Action of the Na ionophore monensin on vascular smooth muscle of guinea-pig aorta

The effects of the Na⁺ ionophore monensin on contractile responses were investigated in guinea-pig aorta in normal and high K⁺ solutions. In normal K⁺ (5.4 mM) solution, monensin (2 × 10⁻⁵ M) produced a rapid increase in tension followed by slow relaxation. This contraction was markedly inhibited by phentolamine (10⁻⁵ M) or prazosin (10⁻⁶ M) and was accompanied by an increase in tritium efflux from tissue preloaded with [³H]norepinephrine. In the presence of phentolamine, monensin (1–2 × 10⁻⁵ M) or ouabain (1−2 × 10⁻⁵ M) caused only a small and slowly developing contraction. Simultaneous application of these agents caused a more rapid and greater contraction. Either monensin or ouabain gradually increased cellular Na⁺ and decreased cellular K⁺ content. When monensin was applied simultaneously with ouabain, there was a rapid increase in cellular Na⁺ and loss of cellular K⁺. In high K⁺ (65.4 mM) solution, monensin (10⁻⁶ M) slightly reduced the increased tension level but when external glucose was omitted monensin markedly inhibited the contraction. A significant decrease in tissue ATP content was observed only when monensin was applied in glucose-free solution. Similarly, hypoxia (N₂ bubbling) markedly inhibited the high K⁺ contraction and decreased the tissue ATP content only in the absence of glucose. These results suggest that monensin produces a neurogenic contraction due to the release of endogenous catecholamines and also produces a myogenic contraction by a decrease in transmembrane Na⁺ and K⁺ gradients when the Na⁺ and -K⁺ pump is inhibited by ouabain, and that monensin inhibits aerobic energy metabolism of vascular smooth muscle.     

Heterogeneity of the effects of monensin on two types of contractions evoked by inhibition of Na+-K+ ATPase in the guinea-pig vas deferens

The interactions between ouabain or K+-free medium and monensin on noradrenaline (NA) release and Ca2+-evoked contractions were assessed in guinea-pig vas deferens. Ouabain (10 microM) produced large, sustained contractions and monensin (1 microM) small, transient contractions. The ouabain-evoked contraction tended to be potentiated in the presence of monensin. The marked contraction induced by ouabain plus monensin was nearly abolished by phentolamine or by treatment with 6-hydroxydopamine. Ouabain caused the release of NA from the tissue, as determined in an HPCL-ECD study. This ouabain-evoked release of NA was enhanced by the simultaneous administration of monensin. High Ca2+ (10 mM) per se, ouabain, monensin or K+-free medium did not elicit contraction of the tissue in the presence of phentolamine. However, 10 mM Ca2+ caused a large contraction 45-65 min after exposure to ouabain or K+-free medium and this contraction was suppressed by monensin. The contraction evoked by Ca2+ in the presence of ouabain was further inhibited by amiloride, an inhibitor of Na+-Ca2+ or Na+-H+ exchange transport, but not by nifedipine, a voltage-dependent Ca2+ antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition by ouabain and veratridine of acetylcholine-evoked phasic contraction in the guinea-pig taenia coli

Summary Inhibitory action of ouabain and veratridine on acetylcholine (ACh)-evoked phasic contraction was examined in the guinea-pig taenia coli. ACh (5×10^–4 m) produced a biphasic (phasic and tonic) contraction. As the phasic contraction, but not the tonic contraction, was resistant to gallopamil (D600, 2 × 10^–7 m), a blocker of voltage-dependent Ca²⁺ channels, all experiments were carried out in the presence of gallopamil at this concentration. Addition of high Ca²⁺ (30 mm) to the solution containing ACh induced a sustained contraction, which increased in the presence of ouabain (10^–5 m), an inhibitor of the Na+-K+ exchange system. On the other hand, the ACh-evoked phasic contraction was suppressed by ouabain 10^–7–(10^–5 m) in a time- and dose-dependent manner. The suppression by ouabain (10^–6 m) of the phasic contraction was transiently potentiated by the addition of veratridine (10^–6 m), an activator of Na⁺ channel. In contrast, the greater suppression by ouabain (10^–5 m) of the contraction was antagonized by amiloride (10^–4 m), a blocker of Na+ channel. This antagonism by amiloride was transiently inhibited in the presence of veratridine. In the absence of ouabain, the amplitude of the phasic contraction was transiently reduced by adding veratridine but was increased by amiloride. In addition, the phasic contraction by ACh increased 80 min after exposure to Na⁺-free isotonic high-K⁺ solution (K⁺, 143 mm), which elicited greater depolarization of the cell membrane. In a fluorescence study with Fura-2, an intracellular free-Ca²⁺ indicator, ACh increased the fluorescence intensity from the tissue by excitative light at 340 nm, which coupled with the phasic contraction. Both the increased fluorescence and tension development were reduced by ouabain. These findings suggest that the ACh-evoked phasic contraction is induced by an increase in intracellular free-Ca²⁺ levels and seems to be inhibited by an elevation of intracellular free-Na⁺ levels. Send offprint requests to: S. Usune at the above address     

Effect of sodium on transmembrane calcium movement in the cat ileal longitudinal muscle

To get a better insight into the existence and the role of a Na?Ca exchange mechanisms in smooth muscle, the effect of Na substitution with sucrose on tension development, cellular Ca uptake and⁴⁵Ca efflux was investigated using isolated cat ileal longtitudinal muscle strips. Experimental results were summarized as follows; 1) Exposure of the cat ileal longitudinal muscle to Na-free solution induced a contraction, and the magnitude of the contraction increased after incubation of the muscle strips with ouabain (2×10^?5 M) for 1 hr. 2) Cellular Ca uptake in Na-free solution increasedin Na content of the Na-loading media, and a linear relationship existed between tissue Na content and cellular Ca uptake for 10 min. 3) After tissues were equilibrated in PSS containing⁴⁵Ca for 2hr, cellular Ca uptake decreased with raising the external Na concentration. 4) Removal of medium Na or inhibition of the Na?K pump decreased the rate of⁴⁵Ca efflux. These results strongly suggested that Na substitution increases cellular Ca uptake and decreases the rate of⁴⁵Ca efflux via a Na?Ca exchange mechanism.     

Mechanism of inhibition of catecholamine release from adrenal medulla by diphenylhydantoin and by low concentration of ouabain (10−10 M)

Summary Catecholamine (CA) release was studied in rat adrenal incubated in vitro. Inhibition of (Na +K)-ATPase either by omission of K⁺ from the incubation medium or by addition of a high concentration of ouabain (10^–3 M) caused increased release of CA from the adrenal. Diphenylhydantoin (DPH) (10^–5 M) inhibited the spontaneous as well as the acetylcholine (10^–4 M)-induced release of CA. However, in K⁺-free medium or in the presence of 10^–3 M ouabain, DPH had no significant effect on CA release. A low concentration of ouabain (10^–10 M) caused a significant inhibition of spontaneous and of acetylcholine-induced release of CA. In a K⁺-free medium ouabain (10^–10 M) had no effect on CA release. DPH (10^–5 M) and a low concentration of ouabain (10^–10 M) caused a significant activation of (Na+K)-ATPase in a membrane fraction of the adrenal medulla. It is suggested that DPH and low ouabain concentrations inhibit CA release from the adrenal by activation of the sodium pump. The possible mechanism involved is discussed.     

The inhibitory effect of X537A on vascular and intestinal smooth muscle contraction

Summary the effects of X537A (Lasalocid) on contractions induced in vascular and intestinal smooth muscles were examined. High K-induced sustained contractions were inhibited by X537A with an IC50 of 2.8·10^–6 M in rabbit aorta and 8.8·10^–8 M in guinea-pig Taenia coli. Changing the Ca concentration in the medium did not modify the effect of X537A. X537A also inhibited the noradrenaline-induced contraction in aorta (IC50=7.0·10^–8 M). In a solution without added glucose, the inhibitory effect of X537A on the K-induced contraction in aorta was augmented (IC50=1.5·10^–8 M). Under hypoxic conditions, the inhibitory effects of X537A on the noradrenaline-induced contractions in aorta and on the K-induced contraction in T. coli were decreased (IC50>10^–5 M and 3.2·10^–6 M, respectively). X537A inhibited the K-induced increase in cellular ⁴⁵Ca content in aorta measured by a modified lanthanum method. However, the IC50 for ⁴⁵Ca uptake (6.6·10^–7 M) was lower than that for K-induced contraction. In both tissues, X537A decreased the ATP content. Oxygen consumption of rat liver mitochondria was inhibited by X537A. From these results, it is concluded that the inhibitory effect of X537A on the noradrenaline-induced contraction in aorta and the K-induced contraction in T. coli, but not the K-induced contractions in aorta, may be due to the inhibition of aerobic metabolism.     

Effects of vanadate on mechanical responses and Na-K pump in vascular smooth muscle

The effects of vanadate on tension and on the Na-K pump in isolated guinea-pig aorta were investigated. Vanadates (NH₄VO₃ or Na₃VO₄ · nH₂O) (10^?3 M) produced a sustained contraction (about 0.5 g) which was not influenced by phentolamine (10^?6 M). In the absence of external Ca, vanadate and norepinephrine (2 × 10^?6 M) induced a small contraction, although high K (45.4 mM) did not. In a Ca-depleted, high K (142.2 mM) solution, vanadate and norepinephrine still caused muscle contraction. D600 (10^?6 M) slightly inhibited the contractions induced by vanadate and norepinephrine, while this agent completely inhibited the contraction induced by high K. Sodium nitroprusside (10^?5 M) strongly inhibited the contractions induced by vanadate and norepinephrine but not the contraction induced by high K. Vanadate produced a contraction in K-free solution with ouabain (10^?4 M). The tissue K content did not change during a 2 h treatment of the muscle with vanadate. Reaccumulation of K following a 3 h treatment of the muscle with K-free solution was inhibited by ouabain but not by vanadate. These results indicate a similarity between the contractions induced by vanadate and by norepinephrine and suggest that the vanadate-induced contraction is not due to an inhibition of the Na-K pump.     

Effects of ouabain on human bronchial muscle in vitro

The effects of ouabain, an inhibitor of the plasmalemmal Na⁺/K⁺-ATPase activity, were examined in human isolated bronchus. Ouabain produced concentration-dependent contraction with –logEC₅₀=7.16±0.11 and maximal effect of 67±4% of the response to acetylcholine (1 mM). Ouabain (10 M)-induced contraction was epithelium-independent and was not depressed by inhibitors of cyclooxygenase and lipoxygenase, antagonists of muscarinic, histamine H₁-receptors and -adrenoceptors, or neuronal Na⁺ channel blockade. The inhibition of ouabain contraction in tissues bathed in K⁺-free medium, and the inhibition by ouabain of the K⁺-induced relaxation confirm that the contractile action of ouabain is mediated by inhibition of Na⁺/K⁺-ATPase. Furthermore, depolarization (16.4±0.9 mV) was observed in human isolated bronchus by intracellular microelectrode recording. Ouabain (10 M)-induced contractions were abolished by a Ca²⁺-free solution but not by blockers of L-type Ca²⁺ channels. In human cultured bronchial smooth muscle cells, ouabain (10 M) produced a sustained increase in [Ca²⁺]_i (116±26 nM) abolished in Ca²⁺-free medium. Incubation with a Na⁺-free medium or amiloride (0.1 mM) markedly inhibited the spasmogenic effect of ouabain thus suggesting the role of Na⁺/Ca²⁺ exchange in ouabain contraction while selective inhibitors of Na⁺/H⁺-antiport, Na⁺/K⁺/Cl^–-antiport, or protein kinase C had no effect. Ouabain (10 M) failed to increase inositol phosphate accumulation in human bronchus. Ouabain (10 M) did not alter bronchial responsiveness to acetylcholine or histamine but inhibited the relaxant effects of isoprenaline, forskolin, levcromakalim, or sodium nitroprusside. These results indicate that ouabain acts directly to produce contraction of human airway smooth muscle that depends on extracellular Ca²⁺ entry unrelated to L-type channels and involving the Na⁺/Ca²⁺-antiporter.     

Early and late contraction induced by ouabain in human umbilical arteries.

1. Ouabain (3 x 10(-7)-10(-4) M) evoked a biphasic contraction in human umbilical artery that consisted of an early and a late contraction. The Ca(2+)-antagonists, verapamil (10(-7)-10(-5)) M), diltiazem (10(-7)-(10(-5)) M) and nifedipine (10(-9)-10(-7)) M) inhibited the early but not the late contraction. Caffeine (5 mM) changed neither the magnitude of the peak of the biphasic contraction nor the time needed to reach it. 2. Sodium concentration reduction (140 to 0 mM, replaced by N-methyl-D-glucamine, NMG) produced dose-dependent contraction of the arterial strip in 2.5 mM Ca2+ solution after the first treatment with verapamil (10(-5) M) and caffeine (5 mM), but not in Ca(2+)-free solution. 3. After prior treatment with verapamil and caffeine, the amplitude of the ouabain (10(-4) M)-induced late contraction varied, depending on the concentration of Ca2+ (0-2.5 mM) in the medium. 4. Amiloride (5 x 10(-5) M-5 x 10(-4) M), an inhibitor of the Na(+)-Ca2+ exchange system, produced dose-dependent inhibition of the late contraction induced by ouabain (10(-5) M) after prior treatment with verapamil and caffeine. 5. The time needed to reach the peak tension induced by monensin (5 x 10(-7) M) together with ouabain (10(-6) M) was less than that with ouabain alone, but the magnitude of the peak tension was not changed. 6. These results suggest that the early and late contractions caused by ouabain respectively produce a Ca2+ influx through voltage-sensitive Ca2+ channels and Ca2+ entry through sodium-calcium (Na(+)-Ca2+) exchange.     

Effects of ouabain and potassium-free solution on the contraction of isolated blood vessels

Ouabain and K-free solution affected the tension development of isolated vessels as follows. In rabbit aorta, ouabain induced a transient contraction followed by a delayed sustained contraction whereas K-free solution was almost ineffective; both ouabain and K-free solution produced a sustained contraction in guinea pig aorta; rat aorta rarely responded to ouabain but developed a contraction in K-free solution. Phentolamine inhibited the transient contraction produced by ouabain in rabbit aorta. In rabbit aorta, , ouabain produced a concentration-dependent potentiation of norepinephrine contraction. In denervated rabbit aorta, however, the potentiation only occurred in the presence of 5 × 10^?7 to 5 × 10^?6 M ouabain. K-free solution inhibited the norepinephrine contraction in denervated rabbit aorta. Ouabain potentiated the norepinephrine contraction in guinea pig aorta but not in rat aorta. It is concluded that ouabain and K-free solution affect contraction both indirectly via the release of endogenous catecholamines and directly by acting on vascular smooth muscle, and that there are species differences in the sensitivity to ouabain.     

Influence of calcium on the effects of okadaic acid and its interaction with caffeine and theophylline in rat myometrium

The effects of okadaic acid (OA), a monocarboxylic acid produced by marine dinoflagellates belonging to the genera Dinophysis and Prorocentrum, and their interactions with theophylline and caffeine were studied on the rat-isolated uterus in a calcium-containing medium and a calcium-free medium in the presence of 10^–3 M EGTA.Okadaic acid (5 × 10^–6 to 5 × 10^–5 M) induced a concentration-dependent contraction of the rat-isolated uterus corresponding, with 5 × 10^–5M, to 142.3±6.1% (n = 7) of the contraction induced by oxytocin 10^–6 M. The time to peak tension was inversely proportional to the maximum effect produced. The contraction was not sustained and was followed by a concentration-dependent decrease in tone. In a Ca²⁺-free medium containing 10^–3 M EGTA the contractile effects of OA were significantly inhibited or reduced. A 30 min pretreatment with theophylline (3 × 10^–3 M) or caffeine (2 × 10^–2 M) significantly reduced, in a Ca²⁺-containing medium, the maximum contractile effect of OA 10^–5 and/or 2 × 10^–5 M and shortened the relative time to peak tension. In a Ca²⁺-free medium containing 10^–3 M EGTA, only the second effect was observed. With a 1 min pretreatment and in a Ca²⁺-containing medium, theophylline 3 × 10^–3 M and caffeine 10^–2 M did not modify the maximum effect of OA 10^–5 M but shortened the time to peak tension. The same concentrations of the xanthines potentiated the E_max of OA 5 × 10^–6 M in the Ca²⁺-containing medium or in a Ca²⁺-free medium containing 10^–3 M EGTA. Okadaic acid 10^–6 M used as 30 min pretreatment versus OA 10^–5 M and 2 × 10^–5 M behaved like caffeine or theophylline.These results suggest that the OA-induced contraction of the rat uterine smooth muscle is partly effected by transmembrane calcium movements which can be inhibited in an O-Ca²⁺–10^–3 M EGTA solution or by theophylline or caffeine. This contraction also involves mobilization of Ca²⁺ from an intracellular pool which is also xanthine-sensitive. The latter effect seems to be important in inducing the contractile effect. This study does not exclude the possibility of other mechanisms being involved in the contraction induced by OA. Correspondence to: M. L. Candenas at the above address in Burjassot-València     

Effects of the potassium channel openers KRN4884 and levcromakalim on the contraction of rat aorta induced by A23187, compared with nifedipine

We examined the different vasodilatory effects of the K⁺ channel openers levcromakalim and 5-amino-N2-[2-(2-chlorophenyl)ethyl]-N-cyano-3-pyridinecarboxamidine (KRN4884), and the Ca²⁺ channel blocker nifedipine in the rat aorta. KRN4884 (10^–10-10^–5 M) and nifedipine (10^–10–10^–5 M) produced concentration-dependent relaxation in the rat aorta precontracted by 25 mM KCl. The K⁺ channel blocker glibenclamide (1 M) inhibited the relaxation induced by KRN4884 but did not influence nifedipine-induced relaxation. KRN 4884 had almost no effect on contraction induced by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl, whereas nifedipine completely relaxed the muscle precontracted by 80 mM KCl. These results indicate that KRN4884 is a K+ channel opener. We investigated the relaxant effects of KRN4884 (10^–10-10^–5 M), levcromakalim (10^–9-10^–5 M) and nifedipine (10^–9-10^–5 M) on A23187 (1 M)-induced contraction. KRN4884 and levcromakalim had a potent relaxant effect but nifedipine only a weak effect on the smooth muscle contracted by A23187. Glibenclamide (1 M) inhibited the relaxation induced by KRN4884 and levcromakalim, but did not influence the nifedipine-induced relaxation. KRN 4884 (1 M) produced a larger relaxation of A23187-induced contraction but had little effect on the increase in intracellular [Ca²⁺] induced by A23187. These results suggest that KRN4884 is a specific K⁺ channel opener and its vasodilating mechanisms involve not only deactivation of Ca²⁺ channels but also a decrease in the Ca²⁺ sensitivity of contractile elements.     

The influence of ouabain on twitch contractions in the presence of veratridine

1. Direct stimulation evoked twitches in mouse diaphragm muscles in presence of 10 microM D-tubocurarine in vitro. Effects of ouabain and their dependence on K+ were examined on the twitch responses and action potentials in the presence and absence of twitch potentiators. 2. Ouabain inhibited twitch contractions only in the presence of veratridine, aconitine and monensin while it had no inhibitory effect on control twitches. The interactions between ouabain and these twitch potentiators depended on the presence of external K+, except in the case of monensin. 3. Removal of Ca2+ from a bathing solution accelerated the potentiating effect of veratridine and the antagonizing effect of ouabain. 4. Caffeine further potentiated the twitches which had been attenuated by ouabain combined with veratridine. 5. Ouabain combined with veratridine consistently decreased resting membrane potentials, action potentials and overshoot potentials and prolonged time to peak of and duration of the muscle action potentials. 6. Tetraethylammonium, 4-aminopyridine, and caffeine produced twitch potentiation which was insensitive to ouabain or the removal of K+. 7. These results suggest that twitch contractions in the presence of activators of sodium channels link with activation of Na+-K+-ATPase. Accumulation of Na+ inside the muscle fibres may uncouple the excitation-contraction system. 8. This uncoupling may not include the caffeine-sensitive process that controls the release of Ca2+ from the sarcoplasmic reticulum. Na+ accumulation may decrease transmembraneous gradient of this ions, thereby causing a reduction in excitation coupled with twitch contraction.     

Die Hemmung des Kalium-und Natrium-Transportes am Erythrocyten durch Glykoside mit gesättigtem Lactonring

Summary Digitoxin, ouabain, digoxin and their derivatives dihydrodigitoxin, dihydro-ouabain, and dihydrodigoxin were investigated in regard to their effects on the Na+ and K+ transport in incubated cold stored guinea-pig erythrocytes.The ion transport is inhibited by 50% in the following concentrations of the different glycosides: digitoxin 3.0 × 10^–7 M, ouabain 1.0 × 10^–6 M, digoxin 1.2 × 10^–6 M.The saturation of the lactone ring decreases the potency of all three glycosides by the same order of magnitude. The potencies of the dihydro-derivatives in relation to their parent compounds are: dihydro-digitoxin 1/22, dihydro-ouabain 1/34, and dihydro-digoxin 1/25.

Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

Inhibitory effect of beauvericin on a high K+-induced tonic contraction in guinea-pig taenia coli

An inhibitory effect of beauvericin, a cyclodepsipeptide, on a high K+-induced contraction in guinea-pig taenia coli was compared with those of verapamil, an organic Ca2+ antagonist, and monensin, an inhibitor of mitochondrial respiration. Beauvericin (10(-5) M), verapamil (5 x 10(-7) M) or monensin (10(-6) M) markedly inhibited the tonic contraction, while these drugs showed less effect on the phasic contraction. Beauvericin at a lower concentration (10(-6) M) competitively inhibited the Ca2+-induced contraction in depolarized muscle, whereas higher concentrations (3 x 10(-6) or 10(-5) M) non-competitively inhibited this contraction. Verapamil (10(-8)-5 x 10(-7) M) competitively inhibited and monensin at a low concentration (10(-7) M) non-competitively inhibited this contraction. A contraction induced by 0.5 mM Ca2+ was inhibited by beauvericin with an IC50 (concentration needed for 50% inhibition) of 2.8 x 10(-7) M, verapamil with an IC50 of 2.9 x 10(-8) M, and nifedipine with an IC50 of 1.8 x 10(-9) M. 10(-6) M CGP 28392, a Ca2+ channel facilitator, increased the IC50 of beauvericin, verapamil or nifedipine. Although the inhibitory effect of monensin (10(-7)-10(-6) M) on the high K+-induced contraction was reduced under hypoxia, the effects of beauvericin (10(-7)-10(-5) M) and verapamil (10(-8)-10(-7) M) were not modified. Beauvericin (10(-5) M) changed neither the intracellular Na+ and K+ contents of the depolarized muscle nor the Ca2+-induced contraction in the chemically skinned taenia coli.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of phthalazinol on smooth muscle cells of the porcine coronary artery and on neuromuscular junction on the guinea-pig vas deferens

Summary In smooth muscle cells of the porcine coronary artery, phthalazinol (10^–5 M) did not modify the membrane potential and the membrane resistance. At a concentration of 10^–4 M or higher, it hyperpolarized the membrane, reduced the membrane resistance and enhanced the rectifying property of the membrane. At the concentration of 10^–5 M, phthalazinol raised the threshold for the induction of a contraction and suppressed nonselectively the amplitude of the contraction evoked by application of high [K]₀, acetylcholine or electrical depolarization of the membrane. Phthalazinol (10^–5–10^–4 M) did not modify the membrane properties of smooth muscle cells from the guinea-pig vas deferens. However, it suppressed the amplitude of the excitatory junction potentials and the facilitation phenomenon produced by repetitive stimulation at various rates. The action potential recorded from the adrenergic nerves was not affected in the presence of phthalazinol (10^–5 and 10^–4 M). The mean amplitude of the miniature excitatory junction potentials (m.e.j.p.s.) was not affected by treatment with phthalazinol (10^–5–10^–4 M), but the rate of which of m.e.j.p.s. appeared was reduced by phthalazinol (>5×10^–5 M). These results indicate that the vasodilator property of phthalazinol may result from a suppression of Ca-mobilization in both the smooth muscle cells and the adrenergic nerve terminals. The former affects the mechanical response directly and the latter leads to an inhibition of noradrenaline release.     

Properties of contractions induced by ouabain and K+-free solution in guinea pig taenia coli

The properties of contractions induced by ouabain and by K depletion were studied in comparison with those induced by other stimulants in guinea pig taenia coli. Ouabain 7.5 × 1o^?6 M and K depletion produced biphasic contractions with similar time courses. Both contractions were slightly inhibited by low (0.25 mM) and high (7.5 mM) Ca concentrations, while the high K contraction was strongly inhibited by low Ca and potentiated by high Ca. Contractions induced by ouabain and K depletion were highly sensitive to lowering of the temperature and abolished at 15°C, although contractions induced by carbachol 1.1 × 10^?7 M and histamine 5 × 10^?7 M were still observed below 15°C. In Cl deficient solution substituted by SO₄ or propionate, contractions induced by ouabain and K depletion were potentiated, while contractions induced by high K, carbachol and histamine were suppressed. These results indicate a striking similarity between contractions induced by ouabain and K depletion suggesting a common mechanism for both contractions.     

Effect of calcium antagonists (ω-conotoxin GVIA,verapamil, gallopamil,diltiazem) on bronchial smooth muscle contractions induced by soman

Summary The effect of the calcium antagonists -conotoxin GVIA, verapamil, gallopamil and diltiazem was investigated on in vitro bronchial smooth muscle contraction in the rat induced by the nerve agent soman. Soman inhibits the acetylcholinesterase activity irreversibly. The effect of the calcium channel antagonists on contractions induced by electrical field stimulation and carbachol was also investigated, in order to elucidate the mechanism by which calcium antagonists inhibit the soman induced contraction.-Conotoxin GVIA reduced the bronchial smooth muscle contraction induced by electrical field stimulation with an almost complete inhibition at approximately 1.0× 10^–6 M. The soman induced contraction was only inhibited by 15% at a concentration of 3.0× 10^–6 M -conotoxin GVIA. The organic calcium antagonists verapamil, gallopamil and diltiazem reduced both electrically and soman induced smooth muscle contraction. Complete inhibition of the contractions induced by soman was achieved at 1.4x 10^–4 M for verapamil and gallopamil, while diltiazem inhibited the contraction to 7% of control at 1.4 × 10^–4 M. Verapamil, gallopamil and diltiazem increased the EC₅₀ for carbachol significantly, while co-conotoxin GVIA had no effect. None of the calcium antagonists had any effect on the maximal contraction induced by carbachol. Verapamil, gallopamil and diltiazem blocked, however, sub-maximal contractions induced by carbachol (10^–7 -10^–5 M) resulting in a right-shift of the dose response curve.The results show that co-conotoxin GVIA inhibits the calcium-dependent release of acetylcholine which causes contraction of airway smooth muscle, while it has no effect on smooth muscle contraction induced by soman. Gallopamil, verapamil and diltiazem inhibit the contraction of bronchial smooth muscle following stimulation of postjunctional muscarinic receptors resulting from inhibition of the acetylcholinesterase activity by soman. Gallopamil and verapamil inhibit the contraction more potently than diltiazem. Correspondence to P. Aas at the above address     

Effect of sea nettle (Chrysaora quinquecirrha) venom on isolated rat aorta

Wan Wan Lin, C. Y. Lee and J. W. Burnett. Effect of sea nettle (Chrysaora quinquecirrha) venom on isolated rat aorta. Toxicon26, 1209–1212, 1988.—The venom from sea nettle (Chrysaora quinquecirrha) (1–10μg/ml) produced an irreversible contraction of the isolated rat aortic ring that was slow in onset, increased with time, and reached maximum in about 10–20 min. The contraction was not inhibited by pretreatment with phenoxybenzamine, atropine, indomethacin, tetrodotoxin, ouabain, low Na⁺ or Na⁺-free medium, however, it was markedly decreased by the Ca²⁺ channel blockers, nifedipine and verapamil. In Ca²⁺-free medium, no increase in tension was produced by the venom. It is concluded that sea nettle venom causes a contraction of the rat aortic ring by increasing Ca²⁺ influx through the voltage-dependent Ca²⁺ channels.     

Species differences in the inhibitory effect of ouabain on high K-induced contraction in the ileal longitudinal muscle

Effects of ouabain on a high K-induced contraction and intracellular Na and K contents of the ileal longitudinal muscle preparations isolated from various animal species was compared. In muscles of various animals (monkey, dog, cat, rabbit, guinea-pig, rat and mouse), the high K-induced contraction was inhibited by verapamil (5 X 10(-8) M) or ouabain (3 X 10(-6) M), and both the inhibitions were antagonized by an increase in external Ca in a competitive manner. Species differences were shown in the ouabain effect but not in the verapamil one. Regarding the sensitivity to ouabain, the muscles were divided into two groups, that is, the muscle from monkey, rabbit, guinea-pig or dog belongs to the high sensitivity group, and that from cat, rat or mouse belongs to the low one. The order of sensitivities of the muscles to ouabain in the relaxation was consistent with that in Na,K-ATPase inhibition, as reported by Repke et al. (1965). Intracellular Na contents of muscles were increased by an addition of ouabain to the high K solution, and the rate and amount of the accumulation of intracellular Na varied in these muscles. Except for the cat muscle, a high correlation was noted between the ouabain-induced relaxation and the amount of intracellular Na accumulation. However, the regression coefficients were variable: 4.1 in the monkey muscle, 2.2 in the rat one, and about 1.0 in the others. That is, the monkey muscle showing the high regression coefficient value probably has a high sensitivity to the inhibition of intracellular Na in the high K-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)