Two types of 'slow waves' in intestinal smooth muscle of cat

A myogenic motor pattern in mice lacking myenteric interstitial cells of Cajal explained by a second coupled oscillator network

The interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are a network of coupled oscillators in the small intestine that generate rhythmic electrical phase waves leading to corresponding waves of contraction, yet rhythmic action potentials and intercellular calcium waves have been recorded from c-kit-mutant mice that lack the ICC-MP, suggesting that there may be a second pacemaker network. The gap junction blocker carbenoxolone induced a "pinstripe" motor pattern consisting of rhythmic "stripes" of contraction that appeared simultaneously across the intestine with a period of ~4 s. The infinite velocity of these stripes suggested they were generated by a coupled oscillator network, which we call X. In c-kit mutants rhythmic contraction waves with the period of X traveled the length of the intestine, before the induction of the pinstripe pattern by carbenoxolone. Thus X is not the ICC-MP and appears to operate under physiological conditions, a fact that could explain the viability of these mice. Individual stripes consisted of a complex pattern of bands of contraction and distension, and between stripes there could be slide waves and v waves of contraction. We hypothesized that these phenomena result from an interaction between X and the circular muscle that acts as a damped oscillator. A mathematical model of two chains of coupled Fitzhugh-Nagumo systems, representing X and circular muscle, supported this hypothesis. The presence of a second coupled oscillator network in the small intestine underlines the complexity of motor pattern generation in the gut.NEW & NOTEWORTHY Physiological experiments and a mathematical model indicate a coupled oscillator network in the small intestine in addition to the c-kit-expressing myenteric interstitial cells of Cajal. This network interacts with the circular muscle, which itself acts as a system of damped oscillators, to generate physiological contraction waves in c-kit (W) mutant mice.

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.

Two independent networks of interstitial cells of cajal work cooperatively with the enteric nervous system to create colonic motor patterns

Normal motility of the colon is critical for quality of life and efforts to normalize abnormal colon function have had limited success. A better understanding of control systems of colonic motility is therefore essential. We report here a hypothesis with supporting experimental data to explain the origin of rhythmic propulsive colonic motor activity induced by general distention. The theory holds that both networks of interstitial cells of Cajal (ICC), those associated with the submuscular plexus (ICC-SMP) and those associated with the myenteric plexus (ICC-MP), orchestrate propagating contractions as pacemaker cells in concert with the enteric nervous system (ENS). ICC-SMP generate an omnipresent slow wave activity that causes propagating but non-propulsive contractions ("rhythmic propagating ripples") enhancing absorption. The ICC-MP generate stimulus-dependent cyclic depolarizations propagating anally and directing propulsive activity ("rhythmic propulsive motor complexes"). The ENS is not essential for both rhythmic motor patterns since distention and pharmacological means can produce the motor patterns after blocking neural activity, but it supplies the primary stimulus in vivo. Supporting data come from studies on segments of the rat colon, simultaneously measuring motility through spatiotemporal mapping of video recordings, intraluminal pressure, and outflow measurements.

Influence of various feeding conditions, the migrating myoelectric complex and cholinergic drugs on antral slow waves in sheep

The presented study was designed to elucidate whether the cholinergic mechanisms control ovine antral slow waves in various physiological conditions, including feeding and various phases of migrating myoelectric complex (MMC). The investigations were carried out on six adult sheep of Polish Merino breed with seven bipolar electrodes surgically implanted onto the antral and small intestinal wall. In the course of chronic experiments, the myoelectric activity was recorded from these regions using the multichannel electroencephalograph. Experiments were performed on 48 h fasted and non-fasted animals. During some of these experiments, sheep were fed with standard fodder. During control experiments 0.15 M NaCl was slowly administered i.v. through the indwelling catheter and during other experiment, hexamethonium bromide (2.0 and 5.0 mg/kg). atropine sulfate (0.02; 0.1; 0.5 and 1.5 mg/kg) and pirenzepine dihydrochloride (0.02; 0.5 and 2.0 mg/kg) were administered i.v. during phase 1-2a or 2b MMC. The drugs were also given in combinations. The recordings were analysed and the antral slow wave amplitudes and frequencies were calculated. Unlike the slow wave amplitude, either feeding or the anticholinergic drugs significantly increased slow wave frequency, especially when the given procedure was started during phase 2b MMC. The most pronounced effects were observed after hexamethonium given alone or in combinations. Thus, the cholinergic system modulates antral slow wave frequency in sheep.

Regulation of slow wave frequency by IP(3)-sensitive calcium release in the murine small intestine

Slow waves determine frequency and propagation characteristics of contractions in the small intestine, yet little is known about mechanisms of slow wave regulation. We propose a role for intracellular Ca(2+), inositol 1,4,5,-trisphosphate (IP(3))-sensitive Ca(2+) release, and sarcoplasmic reticulum (SR) Ca(2+) content in the regulation of slow wave frequency because 1) 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM, a cytosolic Ca(2+) chelator, reduced the frequency or abolished the slow waves; 2) thapsigargin and cyclopiazonic acid (CPA), inhibitors of SR Ca(2+)-ATPase, decreased slow wave frequency; 3) xestospongin C, a reversible, membrane-permeable blocker of IP(3)-induced Ca(2+) release, abolished slow wave activity; 4) caffeine and phospholipase C inhibitors (U-73122, neomycin, and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate) inhibited slow wave frequency; 5) in the presence of CPA or thapsigargin, stimulation of IP(3) synthesis with carbachol, norepinephrine, or phenylephrine acting on alpha(1)-adrenoceptors initially increased slow wave frequency but thereafter increased the rate of frequency decline, 6) thimerosal, a sensitizing agent of IP(3) receptors increased slow wave frequency, and 7) ryanodine, a selective modulator of Ca(2+)-induced Ca(2+) release, had no effect on slow wave frequency. In summary, these data are consistent with a role of IP(3)-sensitive Ca(2+) release and the rate of SR Ca(2+) refilling in regulation of intestinal slow wave frequency.

Coronary artery spasm. Multiple causes and multiple roles in heart disease

Myocardial infarction and sudden cardiac death may be initiated by a sudden intense localized contraction of coronary artery smooth muscle. When this event occurs around a vulnerable eccentric lipid-filled plaque, rupture and extrusion of plaque contents and exposure of collagen occur. This may sometimes be a silent and self-limiting event; other times it leads to thrombus formation. A second wave of spasm due to accumulated platelet and inflammatory mediators may compound the contractile consequences of the initiating event. Spasm involves intrinsic smooth muscle cell electrical mechanisms, hyper-responsive cells, and multiple agonists that synergize their actions, and the involvement of each mechanism varies at different times in the sequence of vascular occlusion. Study of spasm requires vascular systems that adequately model coronary artery responses of the ageing human heart. As previously emphasized, tissues obtained postmortem, and when possible from recipients during heart transplants, must be integral to theory building, alongside animal models, despite the experimental limitations such tissues impose. A multidisciplinary approach, at all levels of vascular physiology and pharmacology, will be necessary to understand coronary motor activity and human heart disease.

Coronary artery spasm and no spasmogens?

Traditionally coronary artery spasm, which is implicated in the pathogenesis of angina, myocardial infarction and sudden death, has been perceived as involving either a powerful spasmogen or larger quantities of a less potent stimulant. The present essay proposes that spasm may occur in the complete absence of any inciting chemical or spasmogen. Sudden intense coronary artery constriction may reflect an abnormality in an intrinsic system of tone regulation involving pacemaker cell discharge, intercellular conduction through gap junctions to adjacent cells, and the cycling of extracellular and bound calcium. Defects in spontaneous tone generation and recovery, may be primary factors in the causation of clinically occurring coronary artery spasm and its sequela.

The cellular network of interstitial cells associated with the deep muscular plexus of the guinea pig small intestine

Systematic examination using electron microscopic montages and serial sections has demonstrated that three types of interstitial cell, namely gap junction-rich cells, glycogen-rich cells and fibroblast-like cells, are densely located along the whole extent of the deep muscular plexus of the guinea pig small intestine. They tend to be distributed in an alternating fashion in the cellular network, connected with muscle cells of the outer, circular layer by means of gap junctions. These three types of interstitial cell show close relations to two types of nerve varicosity: one type is characterized by clear round vesicles with diameters of about 50 nm, and the other by flattened vesicles measuring about 35 nm by 70 nm. Electron-dense patches have been observed at the cytoplasmic side of the axonal membranes. Muscle cells of both inner and outer circular layers also show close relations to these two types of nerve varicosity. These morphological features are discussed with the implication that they may have some regulatory role in intestinal movement.

Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine, with special reference to the interstitial cells of Cajal

Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine were studied by electron microscopy, and three-dimensional cell models were reconstructed from serial ultrathin sections with a computer graphic system. Three types of cells were recognized. The first type was similar in shape to smooth muscle cells, but did not contain an organized contractile apparatus. Many large gap junctions comprising about 4% of the cell surface were present; they connected cells of the first type to each other, to the second type of cell and to smooth muscle cells of the outer circular layer. The second type of cell had a well-demarcated cell body with long slender processes and was characterized by a large amount of glycogen comprising about 9% of the cell volume. The third type of cell was similar to fibroblasts, and contained well-developed Golgi apparatus and rough endoplasmic reticulum. Some of these fibroblast-like cells (a possible subtype) formed small gap junctions. All three types of cells showed close relationships with nerve varicosities. This cellular network consisting of gap-junction-rich cells, glycogen-rich cells and smooth muscle cells may be involved in the pacemaking activity of intestinal movement.

Generation of slow-wave-type action potentials in canine colon smooth muscle involves a non-L-type Ca2+ conductance

1. The hypothesis was addressed that a non-L-type calcium conductance is involved in the generation of the initial part of the slow-wave-type action potential in the canine colon. 2. In the absence of a sodium and chloride gradient (NaCl replaced by glucamine), and in the presence of nitrendipine (in 'glucamine-nitrendipine' Krebs solution), a major portion of the upstroke potential of the slow wave persists at unchanged frequency. 3. In 'glucamine-nitrendipine' Krebs solution, the rate of rise and amplitude of the upstroke potential is reduced by removal of extracellular calcium in a concentration-dependent manner. 4. The rate of rise and the amplitude of the upstroke potential is in a concentration-dependent manner reduced by Ni2+ greater than Cd2+ greater than Co2+ greater than Mg2+. 5. In 'glucamine-nitrendipine' Krebs solution, Ba2+ cannot replace Ca2+ in the generation of the upstroke potential. 6. Positive evidence was obtained for the hypothesis that a non-L-type calcium conductance is involved in the initiation of the slow-wave-type action potential in colonic smooth muscle.

Effect of voltage and cyclic AMP on frequency of slow-wave-type action potentials in canine colon smooth muscle

1. A non-L-type calcium conductance is involved in the generation of the initial part of the slow-wave-type action potential in colonic smooth muscle. The present study addresses the question whether this conductance is voltage or metabolically activated. 2. Current-induced hyperpolarization increased frequency and amplitude of slow waves measured in Krebs solution. 3. The upstroke potential was 'isolated' from the slow wave by superfusion with 'glucamine-nitrendipine' Krebs solution (NaCl was replaced by glucamine, nitrendipine was added). 4. Hyperpolarization up to -100 mV did not affect the upstroke potential frequency and increased its amplitude. Only hyperpolarization further than -100 mV decreased the frequency less than or equal to 20%, and reduced the amplitude less than or equal to 20%. 5. Depolarization did not affect the upstroke potential frequency. 6. Forskolin, but not 1,9-dideoxyforskolin dramatically decreased the upstroke potential frequency, without affecting other parameters including the resting membrane potential. 7. The effect of forskolin was mimicked by dibutyryl cyclic AMP, 8-bromo-cyclic AMP and 3-isobutyl-1-methylxanthine (IBMX), but not extracellular cyclic AMP. 8. The upstroke potential could not be evoked by depolarizing pulses after inhibition of activity by forskolin. 9. The effect of forskolin could be reversed by the calcium ionophore A23187. 10. In summary, voltage changes up to -40 mV and down to -100 mV do not, but changes in intracellular cyclic AMP do affect the frequency of the upstroke potential. 11. It is likely that intracellular metabolic activity, which may include cyclic AMP but not a voltage change, activates the conductance responsible for the generation of the upstroke potential.

Metabolic correlates to pacemaker activity in the smooth muscle of guinea-pig mesotubarium

Oxygen consumption (FO2) and lactate production (Flac) were measured during spontaneous activity in the guinea-pig mesotubarium. During spontaneous contractions FO2 increased to maximally 0.270 +/- 0.025 mumol min-1g-1 (n = 23), followed by a rapid fall immediately upon relaxation. In the relaxed period (5-15 min) between spontaneous contractions FO2 continued to slowly decrease by about 25% towards a final value of 0.150 +/- 0.01 mumol min-1g-1. Flac showed no consistent variation during the relaxed period. Ouabain (10(-6)M) produced a contracture, which was abolished by the Ca2(+)-antagonist felodipine (10(-6)M). In the presence of felodipine, addition of ouabain caused depolarization and a decrease of oxygen consumption by 21% and of lactate production by 31%. Exchange of glucose in the physiological Krebs solution for beta-hydroxybutyrate did not influence spontaneous activity, while subsequent addition of cyanide (2 mM) abolished contractions and caused a hyperpolarization of 15 mV. Blockade of ATP-dependent K+ channels by addition of glibenclamide (10(-7)M) to the relaxed muscle in this situation caused spontaneous contractile activity to reappear. In glucose-containing Krebs solution glibenclamide had no effect on the spontaneous contractile and electrical activity and contractions persisted after addition of cyanide. The relaxing and hyperpolarizing effect of pinacidil could be counteracted by addition of glibenclamide. The results suggest that a decrease in electrogenic Na+/K(+)-pump activity in the course of the relaxed period between contractions could contribute to the pacemaker behaviour. ATP-dependent K+ channels, while having little influence on the spontaneous contractile activity under normal metabolic conditions, could be activated during blockade of aerobic and anaerobic metabolism, leading to inhibition of pacemaker activity.

Role of the sodium pump in pacemaker generation in dog colonic smooth muscle

1. The role of the Na+ pump in the generation of slow wave activity in circular muscle of the dog colon was investigated using a partitioned 'Abe-Tomita' type chamber for voltage control. 2. Blockade of the Na+ pump by omission of extracellular K+, by ouabain, or the combination of 0 mM-Na+ and ouabain, depolarized the membrane up to approximately -40 mV and abolished the slow wave activity. Repolarization back to the control membrane potential by hyperpolarizing current restored the slow wave activity. 3. Slow waves continued to be present in 0 Na+, Li+ HEPES solution. 4. The depolarization induced by the procedures to block Na+ pump activity was associated with an increase in input membrane resistance. 5. Voltage-current relationships show the presence of an inward rectification. 6. Reduction of temperature depolarized the membrane, and decreased the slow wave frequency and amplitude. The slow wave amplitude was restored by repolarization of the membrane. 7. Brief depolarizing pulses evoked premature slow waves. Brief hyperpolarizing pulses terminated the slow waves. 8. We conclude that abolition of slow wave activity by Na+ pump blockade is a direct effect of membrane depolarization and that the Na+ pump is not responsible for the generation of the slow wave. 9. Our results are consistent with the hypothesis that pacemaker activity in smooth muscle is a consequence of membrane conductance changes which are metabolically dependent.

Ionic basis of pacemaker generation in dog colonic smooth muscle

1. The ionic basis of the slow waves in the circular muscle of the dog colon, in particular the ionic conductances involved in their initiation, were investigated by measuring intracellular electrical activity in the Abe-Tomita-type chamber for voltage control. 2. The depolarization that initiates the slow wave activity could be evoked by an increase in inward current and/or by a block of outward current. According to previous work, inward current could be carried by Na+, Cl-, and Ca2+ ions; K+ ions would carry outward current. 3. The Na+ channel blocker tetrodotoxin (5 x 10(-7) M) did not affect the slow wave amplitude nor its rate of rise. After omission of Na+, by replacing Na+ with N-methyl-D-glucamine, large slow waves continued to develop although some changes in slow wave characteristics occurred. 4. Replacement of 91% of the Cl- by isethionate decreased the slow wave frequency and increased the slow wave amplitude. However, NaCl substitution by sucrose increased the slow wave frequency and decreased the slow wave amplitude. 5. Slow wave activity continued to develop after blockade of Ca2+ influx by D600 (10(-6) M) or CoCl2 (1-3 mM). D600 and Co2+ did not affect the membrane potential but reduced the slow wave amplitude and abolished the plateau potential. Slow waves were abolished after omission of extracellular Ca2+ (plus 1 mM-EGTA). This suggests that Ca2+ influx is probably not necessary but extracellular presence of Ca2+ ions is indispensible for the slow wave generation. 6. The combination of 0 Na+, Li+ HEPES solution, by replacing Na+ with Li+, plus D600 depolarized the cells (up to approximately -40 mV) and abolished slow wave activity. This effect was voltage dependent since repolarization caused slow waves to return. 7. Abolition of the slow wave activity was also obtained by current-induced depolarization to approximately -40 mV. However, during high-K+-induced depolarization (to approximately -40 mV) high amplitude (16 mV) slow waves were still present, slowing that the voltage dependence of the slow waves was shifted positively. This effect probably occurs due to modification by extracellular K+ of a voltage-dependent K+ conductance, which would suggest that a K+ conductance is involved in slow wave generation. 8. In conclusion, slow waves are generated by cyclic membrane conductance changes, which are dependent on the presence of extracellular Ca2+ ions and on the membrane potential. Our data are consistent with the hypothesis that slow waves are initiated by the blockade of a K+ conductance.