Ultrastructural characterization of interstitial cells of Cajal associated with the submucosal plexus in the proximal colon of the guinea pig

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Functional properties of submucosal venules in the rat stomach

Venules in the stomach may have intrinsic properties for maintaining active microcirculation drainage even during gastric filling. Properties of spontaneous and nerve-mediated activity of submucosal venules in the rat stomach were investigated. Changes in vasodiameter and intracellular Ca(2+) in venular smooth muscle cells (SMCs) were monitored by video tracking and Fluo-8 Ca(2+) imaging, respectively. Venular SMCs developed synchronous spontaneous Ca(2+) transients and corresponding rhythmic constrictions of the venules. Nominally Ca(2+)-free solution or an L-type Ca(2+) channel blocker (1 μM nifedipine) disrupted the Ca(2+) transient synchrony and abolished spontaneous constrictions. Spontaneous constrictions were also prevented by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase (10 μM cyclopiazonic acid (CPA)), IP3 receptors (100 μM 2-APB) or Ca(2+)-activated Cl(-) channels (100 μM niflumic acid). Transmural nerve stimulation (TNS) induced a long-lasting venular constriction that was abolished by α-adrenoceptor antagonist (1 μM phentolamine), while TNS evoked a sympathetic transient constriction of arterioles that was abolished by a combination of phentolamine and a P2 purinoceptor antagonist (10 μM pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS)). Consistently, P2X1 purinoceptor immunoreactivity was detected in arteriolar but not venular SMCs. Primary afferent nerve stimulation (300 nM capsaicin) caused a venular dilatation by releasing calcitonin gene-related peptide. Thus, Ca(2+) release from the sarcoplasmic reticulum may play a fundamental role in the generation of spontaneous Ca(2+) transients, while electrical coupling amongst venular SMCs via L-type Ca(2+) channel activation appears to be critical for Ca(2+) transient synchrony as well as spontaneous contractions. Sympathetic venular constrictions appear to be exclusively mediated by noradrenaline due to the lack of P2X1 receptor in venular SMCs.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Peristalsis and propulsion of colonic content can occur after blockade of major neuroneuronal and neuromuscular transmitters in isolated guinea pig colon

We recently identified hexamethonium-resistant peristalsis in the guinea pig colon. We showed that, following acute blockade of nicotinic receptors, peristalsis recovers, leading to normal propagation velocities of fecal pellets along the colon. This raises the fundamental question: what mechanisms underlie hexamethonium-resistant peristalsis? We investigated whether blockade of the major receptors that underlie excitatory neuromuscular transmission is required for hexamethonium-resistant peristalsis. Video imaging of colonic wall movements was used to make spatiotemporal maps and determine the velocity of peristalsis. Propagation of artificial fecal pellets in the guinea pig distal colon was studied in hexamethonium, atropine, ω-conotoxin (GVIA), ibodutant (MEN-15596), and TTX. Hexamethonium and ibodutant alone did not retard peristalsis. In contrast, ω-conotoxin abolished peristalsis in some preparations and reduced the velocity of propagation in all remaining specimens. Peristalsis could still occur in some animals in the presence of hexamethonium + atropine + ibodutant + ω-conotoxin. Peristalsis never occurred in the presence of TTX. The major finding of the current study is the unexpected observation that peristalsis can occur after blockade of the major excitatory neuroneuronal and neuromuscular transmitters. Also, the colon retained an intrinsic polarity in the presence of these antagonists and was only able to expel pellets in an aboral direction. The nature of the mechanism(s)/neurotransmitter(s) that generate(s) peristalsis and facilitate(s) natural fecal pellet propulsion, after blockade of major excitatory neurotransmitters, at the neuroneuronal and neuromuscular junction remains to be identified.