Cable properties of smooth muscle

Autonomic neuromuscular junctions

This review traces the history of the discovery and subsequent understanding of smooth muscle cells and their motor innervation. Smooth muscle tissue is made up of thousands of very small, individual, electrically connected, muscle cells. Each axon that enters a smooth muscle tissue branches extensively to form a terminal arbour that comes close to hundreds of smooth muscle cells. The branches of the terminal arbour are varicose, and each varicosity, of which there can be thousands, contains numerous transmitter storage vesicles. However, the probability of an individual varicosity releasing transmitter onto the adjacent muscle cells when an action potential passes is low. Many axons influence each muscle cell, some because they release transmitter close to the cell, and some because the events that they cause in other cells are electrically coupled to the cell under investigation. In tissues where this has been assessed, 20 or more axons can influence a single smooth muscle cell. We present a model of the innervation and influence of neurons on smooth muscle.

Nerve transfer for restoration of lower motor neuron-lesioned bladder function. Part 2: correlation between histological changes and nerve evoked contractions

We determined the effect of pelvic organ decentralization and reinnervation 1 yr later on urinary bladder histology and function. Nineteen canines underwent decentralization by bilateral transection of all coccygeal and sacral (S) spinal roots, dorsal roots of lumbar (L)7, and hypogastric nerves. After exclusions, eight were reinnervated 12 mo postdecentralization with obturator-to-pelvic and sciatic-to-pudendal nerve transfers, then euthanized 8-12 mo later. Four served as long-term decentralized only animals. Before euthanasia, pelvic or transferred nerves and L1–S3 spinal roots were stimulated and maximum detrusor pressure (MDP) recorded. Bladder specimens were collected for histological and ex vivo smooth muscle contractility studies. Both reinnervated and decentralized animals showed less or denuded urothelium, fewer intramural ganglia, and more inflammation and collagen, than controls, although percent muscle was maintained. In reinnervated animals, pgp9.5+ axon density was higher compared with decentralized animals. Ex vivo smooth muscle contractions in response to KCl correlated positively with submucosal inflammation, detrusor muscle thickness, and pgp9.5+ axon density. In vivo, reinnervated animals showed higher MDP after stimulation of L1–L6 roots compared with their transected L7–S3 roots, and reinnervated and decentralized animals showed lower MDP than controls after stimulation of nerves (due likely to fibrotic nerve encapsulation). MDP correlated negatively with detrusor collagen and inflammation, and positively with pgp9.5+ axon density and intramural ganglia numbers. These results demonstrate that bladder function can be improved by transfer of obturator nerves to pelvic nerves at 1 yr after decentralization, although the fibrosis and inflammation that developed were associated with decreased contractile function.

Advances in colonic motor complexes in mice

The primary functions of the gastrointestinal (GI) tract are to absorb nutrients, water, and electrolytes that are essential for life. This is accompanied by the capability of the GI tract to mix ingested content to maximize absorption and effectively excrete waste material. There have been major advances in understanding intrinsic neural mechanisms involved in GI motility. This review highlights major advances over the past few decades in our understanding of colonic motor complexes (CMCs), the major intrinsic neural patterns that control GI motility. CMCs are generated by rhythmic coordinated firing of large populations of myenteric neurons. Initially, it was thought that serotonin release from the mucosa was required for CMC generation. However, careful experiments have now shown that neither the mucosa nor endogenous serotonin are required, although, evidence suggests enteroendocrine (EC) cells modulate CMCs. The frequency and extent of propagation of CMCs are highly dependent on mechanical stimuli (circumferential stretch). In summary, the isolated mouse colon emerges as a good model to investigate intrinsic mechanisms underlying colonic motility and provides an excellent preparation to explore potential therapeutic agents on colonic motility, in a highly controlled in vitro environment. In addition, during CMCs, the mouse colon facilitates investigations into the emergence of dynamic assemblies of extensive neural networks, applicable to the nervous system of different organisms.

Electrophysiology of Syncytial Smooth Muscle

As in other excitable tissues, two classes of electrical signals are of fundamental importance to the functioning of smooth muscles: junction potentials, which arise from neurotransmission and represent the initiation of excitation (or in some instances inhibition) of the tissue, and spikes or action potentials, which represent the accomplishment of excitation and lead on to contractile activity. Unlike the case in skeletal muscle and in neurons, junction potentials and spikes in smooth muscle have been poorly understood in relation to the electrical properties of the tissue and in terms of their spatiotemporal spread within it. This owes principally to the experimental difficulties involved in making precise electrical recordings from smooth muscles and also to two inherent features of this class of muscle, ie, the syncytial organization of its cells and the distributed innervation they receive, which renders their biophysical analysis problematic. In this review, we outline the development of hypotheses and knowledge on junction potentials and spikes in syncytial smooth muscle, showing how our concepts have frequently undergone radical changes and how recent developments hold promise in unraveling some of the many puzzles that remain. We focus especially on computational models and signal analysis approaches. We take as illustrative examples the smooth muscles of two organs with distinct functional characteristics, the vas deferens and urinary bladder, while also touching on features of electrical functioning in the smooth muscles of other organs.

Real-time measurement of biomagnetic vector fields in functional syncytium using amorphous metal

Magnetic field detection of biological electric activities would provide a non-invasive and aseptic estimate of the functional state of cellular organization, namely a syncytium constructed with cell-to-cell electric coupling. In this study, we investigated the properties of biomagnetic waves which occur spontaneously in gut musculature as a typical functional syncytium, by applying an amorphous metal-based gradio-magneto sensor operated at ambient temperature without a magnetic shield. The performance of differentiation was improved by using a single amorphous wire with a pair of transducer coils. Biomagnetic waves of up to several nT were recorded ~1 mm below the sample in a real-time manner. Tetraethyl ammonium (TEA) facilitated magnetic waves reflected electric activity in smooth muscle. The direction of magnetic waves altered depending on the relative angle of the muscle layer and magneto sensor, indicating the existence of propagating intercellular currents. The magnitude of magnetic waves rapidly decreased to ~30% by the initial and subsequent 1 mm separations between sample and sensor. The large distance effect was attributed to the feature of bioelectric circuits constructed by two reverse currents separated by a small distance. This study provides a method for detecting characteristic features of biomagnetic fields arising from a syncytial current.

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.

CaMKII inhibition hyperpolarizes membrane and blocks nitrergic IJP by closing a Cl(-) conductance in intestinal smooth muscle

The ionic basis of nitrergic "slow'" inhibitory junction potential (sIJP) is not fully understood. The purpose of the present study was to determine the nature and the role of calmodulin-dependent protein kinase II (CaMKII)-dependent ion conductance in nitrergic neurotransmission at the intestinal smooth muscle neuromuscular junction. Studies were performed in guinea pig ileum. The modified Tomita bath technique was used to induce passive hyperpolarizing electrotonic potentials (ETP) and membrane potential change due to sIJP or drug treatment in the same cell. Changes in membrane potential and ETP were recorded in the same smooth muscle cell, using sharp microelectrode. Nitrergic IJP was elicited by electrical field stimulation in nonadrenergic, noncholinergic conditions and chemical block of purinergic IJP. Modification of ETP during hyperpolarization reflected active conductance change in the smooth muscle. Nitrergic IJP was associated with decreased membrane conductance. The CAMKII inhibitor KN93 but not KN92, the Cl(-) channel blocker niflumic acid (NFA), and the K(ATP)-channel opener cromakalim hyperpolarized the membrane. However, KN93 and NFA were associated with decreased and cromakalim was associated with increased membrane conductance. After maximal NFA-induced hyperpolarization, hyperpolarization associated with KN93 or sIJP was not seen, suggesting a saturation block of the Cl(-) channel signaling. These studies suggest that inhibition of CaMKII-dependent Cl(-) conductance mediates nitrergic sIJP by causing maximal closure of the Cl(-) conductance.

Smooth muscle Ca(2+) -activated and voltage-gated K+ channels modulate conducted dilation in rat isolated small mesenteric arteries

OBJECTIVE

  To assess the influence of blocking smooth muscle large conductance Ca(2+) -activated K+ channels and voltage-gated K+ channels on the conducted dilation to ACh and isoproterenol.

MATERIALS AND METHODS

  Rat mesenteric arteries were isolated with a bifurcation, triple-cannulated, pressurized and imaged using confocal microscopy. Phenylephrine was added to the superfusate to generate tone, and agonists perfused into a sidebranch to evoke local dilation and subsequent conducted dilation into the feed artery.

RESULTS

  Both ACh- and isoproterenol-stimulated local and conducted dilation with similar magnitudes of decay with distance along the feed artery (2000μm: ∼15% maximum dilation). The gap junction uncoupler carbenoxolone prevented both conducted dilation and intercellular spread of dye through gap junctions. IbTx, TEA or 4-AP, blockers of large conductance Ca(2+) -activated K+ channels and voltage-gated K+ channels, did not affect conducted dilation to either agonist. A combination of either IbTx or TEA with 4-AP markedly improved the extent of conducted dilation to both agonists (2000μm: >50% maximum dilation). The enhanced conducted dilation was reflected in the hyperpolarization to ACh (2000μm: Control, 4±1 mV, n = 3; TEA with 4-AP, 14±3mV, n=4), and was dependent on the endothelium.

CONCLUSIONS

  These data show that activated BK(Ca) and K(V) -channels serve to reduce the effectiveness of conducted dilation.

Pulse-driven magnetoimpedance sensor detection of biomagnetic fields in musculatures with spontaneous electric activity

We measured biomagnetic fields in musculatures with spontaneous electric activity using a pulse-driven magnetoimpedance (PMI) sensor with the sensitivity improved toward a pico-Tesla (pT) level. Due to the sufficiently short operation interval of 1 μs, this magnetic sensor enabled quasi-real time recordings of the magnetic field for biological electric activity. Isolated small musculatures from the guinea-pig stomach, taenia caeci, portal vein and urinary bladder were incubated in an organ bath at a body temperature. The improved PMI sensor mounted approximately 1mm below the preparations detected oscillatory magnetic fields reflecting spontaneous electric activities of musculature preparations. In the taenia caeci, application of tetraethyl ammonium (TEA), a K(+) channel blocker, significantly enhanced the magnetic activity estimated by histogram analysis. Also, in some musculature preparations, simultaneous measurements with electric activity revealed that the observed magnetic activities were attributed to biological electric activity. PMI technology is promising for applications in biology and medicine.

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.

Spontaneous electrical activity in sheep mesenteric lymphatics

BACKGROUND

It has recently become apparent that the lymph pump is an electrical entity that rivals the heart in complexity. Many interesting currents have been demonstrated by voltage clamping isolated lymphatic smooth muscle cells, but until now the role of these currents in the intact syncitium has not been studied.

METHODS AND RESULTS

Intracellular microelectrode recordings were made from smooth muscle of sheep mesenteric lymphatics to investigate the electrophysiological basis of lymphatic pumping. Approximately 50% of the vessels exhibited spontaneous electrical activity, varying from regular oscillations in membrane potential to spike complexes. Spike complexes generally consisted of one or more action potentials superimposed on a slower depolarization or 'plateau' phase and were often preceded by a slow diastolic depolarization or 'pre-potential'. Norepinephrine (5 microM) induced depolarizing events in quiescent preparations. Both agonist-induced oscillations and spike complexes were attenuated or completely abolished by 2-aminoethoxydiphenyl borate (2-APB); 10-100 microM). Cesium (1 mM) reduced the frequency of spontaneous firing by approximately 30% by flattening the pre-potential phase. In addition to having a negative inotropic effect, 10 mM Cs(+) also caused gradual membrane depolarization and prolonged the plateau. 1 microM nifedipine abolished spontaneous events while tetrodotoxin (TTX; 0.5-1 muM) decreased the amplitude and maximum dV/dt of the spike upstroke or stopped activity completely. Spontaneously active segments of lymphatic vessel were inhibited by the chloride channel blocker, anthracene-9-carboxylic acid (9-AC; 250 microM - 1 mM) suggesting that I(Cl(Ca)) plays a significant role in the generation of spontaneous activity in this tissue. Penitrem-A (0.1 microM) did not affect resting membrane potential but increased action potential amplitude and prolonged the plateau, suggesting that calcium-activated potassium current does not make a significant contribution to resting membrane conductance but is important in membrane repolarization following calcium influx during the action potential. In contrast 4-aminopyridine (4-AP; 5 microM) caused significant membrane depolarization, suggesting the existence of an active 4-AP-sensitive current at rest.

CONCLUSIONS

These results demonstrate that the currents found in isolated voltage-clamped cells from sheep mesenteric lymphatics do play a significant role in the shaping of spontaneous electrical activity of the intact syncitium.

Effects of carbenoxolone on syncytial electrical properties and junction potentials of guinea-pig vas deferens

The effects of the putative gap junction blocker carbenoxolone on smooth muscle syncytial properties and junction potentials were studied in guinea pig vas deferens (GPVD). Treatment with 50 muM carbenoxolone reversibly and significantly increased input resistance (R (in)) (by 682.5 +/- 326.0 %, P < 0.05) and abolished cable potentials within 6-7 mins of incubation, without disturbing resting membrane potential. Carbenoxolone reversibly and significantly increased the amplitude of spontaneous excitatory junction potentials (sEJPs) by 96.9 +/- 35.45% (P < 0.05), shifted their amplitude distribution rightwards, and reduced their frequency of occurrence by 58.17 +/- 17.7% (P < 0.05), without altering their time courses. Similarly, carbenoxolone increased the amplitude of evoked excitatory junction potentials (eEJPs) by 17.7 +/- 5.88% and tau (decay) by 19.43 +/- 8.29% (P < 0.05). Our results indicate that carbenoxolone alters the electrical properties and junctional potentials of the GPVD by a mechanism consistent with a relatively specific block of gap junctions. These results suggest that gap junction mediated cell-to-cell communication may significantly modulate the electrical properties and junctional potentials of the GPVD and consequently the physiological functioning of this tissue.

Smooth muscle research: from Edith Bülbring onwards

The properties of smooth muscle are currently being studied extensively. Indeed, the small size of myocytes and the huge range of behaviours they exhibit make them an attractive focus for current research. However, this was not always the case. These properties initially made smooth muscles more difficult to study than the larger specialized striated muscles that were the focus of attention of leading researchers. In the UK, research into the physiology of smooth muscles began in the Pharmacology Department at Oxford, led by Edith Bülbring; her early results attracted much attention and resulted in the formation of an active international smooth muscle research group. Although several areas of current interest in the field of smooth muscle were not tackled by the Oxford group, progression of much of the field has clear links to Bülbring and her group.

Communication between interstitial cells of Cajal and gastrointestinal muscle

Interstitial cells of Cajal (ICC) pace gastrointestinal muscle by initiating slow waves in both muscle layers and appear to be preferred sites for reception of neurotransmitters. ICC of the myenteric plexus (ICC-MP) pace stomach and small intestine, while intramuscular ICC (ICC-IM) receive nerve messages. Recently, ICC-IM have been found to provide regenerative responses to and amplification of pacing messages from ICC-MP, at least in some systems. This review will examine the assumption that gap junctions provide low-resistance contacts for pacing. Structural and functional evidence will be evaluated. Structural, theoretical and experimental difficulties with the gap junctions hypothesis for pacing will be considered. So far little direct evidence about the role of gap junctions in neurotransmission exists, although a structural basis exists. Alternate possibilities for transmission of ICC pacing and neural messages will be examined and suggestions for future research made.

Plasma membrane channels formed by connexins: their regulation and functions

Members of the connexin gene family are integral membrane proteins that form hexamers called connexons. Most cells express two or more connexins. Open connexons found at the nonjunctional plasma membrane connect the cell interior with the extracellular milieu. They have been implicated in physiological functions including paracrine intercellular signaling and in induction of cell death under pathological conditions. Gap junction channels are formed by docking of two connexons and are found at cell-cell appositions. Gap junction channels are responsible for direct intercellular transfer of ions and small molecules including propagation of inositol trisphosphate-dependent calcium waves. They are involved in coordinating the electrical and metabolic responses of heterogeneous cells. New approaches have expanded our knowledge of channel structure and connexin biochemistry (e.g., protein trafficking/assembly, phosphorylation, and interactions with other connexins or other proteins). The physiological role of gap junctions in several tissues has been elucidated by the discovery of mutant connexins associated with genetic diseases and by the generation of mice with targeted ablation of specific connexin genes. The observed phenotypes range from specific tissue dysfunction to embryonic lethality.

Simulation framework for electrophysiological networks: Effect of syncytial properties on smooth-muscle synaptic potentials

A building block-based software framework was developed to simulate electrophysiological networks. The synaptic potentials generated during neurotransmission were simulated in an existing discrete bidomain model of smooth muscle, using cubic, three-dimensional grids of varying sizes. The model is automatically derived and numerically solved, and the results of the simulation agree with previous results obtained analytically. An enhanced model was also proposed, incorporating an additional (junctional) capacitance in the network. The correctness of the model was verified, and the effect of the extra capacitance on the synaptic potentials was explored. It was found that, with a junctional capacitance C(i) of 1.4 x 10(-10) F incorporated, the peak amplitude of the spontaneous excitatory junction potential V(peak) declined by approximately 13% at node 0 and by approximately 37% at node 3x for a system size of 9(3). Similar results were obtained for different system sizes. V(peak) also declined as the junctional capacitance Ci was increased. In a system of size 11(30, a 200-fold increase in C(i) induced a 55% reduction at node 0. It is suggested that the type of modular simulation framework developed here may find general applicability for simulations of other physiological systems.

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.

Electrical coupling between smooth muscle and endothelium in arterioles of the guinea-pig small intestine

Equations describing the steady-state passive electrical properties of arterioles have been derived. The arteriole was modelled as having two thin layers of cells (muscle and endothelium) with strong electrical coupling between cells within a layer and variable coupling between the layers. The model indicated that spread of membrane potential changes was highly dependent on the thickness of cells within the layers. The model was also used to identify the optimal experimental strategy for detecting coupling between the two layers, and experiments were carried out on arterioles from the guinea-pig small intestine. Thickness of the endothelial layer was measured using electron microscopy and was found to be around 0.5 microm. Electrical input resistance was measured in intact arterioles and compared to input resistance of arterioles from which the endothelium had been removed. The experiments confirmed that there was a strong electrical coupling between the muscle and endothelium in these vessels.

Altered electrical activity in colonic smooth muscle cells from dystrophic (mdx) mice

Because the colon from dystrophic (mdx) mice shows an altered motor pattern, probably due to neural disorders, our aim was to examine the electrophysiological properties of muscle cells and the functionality of nitrergic transmission in circular muscle from normal and mdx colon. Normal colonic cells (resting membrane potential [RMP] about -50 mV) showed spontaneous hyperpolarizations (inhibitory junction potentials; IJPs) and cyclic slow depolarizations were sometimes recorded. Mdx colon had a depolarized RMP (about -36 mV) and spontaneous IJPs, but the cyclic activity was never observed. In the normal colon, Nomega-nitro-L-arginine methyl ester (L-NAME) induced depolarization and abolished the cyclic activity. In the mdx colon, L-NAME caused a slight depolarization. Both preparations displayed the same value of RMP in the presence of L-NAME. In normals, neural stimulation induced nonadrenergic, noncholinergic IJPs composed of fast hyperpolarizations followed by a nitrergic slow hyperpolarization, selectively abolished by L-NAME. In the mdx colon the evoked IJPs were composed only of the initial fast hyperpolarization, the nitrergic component being absent. The hyperpolarization to sodium nitroprusside was not significantly different in both preparations. We conclude that the colon from animals lacking in dystrophin displays different electrophysiological features because of an impairment of nitric oxide function.

Origin and propagation of spontaneous excitation in smooth muscle of the guinea-pig urinary bladder

The origin and propagation of waves of spontaneous excitation in bundles of smooth muscle of the guinea-pig bladder were examined using intracellular recording techniques and visualization of the changes in the intracellular calcium concentration ([Ca2+]i). Bladder smooth muscle cells exhibited spontaneous transient increases in [Ca2+]i which originated along a boundary of each smooth muscle bundle and then spread to the other boundary with a conduction velocity of 2.0 1r1r>mm1> s-1. Spontaneous increases in [Ca2+]i were always preceded by action potentials. Nifedipine (10 microM) abolished increases in both [Ca2+]i and action potentials. Caffeine (10 1s1sFmM1F), ryanodine (50 microM) and cyclopiazonic acid (10 microM reduced the amplitude of the associated increases in [Ca2+]i without preventing the generation of action potentials. Spontaneous action potentials had conduction velocities of 40 1t1t>mm 1> s-1 in the axial direction and 1.3 1u1u>mm 1> s-1 in the transverse direction. The electrical length constants of the bundles of muscle were 425 microM in the axial direction and 12.5 microM in the transverse direction. Neurobiotin, injected into an impaled smooth muscle cell, spread more readily to neighbouring cells located in the axial direction than those located in the transverse direction. The spread of neurobiotin was inhibited by 18beta-glycyrrhetinic acid (18beta-GA, 40 microM), a gap junction blocker. Immunohistochemistry for Connexin 43 showed abundant punctate staining on the smooth muscle cell membranes. These results suggested that spontaneous action potentials and associated calcium waves occur almost simultaneously along the boundary of bladder smooth muscle bundles and then propagate to the other boundary probably through gap junctions.

Gap junctions in intestinal smooth muscle and interstitial cells of Cajal

This manuscript reviews gap junctions' roles in control of intestinal motility. Gap junctions (GJs) of small intestine (SmIn) are found between circular muscle (CM) cells, between interstitial cells of Cajal (ICC) of deep muscular plexus (DMP) and between them and adjacent outer circular muscle (OCM). GJs between longitudinal muscle (LM) cells or between cells of inner circular muscle (ICM) have not been reported. Occasional GJs have been reported between ICC of the myenteric plexus (MyP) and rarely between these ICC and adjacent LM or CM cells, or between ICC within CM and smooth muscle cells. In the colon (Co) of several species a special network of ICC lines the inner border of CM, the submuscular plexus (SP). GJs are found between ICCs and between them and CM cells. The ICC of MyP of Co are associated with LM and CM; occasional GJs exist between ICC and each muscle layer. Small GJs are missed by electron microscopy or light microscopic Immunocytochemistry. Therefore, GJ coupling may exist without demonstrated GJs. The consequences for the pacemaking functions of ICC networks of varied densities of GJ between ICC and between ICC of MyP or DMP or of SP and CM are considered. Connexins (Cxs) that compose intestinal GJs may affect coupling, but are incompletely known. Understanding of the role of GJs in coordinating intestinal motility requires knowing: (1) what passes through gap junctions to couple ICC to smooth muscle cells; (2) what Cx with what conductances and what modulatory controls connect ICC and smooth muscle cells; (3) whether smooth muscles can generate slow waves independent of ICC networks; and (4) what happens to motility, slow waves, and IJPs when GJs are selectively uncoupled.

Abnormal response to cholinergic stimulation in the circular muscle layer of the human colon in diverticular disease

BACKGROUND

Diverticular disease is characterized by the occurrence of small herniations of the colonic mucosa, through the external muscle coats of the colon. The muscle wall is thickened, high intraluminal pressures can be recorded, and often constipation develops. The aim of the present study was to investigate whether an abnormality in the electric myogenic control activity could be found to help explain the etiology and symptoms of the disease.

METHODS

Electric activity was studied by extracellular electrodes on tissues from both the circular and the longitudinal muscle of the colon from 12 patients.

RESULTS

In tissues from 10 patients a distinctly abnormal response to cholinergic stimulation was observed. A characteristic development of bursts of action potentials did not develop; instead, slow-wave activity of relatively low frequency was maintained throughout the period of stimulation. This slow-wave activity showed a lack of synchronization.

CONCLUSIONS

The results indicate that, in diverticular disease, local changes in electric activity occur that change the response to cholinergic stimulation. When this happens, development of periodic bursts of action potentials normally associated with propulsive activity do not develop, favoring segmental contractile activity associated with low-frequency slow-wave activity.

Regenerative potentials evoked in circular smooth muscle of the antral region of guinea-pig stomach

1. Slow waves recorded from the circular smooth muscle layer of guinea-pig antrum consisted of two components, an initial component and a secondary regenerative component. Whereas both components persisted in the presence of nifedipine, the secondary component was abolished by a low concentration of caffeine. 2. Short segments of single bundles of circular muscle were isolated and impaled with two microelectrodes. Depolarizing currents initiated regenerative responses which resembled those initiated during normal slow waves. These responses had partial refractory periods of 20-30 s and were initiated about 1 s after the onset of membrane depolarization. 3. The regenerative responses persisted in the presence of either nifedipine or cobalt ions but were abolished by caffeine, BAPTA or cyclopiazonic acid. 4. The observations suggest that depolarizing membrane potential changes trigger the release of Ca2+ from intracellular stores and this causes a depolarization by activating sets of unidentified ion channels in the membranes of smooth muscle cells of the circular layer of guinea-pig antrum.

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.

Slow waves in circular muscle of porcine ileum: structural and electrophysiological studies

The structural and functional bases of pacemaking (slow waves) in porcine ileal circular muscle were studied. The myenteric plexus contained two, structurally distinct types of interstitial cells of Cajal (ICC) interconnected by gap junctions and connected by close contacts to muscle layers. At the deep muscular plexus, ICC were present, not regularly close to nerve axons or in gap junction contact with one another or outer circular muscle, which had many gap junctions. Slow waves (5.2 +/- 2 mV amplitude and 4.6 +/- 0.7 s duration) occurred at 9.9 +/- 1.1 counts/min. Tissue length and time constants were 2.00 +/- 0.3 mm and 111 +/- 37 ms, respectively. Large electrical field-induced hyperpolarizations or depolarizations reduced amplitudes but not frequencies or durations of slow waves; hyperpolarizations progressively reduced inhibitory junction potentials as if the K+ channel opening mediated them. In conclusion, the myenteric plexus ICC of pig ileum, which appears to pace the muscle layers, appears insensitive to voltages applied to the syncytium of circular muscle cells. Limited coupling between ICC and circular muscle or voltage-insensitive pacemaking activity may explain these findings.

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.

Disinhibition during myoelectric complexes in the mouse colon

Intracellular microelectrodes were used to record electrically evoked inhibitory junction potentials (IJPs) and electrotonic potentials during spontaneous cyclical depolarisations (myoelectric complexes, MCs) in the circular muscle layer of mouse colon in vitro. In the presence of nifedipine (1-2 microM) and atropine (1 microM), MCs were recorded every 264 +/- 18 s. Between MCs, single electrical stimuli (15 V, 0.6 ms, every 8 s) elicited IJPs whose amplitudes remained constant. In comparison, during the depolarising phase of MCs, the mean IJP amplitude was reduced by 61 +/- 7%, while during the late plateau and early repolarising phase of MCs, IJP amplitude was increased (up to 20%). NG-nitro-L-arginine (NOLA, 100 microM) abolished the repolarisation phase between MCs, so that the circular muscle remained depolarised and the amplitude of MCs was reduced by 73 +/- 6%. However, the amplitude of evoked IJPs was unaffected, as was the decrease in their amplitude during the depolarising phase of the residual MCs. In the presence of NOLA (100 microM), the further addition of apamin (250 nM) reduced the amplitude of evoked IJPs by approximately half. However, the amplitudes of NOLA- and apamin-resistant IJPs were also attenuated by 82 +/- 5% during the depolarising phase of residual MCs (amplitude: 1.9 +/- 1 mV). However, during this phase, the amplitude of an electrotonic potential (evoked by extracellular current application) was not attenuated. Addition of hexamethonium (500 microM), or tetrodotoxin (TTX) (1.6 microM) to solutions containing NOLA and apamin were without effect on membrane potential, but the residual MCs and the cyclical attenuation in IJP amplitude were abolished. During the intervals between MCs, membrane potential is maintained under tonic inhibition, via spontaneous release of inhibitory neurotransmitter(s), predominantly through nitrergic mechanisms. The cyclical attenuation in the amplitude of the non-nitrergic IJP does not arise from cyclical postjunctional changes in membrane resistance or potential. Moreover, the generation of the depolarising phase of MCs involves the simultaneous suppression of both nitrergic and non-nitrergic inhibitory neurotransmission. It is suggested that MCs arise from presynaptic suppression of ongoing inhibitory neurotransmitter release.

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.

Oxytocin enhances action potentials in pregnant human myometrium--a study with microelectrodes

OBJECTIVE

Our purpose was to quantitatively assess the effects of oxytocin on membrane properties in the pregnant human myometrium.

STUDY DESIGN

Specimens were obtained from the lower uterine segment during cesarean section at term. Electrical activity was recorded from individual cells by a conventional microelectrode method and the membrane functions were analyzed.

RESULTS

Two types of spontaneous action potentials were seen: a long plateau potential and a spike-like action potential. With no change in the resting membrane potential, low concentrations of oxytocin either evoked an action potential with a plateau phase, increased the amplitude and duration of the plateau potential, or increased the frequency of generation of action potentials. Oxytocin also lowered the threshold for evoking an action potential. Higher concentrations depolarized the membrane with an associated reduction in membrane resistance.

CONCLUSION

Oxytocin augments the excitability of pregnant human myometrial cells by multiple actions on the membrane, affecting both frequency and amplitude of action potentials.

Electrophysiological analysis of the actions of pituitary adenylyl cyclase activating peptide in the taenia of the guinea-pig caecum

The actions of pituitary adenylyl cyclase activating peptide (PACAP) on membrane potential and conductance were investigated in the taenia of the guinea-pig caecum. The possible role of PACAP in inhibitory transmission was also investigated. Membrane potentials of smooth muscle cells were measured by intracellular microelectrodes, in the presence of hyoscine and nifidepine (both 10(-6)M. To determine conductance changes, current was passed from external plate electrodes using the technique of Abe and Tomita (1968). PACAP-27 caused a concentration dependent hyperpolarization of the muscle with a maximum of 12-15 mV at 10(-6)M. The hyperpolarization caused by PACAP was associated with a substantial increase in membrane conductance. The hyperpolarization was abolished by apamin (10(-6)M), a blocker of small conductance, calcium-dependent, potassium channels, and was reduced to about 50% by suramin (10(-4)M), which is an antagonist of P2 receptors for purines. The hyperpolarization was not reduced by tetrodotoxin (2 x 10(-6)M), suggesting PACAP acts directly on the muscle. With continued exposure to PACAP, the hyperpolarization decayed back to resting membrane potential after several minutes, possibly due to receptor desensitization. Inhibitory junction potentials (IJPs) were markedly reduced in amplitude in the period of presumed receptor desensitization to PACAP, were abolished by tetrodotoxin, but were not affected by suramin. Apamin abolished the IJP and revealed a small excitatory junction potential. This study implies that PACAP released from nerve fibres in the taenia caeci hyperpolarizes the muscle via an opening of apamin-sensitive potassium channels. The action is probably through type I PACAP receptors.

Tetrodotoxin-sensitive action potentials in smooth muscle of mouse vas deferens

Action potentials were recorded during impalements of some but not all smooth muscle cells of mouse vas deferens in response to both nerve stimulation and intracellular current injection. They were resistant to blockade by nifedipine (0.1-1.0 microM) but were blocked by tetrodotoxin (TTX, 0.2-1.0 microM) when this was added in the presence of nifedipine. It is suggested that voltage-dependent sodium (Na+) channels are present in mouse vas deferens that function to amplify calcium (Ca2+) influx through voltage-dependent Ca2+ channels.

Guinea pig as an animal model for the study of urinary bladder function in the normal and obstructed state

The guinea pig has become an excellent model for the study of mechanical and electrical mechanisms regulating bladder function in the normal and obstructed state. Much preliminary work has been done on the in vitro behavior of the detrusor smooth muscle. The tissue has permitted electrophysiological studies by sucrose gap, microelectrode, and patch clamp technique. Excellent urodynamic studies can be performed under anesthesia. A recent model of bladder obstruction has resulted in a source of tissue which is suitable for electrophysiological analysis of the muscle. Low-cost and simple animal care requirements permit large-scale studies correlating urodynamic, structural, biochemical, contractile, and electrophysiological changes in response to obstruction.

Membrane rectification in single smooth muscle cells from the rat tail artery

Membrane rectification to depolarization was studied by voltage recording with patch electrodes in freshly isolated cells from the rat tail artery. Injection of depolarizing currents elicited electrotonic potentials that developed with a single-exponential time course (time constant of 94.8 ms). When the cell was depolarized beyond -30 mV, delayed rectification was observed. A second type of rectification, characterized by oscillations, was observed when the cell was depolarized positive to +30 mV. The threshold of this rectification and the oscillations were sensitive to changes in intracellular Ca2+. Delayed rectification was more sensitive to 4-aminopyridine but more resistant to tetraethylammonium and charybdotoxin than the Ca(2+)-sensitive rectification. A 4-aminopyridine-sensitive outward current (IK,dr) with a threshold of around -30 mV and a second Ca(2+)-sensitive outward current (IK,Ca) activated at around +30 mV were observed from whole-cell voltage clamp recordings. IK,Ca was blocked by tetraethylammonium and charybdotoxin. An 11-pS and a 122-pS channel, having characteristics similar to IK,dr and IK,Ca respectively, were identified from single-channel recordings. These observations showed how membrane depolarization of vascular smooth-muscle cells was regulated by these two populations of K+ channels under various conditions.

Cholinergic neuromuscular transmission in the longitudinal muscle of the guinea-pig ileum

1. Brief transmural stimuli, 0.5-1 ms, initiated contractions of the longitudinal muscle taken from the guinea-pig ileum that were recorded isometrically. In separate preparations similar stimuli were found to initiate excitatory junction potentials which were recorded using intracellular recording electrodes. All of these responses were abolished by either tetrodotoxin, omega-conotoxin or hyoscine. 2. The contractions produced by increasing [K+]o were blocked by nifedipine, 1 x 10(-7) M; nicardipine, 1 x 10(-7) M; verapamil, 1 x 10(-5) M or diltiazem, 1 x 10(-5) M. In these solutions brief stimuli continued to initiate contractions: this indicates that neuronally released acetylcholine continues to trigger a contraction when muscle voltage-dependent calcium channels appear to have been blocked. 3. When membrane potential recordings were made from the smooth muscle layer, brief transmural stimuli initiated excitatory junction potentials that triggered muscle action potentials. Although muscle action potentials were abolished by low concentrations of a range of organic calcium antagonists, excitatory junction potentials persisted and continued to initiate contractions of reduced amplitude. 4. When the internal concentration of calcium ions, [Ca2+]i, was measured using fura-2, brief transmural stimuli caused an increase in [Ca2+]i. Part of this response, which occurred at a time corresponding to the unblocked excitatory junction potential, persisted in the presence of the organic calcium antagonist nifedipine. 5. Two explanations appear possible. Neuronally released acetylcholine may simultaneously activate non-selective cation channels and cause the release of Ca2+ from an internal store. Alternatively, neuronally released acetylcholine may cause an increase in [Ca2+]i which is separate from that which accompanies the activation of voltage-dependent calcium channels. At this stage there is little other anatomical or electrophysiological evidence to support this view.

Peptide YY stimulates circular muscle contractions of the isolated perfused canine ileum by inhibiting nitric oxide release and enhancing acetylcholine release

Peptide YY (PYY) and neuropeptide Y (NPY), infused into the quiescent isolated perfused canine ileum, dose-dependently increased phasic activity of the circular muscle and decreased tonic output of immunoreactive vasoactive intestinal peptide (VIP) in the venous effluent. The contractions subsided before the prolonged inhibition of VIP output. Motor excitation by PYY, an abundant neuropeptide in this tissue, was reduced by blockade of muscarinic or nicotinic receptors, or inhibition of nitric oxide (NO) synthase, despite continued inhibition of VIP output. A combination of atropine and hexamethonium eliminated the PYY-induced decrement in VIP output and left motor excitation unchanged. Blockade of NO synthase eliminated motility increases under these conditions. Intracellular microelectrode recordings of myenteric plexus-free circular muscle strips found no effect of NPY on the resting membrane potential, or on the field stimulation-induced inhibitory junction potential. Inhibition of VIP release plays no essential role in changing motility. These results suggest that PYY/NPY induce motility by stimulating release of acetylcholine and inhibiting NO release at a locus proximal to but not on nerve terminals.

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

1. In circular muscle strips of the antrum of guinea-pig stomach, the effects of cromakalim were studied on mechanical activity and intracellular membrane potential. 2. Cromakalim inhibited mechanical activity at concentrations higher than 1 microM, accompanied by membrane hyperpolarization and a decrease in membrane resistance. The hyperpolarization was markedly potentiated in K(+)-free solution and was still observed in the absence of Na+. 3. Slow wave electrical activity was relatively resistant to cromakalim. Changes in its amplitude and frequency were not consistent but blockade of slow waves was never observed. In many preparations cromakalim induced spike-like potentials at the top of slow waves, or when spike-like potentials already existed they were potentiated. However, mechanical activity was always inhibited. 4. Inhibition by cromakalim of the phasic contractions associated with the slow waves, could not be reversed by increasing the external K+ concentration (12-30 mM). 5. The results suggest that in guinea-pig stomach muscle mechanical suppression by cromakalim does not simply result from membrane hyperpolarization or from inhibition of slow waves. A clear dissociation was found between the mechanical and electrical activities. Slow waves, particularly their frequency, are relatively insensitive to membrane hyperpolarization.

The effect of experimental urethral obstruction and its reversal on changes in passive electrical properties of detrusor muscle

The passive electrical properties of guinea pig detrusor muscle were studied in order to determine how bladder outflow obstruction and reversal might modify the electrical activity of the bladder and, thus, contractility. Experimental bladder outflow obstruction was produced in guinea pigs and resulted in an increase in bladder weight with a decrease in spontaneous electrical activity, membrane time constant and space constant. In addition, the membrane Na-K pump activity increased with obstruction. Following reversal of obstruction, bladder weight gain associated with obstruction was only partially reversible. The decrease in the membrane time constant induced by obstruction was almost fully reversible following release of obstruction. In contrast, the membrane space constant which reflects spread of current, in addition to spontaneous electrical activity, were only partially reversible. The membrane Na-K pump activity of the detrusor muscle decreased to control levels following reversal of bladder outflow obstruction. There was no significant change in the resting membrane potential of detrusor smooth muscle with either obstruction or following reversal of obstruction. These results suggest, that, the changes in the bladder smooth muscle membrane electrical properties induced by experimental bladder outflow obstruction are only partially reversible following release of obstruction. Furthermore, the results suggest that, the dysfunctional cystometric patterns associated with bladder outflow obstruction might not only be due to changes in detrusor innervation but, fundamental reorganization of the detrusor's electrical syncytium with irreversible suppression of cell-to-cell transfer of electrical activity.