Dependence of electrical slow waves of canine colonic smooth muscle on calcium gradient

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

The effects of mitochondrial inhibitors on Ca2+ signalling and electrical conductances required for pacemaking in interstitial cells of Cajal in the mouse small intestine

Interstitial cells of Cajal (ICC-MY) are pacemakers that generate and propagate electrical slow waves in gastrointestinal (GI) muscles. Slow waves appear to be generated by the release of Ca²⁺ from intracellular stores and activation of Ca²⁺-activated Cl^- channels (Ano1). Conduction of slow waves to smooth muscle cells coordinates rhythmic contractions. Mitochondrial Ca²⁺ handling is currently thought to be critical for ICC pacemaking. Protonophores, inhibitors of the electron transport chain (FCCP, CCCP or antimycin) or mitochondrial Na⁺/Ca²⁺ exchange blockers inhibited slow waves in several GI muscles. Here we utilized Ca²⁺ imaging of ICC in small intestinal muscles in situ to determine the effects of mitochondrial drugs on Ca²⁺ transients in ICC. Muscles were obtained from mice expressing a genetically encoded Ca²⁺ indicator (GCaMP3) in ICC. FCCP, CCCP, antimycin, a uniporter blocker, Ru360, and a mitochondrial Na⁺/Ca²⁺ exchange inhibitor, CGP-37157 inhibited Ca²⁺ transients in ICC-MY. Effects were not due to depletion of ATP, as oligomycin did not affect Ca²⁺ transients. Patch-clamp experiments were performed to test the effects of the mitochondrial drugs on key pacemaker conductances, Ano1 and T-type Ca²⁺ (Ca_V3.2), in HEK293 cells. Antimycin blocked Ano1 and reduced Ca_V3.2 currents. CCCP blocked Ca_V3.2 current but did not affect Ano1 current. Ano1 and Cav3.2 currents were inhibited by CGP-37157. Inhibitory effects of mitochondrial drugs on slow waves and Ca²⁺ signalling in ICC can be explained by direct antagonism of key pacemaker conductances in ICC that generate and propagate slow waves. A direct obligatory role for mitochondria in pacemaker activity is therefore questionable.

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.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

Electrical slow waves in the mouse oviduct are dependent upon a calcium activated chloride conductance encoded by Tmem16a

Myosalpinx contractions are critical for oocyte transport along the oviduct. A specialized population of pacemaker cells-oviduct interstitial cells of Cajal-generate slow waves, the electrical events underlying myosalpinx contractions. The ionic basis of oviduct pacemaker activity is unknown. We examined the role of a new class of Ca(2+)-activated Cl(-) channels (CaCCs)-anoctamin 1, encoded by Tmem16a-in oviduct slow wave generation. RT-PCR revealed the transcriptional expression of Tmem16a-encoded CaCCs in the myosalpinx. Intracellular microelectrode recordings were performed in the presence of two pharmacologically distinct Cl(-) channel antagonists, anthracene-9-carboxylic acid and niflumic acid. Both of these inhibitors caused membrane hyperpolarization, reduced the duration of slow waves, and ultimately inhibited pacemaker activity. Niflumic acid also inhibited propagating calcium waves within the myosalpinx. Slow waves were present at birth in wild-type and heterozygous oviducts but failed to develop by birth in mice homozygous for a null allele of Tmem16a (Tmem16a(tm1Bdh/tm1Bdh)). These data suggest that Tmem16a-encoded CaCCs contribute to membrane potential and are responsible for the upstroke and plateau phases of oviduct slow waves.

Analysis of pacemaker activity in the human stomach

Extracellular electrical recording and studies using animal models have helped establish important concepts of human gastric physiology. Accepted standards include electrical quiescence in the fundus, 3 cycles per minute (cpm) pacemaker activity in corpus and antrum, and a proximal-to-distal slow wave frequency gradient. We investigated slow wave pacemaker activity, contractions and distribution of interstitial cells of Cajal (ICC) in human gastric muscles. Muscles were obtained from patients undergoing gastric resection for cancer, and the anatomical locations of each specimen were mapped by the operating surgeon to 16 standardized regions of the stomach. Electrical slow waves were recorded with intracellular microelectrodes and contractions were recorded by isometric force techniques. Slow waves were routinely recorded from gastric fundus muscles. These events had similar waveforms as slow waves in more distal regions and were coupled to phasic contractions. Gastric slow wave frequency was significantly greater than 3 cpm in all regions of the stomach. Antral slow wave frequency often exceeded the highest frequency of pacemaker activity in the corpus. Chronotropic mechanisms such as muscarinic and prostaglandin receptor binding, stretch, extracelluar Ca(2+) and temperature were unable to explain the observed slow wave frequency that exceeded accepted normal levels. Muscles from all regions through the thickness of the muscularis demonstrated intrinsic pacemaker activity, and this corresponded with the widespread distribution in ICC we mapped throughout the tunica muscularis. Our findings suggest that extracellular electrical recording has underestimated human slow wave frequency and mechanisms of human gastric function may differ from standard laboratory animal models.

Electrical slow waves in the mouse oviduct are dependent on extracellular and intracellular calcium sources

Spontaneous contractions of the myosalpinx are critical for oocyte transport along the oviduct. Slow waves, the electrical events that underlie myosalpinx contractions, are generated by a specialized network of pacemaker cells called oviduct interstitial cells of Cajal (ICC-OVI). The ionic basis of oviduct pacemaker activity is unknown. Intracellular recordings and Ca(2+) imaging were performed to examine the role of extracellular and intracellular Ca(2+) sources in slow wave generation. RT-PCR was performed to determine the transcriptional expression of Ca(2+) channels. Molecular studies revealed most isoforms of L- and T-type calcium channels (Cav1.2,1.3,1.4,3.1,3.2,3.3) were expressed in myosalpinx. Reduction of extracellular Ca(2+) concentration ([Ca(2+)](o)) resulted in the abolition of slow waves and myosalpinx contractions without significantly affecting resting membrane potential (RMP). Spontaneous Ca(2+) waves spread through ICC-OVI cells at a similar frequency to slow waves and were inhibited by reduced [Ca(2+)](o). Nifedipine depolarized RMP and inhibited slow waves; however, pacemaker activity returned when the membrane was repolarized with reduced extracellular K(+) concentration ([K(+)](o)). Ni(2+) also depolarized RMP but failed to block slow waves. The importance of ryanodine and inositol 1,4,5 trisphosphate-sensitive stores were examined using ryanodine, tetracaine, caffeine, and 2-aminoethyl diphenylborinate. Results suggest that although both stores are involved in regulation of slow wave frequency, neither are exclusively essential. The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor cyclopiazonic acid inhibited pacemaker activity and Ca(2+) waves suggesting that a functional SERCA pump is necessary for pacemaker activity. In conclusion, results from this study suggest that slow wave generation in the oviduct is voltage dependent, occurs in a membrane potential window, and is dependent on extracellular calcium and functional SERCA pumps.

Caffeine inhibits nonselective cationic currents in interstitial cells of Cajal from the murine jejunum

Interstitial cells of Cajal (ICC) discharge unitary potentials in gastrointestinal muscles that constitute the basis for pacemaker activity. Caffeine has been used to block unitary potentials, but the ionic conductance responsible for unitary potentials is controversial. We investigated currents in cultured ICC from murine jejunum that may underlie unitary potentials and studied the effects of caffeine. Networks of ICC generated slow wave events under current clamp, and these events were blocked by caffeine in a concentration-dependent manner. Single ICC generated spontaneous transient inward currents (STICs) under voltage clamp at -60 mV and noisy voltage fluctuations in current clamp. STICs were unaffected when the equilibrium potential for Cl- (ECl) was set to -60 mV (excluding Cl- currents) and reversed at 0 mV, demonstrating that a nonselective cationic conductance, and not a Cl- conductance, is responsible for STICs in ICC. Caffeine inhibited STICs in a concentration-dependent manner. Reduced intracellular Ca2+ and calmidazolium (CMZ; 1 microM) activated persistent inward, nonselective cation currents in ICC. Currents activated by CMZ and by dialysis of cells with 10 mM BAPTA were also inhibited by caffeine. Excised inside-out patches contained channels that exhibited spontaneous openings, and resulting currents reversed at 0 mV. Channel openings were increased by reducing Ca2+ concentration from 10(-6) M to 10(-8) M. CMZ (1 microM) also increased openings of nonselective cation channels. Spontaneous currents and channels activated by CMZ were inhibited by caffeine (5 mM). The findings demonstrate that the Ca2+-inhibited nonselective cation channels that generate STICs in ICC are blocked directly by caffeine. STICs are responsible for unitary potentials in intact muscles, and the block of these events by caffeine is consistent with the idea that a nonselective cation conductance underlies unitary potentials in ICC.

Calcium-associated mechanisms in gut pacemaker activity

A considerable body of evidence has revealed that interstitial cells of Cajal (ICC), identified with c-Kit-immunoreactivity, act as gut pacemaker cells, with spontaneous Ca²⁺ activity in ICC as the probable primary mechanism. Namely, intracellular (cytosolic) Ca²⁺ oscillations in ICC periodically activate plasmalemmal Ca²⁺-dependent ion channels and thereby generate pacemaker potentials. This review will, thus, focus on Ca²⁺-associated mechanisms in ICC in the gastrointestinal (GI) tract, including auxiliary organs.

Voltage-gated Ca2+ currents are necessary for slow-wave propagation in the canine gastric antrum

Electrical slow waves determine the timing and force of peristaltic contractions in the stomach. Slow waves originate from a dominant pacemaker in the orad corpus and propagate actively around and down the stomach to the pylorus. The mechanism of slow-wave propagation is controversial. We tested whether Ca(2+) entry via a voltage-dependent, dihydropyridine-resistant Ca(2+) conductance is necessary for active propagation in canine gastric antral muscles. Muscle strips cut parallel to the circular muscle were studied with intracellular electrophysiological techniques using a partitioned-chamber apparatus. Slow-wave upstroke velocity and plateau amplitude decreased from the greater to the lesser curvature, and this corresponded to a decrease in the density of interstitial cells of Cajal in the lesser curvature. Slow-wave propagation velocity between electrodes impaling cells in two regions of muscle and slow-wave upstroke and plateau were measured in response to experimental conditions that reduce the driving force for Ca(2+) entry or block voltage-dependent Ca(2+) currents. Nicardipine (0.1-1 microM) did not affect slow-wave upstroke or propagation velocities. Upstroke velocity, amplitude, and propagation velocity were reduced in a concentration-dependent manner by Ni(2+) (1-100 microM), mibefradil (10-30 microM), and reduced extracellular Ca(2+) (0.5-1.5 mM). Depolarization (by 10-15 mM K(+)) or hyperpolarization (10 microM pinacidil) also reduced upstroke and propagation velocities. The higher concentrations (or lowest Ca(2+)) of these drugs and ionic conditions tested blocked slow-wave propagation. Treatment with cyclopiazonic acid to empty Ca(2+) stores did not affect propagation. These experiments show that voltage-dependent Ca(2+) entry is obligatory for the upstroke phase of slow waves and active propagation.

Kit-like immunoreactivity in the zebrafish gastrointestinal tract reveals putative ICC

Gastrointestinal (GI) motility results from the coordinated actions of enteric neurons, interstitial cells of Cajal (ICC), and smooth muscle cells. The GI tract of the zebrafish has a cellular anatomy that is essentially similar to humans. Although enteric nerves and smooth muscle cells have been described, it is unknown if ICC are present in the zebrafish. Immunohistochemistry and PCR were used determine expression for the zebrafish Kit orthologue in the zebrafish gastrointestinal tract. Cells displaying Kit-like immunoreactivity were identified in the muscular layers of the adult zebrafish gastrointestinal tract. Two layers of Kit-positive cells were identified, one with multipolar cells located between the longitudinal and circular smooth muscle layers and one with simple bipolar cells located deep in the circular muscle layer. Primers specifically designed to amplify mRNA coding for two zebrafish kit genes, kita and kitb, and two kit ligands, kitla and kitlb, amplified the expected transcript from total RNA isolated from zebrafish GI tissues. The Sparse mutant, a kita null mutant, showed reduced contraction frequency and increased size of the GI tract indicating a functional role for kita. These data establish the presence of a cellular network with Kit-like immunoreactivity in the myenteric plexus region of the zebrafish GI tract, adjacent to enteric neurons. Expression of kita and kitb, and the ligands kitla and kitlb, were verified in the adult GI tract. The anatomical arrangement of the Kit-positive cells strongly suggests that they are ICC.

Interstitial cells of cajal as pacemakers in the gastrointestinal tract

In the gastrointestinal tract, phasic contractions are caused by electrical activity termed slow waves. Slow waves are generated and actively propagated by interstitial cells of Cajal (ICC). The initiation of pacemaker activity in the ICC is caused by release of Ca2+ from inositol 1,4,5-trisphosphate (IP3) receptor-operated stores, uptake of Ca2+ into mitochondria, and the development of unitary currents. Summation of unitary currents causes depolarization and activation of a dihydropyridine-resistant Ca2+ conductance that entrains pacemaker activity in a network of ICC, resulting in the active propagation of slow waves. Slow wave frequency is regulated by a variety of physiological agonists and conditions, and shifts in pacemaker dominance can occur in response to both neural and nonneural inputs. Loss of ICC in many human motility disorders suggests exciting new hypotheses for the etiology of these disorders.

Voltage-dependent calcium entry underlies propagation of slow waves in canine gastric antrum

Electrical slow waves in gastrointestinal (GI) muscles are generated by interstitial cells of Cajal (ICC), and these events actively propagate through networks of ICC within the walls of GI organs. The mechanism by which spontaneously active pacemaker sites throughout ICC networks are entrained to produce orderly propagation of slow waves is unresolved. A three-chambered partition bath was used to test the effects of agents that affect metabolism, membrane potential and voltage-dependent Ca(2+) entry on slow wave propagation in canine antral smooth muscle strips. Slow waves evoked by electrical field stimulation actively propagated from end to end of antral muscle strips with a constant latency between two points of recording. When the central chamber of the bath was perfused with low-temperature solutions, mitochondrial inhibitors, reduced extracellular Ca(2+) or blockers of voltage-dependent Ca(2+) channels, active propagation failed. Depolarization or hyperpolarization of the tissue within the central chamber also blocked propagation. Blockade of propagation by reduced extracellular Ca(2+) and inhibitors of dihydropyridine-resistant Ca(2+) channels suggests that voltage-dependent Ca(2+) entry may be the 'entrainment factor' that facilitates active propagation of slow waves in the gastric antrum.

Ionic conductances involved in generation and propagation of electrical slow waves in phasic gastrointestinal muscles

Considerable work has led many to conclude that interstitial cells of Cajal (ICC) are the pacemaker cells of the gastrointestinal (GI) tract. These cells form electrically coupled networks within the pacemaker regions of the GI tract, and ICC are electrically coupled to smooth muscle cells. ICC express unique ion channels that periodically produce inward (pacemaker) currents. Recent work has suggested that the inward current is produced by a calcium (Ca2+)-regulated, nonselective cation conductance. Channels responsible for this conductance oscillate in open probability in response to the periodic drop in intracellular Ca2+ concentration during the slow wave cycle. Pacemaker activity generates slow waves that are propagated actively through ICC networks. Depolarization coordinates the pacemaker activity through the ICC network by activating a dihydropyridine-resistant Ca2+ conductance. Entry of small amounts of Ca2+ into ICC entrains spontaneous pacemaker activity and produces cell-to-cell propagation of slow waves. This review discusses the mechanisms and conductances involved in generation and propagation of electrical slow waves in ICC.

Properties of pacemaker potentials recorded from myenteric interstitial cells of Cajal distributed in the mouse small intestine

Recording of electrical responses from isolated small intestine of mice using conventional microelectrodes revealed two types of potential, a pacemaker potential and a slow wave, both with rapid rising primary components and following plateau components. The rate of rise and peak amplitude were greater for pacemaker potentials than for slow waves, and the plateau component was smaller in slow waves than in pacemaker potentials. Both potentials oscillated at a similar frequency (20-30 min-1). Unitary potentials often discharged during the interval between pacemaker potentials. Infusion of Lucifer Yellow allowed visualization of the recorded cells; pacemaker potentials were recorded from myenteric interstitial cells of Cajal (ICC-MY) while slow waves were recorded from circular smooth muscle cells. Pacemaker potentials were characterized as follows: the primary component was inhibited by Ni2+, Ca2+-free solution or depolarization with high-K+ solution, the plateau component was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), an inhibitor of Ca2+-activated Cl- channels, low [Cl-]o solution or Ca2+-free solution, and the generation of potentials was abolished by co-application of Ni2+and DIDS or by chelating intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM). These results indicate that in the mouse small intestine ICC-MY generate pacemaker potentials with two components in situ; the primary and plateau components may be generated by activation of voltage-dependent Ca2+-permeable channels and Ca2+-activated Cl- channels, respectively. Slow waves are generated in circular smooth muscles via electrotonic spread of pacemaker potentials. These properties of intestinal pacemaker potentials are considered essentially similar to those of gastric pacemaker potentials.

Propagation of slow waves requires IP3 receptors and mitochondrial Ca2+ uptake in canine colonic muscles

In the gastrointestinal (GI) tract electrical slow waves yield oscillations in membrane potential that periodically increase the open probability of voltage-dependent Ca2+ channels and facilitate phasic contractions. Slow waves are generated by the interstitial cells of Cajal (ICC), and these events actively propagate through ICC networks within the walls of GI organs. The mechanism that entrains spontaneously active pacemaker sites throughout ICC networks to produce regenerative propagation of slow waves is unresolved. Agents that block inositol 1,4,5-trisphosphate (IP3) receptors and mitochondrial Ca2+ uptake were tested on the generation of slow waves in the canine colon. A partitioned chamber apparatus was used to test the effects of blocking slow-wave generation on propagation. We found that active propagation occurred along strips of colonic muscle, but when the pacemaker mechanism was blocked in a portion of the tissue, slow waves decayed exponentially from the point where the pacemaker mechanism was inhibited. An IP3 receptor inhibitor, mitochondrial inhibitors, low external Ca2+, and divalent cations (Mn2+ and Ni2+) caused exponential decay of the slow waves in regions of muscle exposed to these agents. These data demonstrate that the mechanism that initiates slow waves is reactivated from cell-to-cell during the propagation of slow waves. Voltage-dependent conductances present in smooth muscle cells are incapable of slow-wave regeneration. The data predict that partial loss of or disruptions to ICC networks observed in human motility disorders could lead to incomplete penetration of slow waves through GI organs and, thus, to defects in myogenic regulation.

Spatial localization and properties of pacemaker potentials in the canine rectoanal region

The present study investigated the spatial organization of electrical activity in the canine rectoanal region and its relationship to motility patterns. Contraction and resting membrane potential (E(m)) were measured from strips of circular muscle isolated 0.5-8 cm from the anal verge. Rapid frequency [25 cycles/min (cpm)] E(m) oscillations (MPOs, 12 mV amplitude) were present across the thickness of the internal anal sphincter (IAS; 0.5 cm) and E(m) was constant (-52 mV). Between the IAS and the proximal rectum an 18 mV gradient in E(m) developed across the muscle thickness with the submucosal edge at -70 mV and MPOs were replaced with slow waves (20 mV amplitude, 6 cpm). Slow waves were of greatest amplitude at the submucosal edge. Nifedipine (1 micro M) abolished MPOs but not slow waves. Contractile frequency changes were commensurate with the changes in pacemaker frequency. Our results suggest that changing motility patterns in the rectoanal region are associated with differences in the characteristics of pacemaker potentials as well as differences in the sites from which these potentials emanate.

Voltage-dependent inward currents of interstitial cells of Cajal from murine colon and small intestine

Electrical slow waves in gastrointestinal (GI) muscles are generated by pacemaker cells, known as interstitial cells of Cajal (ICC). The pacemaker conductance is regulated by periodic release of Ca2+ from inositol 1,4,5-trisphosphate (IP(3)) receptor-operated stores, but little is known about how slow waves are actively propagated. We investigated voltage-dependent Ca2+ currents in cultured ICC from the murine colon and small intestine. ICC, identified by kit immunohistochemistry, were spontaneously active under current clamp and generated transient inward (pacemaker) currents under voltage clamp. Depolarization activated inward currents due to entry of Ca2+. Nicardipine (1 microM) blocked only half of the voltage-dependent inward current. After nicardipine, there was a shift in the potential at which peak current was obtained (-15 mV), and negative shifts in the voltage dependence of activation and inactivation of the remaining voltage-dependent inward current. The current that was resistant to dihydropyridine (I(VDDR)) displayed kinetics, ion selectivity and pharmacology that differed from dihydropyridine-sensitive Ca2+ currents. I(VDDR) was increased by elevating extracellular Ca2+ from 2 to 10 mM, and this caused a +30 mV shift in reversal potential. I(VDDR) was blocked by Ni2+ (100 microM) or mebefradil (1 microM) but was not affected by blockers of N-, P- or Q-type Ca2+ channels. Equimolar replacement of Ca2+ with Ba2+ reduced I(VDDR) without effects on inactivation kinetics. BayK8644 had significantly less effect on I(VDDR) than on I(VDIC). In summary, two components of inward Ca2+ current were resolved in ICC of murine small intestine and colon. Since slow waves persist in the presence of dihydropyridines, the dyhydropyridine-resistant component of inward current may contribute to slow wave propagation.

Physiological study of interstitial cells of Cajal identified by vital staining

Interstitial cells of Cajal (ICC) form networks that intercalate between the enteric nervous system and smooth muscle cells and play a fundamental role in the control of gastrointestinal motility by initiating rhythmic electrical activity. In this report, we used a method to examine the physiological and morphological properties of ICC in living, intact tissues. ACK2, an anti-Kit antibody, was conjugated to a fluorescent probe and used to identify individual ICC for intracellular electrical recordings, to record changes in intracellular calcium concentration using fluorescent dyes and for confocal microscopy. Cyclic changes in intracellular calcium concentration were recorded in ICC with a frequency similar to the electrical slow wave. In addition, injection of a fluorescent dye into single ICC enabled the three-dimensional reconstruction of single myenteric plexus ICC within the intact network. The data show that ICC in intact networks from the myenteric plexus region in living tissues in the guinea-pig antrum exhibit an electrical slow wave, and that intracellular calcium oscillates at a frequency similar to the slow wave.

Electrical activity induced by nitric oxide in canine colonic circular muscle

Nitric oxide generates slow electrical oscillations (SEOs) in cells near the myenteric edge of the circular muscle layer, which resemble slow waves generated by interstitial cells of Cajal (ICCs) at the submucosal edge of this muscle. The properties of SEOs were studied to determine whether these events are similar to slow waves. Rapid frequency membrane potential oscillations (MPOs; 16 +/- 1 cycles/min and 9.6 +/- 0.2 mV) were recorded from control muscles near the myenteric edge. Sodium nitroprusside (0.3 microM) reduced MPOs and initiated SEOs (1.3 +/- 0.3 cycles/min and 13.4 +/- 1.4 mV amplitude). SEOs were abolished by the guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxaline-1-one (10 microM). MPOs were abolished by nifedipine (1 microM), whereas SEO frequency increased and the amount of depolarization decreased. BAY K 8644 (1 microM) prolonged SEOs and reduced their frequency. SEOs were abolished by Ni(2+) (0.5 mM), low Ca(2+) solution (0.1 mM Ca(2+)), cyclopiazonic acid (10 microM), and the mitochondrial uncouplers antimycin (10 microM) and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM). Oligomycin (10 microM) was without effect. These effects are similar to those described for colonic slow waves. Our results suggest that nitric oxide-induced SEOs are similar in mechanism to slow waves, an activity not previously thought to be generated by myenteric pacemakers.

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

In the circular muscle of the guinea-pig gastric antrum, a decrease in the external Na(+) to less than 20 mM produced depolarization of the membrane with transient prolongation of the slow wave. This was followed by a high rhythmic activity. The activity was inhibited by reapplication of Na(+) before recovery. The depolarization in Na(+)-deficient solution was prevented and rhythmic activity continued at about 5/min for at least 6 min by simultaneous removal of K(+), Ca(2+), or Cl(-). After exposure to a Na(+)- and Cl(-)-deficient solution for a few minutes, reapplication of the Na(+) in Cl(-)-deficient solution inhibited generation of the slow wave until Cl(-) reapplication. Similar results were obtained when Na(+) and Cl(-) were reapplied in the absence of K(+) after exposure to a Na(+)-, K(+)-free, and Cl(-)-deficient solution, although the inhibition was weaker than Na(+) reapplication in a Cl(-)-deficient solution. In the presence of furosemide or bumetanide, a strong inhibition of activity was produced by the reapplication of Na(+) and Cl(-) after exposure to an Na(+)- and Cl(-)-deficient solution. A hypothesis is presented that intracellular Ca(2+) concentration ([Ca(2+)](i)) is the most important factor determining the generation and frequency of the slow wave and that [Ca(2+)](i) is regulated by the Na(+) concentration gradient across the plasma membrane. The recovery of the Na(+) concentration gradient by Na(+) reapplication after removal of Na(+) and Cl(-) is mainly controlled by a Na(+)-K(+)-Cl(-) co-transport.

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.

Voltage sensitivity of slow wave frequency in isolated circular muscle strips from guinea pig gastric antrum

In circular muscle preparations isolated from the guinea pig gastric antrum, regular spontaneous electrical activity (slow waves) was recorded. Under normal conditions (6 mM K+), the frequency and shape of the slow waves were similar to those observed in ordinary stomach smooth muscle preparations. When the resting membrane potential was hyperpolarized and depolarized by changing the extracellular K+ concentration (2-18 mM), the frequency of slow waves decreased and increased, respectively. Application of cromakalim hyperpolarized the cell membrane and reduced the frequency of slow waves in a dose-dependent manner. Cromakalim (3 microM) hyperpolarized the membrane, and slow waves ceased in most preparations. In the presence of cromakalim, subsequent increases in the extracellular K+ concentration restored the frequency of slow waves accompanied by depolarization. Also, glibenclamide completely antagonized this effect of cromakalim. In smooth muscle strips containing both circular and longitudinal muscle layers, such changes in the slow wave frequency were not observed. It was concluded that the maneuver of isolating circular smooth muscle altered the voltage dependence of the slow wave frequency.

Spontaneous electrical rhythmicity in cultured interstitial cells of cajal from the murine small intestine

1. Interstitial cells of Cajal (ICC) are pacemaker cells in the small bowel, and therefore this cell type must express the mechanism responsible for slow wave activity. Isolated ICC were cultured for 1-3 days from the murine small intestine and identified with c-Kit-like immunoreactivity (c-Kit-LI). 2. Electrical recordings were obtained from cultured ICC with the whole-cell patch clamp technique. ICC were rhythmically active, producing regular slow wave depolarizations with waveforms and properties similar to slow waves in intact tissues. 3. Spontaneous activity of c-Kit-LI cells was inhibited by reduced extracellular Na+, gadolinium, and reduced extracellular Ca2+. The activity was not affected by nisoldipine. Voltage clamp studies showed rhythmic inward currents that were probably responsible for the slow wave activity. The current-voltage relationship showed that the spontaneous currents reversed at about +17 mV. These observations are consistent with the involvement of a non-selective cation current in the generation of slow waves, but do not rule out contributions from other conductances or transporters. 4. A Ba2+-sensitive inwardly rectifying K+ current in c-Kit-LI cells that may be involved in slow wave repolarization and maintenance of a negative potential between slow waves was also found. Similar pharmacology was observed in studies of intact murine intestinal muscles. 5. Cultured ICC may be a useful model for studying the properties and pharmacology of some of the ionic conductances involved in spontaneous rhythmicity in the gastrointestinal tract.

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 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.

Effect of stretch on calcium channel currents recorded from the antral circular myocytes of guinea-pig stomach

The effect of membrane stretch on voltage-activated Ba2+ current (IBa) was studied in antral circular myocytes of guinea-pig using the whole- cell patch-clamp technique. The changes in cell volume were elicited by superfusing the myocytes with anisosmotic solutions. Hyposmotic superfusate (202 mosmol/l) induced cell swelling and increased peak values of IBa at 0 mV (from -406.6 +/- 45.5 pA to -547.5 +/- 65.6 pA, mean +/- SEM, n = 8) and hyperosmotic superfusate (350 mosmol/l) induced cell shrinkage and decreased peak values of IBa at 0 mV (to -269.5 +/- 39.1 pA, n = 8). Such changes were reversible and the extent of change was dependent on the osmolarity of superfusate. The values of normalized IBa at 0 mV were 1.43 +/- 0.04, 1.30 +/- 0.06, 1.23 +/- 0.04, 1.19 +/- 0.04, 1 and 0. 68 +/- 0.06 at 202, 220, 245, 267, 290 and 350 mosmol/l, respectively (n = 8). IBa was almost completely blocked by nicardipine (5 microM) under hyposmotic conditions. The values of steady-state half-inactivation voltage (-37.7 +/- 3.3 and -36.5 +/- 2.6 mV, under control and hyposmotic conditions, respectively) or the half-activation voltage (-13.6 +/- 2.3 and -13.9 +/- 1.9 mV) of IBa were not significantly changed (P > 0.05, n = 6). Cell membrane capacitance was slightly increased from 50.00 +/- 2.86 pF to 50.22 +/- 2.82 pF by a hyposmotic superfusate (P < 0.05, n = 6). It is suggested that cell swelling increases voltage-operated L-type calcium channel current and that such a property is related to the response of gastric smooth muscle to mechanical stimuli.

Characteristics of the NANC post-stimulus ('rebound') contraction of the urinary bladder neck muscle in sheep

1. Strips of muscle from sheep bladder neck were set up for tension recording and subjected to electrical field stimulation (EFS) to stimulate their intramural nerves. 2. In the presence of atropine (1 microM) and guanethidine (1 microM), the response to 1 Hz EFS was biphasic, characterized by a relaxation during the stimulus period, followed by a post-stimulus contraction. A similar biphasic response was also seen following bolus application of nitric oxide (NO). 3. In the absence of atropine and guanethidine, the relaxations were masked by contractions during stimulation; however, the post-stimulus contraction were unaffected. L-NAME (100 microM) blocked the post-stimulus contractions and L-arginine (1 mM) restored them, suggesting that they were NO-mediated. 4. M&B 22948, a phosphodiesterase inhibitor, prolonged the relaxations and abolished the post-stimulus contractions. This suggests that rapid removal of cyclic GMP is required for post-stimulus contraction to occur. 5. When the number of pulses in the stimulus train was kept constant, the size of the post-stimulus contraction increased as the duration of the preceding period of stimulation increased. Maximal post-stimulus contractions were obtained following stimulation for > 40 s. 6. The L-channel antagonist, nifedipine (1 microM) and verapamil (1 microM), had little effect on the amplitude of the post-stimulus contractions. 7. In contrast, ryanodine-(8 microM) reduced the post-stimulus contractions by over 90%. Caffeine (20 mM) also abolished the post-stimulus contractions and cyclopiazonic acid (CPA, 10 microM) reduced them by 76%. However, in the presence of CPA a slower post-stimulus contraction developed. Nifedipine (1 microM) reduced this by 40%. 8. In conclusion, these results support a role for NO in the post-stimulus contraction of the sheep bladder neck muscle. The post-stimulus contraction depends more on release of intracellular Ca2+, than Ca2+ influx through L-type channels.

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.

Upstroke component of electrical slow waves in canine colonic smooth muscle due to nifedipine-resistant calcium current

1. Electrical slow waves of gastrointestinal smooth muscles are not abolished by organic Ca2+ channel blocking drugs, such as nifedipine or D600. These compounds reduce the amplitude and duration of the plateau phase, but the upstroke phase of slow waves persists. 2. Voltage clamp experiments were performed on isolated circular muscle cells from the canine proximal colon to characterize the dihydropyridine-resistant component of inward current. Inward currents were measured at 25 and 35 degrees C. The higher temperature increased the amplitudes of the transient and sustained phases of the inward current. The voltage dependence of activation and inactivation of the inward current was not significantly changed at 35 vs. 25 degrees C. 3. At 35 degrees C the transient phase of the inward current was reduced but not blocked by nifedipine (10(-6) M). The sustained phase was blocked by nifedipine. 4. The block by nifedipine was voltage dependent, increasing with depolarization. At voltages reached during the upstroke depolarization about 35% of the inward current persisted in the presence of nifedipine (10(-6) M). This may be sufficient inward current to sustain the upstroke depolarization in intact muscles. 5. Nifedipine caused a 20 mV negative shift in the voltage dependence of inactivation suggesting that dihydropyridines may preferentially bind to Ca2+ channels in an inactivated state. 6. Ni2+ (< 100 microM) significantly decreased the transient phase of inward current. A combination of Ni2+ (40 microM) and nifedipine (10(-6) M) blocked all of the inward current at 35 degrees C. Combination of nifedipine (10(-6) M) and Ni2+ (40 microM) blocked slow waves in intact muscles. 7. Bay K 8644 (10(-6) M) increased the amplitude of the transient and sustained components of inward current. On a percentage basis the increase in the sustained component was greater than the increase in the transient component with test potentials in the range of -50 to -20 mV. This may explain why Bay K 8644 preferentially increases the plateau component of slow waves vs. the upstroke component. 8. The findings of this study suggest that the nifedipine resistance of the upstroke depolarization could be due to the voltage dependence of the block of Ca2+ channels by dihydropyridines. Thus a single class of voltage-dependent Ca2+ channels could be responsible for the upstroke and plateau phases of slow waves.