膀胱逼尿肌的组织及生理学概述

膀胱逼尿肌在组织结构、超微结构、收缩功能、舒张功能、自律性及电生理学方面具有自身的特点.在组织结构上,通过平滑肌肌束之间的交叉聚集使膀胱逼尿肌成为一个统一的整体;在超微结构上,通过逼尿肌肌细胞之间的中间连接使平滑肌肌束成为一个有机的组合;在收缩功能上,膀胱逼尿肌的特点是自发性收缩和神经性收缩的动态平衡;在舒张功能上,膀胱逼尿肌的特点是应力性舒张和神经性舒张的协调一致;在自律性方面,膀胱逼尿肌具有产生自发性动作电位的能力以及触发自主性收缩的能力;在电生理方面,膀胱逼尿肌的肌膜去极化引起的动作电位通过作用于钙通道形成Ca2+内流,实现了兴奋与收缩之间的耦联.     

Actomyosin structure in contracting muscle detected by rapid freezing

It is now widely accepted that the ATP-induced active sliding of adjacent thin and thick filaments mediated by myosin heads (cross-bridges) is responsible for muscle contraction. Despite intensive studies, the behaviour of the myosin heads during muscle contraction is still unclear. Recent progress in the rapid freezing electron microscope technique has greatly improved the temporal resolution of the images that can be obtained. Here, we report a new type of actomyosin structure captured by rapid freezing. We have analysed images from thin sections of freeze-substituted rabbit skeletal muscle rapidly frozen during isometric contraction. For comparison, we also studied relaxed and rigor muscles. Our results show that, during isometric contraction, most myosin heads are regularly arrayed along the helix of the actin filaments and that this actomyosin structure appears to be distinct from that observed in rigor muscle.     

Agonist-induced potentiation of acetylcholine sensitivity in denervated skeletal muscle

The entire surface membrane of denervated skeletal muscle is sensitive to the neuromuscular transmitter, acetylcholine (ACh), whereas in innervated muscle only the junctional area is sensitive. It has been proposed that this difference is due to a 'trophic' effect exerted by ACh in innervated muscle to keep the extrajunctional regions of the surface membrane insensitive to its depolarising action. Several studies have demonstrated an agonist-induced potentiation of ACh sensitivity, followed by desensitisation, at the endplate region of normal muscles. The potentiation has been attributed to a cooperative action of ACh on the receptors. Desensitisation of the extrajunctional regions of denervated muscles by ACh has also been described. We now provide evidence that the transmitter itself potentiates the ACh contracture and depolarisation responses of the denervated muscles of the rat in vitro and that it produces this effect by increasing the number of available ACh receptors on the surface membrane.     

Opening of dihydropyridine calcium channels in skeletal muscle membranes by inositol trisphosphate

In many non-muscle cells, D-inositol 1,4,5-trisphosphate (InsP3) has been shown to release Ca2+ from intracellular stores, presumably from the endoplasmic reticulum. It is thought to be a ubiquitous second messenger that is produced in, and released from, the plasma membrane in response to extracellular receptor stimulation. By analogy, InsP3 in muscle cells has been postulated to open calcium channels in the sarcoplasmic reticulum (SR) membrane, which is the intracellular Ca2+ store that releases Ca2+ during muscle contraction. We report here that InsP3 may have a second site of action. We show that InsP3 opens dihydropyridine-sensitive Ca2+ channels in a vesicular preparation of rabbit skeletal muscle transverse tubules. InsP3-activated channels and channels activated by a dihydropyridine agonist in the same preparation have similar slope conductance and extrapolated reversal potential and are blocked by a dihydropyridine antagonist. This suggests that in skeletal muscle, InsP3 can modulate Ca2+ channels of transverse tubules from plasma membrane, in contrast to the previous suggestion that the functional locus of InsP3 is exclusively in the sarcoplasmic reticulum membrane.     

中华稻蝗受精囊的初步观察

应用光镜观察了中华稻蝗受精囊及受精囊管的结构。结果表明，中华蝗受精囊由内向外依次为内膜层、上皮层、底膜、结缔组织、肌肉层和围脏膜，可见许多的排出小管穿过内膜层，开口于囊腔，肌肉层中纵肌环肌交杂排列，并讨论了其结构与功能间的联系。     

Fibre type composition of single motor units during synapse elimination in neonatal rat soleus muscle

Skeletal motor neurones innervate the specialized 'types' of fibres comprising most mammalian muscles in a characteristic fashion: each motor neurone forms a 'motor unit' by innervating a set of fibres all of the same type. Because the type expression of adult muscle fibres is plastic and apparently controlled by their innervation, each motor neurone is thought to impose a common type differentiation on all the fibres in its motor unit. However, the situation in developing muscles cannot be this simple. Muscle fibres in neonates receive synaptic input from several motor neurones and achieve the adult, single innervation only after a period of 'synapse elimination. Despite this polyneuronal innervation, differentiated fibre types are present in neonatal muscles. This means either that the motor neurones polyneuronally innervate fibres in a random fashion and type expression is not determined by innervation or that the polyneuronal innervation is ordered in such a way that each fibre could receive unambiguous instructions for type differentiation. We have investigated these possibilities here by determining the fibre type composition of motor units in neonatal rat soleus muscle. We find that even during the time of polyneuronal innervation each motor neurone confines its innervation to largely one of two fibre types present in the muscle. Therefore, some mechanism during early development segregates the synapses of two groups of soleus motor neurones onto two separate populations of soleus muscle fibres.     

Limitations in the use of actomyosin threads as model contractile systems

Recent studies have suggested that actomyosin threads may provide a useful model for studying the properties of contractile systems. The development of highly sensitive positional feedback transducers has enabled the properties of these threads to be measured reproducibly. Potential applications include such systems as ventricle, smooth muscle and non-muscle preparations, from which it is difficult to obtain suitable fibres for mechanical studies. In addition, studies with chemically modified myosins may provide new insights into the relationships between the biochemical and mechanical events in the cross-bridge cycle. However, there are indications that the mechanical properties of actomyosin threads differ from those of intact fibres in several important respects. For example, contraction velocity is proportional to isometric tension in threads, but is independent of filament density in intact fibres. We have now determined the force-velocity characteristics of actomyosin threads prepared from muscles with known differences in their physiological contraction velocities. No direct relationships could be found between the velocity characteristics of the threads and those of intact muscle. We conclude that the measured velocities of threads reflect properties of the actomyosins other than cross-bridge cycling times, thus severely limiting the usefulness of this technique for comparative purposes.     

从60 cm高度下落到不同硬度的地面下肢关节的力学响应

研究了从60 cm高度下落到2种不同硬度的地面,即木板地面和海绵垫时下肢关节的力学响应.结果表明:人下落到软的地面上时下肢关节的受力、力矩显著减小,关节的角度变化显著增加,从着地到落地停稳所用的时间显著增加.各关节力矩是外力矩与肌肉力矩共同作用的结果,正确的落地技术应该是3个关节的力矩同时减小,掌握正确的落地技术的实质是掌握正确的肌肉被拉伸与收缩的顺序和大小.从60 cm下落着地后,不论是何种地面条件,3个关节的力矩以髋关节力矩为最大,膝和踝关节肌肉是在髋关节控制下起互相协调作用.     

Spectroscopic mapping of voltage sensor movement in the Shaker potassium channel

Voltage-gated ion channels underlie the generation of action potentials and trigger neurosecretion and muscle contraction. These channels consist of an inner pore-forming domain, which contains the ion permeation pathway and elements of its gates, together with four voltage-sensing domains, which regulate the gates. To understand the mechanism of voltage sensing it is necessary to define the structure and motion of the S4 segment, the portion of each voltage-sensing domain that moves charged residues across the membrane in response to voltage change. We have addressed this problem by using fluorescence resonance energy transfer as a spectroscopic ruler to determine distances between S4s in the Shaker K+ channel in different gating states. Here we provide evidence consistent with S4 being a tilted helix that twists during activation. We propose that helical twist contributes to the movement of charged side chains across the membrane electric field and that it is involved in coupling voltage sensing to gating.     

Involvement of dihydropyridine receptors in excitation-contraction coupling in skeletal muscle

The transduction of action potential to muscle contraction (E-C coupling) is an example of fast communication between plasma membrane events and the release of calcium from an internal store, which in muscle is the sarcoplasmic reticulum (SR). One theory is that the release channels of the SR are controlled by voltage-sensing molecules or complexes, located in the transverse tubular (T)-membrane, which produce, as membrane voltage varies, 'intramembrane charge movements', but nothing is known about the structure of such sensors. Receptors of the Ca-channel-blocking dihydropyridines present in many tissues, are most abundant in T-tubular muscle fractions from which they can be isolated as proteins. Fewer than 5% of muscle dihydropyridines are functional Ca channels; there is no known role for the remainder in skeletal muscle physiology. We report here that low concentrations of a dihydropyridine inhibit charge movements and SR calcium release in parallel. The effect has a dependence on membrane voltage analogous to that of specific binding of dihydropyridines. We propose specifically that the molecule that generates charge movement is the dihydropyridine receptor.     

不同时间电刺激对C2C12肌管糖代谢的影响及其机制研究

以小鼠骨骼肌母细胞（C2C12）肌管为研究对象,测定不同时间电刺激对其AMPK、HK-Ⅱ、GS-1基因表达、以及GLUT4基因及蛋白表达等糖转运和代谢相关指标的变化,探讨不同收缩时间对骨骼肌细胞糖代谢的影响,以及这种影响的机制. 骨骼肌细胞的肌糖原消耗随刺激时间延长而增加,为了保证骨骼肌细胞糖消耗的需要,通过AMPK信号通路的调控,其膜对胞外糖转运的能力会随收缩时间增加而增强;骨骼肌细胞内糖原合成酶的合成则由于肌糖原储量的持续下降而增加.     

Molecular dynamics of cyclically contracting insect flight muscle in vivo

Flight in insects--which constitute the largest group of species in the animal kingdom--is powered by specialized muscles located within the thorax. In most insects each contraction is triggered not by a motor neuron spike but by mechanical stretch imposed by antagonistic muscles. Whereas 'stretch activation' and its reciprocal phenomenon 'shortening deactivation' are observed to varying extents in all striated muscles, both are particularly prominent in the indirect flight muscles of insects. Here we show changes in thick-filament structure and actin-myosin interactions in living, flying Drosophila with the use of synchrotron small-angle X-ray diffraction. To elicit stable flight behaviour and permit the capture of images at specific phases within the 5-ms wingbeat cycle, we tethered flies within a visual flight simulator. We recorded images of 340 micros duration every 625 micros to create an eight-frame diffraction movie, with each frame reflecting the instantaneous structure of the contractile apparatus. These time-resolved measurements of molecular-level structure provide new insight into the unique ability of insect flight muscle to generate elevated power at high frequency.     

Detection of K+ and Cl-channels from calf cardiac sarcolemma in planar lipid bilayer membranes

The ionic currents underlying the cardiac action potential are believed to be much more complex than those in nerve. During the cardiac action potential, various membrane channels control the flow of K+, Na+, Ca2+ and Cl- across the sarcolemma of cardiac muscle cells. Thus, it has become increasingly clear that a detailed knowledge of the mechanisms that activate (or inactivate) heart channels is required to understand cardiac excitability. We report here the use of planar lipid bilayer techniques to detect and characterize K+ and Cl- channels in purified heart sarcolemma membrane vesicles. We have identified four different types of channel on the basis of their selectivity, conductance and gating kinetics. We present in some detail the properties of a K+ channel and a Cl- channel. We have tentatively identified the K+ channel with the ix type of current found in Purkinje, myocardial ventricular and atrial fibres. The chloride channel might be related to the transient chloride current found in Purkinje fibres.     

A lethal mutation in mice eliminates the slow calcium current in skeletal muscle cells

Contraction of a vertebrate skeletal muscle fibre is triggered by electrical depolarization of sarcolemmal infoldings termed transverse-tubules (t-tubules), which in turn causes the release of calcium from an internal store, the sarcoplasmic reticulum (SR). The mechanism that links t-tubular depolarization to SR calcium release remains poorly understood. In principle, this link might be provided by the prominent slow calcium current that has been described in skeletal muscle cells of adult frogs and rats. However, blocking this current does not abolish the depolarization-induced contractile responses of frog muscle, and the function of this slow calcium current is unknown. Here we describe measurements of calcium currents in developing skeletal muscle cells of normal rats and mice, and of mice with muscular dysgenesis, a mutation that causes excitation-contraction (E-C) coupling to fail. We find that a slow calcium current is present in skeletal muscle cells of normal animals but absent from skeletal muscle cells of mutant animals. The effect of the mutation is specific to the slow calcium current of skeletal muscle; a fast calcium current is present in developing skeletal muscle cells of both normal and mutant animals, and slow calcium currents are present in cardiac and sensory neurones of mutant animals. We believe this to be the first report of a mutation affecting calcium currents in a multicellular organism. The effects of the mutation raise important questions about the relationship between the slow calcium current and skeletal muscle E-C coupling.     

Single Na+ channel currents observed in cultured rat muscle cells

The voltage- and time-dependent conductance of membrane Na+ channels is responsible for the propagation of action potentials in nerve and muscle cells. In voltage-step-clamp experiments on neurone preparations containing 10(4)-10(7) Na+ channels the membrane conductance shows smooth variations in time, but analysis of fluctuations and other eivdence suggest that the underlying single-channel conductance changes are stochastic, rapid transitions between 'closed' and 'open' states as seen in other channel types. We report here the first observations of currents through individual Na+ channels under physiological conditions using an improved version of the extracellular patch-clamp technique on cultured rat muscle cells. Our observations support earlier inferences about channel gating and show a single-channel conductance of approximately 18 pS.     

Muscle pioneers: large mesodermal cells that erect a scaffold for developing muscles and motoneurones in grasshopper embryos

During embryonic development, muscles differentiate in the appropriate places and motoneurone growth cones find the appropriate muscles; both events occur concurrently and with remarkable specificity. What are the cellular interactions that orchestrate this coordinated development of nerve and muscle? In the development of vertebrate skeletal muscles, motoneurone growth cones arrive in the periphery along stereotyped routes and enter the appropriately located masses of mesodermal cells usually before differentiated muscle fibres appear and before the masses cleave into separate muscles. We find that a similar sequence of events occurs in the grasshopper embryo. We are interested in how mesodermal cells become organized into the appropriate muscles and what guides motoneurone growth cones to their appropriate targets. Fortunately, in the grasshopper embryo the mesodermal cells in the periphery and motoneurones in the central nervous system (CNS) are large, accessible and in many cases individually identifiable from early in their development. We report here the discovery of a class of large mesodermal cells, which we call muscle pioneers, that arise early in development when the embryonic environment is relatively simple and distances short. By their growth and association with particular sites along the ectoderm, the muscle pioneers appear to erect a scaffold for later developing muscles and motoneurone growth cones.     

Myosin phosphorylation, agonist concentration and contraction of tracheal smooth muscle

Myosin phosphorylation plays an important part in excitation--contraction coupling in smooth muscle. Phosphorylation by a Ca2+, calmodulin-dependent kinase stimulates the actin-activated Mg2+-ATPase activity of smooth muscle myosin, suggesting that myosin phosphorylation regulates smooth muscle contraction. This hypothesis is supported by evidence that myosin is phosphorylated during contraction and dephosphorylated during relaxation of intact smooth muscles stimulated with a single agonist concentration. However, there is little information regarding the response to stimulation with various agonist concentrations. As the dose-response relationships for phosphorylation and tension should be similar if myosin phosphorylation does, in fact, regulate smooth muscle contraction, we studied myosin phosphorylation in tracheal smooth muscle stimulated with a broad range of concentrations of the cholinergic agonist, methacholine. The results of these experiments are consistent with the hypothesis that myosin phosphorylation regulates smooth muscle contraction but they indicate a relatively complex relationship between myosin phosphorylation and the generation of isometric tension.     

Cardiac-type excitation-contraction coupling in dysgenic skeletal muscle injected with cardiac dihydropyridine receptor cDNA

There are dihydropyridine (DHP)-sensitive calcium currents in both skeletal and cardiac muscle cells, although the properties of these currents are very different in the two cell types (for simplicity, we refer to currents in both tissues as L-type). The mechanisms of depolarization-contraction coupling also differ. As the predominant voltage-dependent calcium current of cardiac cells, the L-type current represents a major pathway for entry of extracellular calcium. This entry triggers the subsequent large release of calcium from the sarcoplasmic reticulum (SR). In contrast, depolarization of skeletal muscle releases calcium from the SR without the requirement for entry of extracellular calcium through L-type calcium channels. To investigate the molecular basis for these differences in calcium currents and in excitation-contraction (E-C) coupling, we expressed complementary DNAs for the DHP receptors from skeletal and cardiac muscle in dysgenic skeletal muscle. We compared the properties of the L-type channels produced and showed that expression of a cardiac calcium channel in skeletal muscle cells results in E-C coupling resembling that of cardiac muscle.     

Normalization of current kinetics by interaction between the alpha 1 and beta subunits of the skeletal muscle dihydropyridine-sensitive Ca2+ channel.

Purification of skeletal muscle dihydropyridine binding sites has enabled protein complexes to be isolated from which Ca2+ currents have been reconstituted. Complementary DNAs encoding the five subunits of the dihydropyridine receptor, alpha 1, beta, gamma, alpha 2 and delta, have been cloned and it is now recognized that alpha 2 and delta are derived from a common precursor. The alpha 1 subunit can itself produce Ca2+ currents, as was demonstrated using mouse L cells lacking alpha 2 delta, beta and gamma (our unpublished results). In L cells, stable expression of skeletal muscle alpha 1 alone was sufficient to generate voltage-sensitive, high-threshold L-type Ca2+ channel currents which were dihydropyridine-sensitive and blocked by Cd2+, but the activation kinetics were about 100 times slower than expected for skeletal muscle Ca2+ channel currents. This could have been due to the cell type in which alpha 1 was being expressed or to the lack of a regulatory component particularly one of the subunits that copurifies with alpha 1. We show here that coexpression of skeletal muscle beta with skeletal muscle alpha 1 generates cell lines expressing Ca2+ channel currents with normal activation kinetics as evidence for the participation of the dihydropyridine-receptor beta subunits in the generation of skeletal muscle Ca2+ channel currents.     

Comparison of alpha-tropomyosin sequences from smooth and striated muscle

Tropomyosins are a closely related family of proteins with a dimeric alpha-coiled-coil structure. Skeletal isoforms are composed of two types of subunits, alpha and beta which, in turn, are assorted into two main molecular species alpha alpha and alpha beta. Both isoforms are present in different molar ratios in individual skeletal muscle types. In small mammals, however, only alpha-chain is expressed in cardiac muscle. Tropomyosin, in association with the troponin complex (troponin-I, -T and -C) plays a central role in the Ca2+-dependent regulation of vertebrate striated muscle contraction. On the other hand, despite structural similarities with the striated isoforms, the function of this protein in smooth muscle and non-muscle cells remains unknown, because in these cells contraction is thought to be regulated by myosin-linked processes independently of tropomyosin. Here we report the nucleotide sequences of cloned complementary DNAs for rat striated and smooth muscle alpha-tropomyosin. Comparison of the derived amino-acid sequences reveals the existence of tissue-specific peptides that delimit the putative troponin-I and troponin-T binding domains of tropomyosin. S1-nuclease mapping studies reveal the existence of three distinct alpha-tropomyosin messenger RNA isoforms each encoding a different protein; these isoforms are tissue-specific, developmentally regulated and most probably encoded by the same gene.