Novel inhibition of contractility by wortmannin in skeletal muscle

1. The effects of wortmannin and 2-(4-morpholinyl)-8-phenyl-1[4H]-benzopyran-4-one ({"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002), inhibitors of phosphatidylinositol 3-kinase, on the contractile responses of murine skeletal muscle were studied. Wortmannin (10–100 μM) suppressed twitch and tetanic contraction evoked by field stimulation of diaphragm without causing elevation of muscle tone. The inhibition was quasi-irreversible with IC₅₀∼15 μM. In contrast, {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002 increased twitch responses and elevated muscle tone.
2. Wortmannin reversibly depressed the maximal slope of action potential upstroke by ∼40% and inhibited the membrane depolarization and spontaneous burst of action potential induced by crotamine, a polypeptide toxin that activates the Na⁺ channel of skeletal muscle.
3. Wortmannin inhibited contractures evoked by high K⁺, ryanodine and caffeine, but potentiated the contracture induced by rapamycin, which binds to myoplasmic FK506 binding protein, an immunophilin closely associated with the ryanodine receptor. The contractures elicited by cardiotoxin, which disrupts the integrity of sarcolemma and thereby elevates `myoplasmic'' Ca²⁺ level, were suppressed only slightly.
4. In placed left atrium and ventricular strip, wortmannin and {"type":"entrez-nucleotide","attrs":{"text":"LY294002","term_id":"1257998346","term_text":"LY294002"}}LY294002 produced a positive inotropic effect.
5. The results suggest that, in addition to depressing the Ca²⁺ mobilization from sarcoplasmic reticulum, wortmannin exerts a novel inhibitory action on the excitation-contraction coupling in skeletal muscle but not in cardiac muscle.

     

β-Adrenergic potentiation of E–C coupling increases force in rat skeletal muscle

We examined the mechanism(s) which allow terbutaline, a β₂-adrenergic agonist, to increase isometric force in bundles of normal and denervated rat soleus fibers. Terbutaline (10 μmol/L) potentiated tetanic contractions during exposure to 1 mmol/L ouabain, 10 μmol/L nifedipine, or 0.5 mmol/L iodoacetate. Terbutafine induced equivalent increases in submaximal potassium (K⁺) contracture and tetanic force: these effects were mimicked by 2 mmol/L dibutyrl-cyclic AMP. Therefore, terbutaline increased force by a cyclic AMP-dependent mechanism other than enhancement of sodium-pump activity, dihydropyridine sensitive Ca²⁺ currents, glycolysis, or action potentials. Pretreatment with 1 mmol/L caffeine induced submaximal potentiation of peak tetanic force but prevented further potentiation by terbutaline. This suggested that terbutaline did not influence the myofilaments, but acted on the sarcoplasmic reticulum (SR) to increase the myoplasmic Ca²⁺ concentration and hence force production. We speculate that force is potentiated following β-adrenoceptor activation by a cyclic AMP-dependent phosphorylation of Ca²⁺ release channels to facilitate SR calcium release during tetanic stimulation. © 1993 John Wiley & Sons, Inc.     

CHARACTERIZATION OF SINGLE HEART CELL CONTRACTILITY BY RAPID IMAGING

1. A rapid imaging technique with temporal resolution of about 1 ms was employed to describe cell-length changes during the isotonic contraction cycle of adult rat ventricular myocytes at 22 degrees C. 2. Parameters of cell contraction and relaxation were defined and values obtained under control conditions and after treatment with 5 mumol/L verapamil. 3. Over 15 min, verapamil dramatically reduced the maximum shortening attained. This was associated with delay in both excitation-contraction coupling latency and the time at which maximum rate of shortening occurred. However, peak shortening was recorded earlier in the contraction cycle. Total cell-cycle time was abbreviated under the influence of verapamil and maximum rates of shortening and lengthening were depressed in a similar manner. 4. The results highlight the value of improved temporal precision in describing myocyte contractility and validate the use of the parameters defined in the single-cell model for the study of the mechanisms of action of cardiotonic agents.     

PLASMALEMMAL CALCIUM IN CARDIAC EXCITATION-CONTRACTION COUPLING

1. Mammalian heart muscle is extremely sensitive to the external calcium concentration. It reacts to alterations of the external calcium concentration with an immediate adaptation of contractile force. 2. In mammalian heart muscle there is a network of large transverse tubules throughout the cell. These structures are regularly arranged at the level of the sarcomeric Z- and I-lines and increase the cell surface by a factor often. 3. Experimental evidence favours the assumption that the plasmalemma could be the site of a loosely bound superficial Ca fraction which becomes ionized upon depolarization and is again bound upon repolarization of the cardiac cell membrane. 4. A mechanism is discussed which bases the excitation-contraction coupling process on a physicochemical interaction of calcium with membrane phospholipids. The degree of interaction is thought to be governed by the transmembrane electric field, the induced dipole moment of membrane constituents, and proton activity within the membrane.     

Receptor-operated Ca2(+)-permeable nonselective cation channels in vascular smooth muscle: a physiologic perspective

This article summarizes the literature on receptor-operated Ca2(+)-permeable nonselective cation channels in vascular smooth muscle cells. One of these conductances, the P2X1 receptor, is a classic ligand-gated channel, but others are likely to be mediated via G-protein-coupled receptors. The most studied receptor-operated channel in vascular myocytes is the norepinephrine-evoked nonselective cation channel in rabbit portal vein myocytes. The data regarding the transduction mechanisms and biophysical properties of whole-cell and single-channel currents in this preparation are described. The channels have a conductance of 20 to 25 pS and complex kinetic behavior with at least two open and two closed states. These channels are activated by norepinephrine and acetylcholine via G-protein-coupled receptors linked to phospholipase C and by diacylglycerol (DAG). The action of DAG occurs by a mechanism independent of protein kinase C, but other kinases may mediate the responses to norepinephrine and DAG. In addition, activation of tyrosine kinases leads to opening of this channel. Other vasoconstrictors, such as endothelin, vasopressin, serotonin, and angiotensin II, open Ca2(+)-permeable nonselective cation channels, but there may be differences between these conductances and the norepinephrine-evoked channels. A homologue of the transient receptor potential protein (TRPC6) is an essential component of the norepinephrine-activated channel in rabbit portal vein, and it is likely that this family of proteins plays an important role in mediating Ca2+ influx in vascular smooth muscle.     

EFFECTS OF ENDOTHELIN-1 ON THE CONTRACTILITY OF CARDIOMYOCYTES FROM THE SPONTANEOUSLY HYPERTENSIVE RAT

1. Disturbances in cardiovascular responsiveness to endogenous endothelin-1 (ET-1) may play a significant role in the pathogenesis of essential hypertension. In this study the inotropic responses of cardiomyocytes derived from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains to ET-1 (10-¹¹-10-⁸ mol/L) were characterized. Isotonic contraction cycles of ventricular cardiomyocytes isolated from age-matched (11 week) WKY and SHR rats were recorded using a rapid digital imaging technique and evaluated by computation of a range of normalized parameters. 2. The maximum effect of ET-1, eliciting a 60–70% increase in myocyte shortening after 3 min, was observed at 10^-9 mol/L in both strains, and was associated with elevations in the rate of shortening and lengthening, abbreviated latency, contractile cycle prolongation and delayed time to peak shortening. 3. No evidence for a significant strain dependent difference in the relative responsiveness to ET-1 was detected. This finding indicates that altered sensitivity to ET-1 is unlikely to be a major factor underlying the development of hypertension in this model. 4. The distinct nature of the alterations in contractile parameters produced by ET-1 compared with angiotensin II (AII) suggests that the prevailing cellular mechanisms of action of these peptides are different and that ET-1 is not a paracrine or autocrine inotropic intermediate for AII     

Mechanism of action of pentobarbital on the contractile system of isolated frog muscle fibres

The effects of pentobarbital-sodium were studied on single muscle fibres of frog skeletal muscle. In a concentration of 0.5 mM, pentobarbital potentiated the twitch response without significantly affecting the tetanus amplitude. The increase in twitch amplitude was accompanied by a marked increase in the rate of force development. The relaxation phase of both twitch and tetanus was prolonged. These mechanical changes were associated with an increased duration of the action potential. Pentobarbital also increased the amplitude and the rate of force development during the twitch after suppression by dantrolene. The S-shaped curve relating peak contracture tension and log caffeine concentration was shifted to the left by pentobarbital. It is suggested that pentobarbital affects the contractile activity of the muscle fibre; (1) by increasing the rate of release of calcium from its storage sites, (2) by prolonging the duration of the action potential and (3) by inhibiting calcium resequestration by the sarcoplasmic reticulum (SR).     

骨骼肌兴奋收缩偶联与细胞内的钙稳态

背景：在骨骼肌胞浆内,Ca2+是神经兴奋和肌肉收缩之间的重要偶联因子,控制着肌肉收缩的启动和舒张的终止,对骨骼肌的兴奋收缩偶联起着不可或缺的作用,骨骼肌的兴奋收缩偶联与细胞内钙稳态的变化紧密相关。目的：研究近年来骨骼肌细胞内钙调控机制的研究进展及钙稳态与运动之关系的进展。方法：电子检索CNKI数据库、读秀学术搜索等中文数据库和Elsevier SD,Springer Link等英文数据库1980/2010收录的骨骼肌兴奋收缩偶联和钙稳态的相关综述和实验研究报告,分析骨骼肌细胞内钙调控机制的研究进展及钙稳态与运动之关系的研究进展。结果与结论：共纳入骨骼肌兴奋收缩偶联与钙稳态相关文献43篇。骨骼肌的兴奋收缩偶联与细胞内钙稳态之间有着密不可分的关系,但骨骼肌内钙稳态的调控机制还不甚明朗,尚期待大量研究。对运动与肌细胞内钙稳态的关系的研究也仍需要进一步努力,探索适宜适当的运动,探索运动性疲劳的发病机制,有效预防和对抗运动性疲劳及肌肉损伤仍将是研究者的工作重点。