Effects of removal of Na(+) and Cl(-) on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

Extracellular Cl- regulates electrical slow waves and setting of smooth muscle membrane potential by interstitial cells of Cajal in mouse jejunum

NEW FINDINGS

What is the central question of this study? The aim was to investigate the roles of extracellular chloride in electrical slow waves and resting membrane potential of mouse jejunal smooth muscle by replacing chloride with the impermeant anions gluconate and isethionate. What is the main finding and its importance? The main finding was that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in interstitial cells of Cajal (ICCs) is a major contributor to the shape of electrical slow waves. Furthermore, the data confirm that ICCs set the smooth muscle membrane potential and that altering Cl^- homeostasis in ICCs can alter the smooth muscle membrane potential. Intracellular Cl^- homeostasis is regulated by anion-permeable channels and transporters and contributes to excitability of many cell types, including smooth muscle and interstitial cells of Cajal (ICCs). Our aims were to investigate the effects on electrical activity in mouse jejunal muscle strips of replacing extracellular Cl^- (Cl^-_o ) with the impermeant anions gluconate and isethionate. On reducing Cl^-_o , effects were observed on electrical slow waves, with small effects on smooth muscle membrane voltage (E_m ). Restoration of Cl^- hyperpolarized smooth muscle E_m proportional to the change in Cl^-_o concentration. Replacement of 90% of Cl^-_o with gluconate reversibly abolished slow waves in five of nine preparations. Slow waves were maintained in isethionate. Gluconate and isethionate substitution had similar concentration-dependent effects on peak amplitude, frequency, width at half peak amplitude, rise time and decay time of residual slow waves. Gluconate reduced free ionized Ca²⁺ in Krebs solutions to 0.13 mm. In Krebs solutions containing normal Cl^- and 0.13 mm free Ca²⁺ , slow wave frequency was lower, width at half peak amplitude was smaller, and decay time was faster. The transient hyperpolarization following restoration of Cl^-_o was not observed in W/W^v mice, which lack pacemaker ICCs in the small intestine. We conclude that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in ICCs plays a major role in generation or propagation of slow waves. Furthermore, these data support a role for ICCs in setting smooth muscle E_m and that altering Cl^- homeostasis in ICCs can alter smooth muscle E_m .

Conditional genetic deletion of Ano1 in interstitial cells of Cajal impairs Ca2+ transients and slow waves in adult mouse small intestine

Myenteric plexus interstitial cells of Cajal (ICC-MY) in the small intestine are Kit+ electrical pacemakers that express the Ano1/TMEM16A Ca2+-activated Cl- channel, whose functions in the gastrointestinal tract remain incompletely understood. In this study, an inducible Cre-LoxP-based approach was used to advance the understanding of Ano1 in ICC-MY of adult mouse small intestine. KitCreERT2/+;Ano1Fl/Fl mice were treated with tamoxifen or vehicle, and small intestines (mucosa free) were examined. Quantitative RT-PCR demonstrated ~50% reduction in Ano1 mRNA in intestines of conditional knockouts (cKOs) compared with vehicle-treated controls. Whole mount immunohistochemistry showed a mosaic/patchy pattern loss of Ano1 protein in ICC networks. Ca2+ transients in ICC-MY network of cKOs displayed reduced duration compared with highly synchronized controls and showed synchronized and desynchronized profiles. When matched, the rank order for Ano1 expression in Ca2+ signal imaged fields of view was as follows: vehicle controls>>>cKO(synchronized)>cKO(desynchronized). Maintenance of Ca2+ transients' synchronicity despite high loss of Ano1 indicates a large functional reserve of Ano1 in the ICC-MY network. Slow waves in cKOs displayed reduced duration and increased inter-slow-wave interval and occurred in regular- and irregular-amplitude oscillating patterns. The latter activity suggested ongoing interaction by independent interacting oscillators. Lack of slow waves and depolarization, previously reported for neonatal constitutive knockouts, were also seen. In summary, Ano1 in adults regulates gastrointestinal function by determining Ca2+ transients and electrical activity depending on the level of Ano1 expression. Partial Ano1 loss results in Ca2+ transients and slow waves displaying reduced duration, while complete and widespread absence of Ano1 in ICC-MY causes lack of slow wave and desynchronized Ca2+ transients.NEW & NOTEWORTHY The Ca2+-activated Cl- channel, Ano1, in interstitial cells of Cajal (ICC) is necessary for normal gastrointestinal motility. We knocked out Ano1 to varying degrees in ICC of adult mice. Partial knockout of Ano1 shortened the widths of electrical slow waves and Ca2+ transients in myenteric ICC but Ca2+ transient synchronicity was preserved. Near-complete knockout was necessary for transient desynchronization and loss of slow waves, indicating a large functional reserve of Ano1 in ICC.

The effect of SCF and ouabain on small intestinal motility dysfunction induced by gastric cancer peritoneal metastasis

The interstitial cells of Cajal (ICCs) play an important role in maintaining the normal function of gastrointestinal dynamics. In our previous study, we reported that, in advanced gastric cancer, the frequency of bowel movement is always reduced, due in part to the decreased number of ICCs. To investigate the impact of ICCs in gastric cancer, we established a mouse model of gastric cancer peritoneal metastasis using SGC-7901 gastric adenocarcinoma cells and their supernatant. Then, stem cell factor (SCF) and ouabain were used as therapeutic agents to improve gut dynamics. Our data showed that, compared with the normal mice, treatment with SGC-7901 cells and their supernatant led to a significant reduction of the muscle layer thickness, a decreased number of ICCs, broadened gaps between ICCs and surrounding cells, degeneration and necrosis of smooth muscle cells (SMCs), and infiltration of inflammatory cells. In contrast to SGC-7901 cell and supernatant treatment, SCF intervention caused mild submucosal edema and mitochondrial proliferation in the ICCs and SMCs. Additionally, ouabain treatment led to inflammatory cells infiltration into the submucosa and a decreased volume of ICCs. In conclusion, our data illustrated that, under the condition of gastric cancer peritoneal metastasis, the dysfunction of intestinal peristalsis may be related to pathological changes in ICCs. Moreover, we demonstrated that SCF treatment may help to improve intestinal dynamics by regulating the number and function of ICCs.

AqF026 is a pharmacologic agonist of the water channel aquaporin-1

Aquaporin-1 (AQP1) facilitates the osmotic transport of water across the capillary endothelium, among other cell types, and thereby has a substantial role in ultrafiltration during peritoneal dialysis. At present, pharmacologic agents that enhance AQP1-mediated water transport, which would be expected to increase the efficiency of peritoneal dialysis, are not available. Here, we describe AqF026, an aquaporin agonist that is a chemical derivative of the arylsulfonamide compound furosemide. In the Xenopus laevis oocyte system, extracellular AqF026 potentiated the channel activity of human AQP1 by >20% but had no effect on channel activity of AQP4. We found that the intracellular binding site for AQP1 involves loop D, a region associated with channel gating. In a mouse model of peritoneal dialysis, AqF026 enhanced the osmotic transport of water across the peritoneal membrane but did not affect the osmotic gradient, the transport of small solutes, or the localization and expression of AQP1 on the plasma membrane. Furthermore, AqF026 did not potentiate water transport in Aqp1-null mice, suggesting that indirect mechanisms involving other channels or transporters were unlikely. Last, in a mouse gastric antrum preparation, AqF026 did not affect the Na-K-Cl cotransporter NKCC1. In summary, AqF026 directly and specifically potentiates AQP1-mediated water transport, suggesting that it deserves additional investigation for applications such as peritoneal dialysis or clinical situations associated with defective water handling.

Hypoxia differentially modulates the activity of pacemaker and smooth muscle cells in the guinea pig stomach antrum

Effects of hypoxic solution (O(2) tension, 161 +/- 11 mmHg) on electrical responses of the membrane (slow waves), intracellular Ca(2+)-responses measured by Fura-2 fluorescence (Ca-transients) and isometric mechanical responses (phasic contraction) were observed in circular smooth muscles isolated from the guinea-pig stomach antrum. In normoxic solution (O(2) tension, 362 +/- 28 mmHg), muscle cells generated slow waves spontaneously, and switching to hypoxic solution caused an increase in frequency and decrease in duration of slow waves, with no significant change in the resting membrane potential. Hypoxia also reduced the amplitude and duration and increased the frequency of Ca-transients. The increase in frequency of slow waves by hypoxia was prevented by cyclopiazonic acid (CPA) but not by carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), potassium cyanide (KCN) or low-Ca solution. The reduction by hypoxia of the duration of slow waves was prevented by CCCP or KCN but not by CPA or low-Ca solution. Hypoxia resulted in an increase in frequency and decrease in amplitude of phasic contractions, and the changes were prevented by CPA but not by CCCP. These results suggested that in antrum smooth muscle tissues, the increase in frequency of spontaneous activity by hypoxia is related to the enhanced function of the CPA-sensitive internal Ca-stores in pacemaker cells, while the inhibition in amplitude of phasic contractions by hypoxia may be mainly related to the decrease in Ca(2+) release from the CPA-sensitive internal stores in smooth muscle cells. It is concluded that in hypoxic solution, the function of internal Ca(2+) stores is enhanced in ICC-MY and is inhibited in smooth muscle cells in the guinea-pig stomach antrum.

Sodium-activated potassium current in guinea pig gastric myocytes

This study was designed to identify and characterize Na+-activated K+ current (I(K(Na))) in guinea pig gastric myocytes under whole-cell patch clamp. After whole-cell configuration was established under 110 mM intracellular Na+ concentration ([Na+]i) at holding potential of -60 mV, a large inward current was produced by external 60 mM K+([K+]degrees). This inward current was not affected by removal of external Ca2+. K+ channel blockers had little effects on the current (p>0.05). Only TEA (5 mM) inhibited steady-state current to 68+/-2.7% of the control (p<0.05). In the presence of K+ channel blocker cocktail (mixture of Ba2+, glibenclamide, 4-AP, apamin, quinidine and TEA), a large inward current was activated. However, the amplitude of the steady-state current produced under [K+]degrees (140 mM) was significantly smaller when Na+ in pipette solution was replaced with K+- and Li+ in the presence of K+ channel blocker cocktail than under 110 mM [Na+]i. In the presence of K+ channel blocker cocktail under low Cl- pipette solution, this current was still activated and seemed K+-selective, since reversal potentials (E(rev)) of various concentrations of [K+]degrees-induced current in current/voltage (I/V) relationship were nearly identical to expected values. R-56865 (10-20 microM), a blocker of I(K(Na)), completely and reversibly inhibited this current. The characteristics of the current coincide with those of I(K(Na)) of other cells. Our results indicate the presence of I(K(Na)) in guinea pig gastric myocytes.

Subtractive hybridization unravels a role for the ion cotransporter NKCC1 in the murine intestinal pacemaker

In the small intestine, interstitial cells of Cajal (ICC) surrounding the myenteric plexus generate the pacemaking slow waves that are essential for an efficient intestinal transit. The underlying molecular mechanisms of the slow wave are poorly known. Our aim was to identify ICC-specific genes and their function in the mouse jejunum. Suppression subtractive hybridization using two independent ICC-deficient mouse models identified 56 genes putatively downregulated in the muscularis propria compared with wild-type littermates. Differential expression was confirmed by real-time quantitative PCR for the tyrosine kinase receptor KIT, the established marker for ICC, and for the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Immunoreactivity for NKCC1 was detected in myenteric ICC but not in the ICC population located at the deep muscular plexus. NKCC1 was also expressed in enteric neurons and mucosal crypts. Bumetanide, an NKCC1 inhibitor, reversibly affected the shape, amplitude, and frequency of the slow waves. Similar alterations were observed in NKCC1 knockout mice. These data support the hypothesis that NKCC1 expressed in myenteric ICC is involved in the mechanism of slow waves in the murine jejunum.

[Cellular mechanism of the generation of spontaneous activity in gastric muscle]

In gastric smooth muscles, interstitial cells of Cajal (ICC) might be the pacemaker cells of spontaneous activities since ICC are rich in mitochondria and are connected with smooth muscle cells via gap junctions. Several types of ICC are distributed widely in the stomach wall. A group of ICC distributed in the myenteric layer (ICC-MY) were the pacemaker cells of gastrointestinal smooth muscles. Pacemaker potentials were generated in ICC-MY, and the potentials were conducted to circular smooth muscles to trigger slow waves and also conducted to longitudinal muscles to form follower potentials. In circular muscle preparations, interstitial cells distributed within muscle bundles (ICC-IM) produced unitary potentials, which were conducted to circular muscles to form slow potentials by summation. In mutant mice lacking inositol trisphosphate (IP(3)) receptor, slow waves were absent in gastric smooth muscles. The generation of spontaneous activity was impaired by the inhibition of Ca(2+)-release from internal stores through IP(3) receptors, inhibition of mitochondrial Ca(2+)-handling with proton pump inhibitors, and inhibition of ATP-sensitive K(+)-channels at the mitochondrial inner membrane. These results suggested that mitochondrial Ca(2+)-handling causes the generation of spontaneous activity in pacemaker cells. Possible involvement of protein kinase C (PKC) in the Ca(2+) signaling system was also suggested.

Generation of slow waves in the antral region of guinea-pig stomach--a stochastic process

1. Slow waves were recorded from the circular muscle layer of the antral region of guinea-pig stomach. Slow waves were abolished by 2APB, an inhibitor of IP(3)-induced Ca2+ release. 2. When the rate of generation of slow waves was monitored it was found to vary from cycle to cycle around a mean value. The variation persisted after abolishing neuronal activity with tetrodotoxin. 3. When simultaneous recordings were made from interstitial cells in the myenteric region (ICC(MY)) and smooth muscle cells of the circular layer, variations in the rate of generation of slow waves were found to be linked with variations in the rate of generation of driving potentials by ICC(MY). 4. A preparation was devised which consisted of the longitudinal muscle layer and ICC(MY). In this preparation ICC(MY) and smooth muscle cells lying in the longitudinal muscle layer generated driving potentials and follower potentials, synchronously. 5. Driving potentials had two components, a rapid primary component that was followed by a prolonged plateau component. Caffeine (3 mM) abolished the plateau component; conversely reducing the external concentration of calcium ions [Ca2+](o) mainly affected the primary component. 6. Analysis of the variations in the rate of generation of driving potentials indicated that this arose because both the duration of individual driving potentials and the interval between successive driving potentials varied. 7. It is suggested that the initiation of pacemaker activity in a network of ICC(MY) is a stochastic process, with the probability of initiating a driving potential slowly increasing, after a delay, from a low to a higher value following the previous driving potential.