Dihydropyridine-sensitive calcium channels expressed in canine colonic smooth muscle cells

Arecoline hydrobromide enhances jejunum smooth muscle contractility via voltage-dependent potassium channels in W/Wv mice

Gastrointestinal motility was disturbed in W/Wv, which were lacking of interstitial cells of Cajal (ICC). In this study, we have investigated the role of arecoline hydrobromide (AH) on smooth muscle motility in the jejunum of W/Wv and wild-type (WT) mice. The jejunum tension was recorded by an isometric force transducer. Intracellular recording was used to identify whether AH affects slow wave and resting membrane potential (RMP) in vitro. The whole-cell patch clamp technique was used to explore the effects of AH on voltage-dependent potassium channels for jejunum smooth muscle cells. AH enhanced W/Wv and WT jejunum contractility in a dose-dependent manner. Atropine and nicardipine completely blocked the excitatory effect of AH in both W/Wv and WT. TEA did not reduce the effect of AH in WT, but was sufficient to block the excitatory effect of AH in W/Wv. AH significantly depolarized the RMP of jejunum cells in W/Wv and WT. After pretreatment with TEA, the RMP of jejunum cells indicated depolarization in W/Wv and WT, but subsequently perfused AH had no additional effect on RMP. AH inhibited the voltage-dependent K+ currents of acutely isolated mouse jejunum smooth muscle cells. Our study demonstrate that AH enhances the contraction activity of jejunum smooth muscle, an effect which is mediated by voltage-dependent potassium channels that acts to enhance the excitability of jejunum smooth muscle cells in mice.

Adiponectin Exerts Peripheral Inhibitory Effects on the Mouse Gastric Smooth Muscle through the AMPK Pathway

Some adipokines, such as adiponectin (ADPN), other than being implicated in the central regulation of feeding behavior, may influence gastric motor responses, which are a source of peripheral signals that also influence food intake. The present study aims to elucidate the signaling pathways through which ADPN exerts its actions in the mouse gastric fundus. To this purpose, we used a multidisciplinary approach. The mechanical results showed that ADPN caused a decay of the strip basal tension, which was abolished by the nitric oxide (NO) synthesis inhibitor, L-N^G-nitro arginine (L-NNA). The electrophysiological experiments confirmed that all ADPN effects were abolished by L-NNA, except for the reduction of Ca²⁺ current, which was instead prevented by the inhibitor of AMP-activated protein kinase (AMPK), dorsomorphin. The activation of the AMPK signaling by ADPN was confirmed by immunofluorescence analysis, which also revealed the ADPN R1 receptor (AdipoR1) expression in glial cells of the myenteric plexus. In conclusion, our results indicate that ADPN exerts an inhibitory action on the gastric smooth muscle by acting on AdipoR1 and involving the AMPK signaling pathway at the peripheral level. These findings provide novel bases for considering AMPK as a possible pharmacologic target for the potential treatment of obesity and eating disorders.

Modelling Human Colonic Smooth Muscle Cell Electrophysiology

The colon is a digestive organ that is subject to a wide range of motility disorders. However, our understanding of the etiology of these disorders is far from complete. In this study, a quantitative single cell model has been developed to describe the electrical behaviour of a human colonic smooth muscle cell (hCSMC). This model includes the pertinent ionic channels and intracellular calcium homoeostasis. These components are believed to contribute significantly to the electrical response of the hCSMC during a slow wave. The major ion channels were constructed based on published data recorded from isolated human colonic myocytes. The whole cell model is able to reproduce experimentally recorded slow waves from human colonic muscles. This represents the first biophysically-detailed model of a hCSMC and provides a means to better understand colonic disorders.

Impaired contractile responses and altered expression and phosphorylation of Ca(2+) sensitization proteins in gastric antrum smooth muscles from ob/ob mice

Diabetic gastroparesis is a common complication of diabetes, adversely affecting quality of life with symptoms of abdominal discomfort, nausea, and vomiting. The pathogenesis of this complex disorder is not well understood, involving abnormalities in the extrinsic and enteric nervous systems, interstitial cells of Cajal (ICCs), smooth muscles and immune cells. The ob/ob mouse model of obesity and diabetes develops delayed gastric emptying, providing an animal model for investigating how gastric smooth muscle dysfunction contributes to the pathophysiology of diabetic gastroparesis. Although ROCK2, MYPT1, and CPI-17 activities are reduced in intestinal motility disorders, their functioning has not been investigated in diabetic gastroparesis. We hypothesized that reduced expression and phosphorylation of the myosin light chain phosphatase (MLCP) inhibitory proteins MYPT1 and CPI-17 in ob/ob gastric antrum smooth muscles could contribute to the impaired antrum smooth muscle function of diabetic gastroparesis. Spontaneous and carbachol- and high K(+)-evoked contractions of gastric antrum smooth muscles from 7 to 12 week old male ob/ob mice were reduced compared to age- and strain-matched controls. There were no differences in spontaneous and agonist-evoked intracellular Ca(2+) transients and myosin light chain kinase expression. The F-actin:G-actin ratios were similar. Rho kinase 2 (ROCK2) expression was decreased at both ages. Basal and agonist-evoked MYPT1 and myosin light chain 20 phosphorylation, but not CPI-17 phosphorylation, was reduced compared to age-matched controls. These findings suggest that reduced MLCP inhibition due to decreased ROCK2 phosphorylation of MYPT1 in gastric antrum smooth muscles contributes to the antral dysmotility of diabetic gastroparesis.

Investigation of L-type Ca(2+) current in the aganglionic bowel segment in Hirschsprung's disease

BACKGROUND

Studies on animal models of Hirschsprung's disease (HD) suggest that L-type Ca(2+) channels are down-regulated in the aganglionic bowel segment, however, this has yet to be confirmed in HD patients. The objective of this study was to test the hypothesis that L-type Ca(2+) current density is decreased in smooth muscle cells (SMC) obtained from the aganglionic bowel segment of patients with HD in comparison with those from the ganglionic segment.

METHODS

Smooth muscle cells were freshly isolated from colon samples obtained from HD patients undergoing pull-through surgery. L-type Ca(2+) currents were recorded using the perforated patch configuration of the whole cell voltage clamp technique and the expression levels of CACNA1C transcripts (which encode L-type Ca(2+) channels) in the ganglionic and aganglionic bowel segments were compared using real-time quantitative PCR.

KEY RESULTS

All SMC displayed robust currents that had activation/inactivation kinetics typical of L-type Ca(2+) current, were inhibited by nifedipine and enhanced by the L-type Ca(2+) channel agonists FPL 64176 and Bay K 8644. Moreover, FPL 64176 activated currents were also inhibited by nifedipine. However, there was no significant difference in L-type Ca(2+) current density, CACNA1C subunit expression or sensitivity to the pharmacological agents noted above, between SMC isolated from the ganglionic and aganglionic regions of the HD colon.

CONCLUSIONS & INFERENCES

In contrast to studies on genetic animal models of HD, L-type Ca(2+) currents are not down-regulated in the aganglionic bowel segment of HD patients and are therefore unlikely to account for the impaired colonic peristalsis observed in these patients.

Ionic conductances regulating the excitability of colonic smooth muscles

The tunica muscularis of the gastrointestinal (GI) tract contains two layers of smooth muscle cells (SMC) oriented perpendicular to each other. SMC express a variety of voltage-dependent and voltage-independent ionic conductance(s) that develop membrane potential and control excitability. Resting membrane potentials (RMP) vary through the GI tract but generally are within the range of -80 to -40 mV. RMP sets the 'gain' of smooth muscle and regulates openings of voltage-dependent Ca(2+) channels. A variety of K(+) channels contribute to setting RMP of SMC. In most regions, RMP is considerably less negative than the K(+) equilibrium potential, due to a finely tuned balance between background K(+) channels and non-selective cation channels (NSCC). Variations in expression patterns and openings of K(+) channels and NSCC account for differences of the RMP in different regions of the GI tract. Smooth muscle excitability is also regulated by interstitial cells (interstitial cells of Cajal (ICC) and PDGFRα(+) cells) that express additional conductances and are electrically coupled to SMC. Thus, 'myogenic' activity results from the integrated behavior of the SMC/ICC/PDGFRα(+) cell (SIP) syncytium. Inputs from excitatory and inhibitory motor neurons are required to produce the complex motor patterns of the gut. Motor neurons innervate three cell types in the SIP, and receptors, second messenger pathways, and ion channels in these cells mediate postjunctional responses. Studies of isolated SIP cells have begun to unravel the mechanisms responsible for neural responses. This review discusses ion channels that set and regulate RMP of SIP cells and how neurotransmitters regulate membrane potential.

Regulation of basal LC20 phosphorylation by MYPT1 and CPI-17 in murine gastric antrum, gastric fundus, and proximal colon smooth muscles

BACKGROUND

Myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) govern myosin light chain (LC20) phosphorylation and smooth muscle contraction. Rho kinase (ROK) inhibits MLCP, resulting in greater LC20 phosphorylation and force generation at a given [Ca(2+) ](i) . Here, we investigate the role of ROK in regulating LC20 phosphorylation and spontaneous contractions of gastric fundus, gastric antrum, and proximal colon smooth muscles.

METHODS

Protein and phosphorylation levels were determined by western blotting. The effects of Y27632, nicardipine, and GF109203X on phosphorylation levels and contraction were measured.

KEY RESULTS

γ-Actin expression is similar in all three smooth muscles. LC20 and pS19 are highest, but ROK1 and ROK2 are lowest, in antrum and proximal colon smooth muscles. LZ +/- myosin phosphatase targeting subunit 1 (MYPT1), CPI-17, and pT696, pT853, and pT38 are highest in fundus and proximal colon smooth muscles. Myosin phosphatase-rho interacting protein (M-RIP) expression is lowest in fundus, and highest in antrum and proximal colon smooth muscles. Y27632 reduced pT853 in each smooth muscle, but reduced pT696 only in fundus smooth muscles. Nicardipine had no effect on pT38 in each smooth muscle, while GF109203X reduced pT38 in proximal colon and fundus smooth muscles. Y27632 or nicardipine reduced pS19 in proximal colon and fundus smooth muscles. Y27632 or nicardipine inhibited antrum and proximal colon smooth muscle spontaneous contractions, but only Y27632 reduced fundus smooth muscle tone. Zero external Ca(2+) relaxed each smooth muscle and abolished LC20 phosphorylation.

CONCLUSIONS & INFERENCES

Organ-specific mechanisms involving the MLCP interacting proteins LZ +/- MYPT1, M-RIP, and CPI-17 are critical to regulating basal LC20 phosphorylation in gastrointestinal smooth muscles.

Regulation of smooth muscle excitation and contraction

Smooth muscle cells (SMC) make up the muscular portion of the gastrointestinal (GI) tract from the distal oesophagus to the internal anal sphincter. Coordinated contractions of these cells produce the motor patterns of GI motility. Considerable progress was made during the last 20 years to understand the basic mechanisms controlling excitation-contraction (E-C) coupling. The smooth muscle motor is now understood in great molecular detail, and much has been learned about the mechanisms that deliver and recover Ca2+ during contractions. The majority of Ca2+ that initiates contractions comes from the external solution and is supplied by voltage-dependent Ca2+ channels (VDCC). VDCC are regulated largely by the effects of K+ and non-selective cation conductances (NSCC) on cell membrane potential and excitability. Ca2+ entry is supplemented by release of Ca2+ from IP(3) receptor-operated stores and by mechanisms that alter the sensitivity of the contractile apparatus to changes in cytoplasmic Ca2+. Molecular studies of the regulation of smooth muscle have been complicated by the plasticity of SMC and difficulties in culturing these cells without dramatic phenotypic changes. Major questions remain to be resolved regarding the details of E-C coupling in human GI smooth muscles. New discoveries regarding molecular expression that give GI smooth muscle their unique properties, the phenotypic changes that occur in SMC in GI motor disorders, tissue engineering approaches to repair or replace defective muscular regions, and molecular manipulations of GI smooth muscles in animals models and in cell culture will be topics for exciting investigations in the future.

Functional and molecular analysis of L-type calcium channels in human esophagus and lower esophageal sphincter smooth muscle

Excitation of human esophageal smooth muscle involves the release of Ca(2+) from intracellular stores and influx. The lower esophageal sphincter (LES) shows the distinctive property of tonic contraction; however, the mechanisms by which this is maintained are incompletely understood. We examined Ca(2+) channels in human esophageal muscle and investigated their contribution to LES tone. Functional effects were examined with tension recordings, currents were recorded with patch-clamp electrophysiology, channel expression was explored by RT-PCR, and intracellular Ca(2+) concentration was monitored by fura-2 fluorescence. LES muscle strips developed tone that was abolished by the removal of extracellular Ca(2+) and reduced by the application of the L-type Ca(2+) channel blocker nifedipine (to 13 +/- 6% of control) but was unaffected by the inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA). Carbachol increased tension above basal tone, and this effect was attenuated by treatment with CPA and nifedipine. Voltage-dependent inward currents were studied using patch-clamp techniques and dissociated cells. Similar inward currents were observed in esophageal body (EB) and LES smooth muscle cells. The inward currents in both tissues were blocked by nifedipine, enhanced by Bay K8644, and transiently suppressed by acetylcholine. The molecular form of the Ca(2+) channel was explored using RT-PCR, and similar splice variant combinations of the pore-forming alpha(1C)-subunit were identified in EB and LES. This is the first characterization of Ca(2+) channels in human esophageal smooth muscle, and we establish that L-type Ca(2+) channels play a critical role in maintaining LES tone.

Contraction of human colonic circular smooth muscle cells is inhibited by the calcium channel blocker pinaverium bromide

The effects of L-type calcium channel blockers (CCBs) selective for the gastrointestinal tract (pinaverium) or non-selective (nicardipine and diltiazem), were investigated on CCK-, CCh- or KCl-induced contraction of smooth muscle cells (SMC) isolated from the circular muscle layer of normal or of inflamed human colons. In the normal tissue colon, whatever the contractile agent used, CCK-8 (1nM), CCh (1nM) or KCl (20mM), a micromolar concentration of pinaverium significantly inhibited contraction (88.36%, 93.10%, 93.92% inhibition respectively); this effect was concentration-dependent for CCh (IC50 = 0.73 +/- 0.08nM) and for CCK (IC50 = 0.92 +/- 0.12nM). In parallel, both nicardipine and diltiazem inhibit significantly contraction of isolated SMC. In inflamed colons, pinaverium (1 microM) display a significant higher efficacy than diltiazem or nicardipine to reduce cell contraction induced by CCK-8 or by KCl. In addition, RT-PCR experiments were performed to evidence tissue specificity of the L-type calcium channel. They revealed the expression of the messenger of the a-1 subunit L-type calcium channel (binding site of such CCBs), consistent with the expression of the rbC-2 splice variant of the alpha1-C gene.

IN CONCLUSION

(i) the inhibition by calcium channel blockers of agonist-induced contractile activity suggest a modulation of SMC contraction upon extracellular calcium via 'L-type' voltage-dependent calcium channel; (ii) this study provides a rationale for the clinical use of pinaverium in colonic motor disoders affecting the contractility of SMC, since it appeared to decrease the contraction even in pathological situation; and (iii) RT-PCR experiments confirms the presence in human colon SMC of the alpha-1 subunit mRNA of calcium channel.

Novel voltage-dependent non-selective cation conductance in murine colonic myocytes

1. Two components of voltage-gated, inward currents were observed from murine colonic myocytes. One component had properties of L-type Ca(2+) currents and was inhibited by nicardipine (5 x 10(-7) M). A second component did not 'run down' during dialysis and was resistant to nicardipine (up to 10(-6) M). The nicardipine-insensitive current was activated by small depolarizations above the holding potential and reversed near 0 mV. 2. This low-voltage-activated current (I(LVA)) was resolved with step depolarizations positive to -60 mV, and the current rapidly inactivated upon sustained depolarization. The voltage of half-inactivation was -65 mV. Inactivation and activation time constants at -45 mV were 86 and 15 ms, respectively. The half-recovery time from inactivation was 98 ms at -45 mV. I(LVA) peaked at -40 mV and the current reversed at 0 mV. 3. I(LVA) was inhibited by Ni(2+) (IC(50) = 1.4 x 10(-5) M), mibefradil (10(-6) to 10(-5) M), and extracellular Ba(2+). Replacement of extracellular Na(+) with N-methyl-D-glucamine inhibited I(LVA) and shifted the reversal potential to -7 mV. Increasing extracellular Ca(2+) (5 x 10(-3) M) increased the amplitude of I(LVA) and shifted the reversal potential to +22 mV. I(LVA) was also blocked by extracellular Cs(+) (10(-4) M) and Gd(3+) (10(-6) M). 4. Warming increased the rates of activation and deactivation without affecting the amplitude of the peak current. 5. We conclude that the second component of voltage-dependent inward current in murine colonic myocytes is not a 'T-type' Ca(2+) current but rather a novel, voltage-gated non-selective cation current. Activation of this current could be important in the recovery of membrane potential following inhibitory junction potentials in gastrointestinal smooth muscle or in mediating responses to agonists.

Impairment of Ca(2+) mobilization in circular muscle cells of the inflamed colon

This study investigated whether inflammation modulates the mobilization of Ca(2+) in canine colonic circular muscle cells. The contractile response of single cells from the inflamed colon was significantly suppressed in response to ACh, KCl, and BAY K8644. Methoxyverapamil and reduction in extracellular Ca(2+) concentration dose-dependently blocked the response in both normal and inflamed cells. The increase in intracellular Ca(2+) concentration in response to ACh and KCl was significantly reduced in the inflamed cells. However, Ca(2+) efflux from the ryanodine- and inositol 1,4, 5-trisphosphate (IP(3))-sensitive stores, as well as the decrease of cell length in response to ryanodine and IP(3), were not affected. Heparin significantly blocked Ca(2+) efflux and contraction in response to ACh in both conditions. ACh-stimulated accumulation of IP(3) and the binding of [(3)H]ryanodine to its receptors were not altered by inflammation. Ruthenium red partially inhibited the response to ACh in normal and inflamed states. We conclude that the canine colonic circular muscle cells utilize Ca(2+) influx through L-type channels as well as Ca(2+) release from the ryanodine- and IP(3)-sensitive stores to contract. Inflammation impairs Ca(2+) influx through L-type channels, but it may not affect intracellular Ca(2+) release. The impairment of Ca(2+) influx may contribute to the suppression of circular muscle contractility in the inflamed state.

Effects of anion channel antagonists in canine colonic myocytes: comparative pharmacology of Cl-, Ca2+ and K+ currents

1. Volume-Sensitive, Outwardly Rectifying (VSOR) Cl- currents were measured in canine colonic myocytes by whole-cell patch clamp. Decreasing extracellular osmolarity 50 milliosmoles l-1 activated current that was carried by Cl- and 5 - 7 times greater in the outward direction. 2. Niflumic acid, an inhibitor of Ca2+-activated Cl- channels, did not inhibit VSOR Cl- current. Glibenclamide, an antagonist of CFTR, and anthracene-9-carboxylate (9-AC) inhibited current less than 25% at 100 microM. 3. DIDS (4, 4-diisothiocyanato-stilbene-2,2'disulphonate) inhibited VSOR Cl- current more potently than SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate). IC50s were 0.84 and 226 microM, respectively. 4. VSOR Cl- current was strongly inhibited by tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene), an anti-oestrogen compound (IC50=0.57 microM). 5. Gd3+ antagonized VSOR Cl- current more potently than La3+. The IC50 for Gd3+ was 23 microM. In contrast, 100 microM La3+ inhibited current only 35+/-7%. 6. Antagonists of VSOR Cl- current had non-specific effects. These compounds blocked voltage-dependent K+ and Ca2+ currents in colonic myocytes. Tamoxifen (10 microM) and DIDS (10 microM) inhibited L-type Ca2+ current 87+/-7 and 31+/-5%, respectively. Additionally, in the presence of 300 nM charybdotoxin, tamoxifen (1 microM) and DIDS (10 microM) inhibited delayed rectifier K+ current 38+/-8 and 10+/-2%, respectively. 7. The pharmacology of VSOR Cl- channels overlaps with voltage-dependent cation channels. DIDS and tamoxifen inhibited VSOR Cl- equally. However, because DIDS had much less effect on L-type Ca2+ and delayed rectifier K+ channels than did tamoxifen, it might be useful in experiments to investigate the physiological and pathophysiological role of this conductance in whole tissues.

Molecular diversity of voltage-sensitive calcium channels in smooth muscle cells

Voltage-sensitive calcium channels play an important role in the excitation-contraction coupling of smooth muscle. Several subunits form the oligomeric channel complex and determine its functional properties. Therefore a differential distribution of the various channel subunits and their splice forms could contribute to the functional specialization of smooth muscle cells. To test this hypothesis, specific primers were designed to amplify messenger ribonucleic acid (mRNA) from vascular and gastrointestinal smooth muscle of the rabbit by reverse transcription and polymerase chain reaction (RT-PCR). The presence of high- and low-threshold voltage-dependent calcium channels was also examined in a smooth muscle-derived cell line (A7R5). Consistent with the physiologic data, smooth muscle contains mRNA for the pore-forming subunits of high- and low-threshold voltage-dependent calcium channels, alpha-1C and alpha-1G. Three splice variants of the alpha-1C-subunit were identified in smooth muscle. These may affect dihydropyridine binding and the interaction between the alpha-1C and the beta-subunit. In addition, three of the four cloned beta-subunits (beta-1b, beta-2, and beta-3) could be found in all smooth muscle tissues examined. These data demonstrate that various splice forms of the L-type calcium channel exist in smooth muscle tissue. Moreover, these experiments also show for the first time that smooth muscle cells contain mRNA for low-threshold voltage-sensitive calcium channels. Combinations of the pore-forming subunits with one of the three beta-subunits could account for functional differences between smooth muscle cells from distinct regions. A better understanding of the structure and function of these channels may help in our understanding of diseases affecting smooth muscle and help in the development of novel drugs targeting these molecules.

Inflammatory modulation of calcium-activated potassium channels in canine colonic circular smooth muscle cells

BACKGROUND & AIMS

The characteristics of colonic circular smooth muscle slow waves are altered during inflammation. The aim of this study was to examine whether inflammation modulates the open-state probability of Ca2+-activated K+ (KCa) channels in these cells to contribute to these alterations.

METHODS

The experiments were performed on freshly dissociated single smooth muscle cells from the canine colon using standard patch clamp methods. Inflammation was induced by mucosal exposure to ethanol and acetic acid.

RESULTS

Inflammation decreased the open-state probability of large-conductance KCa (BK) channels in the cell-attached and excised inside-out configurations. The voltage sensitivity of the channels was also reduced during inflammation. Inflammation had no significant effect on the large, medium, and small conductances or the unitary current levels of channel openings. However, it decreased the maximum number of simultaneous channel openings. The channels were Ca2+-dependent and were blocked by tetraethylammonium and charybdotoxin in normal and inflamed cells.

CONCLUSIONS

Inflammation decreases the open-state probability of BK channels. This may partially reverse the decrease in duration and amplitude of slow waves and depolarization of membrane potential seen in inflammation.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Ionic conductances in gastrointestinal smooth muscles and interstitial cells of Cajal

Ion channels are the unitary elements that underlie electrical activity of gastrointestinal smooth muscle cells and of interstitial cells of Cajal. The result of ion channel activity in the gastrointestinal smooth muscle layers is a rhythmic change in membrane potential that in turn underlies events leading to organized motility patterns. Gastrointestinal smooth muscle cells and interstitial cells of Cajal express a wide variety of ion channels that are tightly regulated. This review summarizes 20 years of data obtained from patch-clamp studies on gastrointestinal smooth muscle cells and interstitial cells, with a focus on regulation.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

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.

Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCa channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCa current and the activity of KCa channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration. NO (0.5% or 1%) in the bath increased NPo of KCa channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPo of KCa channels. In the on-cell patch recording configuration, methylene blue (1 microM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of KCa channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 microM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activating KCa channels through a cGMP pathway.

Effect of motilin and erythromycin on calcium-activated potassium channels in rabbit colonic myocytes

BACKGROUND & AIMS

Motilin and erythromycin are prokinetic agents that act on the same receptor in gastrointestinal smooth muscle to cause contraction. Both agonists may also cause an increase in outward current. The aim of this study was to determine whether motilin and erythromycin activate calcium-activated potassium (KCa) channels.

METHODS

Freshly dispersed longitudinal smooth muscle cells of the rabbit colon were used to measure whole-cell outward current and single-channel activity using patch clamp recording methods.

RESULTS

Erythromycin and motilin increased a calcium-dependent outward potassium current and increased the open probability of KCa channels of cell- attached patches.

CONCLUSIONS

Erythromycin and motilin activate KCa channels via an intracellular second messenger system. This effect may modulate the increase in contractility caused by these agonists.

Effect of different calcium channel blockers on inhibitory junction potentials and slow waves in porcine ileum

The effect of several calcium channel blockers was evaluated: (i) on spontaneous electrical and mechanical activities and (ii) on the response to electrical field stimulation. The study was carried out on whole-thickness preparation of porcine ileum. Glass microelectrodes were used to record membrane potential from smooth muscle cells. Resting membrane potential was -60 +/- 2mV (n = 18) and preparations generated spontaneous slow waves. Electrical field stimulation (EFS) was applied using different parameters. The amplitude and duration of inhibitory junction potentials (IJPs) increased with EFS strength. IJPs were abolished by tetrodotoxin (1 microM). Nifedipine (1 microM) did not modify the amplitude or duration of IJPs. The frequency of slow waves was not modified, however a slight but significant (p < 0.001) reduction in slow wave duration was observed. Mechanical activity was abolished in presence of nifedipine within approximately 6 min. omega-agatoxin IVA (50 nM) or omega-conotoxin MVIIC (100 nM), respectively a P-type and a Q-type calcium channel blockers, did not modify slow wave and IJP characteristics. In contrast, in presence of omega-conotoxin GVIA (100 nM), a N-type calcium channel blocker, or omega-conotoxin MVIIC (1 microM), IJPs were completely abolished. These data suggest that, in porcine ileum, N-type but not P-,Q- or L-type calcium channels are involved in the release of the non-adrenergic non-cholinergic neurotransmitters mediating IJPs. L-type calcium channels underlie electrical mechanical coupling but are not involved in slow wave generation.

Protein kinase A inhibitors selectively inhibit the tonic contraction of the guinea pig ureter to high potassium

1. We have investigated the effect of various protein kinase A (PKA) inhibitors on the phasic and tonic components of the response to potassium chloride (KCl) in the guinea pig ureter. All experiments were performed in ureters pretreated with capsaicin (10 microM for 15 min) to prevent the release of sensory neuropeptides and in the presence of 1 microM Bay K 8644 to maximize calcium (Ca) entry via voltage-sensitive channels. The addition of 80 mM hypertonic KCl produced maximal shortening of the ureter with distinct phasic and tonic components, the latter further showing a transient and a sustained component. Nifedipine (30 microM for 120 min) totally abolished all the responses to KCl. 2. The selective PKA inhibitor, H89 (10 microM), abolished the tonic response to KCl in about 30 min with minor inhibitory effect on the phasic contraction. This pattern was unchanged when extending the contact time to 120 min. When added 30 min before the next challenge, H89 (1-30 microM) concentration-dependently inhibited the responses to KCl with a preferential inhibitory effect on the tonic contraction. Another PKA inhibitor, H8, produced similar effects at tenfold higher concentrations (10-300 microM) than H89, consistent with the known potency ratio of these isoquinoline derivatives in inhibiting PKA. 3. The potent and nonselective protein kinase inhibitor, staurosporine (10-100 nM) produced an even depression of the various phases of the response to KCl. The selective protein kinase G inhibitor, KT 5823 (10 microM for 60 min) produced only a slight reduction of the sustained tonic response to KCl. The selective protein kinase C inhibitor GF 109,203X (1-3 microM) and the cAMP analog, Rp-cAMPS (300 microM for 60 min) had no effect on the three components of the response to KCl. 4. In the presence of Bay K 8644, electrical field stimulation (10 Hz for 1 sec, 60 V, pulse width 5 ms) produces direct myogenic phasic contractions (twitches) of the ureter which are suppressed by nifedipine (10-30 microM). H8 (up to 30 microM) and H89 (up to 300 microM) had minor effect on the amplitude of twitches, consistent with their poor inhibitory activity on the phasic responses to KCl. 5. In sucrose gap, superfusion with 80 mM hypertonic KCl produced action potentials followed by a sustained depolarization of the membrane: the two electrical responses underlie the phasic and tonic components of contraction to KCl, respectively. H89 (10 microM for 30 min) did not affect the resting membrane potential nor the KCl-evoked action potentials and sustained depolarization. H89 had no effect on the phasic contraction to KCl but markedly depressed (about 65% inhibition) the tonic contraction. 6. The present findings are consistent with the view that phosphorylation by PKA increases the availability of L-type Ca channels in the ureter smooth muscle. Blockade of PKA dissociates the electromechanical coupling between the sustained membrane depolarization produced by KCl and the corresponding sustained increase in tension. The L-type Ca channel responsible for generating action potentials and phasic contractions to KCl are less sensitive to PKA inhibitors than those responsible for the tonic contraction.

Molecular biology of calcium channels

Pharmacological and electrophysiological studies have established that there are multiple types of voltage-gated Ca2+ channels. Molecular biology has uncovered an even greater number of channel molecules. Thus, the molecular diversity of Ca2+ channels has its basis in the expression of many alpha 1 and beta genes, and also in the splice variants produced from these genes. This ability to mix and match subunits provides the cell with yet another mechanism to control the influx of calcium. Future studies will describe new subunits, the subunit composition of each type of channel, and the cloning of new Ca2+ channel types.

Calcium currents in human and canine jejunal circular smooth muscle cells

BACKGROUND & AIMS

Although calcium plays an essential role in intestinal smooth muscle contractile activity, calcium entry pathways in canine and human small intestine are largely unknown. The goal of this study was to characterize calcium channels, a potential entry pathway for calcium, in isolated circular smooth muscle cells of canine and human jejunum.

METHODS

Single freshly dissociated human and canine jejunal circular smooth muscle cells were studied using single-channel and perforated whole-cell patch clamp recordings as well as fluorescence dual wavelength ratio imaging.

RESULTS

An inward whole-cell current was identified that was carried by a 17 pS (80 mmol/L Ba2+) dihydropyridine-sensitive, barium-permeable channel. The current was potentiated by BayK 8644 (1 mumol/L; n = 3; 82% +/- 34%), acetylcholine (1 mumol/L; n = 8; 42% +/- 5%), and erythromycin (1 mumol/L; n = 9; 70% +/- 11%) and was completely blocked by nifedipine (1 mumol/L; n = 6) or diltiazem (200 mumol/L; n = 4). Application of BayK 8644 (1 mumol/L), acetylcholine (1 mumol/L), or erythromycin (1 mumol/L) to Fura-2-loaded smooth muscle cells bathed in Krebs' solution containing 2.54 mmol/L calcium increased intracellular calcium levels.

CONCLUSIONS

A calcium entry pathway was identified in canine and human jejunal circular smooth muscle cells. The pathway was mediated by a dihydropyridine-sensitive calcium channel. The channel allowed the entry of significant amounts of calcium at physiological extracellular calcium concentration.

Effect of Bay K 8644 and ryanodine on the refractory period, action potential and mechanical response of the guinea-pig ureter to electrical stimulation

We have investigated the effect of the dihydropyridine calcium channel agonist, Bay K 8644, and of the plant alkaloid blocker of calcium-induced calcium release (CICR) from the sarcoplasmic reticulum, ryanodine, on the refractory period, action potential and mechanical response of the guinea-pig isolated ureter to electrical stimulation. All experiments were performed in ureters pre-exposed to 10 microM capsaicin to eliminate the inhibitory influence exerted by local release of sensory neuropeptides on ureteral excitability and contraction. In organ bath experiments, electrical field stimulation with parameters which produce direct excitation of ureteral smooth muscle (train of pulses at 10 Hz, 5 ms pulse width, 60 V for 1 s) produced tetrodotoxin- (1 microM) resistant phasic contractions. The response to EFS was abolished by nifedipine (1 nM-3 microM) and was enhanced by Bay K 8644 (1 nM-3 microM). In the presence of Bay K 8644 (1 microM), nifedipine (30 microM) abolished the evoked contractions. Ryanodine (10-100 microM) had no significant effect on the amplitude of evoked contraction. The response of the guinea-pig ureter to direct electrical stimulation of smooth muscle is characterized by a refractory period: at least 40 s interstimulus interval was required to produce a second response in all preparations tested. Bay K 8644 (1 microM) markedly reduced the refractory period of the ureter and a similar effect was observed with ryanodine (100 microM). To further analyze the effect of Bay K 8644 and ryanodine on the refractory period, the response of the ureter was investigated over a 10 s period of stimulation (other parameters as above). In control ureters, continuous stimulation for 10 s produced only one phasic contraction just after the beginning of the train of stimuli. In the presence of Bay K 8644 or ryanodine, more than one phasic contraction developed during a 10 s stimulation, i.e. the refractory period became shorter than the train duration. When both Bay K 8644 and ryanodine were tested on the same preparations, an additive excitatory effect was observed on the mechanical response to electrical stimulation. A slight elevation of KCl concentration (5-10 mM) reduced the refractory period of the ureter as observed with ryanodine or Bay K 8644. Application of KCl (80 mM) produced a biphasic contractile response of the ureter: a series of phasic contractions occurred first, which were then replaced by a slowly developing tonic response. Bay K 8644 (1 microM) enhanced both components of the response to KCl. Ryanodine (10 and 100 microM) markedly prolonged the duration of phasic contractions evoked by KCl and, at 100 microM, slightly (about 25%) reduced the amplitude of tonic contraction.(ABSTRACT TRUNCATED AT 400 WORDS)