Electrical activity of single vascular smooth muscle fibers

Membrane potential and cancer progression

Membrane potential (V_m), the voltage across the plasma membrane, arises because of the presence of different ion channels/transporters with specific ion selectivity and permeability. V_m is a key biophysical signal in non-excitable cells, modulating important cellular activities, such as proliferation and differentiation. Therefore, the multiplicities of various ion channels/transporters expressed on different cells are finely tuned in order to regulate the V_m. It is well-established that cancer cells possess distinct bioelectrical properties. Notably, electrophysiological analyses in many cancer cell types have revealed a depolarized V_m that favors cell proliferation. Ion channels/transporters control cell volume and migration, and emerging data also suggest that the level of V_m has functional roles in cancer cell migration. In addition, hyperpolarization is necessary for stem cell differentiation. For example, both osteogenesis and adipogenesis are hindered in human mesenchymal stem cells (hMSCs) under depolarizing conditions. Therefore, in the context of cancer, membrane depolarization might be important for the emergence and maintenance of cancer stem cells (CSCs), giving rise to sustained tumor growth. This review aims to provide a broad understanding of the V_m as a bioelectrical signal in cancer cells by examining several key types of ion channels that contribute to its regulation. The mechanisms by which V_m regulates cancer cell proliferation, migration, and differentiation will be discussed. In the long term, V_m might be a valuable clinical marker for tumor detection with prognostic value, and could even be artificially modified in order to inhibit tumor growth and metastasis.

Endothelium-derived hyperpolarizing factor and endothelium-dependent relaxations

The endothelial cells inhibit the tone of the underlying vascular smooth muscle by releasing endothelium-derived relaxing factors (EDRF). The existence of at least two such factors, nitric oxide and endothelium-derived hyperpolarizing factor (EDHF), has been demonstrated. EDHF is an as yet unidentified substance that hyperpolarizes vascular smooth muscle cells and causes their relaxation. The contribution of endothelium-dependent hyperpolarization varies along the vascular tree. Particularly in smaller blood vessels, EDHF acts on vascular smooth muscle in cooperation with nitric oxide. Basal release of EDHF is not likely to occur, at least in vitro. The production and/or release of EDHF is regulated by the cytosolic concentration of Ca2+ ions, derived both from the extracellular space and intracellular stores. Calmodulin may be involved in its production and/or release. EDHF hyperpolarizes the vascular smooth muscle by opening K+ channels. The hyperpolarization closes voltage-dependent Ca2+ channels and, as a consequence, EDHF relaxes blood vessels. In the absence of chemical identification of EDHF, it is difficult to assess its contribution to endothelium-dependent relaxations in vivo.

Mechanical and electrical properties of smooth muscle cells and their regulations by endothelium-derived factors in the guinea pig coronary artery

Effects of high K+, acetylcholine (ACh), 9,11-epithio-11,12-methanothromboxane A2 (STA2), thrombin and endothelin were investigated on smooth muscles of the guinea pig coronary artery in intact and endothelium-denuded tissues. In intact tissues, ACh transiently inhibited but ATP produced maintained inhibition of the STA2-induced contraction. However, in the endothelium-denuded tissue, ACh produced contraction and ATP inhibited the STA2-induced contraction. In intact tissues, thrombin produced dual actions on the STA2-induced contraction with an initial relaxation followed by contraction. In endothelium-denuded tissues, thrombin enlarged the STA2-induced contraction without transient relaxation. In intact tissues prepared from both proximal and distal regions, endothelin showed the same dual action as observed with thrombin, whereas higher concentrations of endothelin showed only contraction. In endothelium-denuded tissues, endothelin consistently produced contraction. In intact tissues prepared from proximal and distal regions, ACh produced a biphasic response (initial hyperpolarization and subsequently generated depolarization). The amplitudes of both potential changes occurred in a membrane potential-dependent manner. In endothelium-denuded tissues, ACh depolarized the membrane in both tissues. In intact and endothelium denuded tissues, ATP hyperpolarized the membrane in inverse proportion to the membrane potential level, whereas thrombin and endothelin consistently depolarized the membrane. The results indicate that ACh acts on endothelial and smooth muscle cells, and the former releases both EDRF and EDHF. ATP only acts on smooth muscle cells and hyperpolarizes the membrane. STA2, thrombin and endothelin act on both endothelial and smooth muscle cells. STA2 and endothelin may release EDRF but not EDHF, and thrombin may release EDRF and endothelin.

Viscoelastic behavior of the thoracic aorta of dogs and rabbits

Data are presented which demonstrate no correlation between atrial electrical activity and diameter changes in the descending thoracic aorta of dogs or rabbits. The experiments conducted included bypassing a segment of the descending thoracic aorta in dogs, and production of a variety of arrhythmias without the bypass in dogs and rabbits. We found no evidence of electrical stimulation-induced phasic activity as suggested by other authors.

Effects of vasoactive agents on oxygen uptake in portal venous smooth muscle

The studies concerned the effects of epinephrine, norepinephrine, phenylephrine, acetylcholine and K+ ions, in doses inducing maximal contraction on oxygen consumption of isolated rat portal veins. All the agonists tested stimulated the rate of O2 uptake, whereas high K+ did not. After specific blockade of alpha-adrenoceptors, the effects of epinephrine and norepinephrine on O2 consumption were not affected, whilst those of phenylephrine were abolished, and beta-adrenoceptor blockade suppressed the metabolic effect of epinephrine and norepinephrine. Acetylcholine was the agonist that most elevated O2 consumption; atropine blocked both contractile and metabolic effects of this drug. It appears that there is a clear dissociation between contractile and metabolic actions of some pharmacological agents, and a question is posed concerning phenylephrine and acetylcholine effects. Results with K+ suggest that different metabolic pathways might be involved in the activation and maintenance of drug-induced contraction in smooth muscle.

Electrophysiology and innervation of the smooth muscle of dog inferior vena cava

1. Electrical properties of outer and inner muscles of three portions of dog inferior vena cava and their catecholaminergic innervation were investigated by microelectrode recording and an immunohistochemical technique. 2. There was no difference in the electrical properties of outer and inner muscles of the supradiaphragmatic (portion a) or the infrarenal (portion c) segments which had a quiescent membrane potential, delayed rectification, strong outward going rectification and no action potential. 3. In the longitudinal muscle which made up most of the segment (portion b) between the liver and renal veins, some of the outermost cells fired slow discharges and others action potentials in response to depolarizing current, but no cells from the innermost layer of longitudinal muscle of this portion did so. 4. All smooth muscle portions of the inferior vena cava showed a current spread in the direction of the long axis of the cell. Mean values of space constant of portions a, b and c were 2.25, 1.15 and 0.99 mm, respectively. 5. Noradrenergic nerve terminals were widely distributed in the longitudinal muscle layer of portion b and the circular muscle layer of portion c. Few nerve terminals were seen in any part of portion a. 6. The results suggest that a tendency to repetitive electrical activity was associated with outer smooth muscle aligned longitudinally, though a low space constant may have been associated with noradrenergic innervation.

The fine structure of smooth muscle cells and their relationship to connective tissue in the rabbit portal vein

The smooth muscle of rabbit portal vein was studied by electron microscopy with particular emphasis on the mechanical linkage between the muscle cells and on the distribution of connective tissue. The media of this vein is composed of inner circular and outer longitudinal muscle layers which are orientated almost perpendicularly to each other. The muscle of the inner circular layer shows very irregular contours with much branching and anastomosing of the cytoplasmic processes, which often make membrane contacts with neighbouring cells to form an extensive network of cytoplasmic processes. The muscle cells of the outer longitudinal layer are arranged in densely packed bundles and are spindle-shaped, with no branching processes. Opposing dense areas from neighbouring cells, with variable gap distances (30--100 nm) and close membrane contacts (intermediate junctions) with a gap of 11 nm were observed in both circular and longitudinal muscle layers. In the terminal regions of muscle cells in both circular and longitudinal layers a specialized anchoring structure was present which was closely related to extracellular elastic tissue. Muscle cells in the longitudinal layer showed the most elaborate structure, the tapering end of the muscle cell showing a honeycomb-like structure penetrated by columns of connective tissue compounds. The functional implications of these structures are discussed.

Spontaneous contractile activity of isolated ovarian human vein. A dual influence of prostacyclin (PGI2)

The profile of spontaneous contractions as well as the influences of prostacyclin (PGI2) on the motility of human ovarian veins obtained during the estrogenic phase of the sex cycle were explored. The preparations exhibited a distinct phasic activity, which progressively decreased as isolation time progresses, dissapearing almost completely following more than two hours. PGI2 produced a biphasic influence on quiescent preparations. After the threshold is attained lower concentrations caused depression of tone whereas higher ones enhanced the basal tone and induced phasic contractile cycles. Phentolamine reduced markedly the stimulating influence of PGI2 but had no action on the inhibitory effects, whereas propranolol failed to alter either the excitatory of the depressive action. The results suggest a participation of alpha-adrenoceptive-mediated mechanisms in the stimulatory effect of PGI2. On the other hand, PGI2 may be of importance in the regulation of venous flow and the spontaneous or PGI2-induced contractions could play a role in the counter current mechanism between veins and arteries in the ovarian pedicle.

The effects of acetylcholine on the membrane and contractile properties of smooth muscle cells of the rabbit superior mesenteric artery

1 Effects of acetylcholine (ACh) on the membrane potential and mechanical properties of rabbit superior mesenteric artery were investigated by the use of microelectrode and isometric tension recording methods. The membrane potential was -62.5 +/- 3.0 mV (s.d.). The maximum slope of the membrane depolarization produced by tenfold increase in [K](0) plotted on a log scale was 48 mV. Excess [K](0) and low [K](0) depolarized the membrane and produced contraction (contracture). The minimum depolarization to produce contraction was 10 mV.2 Low concentrations (10 and 100 ng/ml) of ACh hyperpolarized the membrane. Increased concentrations of ACh (1 and 10 mug/ml) hyperpolarized the membrane further in adult rabbit, while increased concentrations of ACh produced a smaller hyperpolarization in young rabbit. These potential changes produced by ACh in immature and adult rabbits were suppressed by treatment with atropine (0.1 mug/ml).3 ACh (10 ng to 1 mug/ml) consistently generated contraction in Krebs solution. However, ACh relaxed the contraction induced by either K(+) or noradrenaline in the adult rabbit, and it enhanced contraction produced by this treatment in the immature rabbit. In Ca-free EGTA solution, the action of ACh on the mechanical response was markedly suppressed, although high concentrations of ACh still evoked contraction. However, treatment with atropine (1 mug/ml) completely prevented these actions of ACh.4 ACh-induced relaxation during either K(+)-induced or noradrenaline-induced contraction was not caused by the hyperpolarization of the membrane.5 It is concluded that ACh possesses dual actions on smooth muscle cells of the rabbit superior mesenteric artery in Krebs solution, i.e. ACh hyperpolarizes the membrane, while it consistently generates contraction. These ACh actions on the muscle cells were modified by aging.

Inhibition by acetylcholine of adrenergic neurotransmission in vascular smooth muscle

Changes in the isometric tension of isolated strips of cutaneous, femoral, mesenteric, pulmonary, and muscle arteries and veins were recorded at 37°C in an organ bath. Acetylcholine (5 x 10 -8 and 10 -7 g/ml) caused relaxation of strips from the saphenous veins, the femoral veins, and all of the arteries after contraction by norepinephrine released from nerve terminals by electrical stimulation (2-5 Hz); in the pulmonary and mesenteric veins, acetylcholine caused a further increase in tension. Pulmonary artery and mesenteric vein strips were incubated with [ 3 H] norepinephrine and mounted for superfusion (3 ml/min) and isometric tension recording. Electrical stimulation increased the tension and the total radioactivity released in both preparations. Acetylcholine (2 x 10 -7 g/ml) depressed the contractions of the pulmonary artery strips but augmented those of the mesenteric vein strips; it diminished the efflux of radioactivity in both, indicating that acetylcholine inhibits adrenergic neurotransmission. In the absence of sympathetic stimulation, acetylcholine (5 x 10 -10 -10 -5 g/ml) caused all vein strips to contract; the most common reaction in artery strips was a slight relaxation (at 10 -9 -10 -8 g/ml) followed by a contraction (at 5 x 10 -8 -10 -5 g/ml). During contractions caused by norepinephrine, acetylcholine caused a further increase in tension in vein strips but a relaxation in artery strips. Atropine abolished the effects of acetylcholine. The results of this study suggest the presence in vascular smooth muscle of both excitatory and inhibitory cholinergic receptors.

Venous relaxation caused by acetylcholine acting on the sympathetic nerves

Experiments were performed to determine whether acetylcholine affects the sympathetic activation of the cutaneous veins of the dog. Changes in isometric tension of saphenous vein strips were recorded at 37°C in an organ bath. Addition of acetylcholine at 10 -11 to 10 -8 g/ml did not affect basal tension, but larger doses (5 x 10 -8 to 5 x 10 -7 g/ml) caused a contraction of the strips which varied from slight to marked. Acetylcholine at 10 -8 to 10 -7 g/ml caused a further increase in tension when it was added to strips already contracted by norepinephrine, tyramine, KCl, or BaCl 2 ; in contrast, similar doses of acetylcholine caused relaxation of strips contracted by liberation of norepinephrine from the nerve terminals by electrical stimulation (1-10 cps). This relaxation was not influenced by propranolol or hexamethonium but was abolished by atropine (10 -8 g/ml). In intact dogs, the lateral saphenous vein was perfused with autologous blood at constant flow. A sustained venoconstriction was induced either by electrical stimulation of the lumbar sympathetic chain or by a continuous infusion of norepinephrine. An infusion of acetylcholine (10 -7 to 10 -6 g/ml min -1 ) relaxed veins constricted by sympathetic stimulation but not those constricted by norepinephrine. Thus, acetylcholine, in doses smaller than those known to have a direct constrictor effect, causes relaxation of cutaneous veins, probably by inhibiting the release of norepinephrine from nerve terminals.

Membrane properties of the smooth-muscle fibres of the guinea-pig portal vein

The membrane activities and the various characteristic constants of the smooth-muscle membrane of the guinea-pig portal vein were investigated with the micro-electrode technique.1. The mean membrane potential was -37 mV. Spontaneous discharges appeared as regular bursts of short trains of spikes alternating with silent periods, as a mixture of single spikes and bursts of spikes appearing continuously, or as regular spikes with low frequency.2. Spontaneous spikes with overshoot were frequently observed. The maximum rate of rise of the spike was 3.7 V/sec. The shapes of the spikes were classified into three different types, i.e. pace-maker type of spike, monophasic spike and spike with a hump during the falling phase.3. Tetrodotoxin (10(-5) g/ml.) did not influence the patterns of the spontaneous train discharges nor the shape of the spike.4. Extracellularly applied outward current elicited spikes which were either monophasic or had a hump on the falling phase. Inward current elicited break excitation of the spike.5. Current-voltage relations, produced by application of inward current pulses to the tissue and measured at various distances from the stimulating partition, were linear.6. The smooth-muscle membrane of portal vein showed cable-like properties. The mean space constant of the membrane was 0.52 mm; the mean time constant of the membrane calculated from the electrotonic potential was 330 msec.7. Conduction velocity of the spike measured by insertion of two micro-electrodes was 0.58 cm/sec.8. The time constant of the foot of the propagated spike was 27 msec. The time constant of the membrane calculated from the time constant of the foot of the spike and the conduction velocity was 310 msec.9. The membrane properties of longitudinal smooth muscle of the portal vein were discussed in comparison with other veins and various visceral smooth muscles.