A novel intramuscular Interstitial Cell of Cajal is a candidate for generating pacemaker activity in the mouse internal anal sphincter.

The internal anal sphincter (IAS) generates phasic contractions and tone. Slow waves (SWs) produced by interstitial cells of Cajal (ICC) underlie phasic contractions in other gastrointestinal regions. SWs are also present in the IAS where only intramuscular ICC (ICC-IM) are found, however the evidence linking ICC-IM to SWs is limited. This study examined the possible relationship between ICC-IM and SWs by recording Ca2+ transients in mice expressing a genetically-encoded Ca2+-indicator in ICC (Kit-Cre-GCaMP6f). A role for L-type Ca2+ channels (CavL) and anoctamin 1 (ANO1) was tested since each is essential for SW and tone generation. Two distinct ICC-IM populations were identified. Type I cells (36% of total) displayed localised asynchronous Ca2+ transients not dependent on CavL or ANO1; properties typical of ICC-IM mediating neural responses in other gastrointestinal regions. A second novel sub-type, i.e., Type II cells (64% of total) generated rhythmic, global Ca2+ transients at the SW frequency that were synchronised with neighbouring Type II cells and were abolished following blockade of either CavL or ANO1. Thus, the spatiotemporal characteristics of Type II cells and their dependence upon CavL and ANO1 all suggest that these cells are viable candidates for the generation of SWs and tone in the IAS.

Rhythmic calcium transients in smooth muscle cells of the mouse internal anal sphincter.

BACKGROUND: The internal anal sphincter (IAS) exhibits slow waves (SWs) and tone that are dependent upon L-type Ca2+ channels (CavL ) suggesting that phasic events (ie, SWs) play a fundamental role in tone generation. The present study further examined phasic activity in the IAS by measuring the spatiotemporal properties of Ca2+ transients (CTs) in IAS smooth muscle cells (SMCs). METHODS: Ca2+ transients were recorded with spinning disk confocal microscopy from the IAS of SM-GCaMP mice. Muscles were pinned submucosal surface up at two different lengths. Drugs were applied by inclusion in the superfusate. KEY RESULTS: Ca2+ transients displayed ongoing rhythmic firings at both lengths and were abolished by nifedipine and the KATP channel activator pinacidil indicating their dependence upon CavL . Like SWs, CTs were greatest in frequency (average 70.6 cpm) and amplitude at the distal extremity and conducted proximally. Removal of the distal IAS reduced but did not abolish CTs. The time constant for clearing cytoplasmic Ca2+ averaged 0.46 seconds and basal Ca2+ levels were significantly elevated. CONCLUSIONS & INFERENCES: The similarities in spatiotemporal and pharmacological properties of CTs and SWs suggest that SW gives rise to CTs while muscle stretch is not required. Elevated relative basal Ca2+ in the IAS is likely due to the inability of cells to clear or sequester Ca2+ between rapid frequency voltage-dependent Ca2+ entry events, that is, conditions that will lead to tone development. The conduction of CTs from distal to proximal IAS will lead to orally directed contractions and likely contribute to the maintenance of fecal continence.

Control of Motility in the Internal Anal Sphincter.

The internal anal sphincter (IAS) plays an important role in the maintenance of fecal continence since it generates tone and is responsible for > 70% of resting anal pressure. During normal defecation the IAS relaxes. Historically, tone generation in gastrointestinal muscles was attributed to mechanisms arising directly from smooth muscle cells, ie, myogenic activity. However, slow waves are now known to play a fundamental role in regulating gastrointestinal motility and these electrical events are generated by the interstitial cells of Cajal. Recently, interstitial cells of Cajal, as well as slow waves, have also been identified in the IAS making them viable candidates for tone generation. In this review we discuss four different mechanisms that likely contribute to tone generation in the IAS. Three of these involve membrane potential, L-type Ca2+ channels and electromechanical coupling (ie, summation of asynchronous phasic activity, partial tetanus, and window current), whereas the fourth involves the regulation of myofilament Ca2+ sensitivity. Contractile activity in the IAS is also modulated by sympathetic motor neurons that significantly increase tone and anal pressure, as well as inhibitory motor neurons (particularly nitrergic and vasoactive intestinal peptidergic) that abolish contraction and assist with normal defecation. Alterations in IAS motility are associated with disorders such as fecal incontinence and anal fissures that significantly decrease the quality of life. Understanding in greater detail how tone is regulated in the IAS is important for developing more effective treatment strategies for these debilitating defecation disorders.

Inhibitory Neural Regulation of the Ca 2+ Transients in Intramuscular Interstitial Cells of Cajal in the Small Intestine.

Gastrointestinal motility is coordinated by enteric neurons. Both inhibitory and excitatory motor neurons innervate the syncytium consisting of smooth muscle cells (SMCs) interstitial cells of Cajal (ICC) and PDGFRα+ cells (SIP syncytium). Confocal imaging of mouse small intestines from animals expressing GCaMP3 in ICC were used to investigate inhibitory neural regulation of ICC in the deep muscular plexus (ICC-DMP). We hypothesized that Ca2+ signaling in ICC-DMP can be modulated by inhibitory enteric neural input. ICC-DMP lie in close proximity to the varicosities of motor neurons and generate ongoing Ca2+ transients that underlie activation of Ca2+-dependent Cl- channels and regulate the excitability of SMCs in the SIP syncytium. Electrical field stimulation (EFS) caused inhibition of Ca2+ for the first 2-3 s of stimulation, and then Ca2+ transients escaped from inhibition. The NO donor (DEA-NONOate) inhibited Ca2+ transients and Nω-Nitro-L-arginine (L-NNA) or a guanylate cyclase inhibitor (ODQ) blocked inhibition induced by EFS. Purinergic neurotransmission did not affect Ca2+ transients in ICC-DMP. Purinergic neurotransmission elicits hyperpolarization of the SIP syncytium by activation of K+ channels in PDGFRα+ cells. Generalized hyperpolarization of SIP cells by pinacidil (KATP agonist) or MRS2365 (P2Y1 agonist) also had no effect on Ca2+ transients in ICC-DMP. Peptidergic transmitter receptors (VIP and PACAP) are expressed in ICC and can modulate ICC-DMP Ca2+ transients. In summary Ca2+ transients in ICC-DMP are blocked by enteric inhibitory neurotransmission. ICC-DMP lack a voltage-dependent mechanism for regulating Ca2+ release, and this protects Ca2+ handling in ICC-DMP from membrane potential changes in other SIP cells.

Relationship between interstitial cells of Cajal, fibroblast-like cells and inhibitory motor nerves in the internal anal sphincter.

Interstitial cells of Cajal (ICC) have been shown to participate in nitrergic neurotransmission in various regions of the gastrointestinal (GI) tract. Recently, fibroblast-like cells, which are positive for platelet-derived growth factor receptor α (PDGFRα(+)), have been suggested to participate additionally in inhibitory neurotransmission in the GI tract. The distribution of ICC and PDGFRα(+) cell populations and their relationship to inhibitory nerves within the mouse internal anal sphincter (IAS) are unknown. Immunohistochemical techniques and confocal microscopy were therefore used to examine the density and arrangement of ICC, PDGFRα(+) cells and neuronal nitric-oxide-synthase-positive (nNOS(+)) nerve fibers in the IAS of wild-type (WT) and W/W ( v ) mice. Of the total tissue volume within the IAS circular muscle layer, 18% consisted in highly branched PDGFRα(+) cells (PDGFRα(+)-IM). Other populations of PDGFRα(+) cells were observed within the submucosa and along the serosal and myenteric surfaces. Spindle-shaped intramuscular ICC (ICC-IM) were present in the WT mouse IAS but were largely absent from the W/W ( v ) IAS. The ICC-IM volume (5% of tissue volume) in the WT mouse IAS was significantly smaller than that of PDGFRα(+)-IM. Stellate-shaped submucosal ICC (ICC-SM) were observed in the WT and W/W ( v ) IAS. Minimum surface distance analysis revealed that nNOS(+) nerve fibers were closely aligned with both ICC-IM and PDGFRα(+)-IM. An even closer association was seen between ICC-IM and PDGFRα(+)-IM. Thus, a close morphological arrangement exists between inhibitory motor neurons, ICC-IM and PDGFRα(+)-IM suggesting that some functional interaction occurs between them contributing to inhibitory neurotransmission in the IAS.

Functional evidence for purinergic inhibitory neuromuscular transmission in the mouse internal anal sphincter.

The neurotransmitter(s) underlying nitric oxide synthase (NOS)-independent neural inhibition in the internal anal sphincter (IAS) is still uncertain. The present study investigated the role of purinergic transmission. Contractile and electrical responses to electrical field stimulation of nerves (0.1-5 Hz for 10-60 s) were recorded in strips of mouse IAS. A single stimulus generated a 28-mV fast inhibitory junction potential (IJP) and relaxation. The NOS inhibitor N(omega)-nitro-l-arginine (l-NNA) reduced the fast IJP duration by 20%. Repetitive stimulation at 2.5-5 Hz caused a more sustained IJP and sustained relaxation. l-NNA reduced relaxation at 1 Hz and the sustained IJP at 2.5-5 Hz. All other experiments were carried out in the presence of NOS blockade. IJPs and relaxation were significantly reduced by the P2 receptor antagonists 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]-2-pyridinyl]azo]-1,3-benzenedisulfonic acid (PPADS) (100 microM), by desensitization of P2Y receptors with adenosine 5'-[beta-thio]diphosphate (ADP-betaS) (10 microM), and by the selective P2Y1 receptor blocker 2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate (MRS2179) (10 microM). Relaxation and IJPs were also significantly reduced by the K(+) channel blocker apamin (1 microM). Removal of extracellular potassium (K(o)) increased IJP amplitude to 205% of control, whereas return of K(o) 30 min later hyperpolarized cells by 19 mV and reduced IJP amplitude to 50% of control. Exogenous ATP (3 mM) relaxed muscles in the presence of TTX (1 microM) and hyperpolarized cells by 15 mV. In conclusion, these data suggest that purinergic transmission significantly contributes to NOS-independent neural inhibition in the mouse IAS. P2Y1 receptors, as well as at least one other P2 receptor subtype, contribute to this pathway. Purinergic receptors activate apamin-sensitive K(+) channels as well as other apamin-insensitive conductances leading to hyperpolarization and relaxation.

Loss of the alpha7 integrin promotes extracellular signal-regulated kinase activation and altered vascular remodeling.

Vascular smooth muscle cell (VSMC) proliferation and migration are underlying factors in the development and progression of cardiovascular disease. Studies have shown that altered expression of vascular integrins and extracellular matrix proteins may contribute to the vascular remodeling observed after arterial injury and during disease. We have recently shown that loss of the alpha7beta1 integrin results in VSMC hyperplasia. To investigate the cellular mechanisms underlying this phenotype, we have examined changes in cell signaling pathways associated with VSMC proliferation. Several studies have demonstrated the mitogen-activated protein kinase signaling pathway is activated in response to vascular injury and disease. In this study, we show that loss of the alpha7 integrin in VSMCs results in activation of the extracellular signal-regulated kinase and translocation of the activated kinase to the nucleus. Forced expression of the alpha7 integrin or use of the mitogen-activated protein kinase kinase 1 inhibitor U0126 in alpha7 integrin-deficient VSMCs suppressed extracellular signal-regulated kinase activation and restored the differentiated phenotype to alpha7 integrin-null cells in a manner dependent on Ras signaling. Alpha7 integrin-null mice displayed profound vascular remodeling in response to injury with pronounced neointimal formation and reduced vascular compliance. These findings demonstrate that the alpha7beta1 integrin negatively regulates extracellular signal-regulated kinase activation and suggests an important role for this integrin as part of a signaling complex regulating VSMC phenotype switching.

Septal interstitial cells of Cajal conduct pacemaker activity to excite muscle bundles in human jejunum.

BACKGROUND & AIMS: Like the heart, intestinal smooth muscles exhibit electrical rhythmicity, which originates in pacemaker cells surrounding the myenteric plexus, called interstitial cells of Cajal (ICC-MY). In large mammals, ICC also line septa (ICC-SEP) between circular muscle (CM) bundles, suggesting they might be necessary for activating muscle bundles. It is important to determine their functional significance, because a loss of ICC in humans is associated with disordered motility. Our aims were therefore to determine the role of ICC-SEP in activating the thick CM in the human jejunum. METHODS: The mucosa and submucosa were removed and muscle strips were cut and pinned in cross-section so that the ICC-MY and ICC-SEP networks and the CM could be readily visualized. The ICC networks and CM were loaded with the Ca(2+) indicator fluo-4, and pacemaker and muscle activity was recorded at 36.5 +/- 0.5( degrees )C. RESULTS: Ca(2+) imaging revealed that pacemaker activity in human ICC-MY can entrain ICC-SEP to excite CM bundles. Unlike the heart, pacemaker activity in ICC-MY varied in amplitude, propagation distance, and direction, leading to a sporadic activation of ICC-SEP. CONCLUSIONS: ICC-SEP form a crucial conduction pathway for spreading excitation deep into muscle bundles of the human jejunum, necessary for motor patterns underlying mixing. A loss of these cells could severely affect motor activity.

The mechanism and spread of pacemaker activity through myenteric interstitial cells of Cajal in human small intestine.

BACKGROUND & AIMS: It has been generally assumed that interstitial cells of Cajal (ICC) in the human gastrointestinal tract have similar functions to those in rodents, but no direct experimental evidence exists to date for this assumption. This is an important question because pathologists have noted decreased numbers of ICC in patients with a variety of motility disorders, and some have speculated that loss of ICC could be responsible for motor dysfunction. Our aims were to determine whether myenteric ICC (ICC-MY) in human jejunum are pacemaker cells and whether these cells actively propagate pacemaker activity. METHODS: The mucosa and submucosa were removed, and strips of longitudinal muscle were peeled away to reveal the ICC-MY network. ICC networks were loaded with the Ca(2+) indicator fluo-4, and pacemaker activity was recorded via high-speed video imaging at 36.5 degrees C +/- 0.5 degrees C. RESULTS: Rhythmic, biphasic Ca(2+) transients (6.03 +/- 0.33 cycles/min) occurred in Kit-positive ICC-MY. These consisted of a rapidly propagating upstroke phase that initiated a sustained plateau phase, which was associated with Ca(2+) spikes in neighboring smooth muscle. Pacemaker activity was dependent on inositol 1,4,5-triphosphate receptor-operated stores and mitochondrial function. The upstroke phase of Ca(2+) transients in ICC-MY appeared to result from Ca(2+) influx through dihydropyridine-resistant Ca(2+) channels, whereas the plateau phase was attributed to Ca(2+) release from inositol 1,4,5-triphosphate receptor-operated Ca(2+) stores. CONCLUSIONS: Each ICC-MY in human jejunum generates spontaneous pacemaker activity that actively propagates through the ICC network. Loss of these cells could severely disrupt the normal function of the human small intestine.

Role of L-type calcium channels and PKC in active tone development in rabbit coronary artery.

The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E-) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor N(omega)-nitro-L-arginine (100 microM) developed active tone, which was enhanced by stretch and reversed by the NO donor sodium nitroprusside (SNP; IC(50)=9 nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu; 10 nM) with the same potency (IC(50)=8 nM), whereas 300 nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF-109203X (IC(50)=1-2 microM) and chelerythrine (IC(50)=4-5 microM) and the classical PKC inhibitor Gö-6976 (IC(50)=0.3-0.4 microM) blocked both active tone and 10 nM PDBu responses with similar potency. Active tone development was associated with depolarization of membrane potential (E(m)) and a shift to the left of the E(m)-vs.-contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 microM) or SNP (30 nM). PDBu (100-300 nM) increased peak L-type calcium channel (Ca(v)) currents in isolated coronary myocytes, and this effect was reversed by chelerythrine (1 microM) or Gö-6976 (200 nM). SNP (500 nM) reduced Ca(v) currents only in the presence of the PKA blocker 8-bromo-2'-O-monobutyryl-cAMPS, Rp isomer (10 microM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent on both enhanced Ca(v) activity and classical PKC activity. Both E(m)-dependent and -independent processes contribute to contraction. Our results suggest that one E(m)-independent process is direct enhancement of Ca(v) current by PKC.

Signaling pathway underlying stimulation of L-type Ca2+ channels in rabbit portal vein myocytes by recombinant Gbetagamma subunits.

In previous studies, we (Callaghan B, Koh SD, and Keef KD, Circ Res 94: 626-633, 2004) have shown that voltage-dependent L-type Ca(2+) channels (Cav) in portal vein myocytes are enhanced when muscarinic M2 receptors are activated with ACh. Current stimulation was coupled to the G protein subunit Gbetagamma along with the downstream mediators phosphatidylinositol-3-kinase (PI3K), protein kinase C (PKC), and c-Src. The present study was designed to determine whether the same second messenger pathway could be identified when exogenous recombinant Gbetagamma subunits are introduced into cells. Smooth muscle myocytes were freshly isolated from rabbit portal vein, and Cav currents were recorded by using the patch-clamp technique. Dialysis of cells with recombinant Gbetagamma (50 nM) significantly increased Cav currents (141%). Nifedipine (1 microM) reduced both control and stimulated currents by approximately 90%. The enhancement of current by Gbetagamma was equivalent to that produced by ACh (142%), whereas the PKC activator phorbol 12,13-dibutyrate (PdBu) gave rise to greater current stimulation (192%). Current stimulation with Gbetagamma, ACh, and PdBu were not associated with changes in the voltage dependence of activation or inactivation. The PI3K inhibitor LY-294002 (20 microM) reduced peak currents by 32% in cells dialyzed with Gbetagamma, whereas the inactive analog LY-303511 resulted in a small but significant reduction in current (12%). The c-Src inhibitor PP2 (1 microM) also significantly reduced currents (34%), whereas the inactive analog PP3 was without effect. These data provide further evidence for the hypothesis that Gbetagamma leads to stimulation of Cav currents in rabbit portal vein myocytes via a signaling pathway that includes PI3K, PKC, and c-Src.

Distribution of interstitial cells of Cajal in tunica muscularis of the canine rectoanal region.

Electrical and mechanical activity of the circular muscle layer in the rectoanal region of the gastrointestinal tract undergoes considerable changes in the site of dominant pacemaking activity, frequency, and waveform shape. The present study was performed to determine whether changes in the structural organization of the circular layer or in the density, distribution, and ultrastructure of interstitial cells of Cajal (ICC) could account for this heterogeneity in electrical and mechanical activities. Light microscopy revealed that the structural organization of the circular muscle layer underwent dramatic morphological changes, from a tightly packed layer with poorly defined septa in the proximal rectum to one of discrete muscle bundles separated by large septae in the internal anal sphincter. Kit immunohistochemistry revealed a dense network of ICC along the submucosal and myenteric borders in the rectum, whereas in the internal anal sphincter, ICC were located along the periphery of muscle bundles within the circular layer. Changes in electrical activity within the circular muscle layer can be partially explained by changes in the structure of the muscle layer and changes in the distribution of ICC in the rectoanal region of the gastrointestinal tract.

Excitatory motor innervation in the canine rectoanal region: role of changing receptor populations.

1. Motor innervation in the canine rectoanal region was examined in isolated strips of the circular muscle layer. Contractile responses to electrical field stimulation began at lower frequencies and were more persistent in the internal anal sphincter (IAS) than in the rectum. 2. Motor innervation to the IAS was almost exclusively sympathetic, since it was blocked by guanethidine (Guan 3 microM) while the response in the proximal rectum was approximately 50% muscarinic, and sensitive to the M(3) selective antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 0.1 microM) and 50% tachykinergic, and sensitive to the neurokinin 2 (NK(2)) receptor antagonist GR 94800 (1 microM). From IAS to rectum there was a gradual shift in the relative contribution of intrinsic and extrinsic neural innervation. 3. Responses to exogenously applied transmitters exhibited a similar pattern to that observed with motor innervation. Norepinephrine (NE) was most potent in the IAS and acetylcholine (ACh) and NK-A were most potent in the proximal rectum. The responses were inhibited by prazosin, 4-DAMP and GR 94800 respectively. 4. A gradient in the density of adrenergic alpha(1), muscarinic and NK(2) receptors also existed from IAS to rectum as determined by measuring the binding of [(3)H]-prazosin, [(3)H]-quinuclidinyl benzilate ([(3)H]-QNB and [(3)H]-SR-48968 to smooth muscle membranes. 5. In summary, these data suggest that the shift in motor innervation in the rectoanal region is achieved in part by changes in receptor populations available for activation by sympathetic and enteric motor neurons.