Effects of cromakalim on the electrical slow wave in the circular muscle of guinea-pig gastric antrum

Pacemaker phase shift in the absence of neural activity in guinea-pig stomach: a microelectrode array study

Gastrointestinal (GI) motility is well organized. GI muscles act as a functional syncytium to achieve physiological functions under the control of neurones and pacemaker cells, which generate basal spontaneous pacemaker electrical activity. To date, it is unclear how spontaneous electrical activities are coupled, especially within a micrometre range. Here, using a microelectrode array, we show a spatio-temporal analysis of GI spontaneous electrical activity. The muscle preparations were isolated from guinea-pig stomach, and fixed in a chamber with an array of 8 x 8 planar multielectrodes (with 300 microm in interpolar distance). The electrical activities (field potentials) were simultaneously recorded through a multichannel amplifier system after high-pass filtering at 0.1 Hz. Dihydropyridine Ca(2+) channel antagonists are known to differentiate the electrical pacemaker activity of interstitial cells of Cajal (ICCs) by suppressing smooth muscle activity. In the presence of nifedipine, we observed spontaneous electrical activities that were well synchronized over the array area, but had a clear phase shift depending on the distance. The additional application of tetrodotoxin (TTX) had little effect on the properties of the electrical activity. Furthermore, by constructing field potential images, we visualized the synchronization of pacemaker electrical activities resolving phase shifts that were measurable over several hundred micrometres. The results imply a phase modulation mechanism other than neural activity, and we postulate that this mechanism enables smooth GI motility. In addition, some preparations clearly showed plasticity of the pacemaker phase shift.

Sulphonylurea receptors differently modulate ICC pacemaker Ca2+ activity and smooth muscle contractility

Appropriate gastrointestinal motility is essential to properly control the body energy level. Intracellular Ca2+ ([Ca2+]i) oscillations in interstitial cells of Cajal (ICCs; identified with c-Kit immunoreactivity) are considered to be the primary mechanism for the pacemaker activity in gastrointestinal motility. In the present study, RT-PCR examinations revealed predominant expression of the type 1 isoform of sulphonylurea receptors (SUR1) in ICCs of the mouse ileum, but expression of SUR2 was predominant in smooth muscle. In cell clusters prepared from the same tissue, smooth muscle contractility and pacemaker [Ca2+]i activity in ICCs were found to be differentially modulated by K(ATP) channel openers and sulphonylurea compounds, in accordance with the expression of SUR isoforms. 1 microM cromakalim nearly fully suppressed the mechanical activity in smooth muscle, whereas ICC pacemaker [Ca2+]i oscillations persisted. Greater concentrations (approximately 10 microM) of cromakalim attenuated pacemaker [Ca2+]i oscillations. This effect was not reversed by changing the reversal potential of K+, but was prevented by glibenclamide. Diazoxide at 30 muM terminated ICC pacemaker [Ca2+]i oscillations, but again treatment with high extracellular K+ did not restore them. These results suggest that SUR can modulate pacemaker [Ca2+]i oscillations via voltage-independent mechanism(s), and also that intestinal pacemaking and glucose control are closely associated with SUR.

Modulation of slow waves by hyperpolarization with potassium channel openers in antral smooth muscle of the guinea-pig stomach

Modulation of spontaneous electrical activities (slow waves, pacemaker potentials and follower potentials) in response to hyperpolarization produced by the ATP-sensitive K+ channel openers (KCOs) pinacidil or nicorandil was investigated in smooth muscle tissues of the guinea-pig stomach antrum. With hyperpolarization, the amplitude of slow waves and follower potentials was reduced and that of pacemaker potentials was increased, with a minor modulation of their frequency. The attenuation of slow waves was associated with an inhibition of the 1st component and abolition of the 2nd component. All these actions of KCOs were antagonized by glibenclamide. An increase in the extracellular K+ concentration prevented the KCO-induced hyperpolarization with partial restoration of slow waves, suggesting that the inhibition was produced mainly by a decrease in membrane resistance. Exposure of tissues to KCOs for a long period of time (> 20 min) resulted in the reappearance of slow waves displaying both 1st and 2nd components. The 2nd component of the slow wave, which displayed a slower recovery, was inhibited again by 5-hydroxydecanoic acid, an inhibitor of mitochondrial ATP-sensitive K+ channels. Noradrenaline hyperpolarized the membrane by activating apamin-sensitive K+ channels and increased the amplitude and frequency of slow waves through activation of alpha 1-adrenoceptors, actions different from those of KCOs. Thus, inhibition of slow waves by KCOs may be primarily related to the decrease in amplitude of a passive electrotonic component, possibly due to a reduction of the input resistance. The hyperpolarization shifted the threshold potential for generation of the 2nd component of slow waves to negative levels, presumably due to modulation of mitochondrial functions.

Smooth muscle and NMR review: an overview of smooth muscle metabolism

Nuclear magnetic resonance (NMR) is a non-invasive technique which allows us to examine the biochemical, physiological and metabolic events occurring inside living tissue; such as vascular and other smooth muscles. It has been found that the smooth muscle metabolism is compartmented such that mitochondrial function fuels contraction and that much glycolytic ATP production is used for membrane pumps. Using NMR we have been able to observe the ATP and phosphocreatine (PCr) concentrations and estimate the ADP concentration, as well as flux through the creatine kinase (CK) system. It has also been found that the smooth muscle metabolism is able to maintain ATP concentration in the absence of mitochondrial function (cyanide inhibition). Therefore, the vessels are able to adapt to metabolic demands as necessary. NMR is versatile in the information it can provide because it has also yielded important contributions with regard to the intracellular pH and ionic status. For example, the intracellular free Mg2+ ([Mg2+]i) can be measured with NMR simultaneously with ATP concentrations and NMR has shown us that the [Mg2+]i is highly protected in the muscle (within confined range), but also responds to the environment around it. In this review we conclude that NMR measurements of smooth muscle research is a useful technique for assessing chronic and acute changes that occur in the tissue and during diseases.

Properties of unitary potentials recorded from myenteric interstitial cells of Cajal distributed in the guinea-pig gastric antrum

Intracellular recordings were made from myenteric interstitial cells of Cajal (ICC-MY) distributed in the guinea-pig gastric antrum to investigate the properties of unitary potentials. In most cells studied, pacemaker potentials with initial fast transient and following plateau components were generated periodically, and intervals between the potentials were quiescent. However, there were few cells (less than 5% of cells examined) which showed discharge of unitary potentials spontaneously in the intervals between pacemaker potentials. The amplitude and frequency of unitary potentials appeared to be random variables, as observed in isolated circular smooth muscle bundles of the guinea-pig gastric antrum. BAPTA-AM (an intracellular Ca2+ chelator) or papaverine (a non-selective phosphodiesterase inhibitor) reduced the discharge frequency of unitary potentials, with associated decrease in the frequency of pacemaker potentials. These agents finally abolished both unitary potentials and pacemaker potentials. In preparations showing no detectable generation of unitary potentials, depolarization of the membrane with high-K solution ([K+]o = 10.6 mM) elicited generation of unitary potentials during intervals between pacemaker potentials. Pinacidil (an opener of K(ATP)-channels) hyperpolarized the membrane and increased the frequency and amplitude of unitary potentials with no alteration to the relationship between the amplitudes of unitary potentials and their half-widths. These results suggest that the elevation of intracellular Ca2+ concentration is causally related to the generation of unitary potentials in pacemaker cells. They are consistent with the proposition that the depolarization produced by a burst of unitary potentials triggers the primary component of pacemaker potentials in ICC-MY, which induces a release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive internal stores and then activates Ca2+-sensitive Cl- -channels to form the plateau component. Similarities and differences in unitary potentials between circular muscle and pacemaker cells are discussed.

The reactivity of serotonin, acetylcholine and kcl-induced contractions to relaxant agents in the rat gastric fundus

The effects of nifedipine, cromakalim, diazoxide, caffeine and sodium nitroprusside (SNP) on acetylcholine, serotonin and KCl-induced contractions were studied in rat stomach fundus. Thus, we aimed to demonstrate how these contractions are modified by the substances acting on Ca (2+)influx and intracellular Ca (2+)stores. Serotonin (10(-9) - 10(-5) M) and KCl (20-80 mM) showed a similar contraction profile, which was slightly different from that of acetylcholine (10(-8)- 3 x 10(-3) M). In the experiments with the incubation of calcium-free/EGTA (0.5 mM) Krebs solution for 20 min, serotonin (3 x 10(-7)M) and KCl (40 mM) did not produce any contraction whereas, 10% of contraction to acetylcholine (3 x 10(-5) M) was still intact. Serotonin-induced contractions were readily reversed by nifedipine (10(-10) - 10(-4) M), cromakalim (10(-9) - 10(-4) M), diazoxide (10(-9) - 10(-4) M), caffeine (10(-5) - 10(-2) M) and SNP (10(-4) M) whereas, acetylcholine-induced contractions showed relative refractoriness to the above relaxant agents. 1 mM caffeine nearly fully inhibited serotonin-induced contraction, but not acetylcholine and high K-induced contractions whereas, 10 mM caffeine completely inhibited all the contractions. The relaxation pattern of nifedipine on serotonin and high K (+)-induced contractions was quite similar. Moreover, nifedipine and cromakalim showed equal dose effectiveness in relaxing acetylcholine and serotonin. The maximum relaxations induced by nifedipine and cromakalim in acetylcholine contractions were 61.51 +/- 6.92 % and 58.97 +/- 7.55 %, respectively. However their maximum relaxations in serotonin and high K (+)-induced contractions were almost 100%. The similarity in myorelaxants properties of cromakalim and nifedipine may relate to the similarity of their effects on calcium influx by a different mechanism of action in rat stomach fundus. In conclusion, acetylcholine-induced contraction is partially mediated both by calcium release from the intracellular Ca (2+) pool and calcium influx via L-type Ca (2+) channels. However, serotonin-induced contraction is possibly triggered by Ca (2+) release from sarcoplasmic reticulum and basically mediated by Ca (2+) influx via L-type Ca (2+)channels.

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.

Effects of iodoacetate 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, the contribution of glycolysis to spontaneous electrical activity, slow wave, was studied. The slow wave could be maintained without a marked change in glucose-free solution for more than 1 h even when treated with iodoacetic acid (IAA, 0.1-0.5 mM). However, reapplication of glucose following the IAA treatment produced clear inhibitory effects on the slow wave. Lactate release from the tissue was reduced to about 10% of the control by IAA (0.1 mM) in the absence of glucose and there was very slow recovery on glucose reapplication. This suggests that IAA did not block glycolysis completely and that the inhibition of slow wave was mainly due to the accumulation of some metabolites. Small electrical activity often remained during the inhibition by IAA and glucose. When the excitability of the smooth muscle was increased by Co(2+) application or Na(+) removal, slow wave-like activity could be generated under the condition in which the slow wave was strongly inhibited by IAA and glucose. These results may be explained by assuming that the accumulation of glycolytic metabolites decreases the excitability of smooth muscle cells and also reduces the driving potential generated in the interstitial cells of Cajal to a subthreshold level for the slow wave in the smooth muscle cells.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

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.

Consequences of metabolic inhibition in smooth muscle isolated from guinea-pig stomach

1. In smooth muscle isolated from the guinea-pig stomach, cyanide (CN) and iodoacetic acid (IAA) were applied to block oxidative phosphorylation and glycolysis, respectively. Effects of IAA on generation of spontaneous mechanical and electrical activities were systematically investigated by comparing those of CN. Spontaneous activity ceased in 10-20 min during applications of 1 mM IAA. On the other hand, application of 1 mM CN also reduced the spontaneous activity, but never terminated it. In the presence of CN the negativity of the resting membrane potential was slightly reduced. 2. When spontaneous activity ceased with IAA, the resting membrane potential was not significantly affected. Also, before ceasing, the amplitude and duration of the spontaneous electrical activity were significantly reduced. The amplitude of the electrotonic potential was, however, not changed by IAA. Further, glibenclamide did not prevent the effects of IAA. These results suggest that, unlike cardiac muscle, activation of metabolism-dependent K+ channels in stomach smooth muscle does not seem to play a major role in reducing and terminating spontaneous activity during metabolic inhibition. 3. Carbachol-induced contraction transiently increased, and subsequently decreased gradually during application of IAA. 4. After 50 min application of IAA, when there was no spontaneous activity, the concentrations of phosphocreatine (PCr) and ATP measured with 31P nuclear magnetic resonance decreased to 60 and 80% of the control, respectively, while inorganic phosphate (Pi) concentration paradoxically fell to below detectable levels. During subsequent prolonged application of IAA, high-energy phosphates steadily decreased. On the other hand, after 50 min CN application, [PCr] and [ATP] decreased to approximately 30 and 80% of the control, respectively, while [Pi] increased by 2.6-fold. 5. In the presence of either CN or IAA, spontaneous mechanical and electrical activities were reduced or eliminated, although amounts of high-energy phosphates sufficient to contract smooth muscle remained. It can be postulated that some mechanism(s) related to energy metabolism, but not including ATP-sensitive K+ channels, plays an important role in generating spontaneous activity in guinea-pig stomach smooth muscle. During metabolic inhibition the energy metabolism-dependent mechanism(s) would preserve high-energy phosphates, and consequently cell viability, by stopping spontaneous activity.

Interstitial cells of Cajal as targets for pharmacological intervention in gastrointestinal motor disorders

Interstitial cells of Cajal (ICCs) have recently been identified as the pacemaker cells for contractile activity of the gastrointestinal tract. These cells generate the electrical 'slow-wave' activity that determines the characteristic frequency of phasic contractions of the stomach, intestine and colon. Slow waves also determine the direction and velocity of propagation of peristaltic activity, in concert with the enteric nervous system. Characterization of receptors and ion channels in the ICC membrane is under way, and manipulation of slow-wave activity markedly alters movement of contents through the gut organs. Here Jan Huizinga, Lars Thuneberg, Jean-Marie Vanderwinden and Jüri Rumessen, suggest that, as ICCs are unique to the gut, they might be ideal targets for pharmacological intervention in gastrointestinal motility disorders, which are very common and costly.

Effects of phosphodiesterase inhibitors on spontaneous electrical activity (slow waves) in the guinea-pig gastric muscle

1. The effects of the phosphodiesterase inhibitors caffeine, theophylline, isobutylmethylxanthine (IBMX) and rolipram on spontaneous electrical activity (slow waves) were studied in the circular muscle of the guinea-pig gastric antrum. 2. All the inhibitors reduced slow wave frequency without changing the membrane potential and the slow wave configuration, but at higher concentrations they blocked the slow waves and caused membrane hyperpolarization. In the presence of the inhibitors a low level of irregular electrical activity could be observed in many preparations. 3. Isoprenaline, forskolin, dibutyryl cAMP and 8-bromo-cAMP all produced effects essentially similar to those of phosphodiesterase inhibitors. K+ (12 mM) and removal of K+ both depolarized the membrane and these were not affected by IBMX (1-3 microM). A decrease in frequency caused by IBMX was also not significantly affected by 12 mM K+ or K+ removal and only partially antagonized by TEA or 4-aminopyridine. 4. These results suggest that an increase in intracellular cAMP inhibits pacemaker activity of slow waves. An increase in K+ conductance does not seem to be a major factor in this inhibition. Slow waves appear to be a compound electrical activity in a group of muscle cells and are likely to be disintegrated by xanthine derivatives.

Sustained contraction produced by caffeine after ryanodine treatment in the circular muscle of the guinea-pig gastric antrum and rabbit portal vein

1. Caffeine inhibited spontaneous mechanical activity at 0.3-1 mM, but produced a tonic contraction at concentrations higher than 3 mM in the circular muscle of the guinea-pig gastric antrum. In the circular muscle of the rabbit portal vein, caffeine at concentrations higher than 1 mM produced an early phasic contraction followed by a small tonic component. The caffeine-induced contraction was abolished by removal of the external Ca2+ more rapidly in the gastric antrum than the portal vein. 2. When the preparations were pretreated with ryanodine (1 microM) a sustained contraction developed on wash-out of caffeine (10 mM) both in the gastric antrum and portal vein. This contraction was not affected by nicardipine (3 microM) or verapamil (3 microM), but was readily abolished by removal of the external Ca2+ or by addition of cobalt (1 mM). Spontaneous electrical activity, the slow wave, in gastric muscles was blocked in the presence of 10 mM caffeine, but reappeared during the sustained contraction. 3. Both the contractions induced directly by caffeine and those produced following caffeine wash-out after ryanodine treatment were accompanied by a maintained increase in intracellular Ca2+ concentration measured with fura-2. 4. The presence or absence of Ca2+ during the application of ryanodine did not affect the ability of caffeine to initiate sustained contractions, provided Ca2+ was present during the exposure to caffeine. 5. It is concluded that caffeine can induce a sustained contraction after ryanodine treatment both in the guinea-pig gastric antrum and rabbit portal vein, by activating a Ca2+ influx pathway insensitive to organic Ca2+ channel blockers. No clear evidence was obtained for involvement of the Ca2+ influx pathway activated through depletion of intracellular Ca2+ stores. A hypothesis is proposed that the plasma membrane of these preparations is similar to the sarcoplasmic reticulum membrane in that Ca2+permeability can be increased almost irreversibly by a combination of caffeine and ryanodine in the presence of the external Ca2+.

Effect of cromakalim and glibenclamide on spontaneous and evoked motility of the guinea-pig isolated renal pelvis and ureter

1. We have investigated the effect of the potassium (K) channel opener, cromakalim, on the spontaneous myogenic activity of the guinea-pig isolated renal pelvis and on myogenic contractions evoked by direct electrical stimulation of the guinea-pig isolated ureter. 2. In the presence of Bay K 8644 (1 microM), electrical stimulation of the guinea-pig ureter (10 Hz for 1 s, pulse width 5 ms, 60 V) produced regular tetrodotoxin-(1 microM) resistant phasic contractions which were suppressed by 3 microM cromakalim. Glibenclamide (0.1-3 microM), 4-aminopyridine (4-AP, 0.1-2 mM) and tetraethylammonium (TEA, 1-10 mM) produced a concentration-dependent inhibition of the effect of cromakalim with the rank order of potency (EC50 in parentheses): glibenclamide (0.64 microM) >> 4-AP (1.11 mM) > TEA (6.6 mM). Apamin (0.1-0.3 microM) was without effect. 3. Cromakalim (0.1-10 microM) produced concentration-dependent inhibition and suppression of spontaneous contractions of the guinea-pig isolated renal pelvis and of evoked contractions of the ureter with EC50 values of 0.71 and 0.47 microM, respectively. 4. Glibenclamide (1 microM) produced a rightward shift of the concentration-response curve to cromakalim in both the renal pelvis and ureter, without producing depression of the maximal inhibitory effect. Glibenclamide did not affect the spontaneous activity of the renal pelvis while it produced a slight enhancement (10-15% increase) of evoked contractions of the ureter. Glibenclamide did not affect the inhibitory action of the adenylate cyclase activator, forskolin, in the renal pelvis or ureter. 5. In electrophysiological experiments (sucrose gap), cromakalim (0.3 and 1 microM) produced hyperpolarization of ureter smooth muscle. Cromakalim also produced a transient suppression of action potentials and accompanying phasic contractions evoked by electrical stimulation. Before suppression of evoked contractions, a shortening of action potential duration was observed concomitant with the developing hyperpolarization produced by cromakalim. A lower concentration (0.1 MicroM) of cromakalim did not affect membrane potential but shortened action potential duration and reduced the evoked contraction.6. Glibenclamide (1 MicroM) inhibited the hyperpolarizing action of cromakalim and prevented its inhibitory action on evoked action potentials and contractions of the ureter. Glibenclamide also produced a slight prolongation of action potential duration and increased the amplitude and duration of the accompanying mechanical response.7. These findings demonstrate that activation of cromakalim- and glibenclamide-sensitive K channels produces a powerful mechanism for regulation of pyeloureteral motility and suppression of latent pacemakers of the ureter in guinea-pig. Glibenclamide-sensitive K channels take part in determining action potential shape and duration in the guinea-pig ureter.