The effect of tetraethylammonium chloride on calcium fluxes in smooth muscle from rabbit main pulmonary artery

Acute inhibition of spontaneous uterine contractions by an estrogenic polychlorinated biphenyl is associated with disruption of gap junctional communication

An estrogenic polychlorinated biphenyl, 4-hydroxy-2',4', 6'-trichlorobiphenyl (4-OH-TCB), inhibits oscillatory uterine contractions immediately. Because increased gap junction formation is associated with the development of synchronized uterine contractions at term, we examined whether the inhibitory effect of 4-OH-TCB on spontaneous oscillatory contractions was due to the disruption of gap junctional communication. The effect of 4-OH-TCB on gap junctional communication was determined by intercellular Lucifer yellow dye transfer in primary cultures of myometrial myocytes isolated from midgestation rats. Intercellular dye transfer was inhibited by 4-OH-TCB or 17beta-estradiol in a concentration-dependent manner. The inhibitory effect of 4-OH-TCB on intercellular dye transfer was reversed by tetraethylammonium (TEA). To examine effects on uterine contraction, longitudinal uterine strips were excised from midgestation rats and placed in muscle baths for isometric force measurement. Spontaneous uterine oscillation was suppressed by 4-OH-TCB or 17beta-estradiol. The inhibitory effects of 4-OH-TCB and 17beta-estradiol on spontaneous oscillations were counteracted by TEA but were not affected by a calcium ionophore (A23187) or a calcium-dependent potassium channel blocker (apamin). These results suggest that the acute inhibition of spontaneous oscillatory contractions by an estrogenic polychlorinated biphenyl may result from the disruption of intercellular communication.

Rhythmic contractions in isolated small arteries of rat: role of K+ channels and the Na+,K(+)-pump

Small mesenteric arteries from Wistar rats display rhythmic tension oscillations, associated with oscillations in membrane potential, when stimulated with noradrenaline. The purpose of this study was to investigate the role of potassium conductance and Na+, K(+)-pump activity in the generation of these oscillations. The effect on the rhythmic contractions of several agents, interacting with K+ channels, was studied. Application of apamin, pinacidil or glibenclamide did not affect the rhythmic activity. Tetraethylammonium (TEA) increased the frequency of the rhythmic contractions, while application of 4-aminopyridine (4-AP) increased the amplitude by approximately 50%, with no changes in frequency. Ba2+, on the other hand, impaired the rhythmic contractions or converted them to irregular oscillations in the presence of functional endothelium, but did not affect oscillations in endothelium-denuded vessels. Ouabain or exposure to K(+)-free solution, procedures known to inhibit the Na+,K(+)-pump, abolished the rhythmic contractions. This effect was immediate, suggesting that it was due to elimination of the electrogenic action of the Na+,K(+)-ATPase, rather than to a change in intracellular ion concentrations. Exposure to an extracellular potassium concentration of more than 20 mM also inhibited the oscillation activity. The results suggest that the oscillations are not caused by, but may be modulated by, variations in potassium conductance. The Na+,K(+)-pump seems to play an important role in the generation of rhythmic contractions in these vessels.

Effects of tetraethylammonium and 4-aminopyridine on outward currents and excitability in canine tracheal smooth muscle cells

1. The effects of tetraethylammonium (TEA) and 4-aminopyridine (4-AP) on membrane currents and on single channel K currents in smooth muscle cells isolated from canine trachea were examined by use of tight seal whole cell- and patch-clamp techniques. 2. Depolarizing current applied through a recording pipette did not elicit an action potential under current clamp. A strong outward rectification was observed. 3. In most cells under voltage-clamp, only an outward current was observed upon depolarization from -60 mV when a pipette solution contained mainly KCl. The outward current consisted of three components; a large initial transient, a following sustained component and an additional component of irregular small transients on the sustained one. The two transient components were almost abolished when extracellular and pipette solutions contained 2.2 mM Cd2+ (0 mM Ca2+) and 10 mM EGTA, respectively. The sustained component was well maintained under these conditions. 4. TEA at low concentrations (less than 1 mM) effectively decreased the transient components and made the outward current smooth; it also suppressed the sustained component at higher concentrations. In outside-out patches, external 1 mM TEA reduced the single channel conductance of Ca-activated K channels by about 87% whereas 3 mM 4-AP did not. 4-AP at low concentrations (less than 3 mM) selectively reduced the sustained component of the outward current. 5. A Ca current recorded after the suppression of outward current by internal Cs+ had a peak of approximately 200 pA at +10 mV (holding potential: -60 mV). The half inactivation voltage in the steady-state was approximately -30 mV. 6. Simultaneous application of 1 mM TEA and 4-AP reduced the outward current and unmasked a Ca current. Under these conditions, an action potential with overshoot was easily elicited under current clamp. 7. It is concluded that the low excitability of canine tracheal smooth muscle cell upon depolarization is due to a large outward K current which consists of Ca-dependent and Ca-independent components. The peak amplitude of the Ca current is similar to that in highly excitable smooth muscle cells such as those of the ureter.

Early effects of tetraethylammonium chloride on the contractile properties of isolated rabbit basilar arteries

The acute vascular effects of tetraethylammonium chloride (TEA) were examined on annular segments of rabbit basilar arteries. Contractions induced by the potassium channel blocker were compared with those obtained for potassium chloride, 5-hydroxytryptamine (5-HT) and norepinephrine (NE). The greater magnitude of the contractions was of the following order: [K+] greater than 5-HT greater than TEA greater than NE. High concentrations of TEA alone (10(-2) M) generated spontaneous oscillatory contractions in cerebral vessels that were normally quiescent. Low concentrations of TEA (10(-8)-10(-6) M), which had no vasomotor properties per se, enhanced the contractile response of submaximal concentrations of 5-HT (10(-7) M) and NE (3 X 10(-6) M) and attenuated the contraction produced by 60 mM [K+]. An increased vascular response to the amines was still evident up to 3 h after the addition of TEA despite frequent rinsing with fresh buffer solutions. On arteries precontracted with TEA (10(-2) M), but not high [K+], the subsequent addition of 5-HT (10(-7) M) still induced a powerful constriction. Repeated concentration-response curves for [K+] were reproducible and, in the presence of TEA (10(-8) or 10(-6) M), the curve was displaced to the right in a competitive manner. A higher concentration of TEA (10(-4) M) was devoid of any blocking properties on the [K+]-induced response whereas, at 10(-3) M TEA, the response was potentiated, as evidenced by a shift of the curve to the left. Interactions between TEA and the cumulative response to 5-HT were difficult to interpret. Repeated exposures of the artery to 5-HT resulted in an increased maximal response with each determination (EAm = 127 +/- 9% and 149 +/- 14% of control values following the second and third applications, respectively). With TEA (10(-6) M), the increase in the maximal contractile effect noted previously was not observed. Contractions induced by single concentrations of TEA (10(-2) M) or [K+] (60 mM) were calcium dependent, were abolished completely in a calcium-free medium, and were depressed by the calcium antagonist nimodipine. 5-Hydroxytryptamine-induced contractions (10(-5) M) were less sensitive to withdrawal of calcium from the extracellular medium (31 +/- 6% relative to the maximal response at 4 mM calcium). Hence, an acute reduction in potassium conductance in cerebrovascular smooth muscle produced by TEA has complex, concentration-dependent effects and reproduces only part of the spectrum of effects of cisternal injection of blood on cerebrovascular reactivity.

Calcium influx and vascular reactivity in systemic hypertension

Numerous studies have focused on functional vascular changes that characterize the hypertensive state. Recent evidence that suggests that increased vascular reactivity in hypertension is due to changes in the delivery of activator Ca++ through channels in the cell membrane will be reviewed. The primary evidence supporting this hypothesis comes from studies that characterize the effects of Ca++-free solution and calcium channel blockers on contractile properties of isolated vascular smooth muscle. In the present study, experiments were performed to investigate the role of Ca++ influx in vascular contractions produced by interventions that cause membrane depolarization. Isometric tension development in helical strips of carotid arteries from stroke-prone spontaneously hypertensive rats in response to elevated K+ and tetraethylammonium chloride was greater than that in carotid arteries from Wistar-Kyoto normotensive rats. The rate of tension development to K+-free solution in carotid arteries from stroke-prone spontaneously hypertensive rats was faster than in Wistar-Kyoto normotensive rat arteries. Contractile responses to all 3 depolarizing interventions were reduced in arterial strips incubated in Ca++-free solution containing the chelator ethylene glycol bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid and in arterial strips treated with the Ca++ channel blocker verapamil. These results are consistent with the hypothesis that constrictor stimuli that produce membrane depolarization cause an opening of Ca++ channels in the plasma membrane that are sensitive to the organic channel blockers. Further, a change in Ca++ permeability or membrane depolarizing mechanisms contributes to increased contractile responsiveness in carotid arteries of stroke-prone spontaneously hypertensive rats.

Myorelaxant effect of diazepam. Interactions with neuromuscular blocking agents and cholinergic drugs

Experiments were performed, using electrophysiological techniques, to investigate the effects and interactions of diazepam with commonly-used neuromuscular blocking agents and cholinergic agonists in chick and rat isolated nerve-muscle preparations. The results showed that in low concentrations (3.4-340 mumol/l, i.e. 1-100 micrograms ml-1) diazepam increased, in a dose-dependent manner, the twitch contractions in response to motor nerve stimulation of the chick and rat. High concentrations of diazepam (greater than 340 mumol/l) decreased the twitch tension and greatly reduced the contractures produced by acetylcholine (ACh) and tetraethylammonium (TEA) in the chick skeletal muscle. However, in the concentration range studied (close to a therapeutic dose of 0.15 mg kg-1), diazepam had no significant effect on the neuromuscular blockade produced either by tubocurarine or by succinylcholine. It was concluded that diazepam may either increase or decrease the twitch tension, in rat and chick skeletal muscle, the effect being dependent on the concentrations used. The mechanism of action of diazepam may depend on an increasing intracellular calcium concentration, which is directly involved in the increase of the twitch tension by low concentrations of diazepam.

Calcium kinetics in vascular smooth muscle

The accumulation, binding, and mobilization of Ca++ in vascular smooth muscle directly affects intracellular free Ca++ levels and contractility. Techniques have been developed to delineate Ca++ uptake and efflux parameters in isolated vessels. Similar Ca++-related components are present in different types of vessels, but their relative importance for induction and maintenance of tension differ.

Assessment of the presynaptic effect of atracurium. Train-of-four and tetanic stimulation in in vitro preparations

The present experiments were designed to study the effect of atracurium on the contractile responses produced by repetitive motor nerve stimulation and by depolarizing drugs in frog, chick and rat skeletal muscle-nerve preparations, using electrophysiological and neurochemical techniques. The presynaptic effect of atracurium was assessed using train-of-four (2 Hz) and tetanic stimulation (50 Hz) in isolated chick and rat neuromuscular junctions. A further measure of the presynaptic effect of atracurium was examined by studying its effect on the uptake of labelled choline, 3H-methylcholine, in the control preparations. The effect of atracurium on postjunctional contractile responses of the chick isolated biventer cervicis skeletal muscle was studied using cholinergic drugs such as acetylcholine and tetraethylammonium. The results indicated that in addition to its postjunctional competitive and non-depolarizing blocking effect, atracurium may have a prejunctional inhibitory action at the neuromuscular junction. Atracurium reduced all the contractile responses produced by both electrical and chemical stimulation.

Possible involvement of substance P in the contraction produced by periarterial nerve stimulation in the rat ileum

The effect of substance P (SP) on the contractile responses produced by periarterial (mesenteric) nerve stimulation was studied in the rat isolated ileum. Periarterial nerve stimulation at 1-50 Hz, with 10 V (maximum) and 0.2 msec pulse duration, for 15-20 sec, produced frequency-dependent contractions in the rat ileum. In the presence of guanethidine (10 microM) and 6-hydroxydopamine (1 microM), to block noradrenergic responses, periarterial nerve stimulation at 1-20 Hz still produced small contractions which were reduced by atropine (1 microM) and morphine (1 microM). In the presence of atropine, morphine, guanethidine and 6-hydroxydopamine, the contraction produced by periarterial nerve stimulation was readily abolished by tetrodotoxin (1 microM), capsaicin (3.3 microM) and an SP-antagonist (SPA1, 10 microM). SP in low concentrations (0.01-1.0 microM) potentiated the contractions produced by periarterial nerve stimulation at 1-2 Hz by 20-30%. High concentrations of SP (1.0-10.0 microM) reduced the contractile response by 40-50%. Indomethacin (2.8 microM) amd mepyramine (1 microM) had no effect on these responses. When the mesenteric nerve supply to the gut was cut, periarterial nerve stimulation produced no contraction in the rat ileum. However, SP in low concentrations, still produced small contractions which were abolished by an SP-antagonist but not by tetrodotoxin. SP in low concentrations, slightly increased the contractions produced by ACh (0.5-50 microM) or TEA (2.4-12 mM). High concentrations of SP significantly reduced the ACh and TEA-induced contractions in the rat ileum.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of some intravenous anaesthetics on the contractile responses produced in the chick biventer cervicis skeletal muscle

The effects of some intravenous anaesthetics, in low and high concentrations, on the contractile responses produced in the chick biventer cervicis skeletal muscle were studied using electrophysiological and pharmacological techniques. The anaesthetics used were methohexitone sodium, thiopentone, ICI 35 868 (propofol, diprivan), althesin and etomidate. The results showed that all these anaesthetics had a common mode of action at the chick neuromuscular junction. In low concentrations, they increased the amplitude of the indirectly-elicited twitch and tetanic contractions, whereas in high concentrations they reduced the evoked contractions and the contractures produced by depolarizing agents such as acetylcholine or tetraethylammonium. The results indicated that, in low concentrations, anaesthetics may stimulate skeletal muscle whereas in high concentrations they reduce the sensitivity of the postjunctional membrane to depolarizing drugs. The possibility that anaesthetics may block endplate ion channels was also discussed.

The effect of substance P on the mechanical and electrical responses of frog, chick and rat skeletal muscle

The effect of substance P (SP) on the electrical and contractile responses of skeletal muscles of frog, chick and rat were studied using electrophysiological techniques. In low concentrations, SP increased the amplitude of twitch and tetanic contractions in response to motor nerve stimulation of frog, chick and rat preparations, increased the amplitude and duration of frog sciatic nerve compound action potential and reduced the contracture responses produced by acetylcholine (ACh) or tetraethylammonium (TEA) in the chick skeletal muscle. In all these actions, the effect of SP was calcium-dependent. The results provide evidence that SP had a dual action; a prejunctional facilitatory effect by causing a further release of ACh and/or Ca2+ entry or release, and postjunctional effect by reducing the sensitivity of the postjunctional membrane to depolarizing drugs.

Hyperpolarization produced by methohexitone in the chick biventer cervicis skeletal muscle

The effect of methohexitone on endplate potential and on depolarizations produced by acetylcholine (ACh) and tetraethylammonium (TEA) was studied in the isolated chick biventer cervicis skeletal muscle using a sucrose-gap recording technique. Methohexitone (5.5-180 uM) produced concentration-dependent hyperpolarization in the chick skeletal muscle without producing an initial contracture. Methohexitone had a differential effect on the depolarizations produced by ACh (5.5 uM- 11 mM) and TEA (0.95- 48 mM); it greatly reduced the ACh whereas it increased the TEA-induced responses. It was suggested that methohexitone had a dual postjunctional action, but a presynaptic effect was not ruled out.

Effect of lignocaine on the responses produced by depolarizing agents at the chick neuromuscular junction

The effect of lignocaine on the depolarization and contracture responses produced by acetylcholine (ACh) or tetraethylammonium (TEA) was studied in the isolated chick biventer cervicis (BVC) nerve-muscle preparation using the moving fluid electrode technique. Lignocaine (37 mumol X 1-1-1.8 mmol X 1-1) produced concentration-dependent contractures in the chick BVC muscle. The contractures produced by lignocaine were not accompanied by membrane depolarizations. ACh (5.5 mumol X 1-1-11.0 mmol X 1-1) and TEA (0.48-24.0 mmol X 1-1) produced concentration-dependent depolarizations and contractures in the chick BVC muscle. Lignocaine (0.93 mmol X 1-1) greatly reduced the responses produced by ACh, while it markedly increased the contracture responses produced by TEA. The depolarizations produced by TEA were reduced in lignocaine (0.93 mmol X 1-1). It was concluded that the local anaesthetic lignocaine produces contractures in the chick BVC muscle. Lignocaine potentiates the contractures produced by TEA, while it greatly reduces those produced by ACh.

The effects and interactions of caffeine, lignocaine and carbachol at the chick neuromuscular junction

The effects and interactions of lignocaine, carbachol and caffeine on the contractile response and endplate potential were investigated in the isolated chick biventer cervicis nerve-muscle preparation using a moving fluid electrode technique. Lignocaine (37 microM-1.8 mM) produced a dose-dependent contracture response that was not related to an endplate depolarization and appeared to inhibit non-competitively the usual depolarization and contractile response to carbachol (5.5 microM-0.8 mM). In contrast, lignocaine enhanced the contractile response to caffeine (0.54-10.8 mM) possibly by lowering the threshold and mainly by increasing the maximal response. The peak effects occurred at the same caffeine concentration regardless of the presence of lignocaine. The possibility that both lignocaine and caffeine may produce contractures in the chick muscle by acting on intracellular calcium concentration is discussed.

The neuromuscular effects of lignocaine

The local anesthetic drug lignocaine, in concentrations which completely block both nerve and muscle action potentials, produces concentration-dependent contractures in the vertebrate skeletal and smooth muscle fibres. The contractures produced by lignocaine were not related to membrane depolarization. Although lignocaine greatly reduced the depolarizations and contractures produced by depolarizing agents, such ACh or carbachol, it had a differential effect on the responses produced by tetraethylammonium (TEA) at the chick neuromuscular junction. Lignocaine reduced the TEA-induced depolarizations while it markedly enhanced the contracture responses. In the absence of membrane depolarization, the contractures produced by lignocaine may be attributed to an effect on release of intracellular calcium (Ca2+) from the sarcoplasmic reticulum (SR). The results suggested a multiple site of action of lignocaine at the vertebrate neuromuscular junction.

Dissociation of K+-induced tension and cellular Ca2+ retention in vascular and intestinal smooth muscle in normoxia and hypoxia

In experiments on smooth muscle preparations of rabbit aorta and guinea pig taenia coli, replacement of the external Na+ with K+ produced sustained contraction. When external K+ concentration was increased, cellular Ca2+ retention as measured by a modified lanthanum technique increased. However, when K+ concentration was above 80 mM, the tension decreased despite an increase in Ca2+ retention. Maximum amount of Ca2+ retained was 1280 nmol/g in aorta and 980 nmol/g in taenia coli while the control values for both tissues were approximately 430 nmol/g when the external Ca2+ concentration was 2.5 mM. Under hypoxia (N2 aeration), sustained contraction was induced by 80 mM K+ in aorta and by 45.4 mM K+ (and 55 mM glucose) in taenia coli. However, no increase in the cellular Ca2+ retention was observed under these conditions. During the K+-induced sustained contraction in aorta, introduction of N2 transiently increased, while readmission of O2 transiently decreased the muscle tension. In taenia coli, the introduction of N2 decreased the sustained contractile tension probably because of an ATP deficiency, while the readmission of O2 further decreased the tension transiently. From these results, it is concluded that, in the presence of a high concentration of K+, external Ca2+ enters the cell and activates the contractile machinery. A part of the cellular Ca2+ is taken up by mitochondria under normoxic but not under hypoxic conditions.

Involvement of a Na-Ca exchange mechanism in contraction induced by low-Na solution in isolated guinea-pig aorta

The possibility of involvement of a Na-Ca exchange mechanism in the contractile responses induced by a reduction of external Na concentration ([Na]0) has been studied in isolated guinea-pig aorta. Low-Na (11.9 mM) solution (Li-substituted) produced a contraction in ouabain-treated muscles in the presence of phentolamine (10(-6) M). The magnitude of the contraction was dependent on the duration of the pretreatment with ouabain (2 x 10(-5) M). Ca-free solution, but not verapamil (10(-6) M), abolished the contraction induced by low-Na solution. The muscles were loaded with various amounts of Na by incubating the tissue with ouabain and varying [Na]0 (11.9--148.7 mM) in the absence of Ca. The magnitude of the contractions induced in these muscles by low-Na solution containing Ca (2.25 mM) was dependent on the cellular Na content. Loss of cellular Na into low-Na solution followed a single exponential time course and the rate coefficient of Na-loss in the presence of external Ca was about twice as great as in the absence of Ca. Cellular 45Ca uptake in low-Na solution was significantly greater in Na-loaded tissues (pretreated with ouabain for 3 h) than in normal tissues. The 45Ca uptake in low-Na solution was not inhibited by verapamil. These results suggest that the contraction induced by low-Na solution is caused by a Ca influx which is dependent on internal Na (a Na-Ca exchange mechanism).

4-Aminopyridine does not increase m.e.p.p. frequencies at junctions depolarized by potassium

4-aminopyridine (4-AP) is known to produce large increases in quantal acetylcholine release from stimulated motor nerve terminals. It has been suggested that the drug might act directly on Ca2+ channels to increase Ca2+ influx. This possibility was tested at frog neuromuscular junctions depolarized in elevated [K+]out. The 4-AP did not increase miniature end-plate potential frequencies. Also, 4-AP did not alter the increase in frequency that follows a rise in [Ca2+]out at a depolarized junction. Therefore, under these conditions, 4-AP does not appear to change Ca2+ entry into or elimination from the nerve terminal. The results support the hypothesis that 4-AP acts by lengthening the nerve terminal action potential.

Effects of tetraethylammonium chloride on contractile, membrane and cable properties of rabbit artery muscle

1. Two types of effects of tetraethylammonium chloride (TEA) have been found in the smooth muscle cells of the rabbit main pulmonary artery. (a) With rapid onset of action TEA depolarizes the cell membrane, increases the membrane resistance, causes anomalous rectification and occasionally spike potentials in response to externally applied depolarizing current pulses and produces tonic contractions. (b) During prolonged (greater than 30 min) incubation in TEA phasic contractions develop progressively and the vascular strips respond to electrical stimulation with synchronized and powerful contractions. 2. There is a linear relationship between log concentration TEA and depolarization over the range of 10-100 mM-TEA. TEA (10 and 30 mM) raises the membrane resistance and decreases the core resistance. The latter effect appeared to develop more slowly than the former. 3. During short exposure to TEA part of the smooth muscle cells respond to depolarizing current pulses with spike potentials of variable amplitude and duration. These spikes are very sensitive to inhibition by verapamil or nickel chloride but are not affected by tetrodotoxin. The amplitude of electrotonic potentials, increased by TEA, is slightly further elevated by verapamil or nickle chloride. 4. TEA (10 mM) increases the mechanical response to low and intermediate potassium concentrations but has no effect on maximal contractions to high potassium. The slope of the line relating log potassium concentration to membrane potential is decreased by TEA. 5. TEA (10 mM) shifts the concentration response curve for the contractile effect of noradrenaline to the left and increases the maximum of noradrenaline-induced contractions. In the presence of TEA, noradrenaline reduces the membrane potential to markedly lower values than under control conditions. 6. It is concluded that the rapidly occurring effects of TEA on the vascular smooth muscle cells of the rabbit main pulmonary artery are a decrease in potassium and an increase in calcium conductance. The latter effect may be related to a blockade of potassium channels; however, we cannot rule out the possibility that TEA affects calcium conductance independent of its presumed action on potassium channels. The slowly developing effects of TEA may be ascribed to the formation of gap junctions and/or (less likely) to an intracellular accumulation of TEA.