Slow Waves Of Membrane Potential Research Articles

Interstitial cells of Cajal link excitatory and nitrergic inhibitory neurotransmission with slow-wave activity in the intestine

Interstitial cell of Cajal (ICC) form a cellular network that is embedded in the musculature of the gastrointestinal tract. Previous studies provided clear evidence that ICC generate a rhythmic pacemaker current which manifests itself as slow-waves in membrane potential of smooth muscle cells resulting in rhythmic bowel contractions. However, a role of ICC in the transmission of excitatory and inhibitory signals from enteric neurons to smooth muscle cells is highly controversial and remains an outstanding question. This is mainly due to the general lack of models and systems to study ICC function in adult animals at the level of genetics. The aim of this study was to investigate the role of ICC for excitatory and inhibitory nitrergic neurotransmission in the gut.

The unimodal distribution of sub–threshold, ongoing activity in cortical networks

The characterization of the subthreshold, ongoing activity in cortical neurons has been the focus of numerous studies. This activity, described as spontaneous slow waves in membrane potential, has been observed in a span of species in diverse cortical and subcortical areas. We here characterized membrane potential fluctuations in motor and the frontal association cortices cortical neurons of ketamine–xylazine anesthetized rats. We recorded from 95 neurons from a range of cortical depths to unravel the network and cellular mechanisms that shape the subthreshold ongoing spontaneous activity of these neurons. We define a unitary event that generates the subthreshold ongoing activity: giant synaptic potentials (GSPs). These events have a duration of 87 ± 50 ms and an amplitude of 19 ± 6.4 mV. They occur at a frequency of 3.7 ± 0.8 Hz and involve an increase in conductance change of 22 ± 21%. GSPs are mainly due to excitatory activity that occurs throughout all cortical layers, unaffected by the intrinsic properties of the cells. Indeed, blocking the GABAA receptors, a procedure that had a profound effect on cortical activity, did not alter these unitary events. We propose that this unitary event is composed of individual, excitatory synaptic potentials that appear at different levels of synchrony and that the level of synchrony determines the shape of the subthreshold activity.

The search for the origin of rhythmicity in intestinal contraction; from tissue to single cells.

More than a century ago, rhythmic propulsive contractile activity was observed in the intestine after blockade of nerve conduction, thus demonstrating a form of peristalsis that appeared to be under myogenic control. During this century, light and electron microscopic investigations provided the hypothesis that interstitial cells of Cajal (ICC) could be the cells of origin for this rhythmicity. In recent years, physiological studies demonstrated a link between the presence of electrical slow wave activity and the presence of ICC. The recognition that the ICC cell membrane harbours the Kit protein sparked rapid advancement in ICC research, and has been essential in the identification of ICC in tissue and in culture through Kit immunohistochemistry and kit mRNA reverse transcriptase polymerase chain reaction (RT-PCR). With these techniques, electrophysiology was carried out on positively identified single ICC in culture. These methods revealed that single ICC generate spontaneous rhythmic inward currents and slow waves in membrane potential, thus providing strong evidence that ICC generate the electrical pacemaker activity for the gut musculature.

Generation of slow waves in membrane potential is an intrinsic property of interstitial cells of Cajal.

To reveal the unique intrinsic properties of interstitial cells of Cajal (ICC), morphological and electrophysiological characteristics of isolated ICC from the adult mouse small intestine were investigated and compared with those of smooth muscle cells. All typical ultrastructural features of in situ ICC were evident in isolated ICC throughout the isolation procedure and short-term culture. With the use of the nystatin perforated patch-clamp technique, ICC demonstrated spontaneous voltage oscillations that were not abolished by hyperpolarization nor by L-type calcium channel blockers. This rhythmic activity occurred at room temperature at a frequency of 13.9 +/- 11.2 cycles/min, with an amplitude of 13.4 +/- 11.2 mV at membrane potentials from -20 to -70 mV. Smooth muscle cells from the same culture only generated voltage-sensitive action potentials above the threshold potential of -35 mV. Hyperpolarization as well as the addition of L-type calcium channel blockers abolished the action potentials. In whole cell voltage-clamp recordings from ICC, a large noninactivating outward current was observed to be activated (5% threshold) at -49.6 mV with a half-activation voltage of -18.7 mV and slope factor of 9.9 mV. In contrast, in smooth muscle cells, smaller outward currents with distinctive transient outward currents were present. In conclusion, the generation of L-type calcium channel blocker-insensitive slow waves in membrane potential is a unique intrinsic property of ICC.

Interstitial cells of Cajal generate a rhythmic pacemaker current.

Networks of interstitial cells of Cajal embedded in the musculature of the gastrointestinal tract are involved in the generation of electrical pacemaker activity for gastrointestinal motility. This pacemaker activity manifests itself as rhythmic slow waves in membrane potential, and controls the frequency and propagation characteristics of gut contractile activity. Mice that lack a functional Kit receptor fail to develop the network of interstitial cells of Cajal associated with Auerbach's plexus in the mouse small intestine and do not generate slow wave activity. These cells could provide an essential component of slow wave activity (for example, a biochemical trigger that would be transferred to smooth muscle cells), or provide an actual pacemaker current that could initiate slow waves. Here we provide direct evidence that a single cell, identified as an interstitial cell of Cajal by light microscopy, electron microscopy and expression of Kit mRNA, generates spontaneous contractions and a rhythmic inward current that is insensitive to L-type calcium channel blockers. Identification of the pacemaker of gut motility will aid in the elucidation of the pathophysiology of intestinal motor disorders, and provide a target cell for pharmacological treatment.

A minimum of fuel is necessary for tolbutamide to mimic the effects of glucose on electrical activity in pancreatic beta-cells.

Glucose stimulation of pancreatic beta-cells triggers electrical activity (slow waves of membrane potential with superimposed spikes) that is best monitored with intracellular microelectrodes. Closure of ATP-sensitive K+ channels underlies the depolarization to the threshold potential and participates in the increase in electrical activity produced by suprathreshold (>7 mM) concentrations of glucose, but it is still unclear whether this is the sole mechanism of control. This was investigated by testing whether blockade of ATP-sensitive K+ channels by low concentrations of tolbutamide is able to mimic the effects of glucose on mouse beta-cell electrical activity even in the absence of the sugar. The response to tolbutamide was influenced by the duration of the perifusion with the low glucose medium. Tolbutamide (25 microM) caused a rapid and sustained depolarization with continuous activity after 6 min of perifusion of the islet with 3 mM glucose, and a progressive depolarization with slow waves of the membrane potential after 20 min. In the absence of glucose, the beta-cell response to tolbutamide was a transient phase of depolarization with rare slow waves (6 min) or a silent, small, but sustained, depolarization (20 min). Readministration of 3 mM glucose was sufficient to restore slow waves, whereas an increase in the glucose concentration to 5 and 7 mM was followed by a lengthening of the slow waves and a shortening of the intervals. In contrast, induction of slow waves by tolbutamide proved very difficult in the absence of glucose, because the beta-cell membrane tended to depolarize from a silent level to the plateau level, at which electrical activity is continuous. Azide, a mitochondrial poison, abrogated the electrical activity induced by tolbutamide in the absence of glucose, which demonstrates the influence of the metabolism of endogenous fuels on the response to the sulfonylurea. The partial repolarization that azide also produced was reversed by increasing the concentration of tolbutamide, but reappearance of the spikes required the addition of glucose. It is concluded that inhibition of ATP-sensitive K+ channels is not the only mechanism by which glucose controls electrical activity in beta-cells.

Adenosine triphosphate-sensitive K+ channels may not be the sole regulators of glucose-induced electrical activity in pancreatic B-cells.

Stimulation of insulin release by glucose requires Ca2+ influx in pancreatic B-cells. This influx occurs during phases of electrical activity (slow waves of membrane potential with superimposed spikes) that can be monitored with intracellular microelectrodes. It has been suggested that closure of ATP-sensitive K+ channels contributes to the increase in electrical activity (and, hence, in Ca2+ influx and insulin release) produced by suprathreshold (greater than 7 mM) concentrations of glucose. If this is the sole mechanism of control, the decrease in electrical activity that follows a decrease in glucose concentration should be mimicked by opening these ATP-sensitive K+ channels. This was achieved by diazoxide, which selectively and directly acts at the channel level (without decreasing B-cell metabolism), and azide, which indirectly opens the channels by inhibiting mitochondrial ATP production. Stepwise lowering of the glucose concentration from 15 to 8 mM progressively decreased electrical activity in B-cells. This decrease was characterized by a shortening of the slow waves and a lengthening of the intervals between the slow waves, with little change in slow wave frequency. Similar changes followed the addition of azide (250-750 microM) to a medium containing 15 mM glucose. In contrast, in the presence of 15 mM glucose, diazoxide (5-20 microM) considerably increased the interval duration, but did not shorten the slow waves, thus causing a marked fall in slow wave frequency. In B-cells persistently depolarized by 30 mM glucose, diazoxide restored slow waves and intervals that were much longer than those recorded when the same cells were stimulated by 15 mM glucose. In conclusion, decreasing mitochondrial ATP production with azide is more able to reproduce the effects of a decrease in glucose concentration on electrical activity in B-cells than a direct pharmacological opening of ATP-sensitive K+ channels with diazoxide. This suggests that ionic channels other than ATP-sensitive K+ channels are under metabolic control and may contribute to the regulation of electrical activity by glucose.

Glucose modulation of spike activity independently from changes in slow waves of membrane potential in mouse B-cells.

In mouse B-cells glucose induces a typical electrical activity consisting of slow waves of the membrane potential with spikes superimposed on the plateau. As the concentration of glucose is raised the number of spikes per minute increases. However, this increase could simply be due to the concomitant lengthening of the slow waves. We thus investigated whether glucose can influence spike activity when no slow waves occur. Persistent depolarization to the plateau potential was achieved at 3 mM glucose by tolbutamide or at 10 mM glucose by low Ca2+, by arginine or by ouabain. Under all these conditions, raising the concentration of glucose increased the spike frequency without changing the plateau potential. Similar effects were produced by tolbutamide which does not affect B-cell metabolism but directly blocks K+-ATP channels. The spike frequency could also be increased by arginine, which, however, consistently depolarized the membrane. In conclusion, spike activity in B-cells can be influenced by glucose independently from changes in slow wave duration. This indicates that some K+-ATP channels, a target for both glucose and tolbutamide, are still open when the membrane is depolarized at the plateau, or that these two agents share another yet unidentified target involved in spike generation.

ATP-sensitive K+ channels may control glucose-induced electrical activity in pancreatic B-cells

Tolbutamide and diazoxide, which respectively decrease and increase the activity of ATP-sensitive K+ channels, were used to investigate whether these channels play a role in the control of glucose-induced electrical activity (slow waves with spikes) in pancreatic B-cells. Addition of tolbutamide to a medium containing 10 mM glucose largely mimicked the effects of a rise in glucose concentration from 10 to 15 mM on electrical activity, ionic fluxes and insulin release. Tolbutamide was still active in the presence of maximally effective concentrations of glucose (30-40 mM). Diazoxide restored slow waves of membrane potential in B-cells which were persistently depolarized by 30 mM glucose. It is concluded that ATP-sensitive K+ channels are still operative at high glucose concentrations. These channels could thus be one target on which physiological concentrations of glucose act to regulate electrical activity in B-cells and, hence, insulin release.

Helix neurons invovled in control of rhythmic movement in the pneumostome

Three neurons capable of generating coordinated bursting activity or synchronized slow-wave fluctuations in membrane potential (MP) were identified in the left parietal ganglion ofHelix pomatia. The function of these units contributes to regulating rhythmic opening and closing movements in the pneumostome. Both bursting activity and slow-wave neuronal MP synchronize with rhythmic movements of the pneumostome. Onset of bursting activity and fluctuations in MP on the one hand or suppression of these effects due to different influences on the cells on the other leads to initiation or extinction of pneumostome movements respectively. These neurons do not exhibit endogenous bursting activity but do produce a fairly high rate of firing activity without bursting pattern and without slow-waves in MP in isolation. Bursting activity occurs in these neurons in the intact central nervous system (CNS) as a result of gigantic synchronized IPSP in some cases and due to the aforementioned slow waves in MP and in others. No direct chemically- or electrically-operated synaptic connections exist between the three cells. Serotonin triggers both waves in MP and bursting activity in all three neurons in the intact CNS and exerts a pronounced hyperpolarizing action on each of these factors in isolation.

Inosine partially mimics the effects of glucose on ionic fluxes, electrical activity, and insulin release in mouse pancreatic B-cells

The purine ribonucleoside inosine is known to be metabolized in islet cells (its ribose moiety feeds into the pentose-phosphate cycle) and stimulate insulin release, but the mechanisms of this stimulation have not been established. These were investigated with mouse islets. In the absence of glucose, 5 mM inosine decreased 86Rb+ efflux from islet cells, depolarized the B-cell membrane, induced electrical activity (slow waves of membrane potential with bursts of spikes on the plateau), accelerated 45Ca2+ efflux and stimulated insulin release with the same efficiency as 10 mM glucose. Raising the concentration of inosine to 20 mM only had a slight further effect and, in particular, failed to cause persistent depolarization of the B-cell membrane. The electrical activity triggered by inosine was blocked by cobalt, and the stimulation of 45Ca2+ efflux and insulin release was abolished in a Ca2+-free medium. The effects of 10 mM glucose on electrical activity, 45Ca2+ efflux and insulin release were augmented by as little as 0.5 mM inosine. All effects of inosine were abolished by an inhibitor of nucleoside transport (nitrobenzylthioguanosine) and markedly impaired by inhibitors of nucleoside phosphorylase (formycin B) or of glycolysis (iodoacetate). In conclusion, inosine metabolism in B-cells induces insulin release by triggering the same sequence of events as glucose metabolism: a decrease of K+ permeability of the B-cell membrane, leading to depolarization and activation of voltage-dependent Ca channels.

Effects of a calcium channel agonist on the electrical, ionic and secretory events in mouse pancreatic B-cells

The changes in pancreatic B-cell function produced by a Ca channel agonist, the dihydropyridine derivative CGP 28392, have been studied with mouse islets. CGP 28392 (5 μM) modified the electrical activity induced in B-cells by 10 mM glucose: the duration and the amplitude of the slow waves of membrane potential increased, but the overall spike activity decreased. Simultaneously, CGP 28392 markedly increased insulin release and 45Ca2+ efflux, and slightly accelerated 86Rb+ efflux from islet cells. These latter effects were abolished by omission of extracellular Ca2+. Qualitatively similar changes were observed at 15 mM glucose, whereas CGP 28392 was ineffective at 3 mM glucose. These results strongly suggest that an influx of Ca2+ contributes to the slow waves of membrane potential triggered by glucose, and underline the importance of this influx of Ca2+ for the control of insulin release by the sugar.

The electrogenic sodium-potassium pump of mouse pancreatic B-cells.

1. The contribution of the sodium pump to the membrane potential of mouse pancreatic B-cells was studied with micro-electrodes.2. In 0 or 3 mM-glucose, ouabain rapidly (within 2 min) depolarized the B-cell membrane by an average of 7 mV, whereas K omission hyperpolarized it markedly.3. In 6 or 7 mM-glucose, ouabain still produced depolarization, but K omission had no consistent effect. Both induced electrical activity in certain cells.4. In 10 mM-glucose, withdrawal of ouabain or K re-introduction caused a transient hyperpolarization with suppression of electrical activity. Duration and amplitude of the hyperpolarization increased with the time of pump blockade and with the concentration of ouabain.5. The hyperpolarization following K re-admission was abolished by ouabain and that following ouabain withdrawal was prevented by K omission. Re-admission of various K concentrations showed that the hyperpolarization was not due to depletion of K just outside of the membrane.6. In 10 mM-glucose, the membrane potential of B-cells exhibited repetitive slow waves with bursts of spikes on the plateau. These electrical events were modified by ouabain in a dose-dependent manner. The frequency of the slow waves augmented markedly because of an increase in the slope of the pre-potential and a shortening of the intervals; the slope of their repolarization phase decreased, but their duration was not changed.7. Omission of K increased the slope of the pre-potential and the frequency of the slow waves. It also accelerated their repolarization phase and reduced their duration, likely because of the increase in driving force for K efflux. Increasing K concentration to 8 mM slowed the repolarization phase and lengthened the slow waves without changing their frequency.8. Even when K permeability of the B-cell membrane was increased by high extracellular Ca, ouabain and K omission augmented the frequency of the slow waves.9. In 0 or 10 mM-glucose, ouabain increased (86)Rb(+) efflux from perifused islets, whereas K omission decreased it. In 10 mM-glucose, a marked decrease in (86)Rb(+) efflux accompanied ouabain withdrawal and K re-introduction. The hyperpolarization is thus not due to an increase in K permeability.10. It is concluded that, in pancreatic B-cells, the sodium pump is truly electrogenic, contributes to the resting potential and modulates the slow waves of membrane potential induced by glucose. Rapid changes in insulin release occurring upon inhibition or activation of the sodium pump may thus be due to the changes in B-cell membrane potential.

Nature of the intestinal slow-wave frequency gradient.

Nature of the intestinal slow-wave frequency gradient.

ACTION CURRENTS IN STOMACH AND INTESTINE

ArticleACTION CURRENTS IN STOMACH AND INTESTINEWalter C. AlvarezWalter C. AlvarezFrom the George Williams Hooper Foundation for Medical Research, University of California Medical School, San FranciscoPublished Online:01 Jan 1922https://doi.org/10.1152/ajplegacy.1922.58.3.476MoreSectionsPDF (3 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformationCited ByConditional genetic deletion of Ano1 in interstitial cells of Cajal impairs Ca2+ transients and slow waves in adult mouse small intestineJohn Malysz, Simon J. Gibbons, Siva A. Saravanaperumal, Peng Du, Seth T. Eisenman, Chike Cao, Uhtaek Oh, Dieter Saur, Sabine Klein, Tamas Ordog, and Gianrico Farrugia1 March 2017 | American Journal of Physiology-Gastrointestinal and Liver Physiology, Vol. 312, No. 3Origin and propagation of the slow wave in the canine stomach: the outlines of a gastric conduction systemWim J. E. P. Lammers, Luc Ver Donck, Betty Stephen, Dirk Smets, and Jan A. J. Schuurkes1 June 2009 | American Journal of Physiology-Gastrointestinal and Liver Physiology, Vol. 296, No. 6Generation of slow waves in membrane potential is an intrinsic property of interstitial cells of CajalJonathan C. F. Lee, Lars Thuneberg, Irene Berezin, and Jan D. Huizinga1 August 1999 | American Journal of Physiology-Gastrointestinal and Liver Physiology, Vol. 277, No. 2A 100-year perspective on gastrointestinal motilityJ. H. Szurszewski1 March 1998 | American Journal of Physiology-Gastrointestinal and Liver Physiology, Vol. 274, No. 3In vitro analysis of rat intestinal wall movements at rest and during propagated contraction: a new methodThierry Benard, Michel Bouchoucha, Michel Dupres, and Paul-Henri Cugnenc1 October 1997 | American Journal of Physiology-Gastrointestinal and Liver Physiology, Vol. 273, No. 4 More from this issue > Volume 58Issue 3January 1922Pages 476-493 Copyright & PermissionsCopyright © 1922 by American Physiological Societyhttps://doi.org/10.1152/ajplegacy.1922.58.3.476History Received 29 July 1921 Published online 1 January 1922 Published in print 1 January 1922 Metrics