Excitation-contraction coupling in the smooth muscle of the femoral artery under the effects of noradrenaline

Noradrenaline (5 x 10(-8) - 10(-5) M) induced a dose-dependent contraction of muscle strips from rabbit femoral artery. At concentrations higher than 10(-7) M noradrenaline evoked also a depolarization of smooth muscle cells due to an increase in sodium and/or chloride permeability of the membrane. Repolarization of the membrane to original level by inwardly applied current resulted in restoration of membrane resistance and partial relaxation of noradrenaline-evoked contraction. The same part of contraction was also blocked by verapamil. In calcium-free EGTA-containing solution noradrenaline induced only a small transient contraction. These findings indicate that noradrenaline-activated sodium (or chloride) permeability is voltage dependent. Noradrenaline evoked contraction is activated by calcium ions entered the cell through receptor-operated and partly through voltage-operated calcium channels.     

Chemosensitivity of single smooth muscle cells to acetylcholine, noradrenaline, and histamine in vitro

Electrical responses to acetylcholine, noradrenaline, and histamine were recorded from solitary smooth muscle cells. Iontophoresis of each transmitter elicited three fast responses: a hyperpolarization, a depolarization, or a biphasic hyperpolarization-depolarization. Each transmitter activated a specific receptor since responses were specifically blocked by antagonists, two transmitters elicited different responses in solitary cells, and desensitization of response to one transmitter did not cause desensitization of responses to other transmitters. Responses were due to increased ion conductances since input resistance decreased during responses and reversal potentials were measured for depolarizing responses (-5 mV) and hyperpolarizing responses (-60 mV). Regional differences in transmitter sensitivity were mapped on solitary cells. Biphasic responses were due to simultaneous activation of receptors mediating hyperpolarizing responses and receptors mediating depolarizing responses which were segregated in the cell membrane. Noradrenaline enhanced action potential amplitude by regulation of voltage-dependent ion conductances. Finally, noradrenaline and histamine elicited periodic hyperpolarizing potentials, which may be due to increased intracellular Ca++.     

Electro- and pharmacomechanical coupling in the smooth muscle cells of the rabbit ear artery

A contraction of the rabbit ear artery can be induced by depolarizing the cells with a K-rich solution if Ca is present. 10(-9)-10(-6) M noradrenaline and 10(-8)-10(-7) M histamine cause a contraction of this tissue without modifying the membrane potential. If the histamine concentration exceeds 10(-7) M some depolarization of the membrane also occurs. Both noradrenaline and histamine also induce a contraction in Ca-free medium, even if La is present. None of these stimuli produces action potentials or fluctuations of the membrane potential. Besides these tonic contractions, the ear artery can also produce phasic contractions when 10 mM TEA is added to the medium. Such contractions are caused by the appearance of action potentials which are Ca dependent and which are similar to those appearing in visceral smooth muscle. A study of 45Ca fluxes has revealed that K depolarization and noradrenaline cause only a small increase in 45Ca uptake by the cells, while noradrenaline also releases cellular Ca, even in Ca-free medium. A comparison of tension development and 45Ca release induced by noradrenaline in Ca-free medium suggests that Ca extrusion could be very efficient in the rabbit ear artery and that it could play a direct role in its relaxation.     

Ionic mechanism of the stimulatory effect of noradrenaline on smooth muscle cells of the pulmonary artery

Noradrenaline (NA) in a concentration of 5 X 10(-6) M produces depolarization of smooth muscle cells of the rabbit pulmonary artery and reduction of membrane resistance followed by contraction and increased excitability of muscle cells. Experiments with repolarization of the membrane exposed to NA in normal and Ca-free Krebs solutions have shown that activation of the NA-induced contraction is mainly due to Ca++ entering the cells through NA-sensitive potential-dependent Ca-channels. The NA-induced depolarization results from an initial decrease of K-permeability of the membrane subsequent increase of the permeability of NA-sensitive potential-dependent channels for Na+ and/or Cl-, which provides further depolarization of the membrane. Depolarization ceases after becoming sufficient for activation of potential-dependent non-inactivated K-channels. Voltage clamp experiments have shown that the NA-induced increased excitability is related to a reduction of slow, particularly of fast component of outward current, whose early activation prevents the development of regenerative process of action potential generation under normal conditions.     

Tetrodotoxin-resistant electric activity in chick skeletal muscle cells differentiated in vitro

The embryonic chick skeletal muscle cells differentiated in cell culture from trypsin-dissociated myoblasts produce a spike response which is tetrodotoxin-sensitive. It has been found that many cells also produce a plateau response which is resistant to tetrodotoxin. The plateau response frequently occurs even in the muscle cells which do not normally exhibit the spike response. During the plateau response membrane resistance is greatly reduced below its resting value. The current-voltage relation in muscle cells with the plateau response is always S-shaped. It is suggested that the plateau arises from a voltage-dependent increase in permeability to external cations whose influx produce the maintained depolarization, and from low level of repolarizing potassium outflux. The plateau response is sensitive to manganese ions. This finding, together with resistibility to tetrodotoxin, suggests that calcium ions are the dominant carriers for the depolarizing current.     

Effect of serotonin and histamine on the cellular contractile activity of the internal carotid artery]

In the experiments on isolated segments of the canine internal carotid artery it was shown that serotonin (5.10(-11)--5.10(-5) g/ml) activated the contractions. Histamine (5.10(-9) g/ml) induced dilation of isolated segments and its higher concentrations produced the contractile responses. Serotonin and histamine were shown to induce the contractions of the depolarized smooth vascular muscle. It is suggested that serotonin activates the inflow of extracellular calcium ions and histamine activates both the inflow of extracellular and the cut-flow of the intracellular calcium ions.     

去极化时Ca~(2+) 内流引起蛋白激酶C_α浆膜转位

 为观察胞外Ca2 + 内流和肌浆网Ca2 + 释放两种来源的Ca2 + 对cPKCα转位激活的影响 ,揭示PKC在去极化 nAChR转录偶联中的作用 ,构建了pPKCα EGFP N1融合蛋白真核基因表达载体 .转染C2C1 2肌细胞后 ,采用激光共聚焦显微镜记录了KC1或咖啡因处理所引起的细胞Ca2 + 波变化及PKCα GFP融合蛋白在细胞内的分布 .结果提示 ,只有用KC1处理引起细胞膜去极化时 ,伴随Ca2 +内流 ,才能观察到PKCα GFP绿色荧光在细胞内发生的细胞浆至细胞膜分布变化 .然而 ,采用肌浆网Ca2 + 通道激动剂咖啡因刺激肌细胞 ,使肌浆网中Ca2 + 释放 ,未见PKCα GFP绿色荧光在浆、膜分布发生任何变化 .结果提示 ,去极化时外Ca2 + 内流可引起PKCα转位激活 ,肌浆网Ca2 + 释放对PKCα的转位激活没有影响 .     

Effects of calcium ions on electrical responses of gastric gland cells to histamine and pentagastrin

The effect of Ca ions on electrical responses of gastric gland cells on histamine and pentagastrin was investigated using intracellular glass microelectrodes. It was established that in low-calcium solutions hyperpolarization induced by these secretagogues was diminished. In calcium-free solutions and in solutions with blockers of the calcium current hyperpolarization induced by histamine and pentagastrin was not observed. It was suggested that external calcium ions are necessary for hyperpolarization responses to histamine and pentagastrin actions on gastric gland cells to occur.     

Modulation of solute permeability in microvascular endothelium

Modulation of macromolecular permeability involves creation of venular leaks in response to receptor-operated mechanisms in the endothelial cell membrane elicited by various autacoids (histamine, serotonin, bradykinin). Reversible modulations may occur within seconds in response to specific agents, which indicates receptor-mediated events that act via the endothelial cells' contractile apparatus, leading to subtle changes in junctional microtopography and allowing faster passage of small solutes. This mechanism probably involves activation of the actin-myosin system in endothelial cells. Ca2+ is an important signal substance, as reflected in the permeability-increasing effect of calcium ionophores. The junctional control system may share functional similarities with the contractile system in various types of muscle cells, in particular, smooth muscle. This suggests a function for the extensive vesicular invaginations of the plasmalemmal membrane present in endothelial cells. Rather than being a system to carry macromolecules across the endothelium, its physiological role may be to regulate free cytosolic calcium concentration. It is reminiscent of similar membrane invaginations found in muscle cells. Thus intracellular free calcium may be regulated by a combination of energy-requiring extrusion and passive influx through receptor-operated calcium channels located in the invaginated vesicular membranes, with short diffusion distances to the actin-myosin filaments in the cytoplasm.     

Contractile capacity of isolated flaps from the aorta of rats with stable arterial hypertension caused by the prolonged administration of cerebrosides

A study was made of the contractility of smooth muscle cells of abdominal aorta strips of noninbred white rats with stable arterial hypertension induced by protracted intraperitoneal injection of cerebrosides isolated from cattle brain. It was demonstrated that as compared with normotensive animals, smooth muscle cells of the animals' arteries are characterized by the increased influx of extracellular calcium via slow potential-dependent calcium channels and hypersensitivity to noradrenaline and serotonin.     

The control of chick myoblast fusion by ion channels operated by prostaglandins and acetylcholine

Chick myoblast fusion in culture was investigated using prostanoid synthesis inhibitors to delay spontaneous fusion. During this delay myoblast fusion could be induced by prostaglandin E1 (PGE1), by raising extracellular potassium and by addition of carbachol. Carbachol-induced fusion, but not PGE-induced fusion, was prevented by the acetylcholine receptor blocker alpha-bungarotoxin. Fusion induced by any of these agents was prevented by the Ca channel blockers lanthanum and D600. The threshold for potassium-induced fusion was 7-8 mM; maximal fusion occurred at 16-20 mM. Low extracellular potassium inhibited spontaneous fusion. Intracellular potassium in fusion competent myoblasts was 101 m-moles/l cell. Calcium flux measurements demonstrated that high potassium increased calcium permeability in fusion-competent myoblasts. A 30-s exposure to high potassium or PGE1 was sufficient to initiate myoblast fusion. Anion-exchange inhibitors (SITS and DIDS) delayed spontaneous myoblast fusion and blocked fusion induced by PGE1 but not carbachol. Blocking the acetylcholine receptor shifted the dose-response relation for PGE-induced fusion to higher concentrations. PGE1-induced fusion required chloride ions; carbachol-induced fusion required sodium ions. Provided calcium channels were available, potassium always induced fusion. We conclude that myoblasts possess at least three, independent pathways, each of which can initiate myoblast fusion and that the PGE-activated pathway and the acetylcholine receptor-activated pathway act synergistically. We suggest that fusion competent myoblasts have a high resting membrane potential and that fusion is controlled by depolarization initiated directly (potassium), by an increase in permeability to chloride ions (PGE), or by activation of the acetylcholine receptor (carbachol); depolarization triggers a rise in calcium permeability. The consequent increase in intracellular calcium initiates myoblast fusion.     

Cyclic nucleotide involvement in histamine release from mast cells--a reevaluation

Secretory events in cells in general are accompanied by increased levels of cyclic AMP. In mast cells, however, the pattern is reversed. Thus histamine release is associated with a fall in cAMP. It has been suggested that the lowered levels of cAMP lead to an increase in membrane permeability towards calcium and that an influx of such ions triggers the release mechanisms. It has further been reported that high levels of cAMP inhibit histamine release by decreasing the permeability. However, evidence has now accumulated indicating that this general concept is far too simplistic. Studies are reviewed which imply that there is little or no correlation between histamine release and intracellular levels of cyclic nucleotides. A new working hypothesis with respect to the role of these nucleotides in mast cell secretion is proposed.     

Regulation of beta-adrenergic receptors and calcium channel agonist binding sites in cultured human embryonal smooth muscle cells

Recent electrophysiological studies with cell membrane patches of cardiac myocytes and an electrically excitable cell line derived from rat pituitary tumor suggested that voltage activated calcium channels must be phosphorylated to respond to membrane depolarization (Armstrong and Eckert 1986; Trautwein and Kameyama 1986). In view of the "phosphorylation hypothesis" we investigated the adenylate-cyclase activity, the characteristics of beta-adrenergic and calcium channel agonist binding sites in control and desensitized (exposure to isoproterenol) human embryonal cells (HEC), and in fragmented membrane preparations of canine coronary smooth muscle. Our results suggest that down-regulation of the membrane-bound beta-adrenergic receptors, induced by isoproterenol in human embryonal cells and also in adult canine vascular tissue, results in physical translocation of beta-adrenergic binding sites into the light membrane fraction. This phenomenon is accompanied with an increased intracellular concentration of cAMP in and an increased binding of the calcium channel agonist (3H) BAYK 8644 to both HEC and canine smooth muscle membrane preparations. It could be concluded that phosphorylation of beta-adrenergic receptors regulates not only the beta subcellular distribution of the beta receptors but also the availability of calcium channel agonist binding sites in the cellular membrane.     

Actions of histamine on muscle and ganglia of the guinea pig gallbladder

Histamine is an inflammatory mediator present in mast cells, which are abundant in the wall of the gallbladder. We examined the electrical properties of gallbladder smooth muscle and nerve associated with histamine-induced changes in gallbladder tone. Recordings were made from gallbladder smooth muscle and neurons, and responses to histamine and receptor subtype-specific compounds were tested. Histamine application to intact smooth muscle produced a concentration-dependent membrane depolarization and increased excitability. In the presence of the H(2) antagonist ranitidine, the response to histamine was potentiated. Activation of H(2) receptors caused membrane hyperpolarization and elimination of spontaneous action potentials. The H(2) response was attenuated by the ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide in intact and isolated smooth muscle. Histamine had no effect on the resting membrane potential or excitability of gallbladder neurons. Furthermore, neither histamine nor the H(3) agonist R-alpha-methylhistamine altered the amplitude of the fast excitatory postsynaptic potential in gallbladder ganglia. The mast cell degranulator compound 48/80 caused a smooth muscle depolarization that was inhibited by the H(1) antagonist mepyramine, indicating that histamine released from mast cells can activate gallbladder smooth muscle. In conclusion, histamine released from mast cells can act on gallbladder smooth muscle, but not in ganglia. The depolarization and associated contraction of gallbladder smooth muscle represent the net effect of activation of both H(1) (excitatory) and H(2) (inhibitory) receptors, with the H(2) receptor-mediated response involving the activation of K(ATP) channels.     

Neuropeptides modulate the transmitter-induced glycogenolysis and gluconeogenesis in leech segmental ganglia

The effects of four neuropeptides (AKH, FMRFamide, proctolin, VIP) on the alterations in glycogen levels induced by other transmitters were studied in isolated leech segmental ganglia. With the exception of FMRFamide, which produced a small increase (14%) in glycogen, the peptides were by themselves without effect on the glycogen levels. Proctolin abolished the glycogenolytic effects of 5-HT, dopamine and octopamine but not histamine. AKH in combination with ACh produced glycogenolysis although each by themselves were ineffective. AKH modified the effects of other transmitters in different ways i.e. by reduction or reversal of effect. VIP and noradrenaline produced an increase in glycogen (cf. noradrenaline alone which decreased glycogen), but did not modify the effects of the other transmitters. FMRFamide produced a complex variety of modulatory effects on the other transmitters. It is concluded that the glycogen stores in leech ganglia, which are localized principally in the glial cells, may be controlled in complex ways by the different combinations of monoamines, amino acids and neuropeptides.     

Mechanism of the effect of dibasol on the contractile and electric activities of the smooth muscle of the portal vein

The effects of dibasol on spontaneous electrical and contractile activities as well as on the reactions evoked by hyperkalemic solution and noradrenaline were studied in smooth muscle of rabbit portal vein. It was shown that dibasol blocked the potential-operated influx Ca2+ into smooth muscle cells. The noninactivating calcium channels were found to be more sensitive to dibasol than inactivating ones. Significant part of the tonic contraction induced by noradrenaline was resistant to dibasol suggesting its weak effect on Ca2+ influx through calcium channels operated by alpha 1-adrenoceptors. It is supposed that vasodilative effect of dibasol is associated with blocking the influx Ca2+ through potential-operated noninactivating calcium channels into smooth muscle cells.     

Electric Dipole Theory of Chemical Synaptic Transmission

In this paper we propose that chemicals such as acetylcholine are electric dipoles which when oriented and arranged in a large array could produce an electric field strong enough to drive positive ions over the junction barrier of the post-synaptic membrane and thus initiate excitation or produce depolarization. This theory is able to explain a great number of facts such as cleft size, synaptic delay, nonregeneration, subthreshold integration, facilitation with repetition, and the calcium and magnesium effects. It also shows why and how acetylcholine could act as excitatory or inhibitory transmitters under different circumstances. Our conclusion is that the nature of synaptic transmission is essentially electrical, be it mediated by electrical or chemical transmitters.     

Inhibition of neuromuscular transmission in the guinea-pig saphenous artery by atriopeptin II

In the guinea-pig saphenous artery, stimulation of perivascular nerves elicited contraction and two types of synaptic potentials: the excitatory junction potential and the slow depolarization. The synaptic potentials were inhibited by atriopeptin II but not by sodium nitroprusside. Exogenous noradrenaline induced membrane depolarization and contraction, and both sodium nitroprusside and atriopeptin II inhibited the contraction but not the depolarization. These results suggest that atriopeptin II has an inhibitory effect both presynaptically at the nerve terminals and postsynaptically on the vascular smooth muscle cells.