATP in nonadrenergic, noncholinergic (NANC) relaxations was examined in the Wistar rat jejunum. Electrical field stimulation (EFS) induced NANC relaxation of longitudinal muscle of the jejunal segments in a frequency-dependent manner. A purinoceptor antagonist, adenosine 3'-phosphate 5'-phosphosulfate (A3P5PS, 100 muM) inhibited the relaxation: relaxations induced by EFS at lower or higher frequencies were either completely or partially inhibited, respectively. After the jejunal segments had been desensitized to ATP, the relaxations were decreased to the same extent as those inhibited by A3P5PS. An inhibitor of small conductance Ca(2+)-activated K(+) channels (SK channels), apamin (100 nM), completely inhibited EFS-induced relaxations. Treatment of the segments with an inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin (1 muM), significantly inhibited the relaxations. The exogenous ATP-induced relaxation of longitudinal muscle occurred with a concomitant decrease in intracellular Ca(2+) levels. Apamin and thapsigargin abolished these ATP-induced responses. A3P5PS significantly inhibited the inhibitory junction potentials which were induced in the longitudinal muscle cells. In addition, apamin significantly inhibited the hyperpolarization that was induced by exogenous ATP in the cells. These findings in the Wistar rat jejunum suggest that ATP participates in the NANC relaxation via activation of SK channels induced by Ca(2+) ions that are released from the thapsigargin-sensitive store site.

Role of protein kinase C in the excitatory action of cholinergic nerve stimulation on spontaneous activity of circular smooth muscle isolated from the guinea-pig stomach antrum

Following inhibition of NO production with nitroarginine, circular muscle isolated from the guinea-pig gastric antrum generated periodic slow potentials and unitary potentials. Transmural nerve stimulation (TNS) during the interval between slow potentials evoked an apamin-sensitive inhibitory junction potential (IJP) followed by an atropine-sensitive depolarization; the latter was either a transient depolarization with enhanced generation of unitary potentials or a slow potential. After inhibition of unitary potentials and slow potentials with 1 mM caffeine, TNS evoked an IJP and subsequent cholinergic depolarization, the latter developing slowly and lasting for about 10 s. TNS was unable to elicit a slow potential until a certain period of time had elapsed following the cessation of a slow potential. The period during which TNS could not evoke slow potentials (termed the high-threshold period) was about 10 s, and this period was increased by chelerythrine and decreased by phorbol esters. It is concluded that cholinergic nerve-mediated excitation of gastric muscle involves the activation of protein kinase C (PKC), and that the high-threshold period, during which the generation of slow potentials by TNS is inhibited, may be a consequence of reduced activity of PKC.

Muscarinic modulation of nitrergic neurotransmission in guinea-pig gastric fundus

Muscarinic receptor activation by (4-Hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride (McN-A-343) was investigated both on NADPH-d staining and on electrically induced responses in guinea-pig gastric fundus. McN-A-343 (10 micromol L(-1)) significantly increased the optical density of NADPH-d positive neurones, while blockade of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NA) decreased it, suggesting facilitation of nitric oxide (NO) production. Electrical field stimulation (EFS; 2 Hz, 0.2 ms, supramaximal current intensity, 10 s train duration) elicited on-contraction followed by off-relaxation in the circular muscle strips. McN-A-343 (10 micromol L(-1)) transformed the EFS-evoked response from on-contraction into on-relaxation, which was neurogenic, tetrodotoxin-sensitive and hexamethonium-resistant. L-NA partly reduced the EFS-evoked relaxation, revealing two components: a nitrergic and a non-nitrergic one. The effect of McN-A-343 on the amplitude of the EFS-evoked relaxation was not changed by the M(3) receptor antagonist para-fluoro-hexahydro-sila-difenidol hydrochloride, but was significantly enhanced by M(1) receptor blockade with telenzepine. In the presence of telenzepine, the L-NA-dependent nitrergic component of the EFS-induced relaxation predominates. We suggest that cholinergic receptor activation has a dual effect on nitrergic neurotransmission: (i) stimulation of NOS by muscarinic receptor(s) different from M(1) and M(3) subtype, (ii) prejunctional inhibition of NO-mediated relaxation via M(1) receptors. In addition, M(1) receptors may facilitate the non-nitrergic relaxation.

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.

Involvement of intramuscular interstitial cells in nitrergic inhibition in the mouse gastric antrum

Intracellular recordings were made from isolated bundles of the circular muscle layer of mouse gastric antrum and the responses evoked by stimulating intrinsic nerve fibres were examined. Transmural nerve stimulation evoked a fast inhibitory junction potential (fast-IJP) which was followed initially by a smaller amplitude long lasting inhibitory junction potential (slow-IJP) and a period of excitation. The excitatory component of the response was abolished by atropine, suggesting that it resulted from the release of acetylcholine and activation of muscarinic receptors. Fast-IJPs were selectively reduced in amplitude by apamin and slow-IJPs were abolished by N(omega)-nitro-L-arginine. Slow-IJPs were associated with a drop in membrane noise, suggesting that inhibition resulted from a reduced discharge of unitary potentials by intramuscular interstitial cells of Cajal (ICC(IM)). The chloride channel blocker, anthracene-9-carboxylic acid, reduced the discharge of membrane noise in a manner similar to that detected during the slow-IJP. When recordings were made from the antrum of W/W(V) mice, which lack ICC(IM), the cholinergic and nitrergic components were absent, with only fast-IJPs being detected. The observations suggest that neurally released nitric oxide selectively targets ICC(IM) causing a hyperpolarization by suppressing the discharge of unitary potentials.

Neurotransmitters involved in the fast inhibitory junction potentials in mouse distal colon

We investigated, in murine colon circular muscle, the role of adenosine 5'-triphosphate (ATP) and pituitary adenylate cyclase activating peptide (PACAP) as inhibitory neurotransmitters of the fast component of nerve-evoked inhibitory junction potential (fast IJP). Fast IJP was antagonised by apamin or suramin, abolished by desensitisation with the P2Y receptor agonist, adenosine 5'-O-2-thiodiphosphate (ADPbetaS), unaffected by desensitisation with P2X receptor agonist, alpha,beta-methylene ATP (alpha,beta-meATP), and reduced by PACAP-(6-38), a PACAP receptor antagonist. ATP induced membrane hyperpolarization resistant to tetrodotoxin, N(omega)-nitro-L-arginine methyl ester (L-NAME) or PACAP-(6-38), but antagonised by apamin, suramin, P2X and P2Y receptor desensitisation. PACAP-(1-27) caused membrane hyperpolarization antagonised by PACAP-(6-38), apamin and P2Y receptor desensitisation, reduced by tetrodotoxin, but not affected by L-NAME and by P2X receptor desensitisation. Therefore, in murine colon circular muscle, an ATP-like endogenous P2Y purinoceptor ligand is the final nonadrenergic, noncholinergic (NANC) inhibitory mediator involved in the generation of fast IJP. A neuromodulator role of PACAP in the inhibitory pathway is supposed.

Nitric oxide inhibits smooth muscle responses evoked by cholinergic nerve stimulation in the guinea pig gastric fundus

In circular smooth muscle tissues of the guinea pig gastric fundus, transmural nerve stimulation (TNS) evoked an atropine-sensitive cholinergic excitatory junction potential (e.j.p.) and, after inhibiting the e.j.p. with atropine, an apamin-sensitive nonadrenergic noncholinergic (NANC) inhibitory junction potential (i.j.p.). The amplitude of e.j.p.s was similar when the frequency of TNS was low (<0.5 Hz), but it decreased successively (depression phenomenon) when the frequency was high (>1 Hz). The depression phenomenon was attenuated after inhibiting the production of nitric oxide (NO) with N(omega)-nitro-L-arginine (NOLA), but was not altered by inhibiting the i.j.p. with apamin. The e.j.p.s were increased in amplitude by the inhibition of cholinesterase activity, but they were decreased by NO produced from SNP with no alteration of their depression phenomenon. Isometric twitch contractions were depressed during high-frequency TNS. NOLA caused an increase in the amplitude of twitch contractions and the attenuation of their depression that changed the transient contraction produced by high-frequency TNS (1 Hz) to a tetanic one. SNP reduced the amplitude of twitch contractions, with no alteration of the depression phenomena. Contractions produced by low concentrations of acetylcholine, but not by high concentrations, were attenuated by SNP, with no alteration of the membrane depolarization. The results suggest that NO produced during TNS has inhibitory actions on cholinergic transmission; the depression of e.j.p.s is mainly prejunctional events, and the depression of mechanical responses is mainly postjunctional events.

Inhibitory actions of indomethacin on electrical and mechanical responses produced by nerve stimulation in circular smooth muscle of the guinea-pig gastric fundus

The effects of indomethacin on electrical and mechanical responses produced by transmural nerve stimulation (TNS) were investigated in isolated circular smooth muscle of the guinea-pig gastric fundus. TNS evoked a cholinergic excitatory junction potential (e.j.p.). The e.j.p.s were inhibited by 1-10 microM indomethacin, in a concentration-dependent manner, with no marked alteration of the resting membrane potential. Exogenously applied acetylcholine caused a depolarization of the membrane that was not altered by indomethacin. TNS evoked a cholinergic twitch contraction at low frequencies (0.1 Hz). A train of TNS's at high frequency (1 Hz) produced a transient contraction with a subsequent sustained relaxation. Indomethacin reduced the resting tension and inhibited these TNS-induced contractions. Application of Nomega-nitro-L-arginine (NOLA), an inhibitor of nitric oxide (NO) synthesis, increased the amplitude of twitch contractions, and altered transient contractions to tetanic contractions during TNS at a frequency of 1 Hz, also with an increased amplitude. In the presence of NOLA, indomethacin (5 microM) again reduced the resting tension and inhibited TNS-induced contractions. This inhibition was greater for twitch contractions than for tetanic contractions. Nifedipine reduced the TNS-induced contractions, while addition of indomethacin further reduced the amplitude of contractions. Contractions produced by low concentrations of acetylcholine (0.1 microM) were inhibited by indomethacin, while those produced by 1 microM were not. These results indicate that the inhibitory actions of indomethacin on TNS-induced contractions do not involve enhanced production of NO or selective inhibition of voltage-gated Ca-channels. Prejunctional autoregulatory mechanisms may also not be altered by indomethacin. As indomethacin inhibits the enzyme cyclooxygenase, it is speculated that endogenously produced prostaglandins exert excitatory actions on gastric smooth muscle, and act mainly postjunctionally to facilitate spontaneous and neurogenic electrical and mechanical activity.

Mechanical responses evoked by nerve stimulation in gastric muscles of mouse lacking inositol trisphosphate receptor

Alteration of mechanical responses elicited by transmural nerve stimulation (TNS) was investigated in pylorus muscle of stomach isolated from mutant mice lacking expression of IP, type-1 receptor. In wild and mutant mice. TNS inhibited spontaneous contractions and generated an off-response at the cessation. The effects of inhibitors of neurotransmission revealed that in wild mice, acetylcholine and nitric oxide were involved as excitatory and inhibitory mediators, respectively. In mutant mice, a lack of nitroxidergic component with associated attenuation of cholinergic transmission was found. The off-response was inhibited by apamin in both mice. In mutant mice, spantide-sensitive excitatory response appeared in the presence of apamin. Acetylcholine and substance P enhanced while noradrenaline and sodium nitroprusside inhibited spontaneous contractions, in both wild and mutant mice; the actions were weaker in mutant mice than in wild mice for any agonists. The results indicate that pylorus smooth muscles receive cholinergic excitatory and nitroxidergic and non-adrenergic non-cholinergic inhibitory projections, and a lack of IP, type-1 receptor results in an impairment of cholinergic and nitroxidergic components, with no alteration of non-adrenergic non-cholinergic inhibitory projections. In addition, the mutation induces a substance P projection which is not detected in wild mice.

Voltage dependency of the frequency of slow waves in antrum smooth muscle of the guinea-pig stomach

The effects of membrane depolarization on the frequency of spontaneous activities were investigated in circular smooth muscle of the guinea-pig antrum attached with (intact tissue) or without longitudinal muscles (circular tissue). Both types of tissue were spontaneously active; the intact tissues generated slow wave and circular tissues generated regenerative potential. The latter but not the former was abolished by caffeine. Increasing K(+) concentrations depolarized the membrane and reduced the amplitude and interval between spontaneous activities in both tissues; the amplitude was reduced linearly with depolarization and disappeared at about -35 mV; the interval was reduced successively with depolarization and reached a stable value (about 8 s) at about -45 mV. The depolarization and reduction in amplitude and interval of spontaneous activities induced by high K(+) solution were not altered by atropine, nitroarginine, or apamin in either tissue, suggesting that these changes did not involve the effects of neurotransmitters. The depolarization of the membrane by electrical stimulation also reduced the amplitude and interval of spontaneous activities in both tissues, in a potential-dependent way. The absolute refractory period for generation of the evoked regenerative potential was about 8 s, and the relative refractory period was 8--12 s. The results indicate that the frequency of slow waves increases with a depolarization of the membrane up to -45 mV, irrespective of the presence of caffeine-insensitive components. A depolarization of the membrane above -45 mV does not further increase the frequency of slow waves, possibly because of the refractory period for the generation of slow waves.

The P(2)-purinoceptor antagonist suramin is a competitive antagonist at vasoactive intestinal peptide receptors in the rat gastric fundus

The P(2)-purinoceptor antagonist, suramin, was used to investigate the possible involvement of adenosine 5'-triphosphate (ATP) in the inhibitory non-adrenergic non-cholinergic (NANC) innervation of the rat gastric fundus. ATP (1-30 microM) produced biphasic responses consisting of concentration-dependent relaxations followed by concentration-dependent contractions. Suramin (200 microM) significantly reduced relaxations and abolished contractions to ATP. Under NANC conditions, electrical field stimulation (EFS) induced frequency-dependent relaxations. Suramin (200 microM) and the peptidase alpha-chymotrypsin (1 u ml(-1)) had the same effects on EFS-induced relaxations: their duration was reduced, but their magnitude was unaffected. Cumulative relaxations to vasoactive intestinal peptide (VIP; 0.1-100 nM), and to the VIP analogue pituitary adenylate cyclase activating peptide 1-27 (PACAP; 0.2-100 nM), were almost completely abolished by alpha-chymotrypsin (1 u ml(-1)), and were inhibited by suramin (3-200 microM) in an apparently competitive manner. Schild plot analysis indicated that suramin had pA(2) values of 5.1+/-0.2 (Hill slope=0.9+/-0.2) and 5.6+/-0.1 (Hill slope=1.0+/-0.1), against VIP and PACAP, respectively. Concentration-dependent relaxations to nitric oxide (1-30 microM) and cumulative relaxations to isoprenaline (0.1-300 nM) were not affected by suramin (200 microM). No conclusions can be made regarding the possible involvement of ATP in EFS-induced NANC relaxations. The results suggest that suramin acts as a competitive antagonist at VIP receptors in the rat gastric fundus.

Properties of gastric smooth muscles obtained from mice which lack inositol trisphosphate receptor

Membrane potential recordings, made from the circular smooth muscle layer of the gastric antrum taken from mutant mice which lacked the inositol trisphosphate (InsP3) type 1 receptor, were compared with those obtained from the stomach of control (wild-type) mice. Immunostaining of gastric muscles indicated that the distribution and form of c-kit positive cells were similar in wild-type and mutant mice. Smooth muscles from wild-type mice generated slow waves that in turn initiated spike potentials, while those from mutant mice were either quiescent or generated irregular bursts of spike potentials. In the presence of nifedipine, slow waves with reduced amplitude were generated in wild-type mice, while all electrical activity was abolished in mutant mice. Acetylcholine depolarized and sodium nitroprusside hyperpolarized the membrane in muscles from both types of mice, being more effective in wild-type mice. Noradrenaline produced similar hyperpolarizations in both types of mice. Transmural nerve stimulation evoked inhibitory junction potentials (IJPs) in both wild-type and mutant mice. In wild-type mice, the IJPs were reduced in amplitude by nitroarginine and converted to a cholinergic excitatory junction potential (EJP) by apamin. In mutant mice, the IJPs were unaffected by nitroarginine or atropine but were abolished by apamin. It is concluded that in antral smooth muscle, the expression of InsP3 type 1 receptors may be causally related to the generation of slow waves but not to the generation of action potentials. A lack of InsP3 receptors attenuates cholinergic excitatory and nitrergic inhibitory responses but does not alter the response to noradrenaline.

Neuronal control of the gastric sling muscle of the guinea pig

The gastric sling (oblique) muscle (GSM), located close to the lower esophageal sphincter (LES), is involved in gastric motor function and may cooperate with the LES in controlling propulsion between the esophagus and stomach. Neuronal pathways and transmission to the GSM were investigated in isolated esophagus-stomach preparations by using intracellular recording with the focal electrical stimulation and neuroanatomical tracing method. Focal stimulation on the GSM evoked inhibitory junction potentials (IJPs) that were reduced to 45% by 100 microM N-nitro-L-arginine and subsequently blocked by 0.5 microM apamin, thereby unmasking excitatory junction potentials (EJPs), which were abolished by 1 microM hyoscine. Vagal and esophageal stimulation evoked IJPs that were blocked by 100 microM hexamethonium. Vagal stimulation also evoked EJPs after blockade of IJPs. Application of 1,1'-didodecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate to the GSM labeled muscle motor neurons located in the stomach mainly close to the GSM, with a few neurons (2%) in the esophagus. The majority (79%) of labeled neurons were immunoreactive for choline acetyltransferase and, hence, excitatory motor neurons. Inhibitory motor neurons (nitric oxide synthase immunoreactive; 15%) were clustered in the midline near the gastroesophageal region. These results demonstrate that the GSM is innervated primarily by gastric excitatory and inhibitory motor neurons and some esophageal neurons. Both excitatory (acetylcholine) and inhibitory (nitric oxide and apamin-sensitive component) transmission can be activated via vagal-enteric pathways.

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.

Properties of inhibitory junctional transmission in smooth muscle of the guinea pig lower esophageal sphincter

Inhibitory neurotransmission in guinea pig lower esophageal sphincter (LES) muscles was investigated by using electrophysiological methods. Transmural nerve stimulation (TNS) initiated an inhibitory junction potential (i.j.p.); the amplitude increased 35% by atropine (10(-6) M) and converted to a muscarinic excitatory junction potential (e.j.p.) by apamin (10(-7) M) plus Nomega-nitro-L-arginine (L-NNA, 10(-5) M). In atropinized tissue, the i.j.p. amplitude was reduced 58% by guanethidine (5 x 10(-6) M), 41% by L-NNA (10(-5) M), 57% by suramin (10(-4) M), and it was abolished by apamin (10(-7) M), suggesting that this potential was produced by ATP and nitric oxide (NO) released from adrenergic and nitrergic nerves, respectively, through the activation of Ca2+-sensitive K+ channels. Hyperpolarizations produced by ATP and NO were inhibited by apamin. The i.j.p. amplitude was reduced after desensitizing the membrane with ATP. In atropinized tissue, TNS produced a relaxation that was reduced 15% by guanethidine (5 x 10(-6) M), 50% by L-NNA (10(-5) M), and 30% by apamin (10(-7) M). Thus the LES receives cholinergic excitatory and adrenergic and nitrergic inhibitory innervations; the latter two components contribute evenly to the i.j.p. generation. The relaxation is mainly produced by NO in a membrane potential-independent way.

Effects of suramin on electrical and mechanical activities in antrum smooth muscle of the guinea-pig stomach

The effects of suramin on electrical and mechanical responses produced by adenosine triphosphate (ATP), acetylcholine and transmural nerve stimulation were observed in antrum smooth muscle isolated from the guinea-pig stomach. Suramin (>10(-6) M) inhibited the non-adrenergic non-cholinergic inhibitory junction potential, with no alteration of the resting membrane potential, slow wave and the ATP-induced responses (hyperpolarization and inhibition of slow waves). The amplitude, but not the frequency, of spontaneous rhythmic contraction was inhibited by suramin (>10(-5) M), with no alteration of electrical responses of the membrane. Transmural nerve stimulation elicited cholinergic excitatory and non-adrenergic non-cholinergic inhibitory responses on the spontaneous contraction, and suramin inhibited only the latter. Suramin did not alter the ATP-induced inhibition of spontaneous contraction. The contractions produced by low concentrations (<10(-7) M), but not high concentrations (10(-6) - 10(-5) M), of acetylcholine were inhibited by suramin. It is concluded that in smooth muscle of the guinea-pig antrum, suramin inhibits contractions produced spontaneously and by low concentrations of acetylcholine, with no relation to the electrical responses of the membrane. Parallel inhibition by suramin of the electrical and mechanical responses elicited by excitation of non-adrenergic non-cholinergic inhibitory nerves may not be causally related to the inhibition of ATP-receptors.

Neuronal pathways and transmission to the lower esophageal sphincter of the guinea Pig

BACKGROUND & AIMS

The lower esophageal sphincter (LES) normally controls the opening and closing of the gastroesophageal junction to resist gastric reflux but allow swallowing. Neuronal pathways controlling the guinea pig LES were investigated anatomically and physiologically in isolated preparations.

METHODS

Intracellular recording from the LES with focal electrical stimulation and retrograde and anterograde neuronal tracing were used.

RESULTS

Electrical stimulation on the LES evoked inhibitory junction potentials (IJPs), which were reduced by 60% by 100 micromol/L N-nitro-L-arginine and subsequently blocked by 0.5 micromol/L apamin, unmasking excitatory junction potentials, which were abolished by 1 micromol/L hyoscine. Esophageal or vagal stimulation evoked IJPs, which were blocked by 100 micromol/L hexamethonium. Focal stimulation of the upper stomach evoked IJPs at 5-8 of 20 stimulation sites, which were abolished by cutting between the stimulation site and sphincter. Application of 1,1'-didodecyl-3,3,3', 3'-tetramethyl indocarbocyanine perchlorate (DiI) to the gastric sling muscle anterogradely labeled many motor axons in the sling muscle but few in the LES, confirming that the two muscles are separately innervated. DiI on the esophagus labeled nerve fibers, but not cell bodies, in the upper stomach.

CONCLUSIONS

The inhibitory motor neurons of the LES receive inputs from the vagus nerve, esophagus, and upper stomach.

Role of sarcoplasmic reticulum in the myorelaxant activity of nitric oxide donors in guinea pig gastric fundus

The relaxant effect of two nitric oxide (NO) donors: sodium nitroprusside and 3-morpholino-sydnonimine (SIN-1) on circular smooth muscle strips isolated from guinea pig gastric fundus was studied with the view to elucidating the mechanism, which underlies the NO-induced relaxation of this tissue. Both sodium nitroprusside (10(-9)-10(-5) M) and SIN-1 (10(-9)-10(-4) M) suppressed the spontaneous fundus tone and hyperpolarized the muscle cells by about 5 mV. They antagonized the acetylcholine (10(-6) M)-induced tone and exerted their relaxant effects even when Ca2+ influx into the cells was triggered through the Na+/Ca2+ exchanger. Sodium nitroprusside and SIN-1 antagonized the contraction induced by cyclopiazonic acid (10(-5) M), a specific inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. In the presence of high concentrations of sodium nitroprusside or SIN-1, cyclopiazonic acid (10(-5) M) exerted only a slight if any contractile effect. After the complete relaxation induced by sodium nitroprusside or SIN-1, the K+-channel blockers, tetraethylammonium, apamin and charybdotoxin, as well as the Ca2+ ionophore, A 23187, induced high-amplitude contractions, suggesting that the Ca2+ sensitivity of the contractile myofilaments was not affected. The results suggest that NO, released from NO donors increases the sarcoplasmic reticulum Ca2+ uptake thereby enhancing the vectorial sarcoplasmic reticulum Ca2+ release toward the plasmalemma to elicit membrane hyperpolarization and relaxation in guinea pig gastric fundus.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Alteration of the properties of gastric smooth muscle in the genetically hyperglycemic OLETF rat

Membrane responses were recorded from isolated gastric smooth muscle of Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO) rats, using microelectrode techniques. At the age of 68-76 weeks, the blood sugar level was 181 mg/dl in LETO rats and 350 mg/dL in OLETF rats. In both rats, the membrane potential was stable in fundus muscle and spontaneously active with generation of slow waves in antrum muscle. The resting membrane potential was about - 46 mV in fundus and - 55 mV in antrum muscles of LETO rats, and the values were 3-7 mV lower in OLETF rats. The slow waves were generated regularly in LETO rats, while they were irregular and of small amplitude in OLETF rats. Transmural nerve stimulation evoked a cholinergic excitatory junction potential and following inhibitory junction potential in LETO rats, and only an inhibitory junction potential of smaller size was generated in most of OLETF rats. The acetylcholine-induced depolarization was greater in OLETF than in LETO rats. The level of hyperpolarization produced by noradrenaline was similar between OLETF and LETO rats. Thus, the reduction of the resting membrane potential, weakening of spontaneous activity, impairment of cholinergic transmission and cholinergic supersensitivity were associated with hyperglycemia. These alterations were considered due to the development of diabetes mellitus.