Changes in cytosolic CA2+ and contraction induced by various stimulants and relaxants in canine tracheal smooth muscle

Imidazole-induced contractions in bovine tracheal smooth muscle are not dependent on the cAMP pathway

The mechanism of imidazole-induced contraction on the bovine tracheal smooth muscle was investigated. Imidazole induced muscle contraction in a concentration-dependent manner on bovine, porcine and guinea-pig tracheas, but not in rat or mouse. In bovine tracheas, imidazole was cumulatively applied and induced muscle tension and increasesd intracellular Ca²⁺ level in a concentration -dependent manner. Imidazole, even at 300 µM, the concentration at which maximum contractile response occurs, did not significantly increase in cAMP content relative to control. Atropine inhibited imidazole-induced contraction at a concentration- dependent manner and pretreatment of hemicholinium-3 almost abolished imidazole-induced contraction. Conversely, pretreatment of tripelennamine, indomethacin or tetrodotoxin did not affect imidazole-induced contraction. Acetylcholine or eserine induced contraction in bovine, porcine, guinea pig, rat and mice trachea in a concentration-dependent manner. However, there was little difference in the rank order of maximum contraction of these agents. Imidazole-induced contraction was greater in bovine trachea compared to the other species tested. Further, cAMP did not appear to play a role in imidazole-induced contraction, suggesting other mechanisms, such as the release of endogenous acetylcholine.

RhoA, a possible target for treatment of airway hyperresponsiveness in bronchial asthma

Airway hyperresponsiveness to nonspecific stimuli is one of the characteristic features of allergic bronchial asthma. An elevated contractility of bronchial smooth muscle has been considered as one of the causes of the airway hyperresponsiveness. The contraction of smooth muscles including airway smooth muscles is mediated by both Ca²+-dependent and Ca²+-independent pathways. The latter Ca²+-independent pathway, termed Ca²+ sensitization, is mainly regulated by a monomeric GTP-binding protein, RhoA, and its downstream target Rho-kinase. In animal models of allergic bronchial asthma, an augmented agonist-induced, RhoA-mediated contraction of bronchial smooth muscle has been suggested. The RhoA/Rho-kinase signaling is now proposed as a novel target for the treatment of airway hyperresponsiveness in asthma. Herein, we will discuss the mechanism of development of bronchial smooth muscle hyperresponsiveness, one of the causes of the airway hyperresponsiveness, based on the recent studies using animal models of allergic bronchial asthma and/or cultured airway smooth muscle cells. The possibility of RhoA as a therapeutic target in asthma, especially airway hyperresponsiveness, will also be described.

Roles of stretch-activated cation channel and Rho-kinase in the spontaneous contraction of airway smooth muscle

In guinea pigs, it is well-known that mechanical stretch of airway smooth muscle exhibits spontaneous tone which is mediated by cyclooxygenase (COX) activation. We tested the hypothesis that this spontaneous contraction of airway smooth muscle is mediated by stretch-activated non-selective cation channels and the Rho/Rho-kinase pathway, as well as COX-2 using a pharmacological approach. Isometric force and intracellular Ca(2+) concentrations ([Ca(2+)](i)) were assessed in isolated guinea pig tracheal smooth muscle tissues. The samples were stretched to a given level and the muscle behavior was monitored under isometric conditions. We observed an increase in [Ca(2+)](i) and subsequent force generation over a 15-min period. The augmented [Ca(2+)](i) and spontaneous contraction due to the stretch were markedly attenuated by application of Gd(3+), an inhibitor of stretch-activated channels, and removal of extracellular Ca(2+). In contrast, nifedipine only had a mild inhibitory effect on the contraction. (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexane-carboxamide (Y-27632; a Rho-kinase inhibitor) abolished the spontaneous contraction with no changes in [Ca(2+)](i). Simvastatin, which down-regulates Rho activity, also significantly inhibited the contraction. Moreover, indomethacin, an inhibitor of COX-1 and -2, and N-[2-(cyclohexyloxy)-4-nitrophenyl]-methanesulfonamide (NS-398; a COX-2 inhibitor) abolished the stretch-induced contraction without affecting [Ca(2+)](i), whereas the inhibitory effect of 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole (SC560; a COX-1 inhibitor) on the contraction was much less. These findings demonstrated that Ca(2+) entry via stretch-activated channels, the Rho/Rho-kinase pathway, and COX-2 are involved in the mechanotransduction in guinea pig tracheal smooth muscle. Additionally, while the Rho/Rho-kinase pathway and COX-2 regulate the spontaneous contraction independently of [Ca(2+)](i), COX-1 is not involved in the stretch-induced force generation.

Involvement of reduced sensitivity to Ca in beta-adrenergic action on airway smooth muscle

BACKGROUND

It is well known that beta-adrenoceptor agonists (beta-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+](i)). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the beta-adrenergic relaxation.

OBJECTIVE

This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in beta-adrenergic relaxation (Ca(2+)-independent relaxation by beta-agonists). Methods Isometric tension and [Ca2+](i) were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+](i) were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.

RESULTS

The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+](i). In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+](i.) This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca(2+) channels, was associated with that in [Ca2+](i). In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+](i). The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase. Conclusion beta-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, G(s)-direct action is more potent in Ca(2+)-independent relaxation by beta-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by beta-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting beta-agonists in acute asthma management.

Modulation of the Ca2+ sensitivity of airway smooth muscle cells in murine lung slices

To investigate the phenomenon of Ca(2+) sensitization, we developed a new intact airway and arteriole smooth muscle cell (SMC) "model" by treating murine lung slices with ryanodine-receptor antagonist, ryanodine (50 microM), and caffeine (20 mM). A sustained elevation in intracellular Ca(2+) concentration ([Ca(2+)](i)) was induced in both SMC types by the ryanodine-caffeine treatment due to the depletion of internal Ca(2+) stores and the stimulation of a persistent influx of Ca(2+). Arterioles responded to this sustained increase in [Ca(2+)](i) with a sustained contraction. By contrast, airways responded to sustained high [Ca(2+)](i) with a transient contraction followed by relaxation. Subsequent exposure to methacholine (MCh) induced a sustained concentration-dependent contraction of the airway without a change in the [Ca(2+)](i). During sustained MCh-induced contraction, Y-27632 (a Rho-kinase inhibitor) and GF-109203X (a protein kinase C inhibitor) induced a concentration-dependent relaxation without changing the [Ca(2+)](i). The cAMP-elevating agents, forskolin (an adenylyl cyclase activator), IBMX (a phosphodiesterase inhibitor), and caffeine (also acting as a phosphodiesterase inhibitor), exerted similar relaxing effects. These results indicate that 1) ryanodine-caffeine treatment is a valuable tool for investigating the contractile mechanisms of SMCs while avoiding nonspecific effects due to cell permeabilization, 2) in the absence of agonist, sustained high [Ca(2+)](i) has a differential time-dependent effect on the Ca(2+) sensitivity of airway and arteriole SMCs, 3) MCh facilitates the contraction of airway SMCs by inducing Ca(2+) sensitization via the activation of Rho-kinase and protein kinase C, and 4) cAMP-elevating agents contribute to the relaxation of airway SMCs through Ca(2+) desensitization.

Effect of cigarette smoke exposure in vivo on bronchial smooth muscle contractility in vitro in rats

Cigarette smoking is a risk factor for the development of airway hyperresponsiveness and chronic obstructive pulmonary disease. Little is known concerning the effect of cigarette smoking on the contractility of airway smooth muscle. The current study was performed to determine the responsiveness of bronchial smooth muscles isolated from rats that were subacutely exposed to mainstream cigarette smoke in vivo. Male Wistar rats were exposed to diluted mainstream cigarette smoke for 2 h/d every day for 2 wk. Twenty-four hours after the last cigarette smoke exposure, a marked airway inflammation (i.e., increases in numbers of neutrophils, lymphocytes, and macrophages in bronchoalveolar lavage fluid and peribronchial tissues) was observed. In these subacutely cigarette smoke-exposed animals, the responsiveness of isolated intact (nonpermeabilized) bronchial smooth muscle to acetylcholine, but not to high K+ -depolarization, was significantly augmented when compared with the air-exposed control group. In alpha-toxin-permeabilized bronchial smooth muscle strips, the acetylcholine-induced Ca2+ sensitization of contraction was significantly augmented in rats exposed to cigarette smoke, although the contraction induced by Ca2+ was control level. Immunoblot analyses revealed an increased expression of RhoA protein in the bronchial smooth muscle of rats that were exposed to cigarette smoke. Taken together, these findings suggest that the augmented agonist-induced, RhoA-mediated Ca2+ sensitization may be responsible for the enhanced bronchial smooth muscle contraction induced by cigarette smoking, which has relevance to airway hyperresponsiveness in patients with chronic obstructive pulmonary disease.

Effects of Repeated Antigen Exposure on Endothelin-1–Induced Bronchial Smooth Muscle Contraction and Activation of RhoA in Sensitized Rats

Changes in endothelin-1 (ET-1)-induced contraction and activation of RhoA in bronchial smooth muscle of repeatedly antigen-challenged rats, which exhibit marked airway hyperresponsiveness (AHR), were examined. The ET-1-induced contraction of bronchial smooth muscle was significantly enhanced in the repeatedly antigen-challenged group. In normal control animals, ET-1 induced time- and concentration-dependent translocation of RhoA to the plasma membrane, indicating activation of RhoA by ET-1 in rat bronchial smooth muscle. The level of ET-1-induced RhoA translocation was increased much more markedly in the AHR group than in the control animals. It is suggested that the augmented activation of RhoA observed in the hyperresponsive bronchial smooth muscle might be responsible for the enhanced ET-1-induced contraction of bronchial smooth muscle in AHR rats.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.

The role of RhoA-mediated Ca2+ sensitization of bronchial smooth muscle contraction in airway hyperresponsiveness

Smooth muscle contraction is mediated by Ca2+-dependent and Ca2+-independent pathways. The latter Ca2+-independent pathway, termed Ca2+ sensitization, is mainly regulated by a monomeric GTP binding protein RhoA and its downstream target Rho-kinase. Recent studies suggest a possible involvement of augmented RhoA/Rho-kinase signaling in the elevated smooth muscle contraction in several human diseases. An increased bronchial smooth muscle contractility, which might be a major cause of the airway hyperresponsiveness that is a characteristic feature of asthmatics, has also been reported in bronchial asthma. Here, we will discuss the role of RhoA/Rho-kinase-mediated Ca2+ sensitization of bronchial smooth muscle contraction in the pathogenesis of airway hyperresponsiveness. Agonist-induced Ca2+ sensitization is also inherent in bronchial smooth muscle. Since the Ca2+ sensitization is sensitive to a RhoA inactivator, C3 exoenzyme, and a Rho-kinase inhibitor, Y-27632, the RhoA/Rho-kinase pathway is involved in the signaling. It is of interest that the RhoA/Rho-kinase-mediated Ca2+ sensitization of bronchial smooth muscle contraction is markedly augmented in experimental asthma. Moreover, Y-27632 relaxes the bronchospasm induced by contractile agonists and antigens in vivo. Y-27632 also has an ability to inhibit airway hyperresponsiveness induced by antigen challenge. Thus, the RhoA/Rho-kinase pathway might be a potential target for the development of new treatments for asthma, especially in airway hyperresponsiveness.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

ML-9, a myosin light chain kinase inhibitor, reduces intracellular Ca2+ concentration in guinea pig trachealis

We investigated the effects of ML-9 [1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine], a myosin light chain kinase (MLCK) inhibitor, on intracellular Ca2+ concentration ([Ca2+]i), contraction induced by high K+ and an agonist, and capacitative Ca2+ entry in fura-2-loaded guinea pig tracheal smooth muscle. ML-9 inhibited both the increase in [Ca2+]i and the contraction induced by 60 mM K+, 1 microM methacholine or 1 microM thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. However, another MLCK inhibitor, wortmannin (3 microM), inhibited the contraction elicited by these stimuli without affecting [Ca2+]i. Under the condition that the thapsigargin-induced contraction was fully suppressed by 3 microM wortmannin, 30 microM ML-9 caused a further decrease in [Ca2+]i. The inhibitory effects of ML-9 on [Ca2+]i and the contraction elicited by methacholine were similar to those of SKF-96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride), a Ca2+ channel blocker. These results indicate that ML-9 acts as a potent inhibitor of Ca2+-permeable channels independently of MLCK inhibition in tracheal smooth muscle.

Possible role of the protein kinase C/CPI-17 pathway in the augmented contraction of human myometrium after gestation

1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (PDBu, 1 microm) induced sustained contractions with no increase in [Ca2+]i in nonpregnant and pregnant human myometria. The contractile effects of PDBu in pregnant myometrium were much greater than those in nonpregnant myometrium, and the contractions in pregnant myometrium were accompanied by an increase in myosin light chain (MLC) phosphorylation at Ser19. 2. The contraction induced by PDBu in pregnant myometrium was inhibited by the inhibitors of conventional PKC isoforms, bisindolylmaleimides and indolocarbazole, such as Go6976, Go6983, and Go6850 (1 microM). LY333531 (1 microM), a specific inhibitor of PKC beta, also inhibited the PDBu-induced contraction in the pregnant myometrium. 3. In the pregnant myometrium permeabilized with alpha-toxin, PDBu increased the contractions induced at fixed Ca2+ concentration (0.3 microM) both in nonpregnant and pregnant myometria, indicating Ca2+ sensitization of contractile elements. 4. Western immunoblot analysis indicated that pregnant myometrium contained PKC isozymes such as conventional PKC (alpha, beta, gamma), novel PKC (delta, epsilon, theta), and atypical PKC (zeta but not iota and lambda). RT-PCR and real-time RT-PCR analysis indicated that, among the conventional PKC, the levels of mRNA of beta isoform in pregnant human myometrium were greater than those in nonpregnant myometrium. 5. CPI-17 is a substrate for PKC, and the phosphorylated CPI-17 is considered to inhibit myosin phosphatase. The levels of CPI-17 mRNA and protein expression were also greater in the pregnant myometrium. 6. These results suggest that the PKC-mediated contractile mechanism is augmented in human myometrium after gestation, and that this augmentation may be attributable to the increased activity of the beta PKC isoform and CPI-17.

The Non-neuronal cholinergic system: an emerging drug target in the airways

The non-neuronal cholinergic system is widely expressed in human airways. Choline acetyltransferase (ChAT) and/or acetylcholine are demonstrated in more or less all epithelial surface cells (goblet cells, ciliated cells, basal cells), submucosal glands and airway smooth muscle fibres. Acetylcholine is also demonstrated in the effector cells of the immune system (lymphocytes, macrophages, mast cells). Epithelial, endothelial and immune cells express nicotinic and muscarinic receptors. Thus the cytomolecule acetylcholine can contribute to the regulation of basic cell functions via auto-/paracrine mechanisms (proliferation, differentiation, ciliary activity, secretion of water, ions and mucus, organization of the cytoskeleton, cell-cell contact). Acetylcholine also modulates immune functions (release of cytokines; proliferation, activation and inhibition of immune cells). Preliminary experimental evidence suggests that mucosal inflammation may be associated with raised acetylcholine levels, impairing cell and organ homeostasis. It should be considered that anti-muscarinic drugs which are applied for the treatment of chronic airway diseases antagonize the effect of both neuronal and non-neuronal acetylcholine. Non-neuronal acetylcholine, however, is still active, possibly directly within the cell cytosol and also via nicotinic receptors localized on various non-neuronal cells. It is an essential task to clarify the pathophysiological role of the non-neuronal cholinergic system in more detail to develop new drugs which can target the synthesis, release, inactivation and cellular activity of non-neuronal acetylcholine.

Sevoflurane inhibits contraction of uterine smooth muscle from pregnant rats similarly to halothane and isoflurane

PURPOSE

The present study was designed to clarify the direct effects of the volatile anesthetics halothane, isoflurane and sevoflurane on oxytocin-induced uterine smooth muscle contraction from pregnant rats.

METHODS

Longitudinal smooth muscle layers were obtained from pregnant rats. Intracellular concentration of free Ca++ ([Ca++](i)) was measured, using a fluorescence technique, simultaneously with muscle tension. Inward Ba++ current (I(Ba)) through voltage-dependent Ca++ channels (VDCCs) was measured using a whole cell patch clamp technique. After incubation with 20 nM oxytocin, halothane, isoflurane or sevoflurane (1, 2, and 3%) was introduced into the tissue bath.

RESULTS

All volatile anesthetics significantly inhibited muscle contraction concomitant with a decrease in [Ca++](i). Volatile anesthetics also inhibited the peak I(Ba). When the anesthetic concentrations were expressed as multiples of minimum alveolar concentrations, there were no differences in the inhibitory potencies of the three volatile agents tested for muscle tension and VDCC.

CONCLUSIONS

Volatile anesthetics halothane, isoflurane and sevoflurane reduce the oxytocin-induced contraction of pregnant uterine smooth muscle. Inhibition of the contraction by the volatile anesthetics is due, at least in part, to the decrease in [Ca++](i), and the decrease in [Ca++](i) may be mediated by inhibition of VDCC activity.

Effects of Y-27632 on acetylcholine-induced contraction of intact and permeabilized intrapulmonary bronchial smooth muscles in rats

In the present study, the effects of a selective Rho-associated coiled-coil forming protein kinase (ROCK) inhibitor, Y-27632 [(+)-(R)-trans-4-(1-aminoethyl)-(4-pyridyl)cyclohexanecarboxamide dihydrochloride] on acetylcholine-induced contraction and Ca(2+) sensitization of rat bronchial smooth muscle were examined. Intact and beta-escin-permeabilized muscles of the third branch of intrapulmonary bronchi were used. In intact muscles, Y-27632 (10(-6)-10(-4) M) concentration-dependently inhibited acetylcholine-induced contractile responses. In acetylcholine (10(-3) M)-precontracted intact muscles, the maximal relaxation (about 50% inhibition of contraction) was obtained by a concentration of 10(-4) M Y-27632, which had no effect on the resting tone. In beta-escin-permeabilized muscles, addition of acetylcholine (10(-5)-10(-3) M) plus GTP (100 microM) induced a further contraction, i.e., Ca(2+) sensitization at a constant Ca(2+) concentration of pCa=6.0. The acetylcholine-induced Ca(2+) sensitization was completely blocked in the presence of 10(-4) M Y-27632, whereas the Ca(2+)-induced contraction itself was not affected by Y-27632. Immunoblot study revealed the expression of ROCK-I and ROCK-II proteins in the intrapulmonary bronchi of rats. These findings suggest that Y-27632 dilates acetylcholine-mediated contraction of rat bronchial smooth muscle by inhibiting RhoA/ROCK-mediated Ca(2+) sensitization.

Augmented acetylcholine-induced translocation of RhoA in bronchial smooth muscle from antigen-induced airway hyperresponsive rats

Acetylcholine (ACh)-induced translocation of RhoA in bronchial smooth muscle of repeatedly antigen-challenged rats that have a marked airway hyperresponsiveness (AHR) was examined. ACh induced time- and concentration-dependent translocation of RhoA to the plasma membrane, indicating an activation of RhoA in bronchial smooth muscle. The level of ACh-induced RhoA translocation was further increased markedly in the AHR group as compared to that in the control group. It is suggested that the augmented activation of RhoA observed in the hyperresponsive bronchial smooth muscle might be responsible for the enhanced ACh-induced Ca(2+) sensitization of bronchial smooth muscle contraction associated with AHR.

Possible involvement of Rho kinase in Ca2+ sensitization and mobilization by MCh in tracheal smooth muscle

We examined the effects of Rho kinase on contraction and intracellular Ca2+ concentration ([Ca2+](i)) in guinea pig trachealis by measuring isometric force and the fura 2 signal [340- to 380-nm fluorescence ratio (F340/F380)]. A Rho kinase inhibitor, Y-27632 (1-1,000 microM), inhibited methacholine (MCh)-induced contraction, with a reduction in F340/F380 in a concentration-dependent manner. The values of EC(50) for contraction and F340/F380 induced by 1 microM MCh with Y-27632 were 27.3 +/- 5.1 and 524.1 +/- 31.0 microM, respectively. With 0.1 microM MCh, the values for these parameters were decreased to 1.0 +/- 0.1 and 98.2 +/- 6.2 microM, respectively. Tension-F340/F380 curves for MCh indicated that Y-27632 caused an ~50% inhibition of MCh-induced contraction, without a reduction in F340/F380. These effects of Y-27632 were not inhibited by a protein kinase C inhibitor, GF-109203X. Our results indicate that inhibition of Rho kinase attenuates both Ca2+ sensitization and [Ca2+](i).

Inhibitory effects of verapamil and nitroglycerin on contraction and cytosolic Ca2+ levels in cerebrovascular smooth muscle during chronic cerebral vasospasm

The mechanisms of the inhibitory effects of verapamil and nitroglycerin on vasospasm were investigated by measuring cytosolic Ca2+ level ([Ca2+]i) and muscle tension in 28 normal specimens and 28 spastic vascular specimens of smooth muscle. Experimental vasospasm was produced by the two-hemorrhage method in the canine basilar artery. [Ca2+]i and tension were recorded simultaneously with a fluorimeter using fura-2. High K+ concentration (72.4 mM) and U-46619 (thromboxane A2 analogue, 10(-8) M) were used as stimulants, and the inhibitory effects of verapamil or nitroglycerin on muscle contraction and increased [Ca2+]i were examined. Verapamil inhibited [Ca2+]i and contraction in high K+ concentration-stimulated arteries. Verapamil inhibited [Ca2+]i more strongly than contraction in U-46619-stimulated arteries. There were no significant differences in the effects of verapamil in the control and vasospasm groups. Nitroglycerin inhibited contraction with little effect on [Ca2+]i in high K+ concentration-stimulated arteries in both the control and vasospasm groups. Nitroglycerin inhibited contraction with little effect on [Ca2+]i in U-46619-stimulated arteries and the inhibitory effect was weaker in the vasospasm group than in the control group. The inhibitory effects of verapamil on muscle tension and [Ca2+]i in vasospastic vessels were as strong as those in normal vessels. In contrast, the inhibitory effects of nitroglycerin were reduced in vasospastic vessels. Increased Ca2+ sensitivity in vasospastic vessels may have reduced the inhibitory effects of nitroglycerin.

Interaction between volatile anesthetics and hypoxia in porcine tracheal smooth muscle

We investigated the direct interaction between the volatile anesthetics, isoflurane and sevoflurane, and hypoxia in porcine tracheal smooth muscle in vitro by simultaneously measuring muscle tension and intracellular concentration of free Ca(2+) ([Ca2+]i). Muscle tension was measured by using an isometric transducer, and [Ca2+]i was measured by using fura-2, an indicator of Ca2+. Under the condition of bubbling with 95% O2/5% CO2, [Ca2+]i was increased by 1 microM carbachol with a concomitant contraction. Volatile anesthetics significantly inhibited both carbachol-induced muscle contraction and increase in [Ca2+]i. Hypoxia bubbled with 95% N(2)/5% CO2 inhibited the muscle contraction by 30% with an increase in [Ca2+]i by 20%. Exposure to hypoxia substantially enhanced the inhibitory effects of these anesthetics on carbachol-induced muscle contraction, whereas the decreases in [Ca2+]i were significantly prevented by hypoxia. Under Ca2+-free conditions, hypoxia significantly decreased the muscle contraction by 20%; however, it still increased [Ca2+]i by 15%. Exposure to the anesthetics significantly enhanced the inhibitory effect of hypoxia on the muscle contraction; however, it appeared to have little effect on [Ca2+]i. Hypoxia inhibits airway smooth muscle contraction independently of intracellular Ca2+, and it substantially potentiates the inhibitory effects of volatile anesthetics on airway smooth muscle contraction.

IMPLICATIONS

Hypoxia inhibits agonist-induced tracheal smooth muscle contraction with an increase in free Ca2+ [Ca2+]i, which comes from intracellular Ca2+ stores. Hypoxia also potentiates the inhibitory effect of volatile anesthetics on airway smooth muscle contraction. Conversely, there is a possibility that the treatment of asthmatic patients with oxygen partially attenuates the inhibitory effect of volatile anesthetics on airway smooth muscle contractility.

Involvement of K(+) channel in procainamide-induced relaxation of bovine tracheal smooth muscle

The relaxant effect of procainamide, a class Ia antiarrhythmic agent, was examined in bovine tracheal smooth muscle. Procainamide produced concentration-dependent decreases in tension and full relaxation in the preparations contracted with methacholine (0.3 microM). By comparison, in preparations contracted with 40 mM K(+), procainamide had only slight relaxant effects. The relaxant effects of cromakalim and salbutamol on 40 mM K(+)-contracted preparations were significantly (P<0.01) smaller than those on 0.3 microM methacholine-contracted ones. On the other hand, the concentration-response relationships for quinidine, lidocaine, mexiletine and propafenone were not so dramatically different between 0.3 microM methacholine- and 40 mM K(+)-contracted preparations. Tetraethylammonium (300 microM), iberiotoxin (30 nM) and Ba(2+) (1 mM) significantly (P<0.05) attenuated the relaxant effects of procainamide on methacholine-induced contractions, whereas apamin (100 nM), 4-aminopyridine (300 microM), and glibenclamide (10 microM) did not affect them. The inhibitory effect of a combination of iberiotoxin and Ba(2+) was greater than that of iberiotoxin or Ba(2+) alone (P<0.01). These results suggest that the activation of at least two types of K(+) (maxi-K(+) and inward rectifier K(+)) channels contributes to the procainamide-induced relaxation of bovine tracheal smooth muscle.

Tumor necrosis factor-alpha augments contraction and cytosolic Ca(2+) sensitivity through phospholipase A(2) in bovine tracheal smooth muscle

To elucidate the effects of tumor necrosis factor-alpha (TNF-alpha) on tracheal smooth muscle contraction, we simultaneously measured isometric tension and intracellular Ca(2+) ([Ca(2+)](i)) in fura 2-loaded muscle strips. Smooth muscle force generation was evaluated in a high potassium (K(+); 20.0-80.0 mM) solution and with acetylcholine (3 nM-10 microM ). TNF-alpha (1-100 ng/ml) did not directly contract muscle strips. The contractile response to acetylcholine was enhanced after application of 10 ng/ml of TNF-alpha for 30 min but not the response of [Ca(2+)](i). The contractile response and the response of [Ca(2+)](i) to a high K(+) solution were not altered after application of TNF-alpha. The [Ca(2+)](i)-tension curve indicated that TNF-alpha enhanced the responsiveness of tracheal smooth muscle through the acetylcholine-mediated Ca(2+) sensitivity of intracellular contractile elements. The augmentation of the acetylcholine concentration-response curves for muscle tension in the presence of TNF-alpha (10 ng/ml) was inhibited in part after application of manoalide, a phospholipase A(2) inhibitor. We conclude that a low concentration of TNF-alpha enhances smooth muscle responsiveness to acetylcholine by agonist-mediated Ca(2+) sensitivity facilitated by phospholipase A(2).

S-nitrosoglutathione-induced decrease in calcium sensitivity of airway smooth muscle

The purpose of this study was to examine whether the nitric oxide donor S-nitrosoglutathione (GSNO) relaxes canine tracheal smooth muscle (CTSM) strips by decreasing Ca(2+) sensitivity [i.e., the amount of force for a given intracellular Ca(2+) concentration ([Ca(2+)](i))]. We further investigated whether GSNO decreases Ca(2+) sensitivity by altering the relationship between regulatory myosin light chain (rMLC) phosphorylation and [Ca(2+)](i) and the relationship between force and rMLC phosphorylation. GSNO (100 microM) relaxed intact CTSM strips contracted with 45 mM KCl by decreasing Ca(2+) sensitivity in comparison to control strips without significantly decreasing [Ca(2+)](i). GSNO reduced the amount of rMLC phosphorylation for a given [Ca(2+)](i) but did not affect the relationship between isometric force and rMLC phosphorylation. These results show that in CTSM strips contracted with KCl, GSNO decreases Ca(2+) sensitivity by affecting the level of rMLC phosphorylation for a given [Ca(2+)](i), suggesting that myosin light chain kinase is inhibited or that smooth muscle protein phosphatases are activated by GSNO.

Augmented acetylcholine-induced, Rho-mediated Ca2+ sensitization of bronchial smooth muscle contraction in antigen-induced airway hyperresponsive rats

Treatment with acetylcholine (ACh) of a beta-escin-permeabilized intrapulmonary bronchial smooth muscle of the rat induced force when the Ca2+ concentration was clamped at 1 microM. The ACh-induced Ca2+ sensitization of myofilaments was significantly greater in antigen-induced airway hyperresponsive rats than in control rats. The ACh-induced Ca2+ sensitization was completely blocked by treatment with Clostridium botulinum C3 exoenzyme, an inactivator of Rho family of proteins. Moreover, the protein level of RhoA in the intrapulmonary bronchi was significantly increased in the airway hyperresponsive rats. Thus, increased airway smooth muscle contractility observed in asthmatics may be related to augmented agonist-induced, Rho-mediated Ca2+ sensitization of myofilaments.

Relaxation of contracted rabbit tracheal and human bronchial smooth muscle by Y-27632 through inhibition of Ca2+ sensitization

The mechanism of Ca2+ sensitization of contraction has not been elucidated in airway smooth muscle (SM). To determine the role of a small G protein, rhoA p21, and its target protein, rho-associated coiled coil-forming protein kinase (ROCK), in receptor-coupled Ca2+ sensitization of airway SM, we studied the effect of (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl)cyclohexane carboxamide dihydrochloride, monohydrate (Y-27632), a ROCK inhibitor, on isometric contractions in rabbit tracheal and human bronchial SM. Y-27632 completely reversed 1 microM carbachol (CCh)-induced contraction of intact trachea with a concentration producing half-maximum inhibition of effect (IC50) of 1.29 +/- 0.2 microM (n = 5). Although 4beta-phorbol 12,13-dibutyrate (1 microM)-induced Ca2+ sensitization was relatively resistant to Y-27632 in alpha-toxin-permeabilized trachea, CCh (100 microM) plus guanosine triphosphate (GTP) (3 microM)- and guanosine 5'-O-(3'-thiotriphosphate) (10 microM)-induced contractions were relaxed completely by Y-27632 with IC50 of 1.44 +/- 0.3 (n = 6) and 1.15 +/- 0.3 microM (n = 6). Endothelin-1 (1 microM) plus GTP (3 microM)- developed force was also reversed by Y-27632 with IC50 of 4. 10 +/- 1.1 microM (n = 6) in the alpha-toxin-permeabilized bronchus. Both the rabbit and human SM expressed rhoA p21, ROCK I, and its isoform ROCK II. Collectively, rho/ROCK-mediated Ca2+ sensitization plays a central role in the sustained phase of airway SM contraction, and selective inhibition of this pathway may become a new strategy to resolve airflow limitation in asthma.

The direct effects of heparin and protamine on canine tracheal smooth muscle tone

Heparin and protamine are used for cardiopulmonary bypass in cardiac surgery; however, the direct effects and mechanisms of these drugs on airway smooth muscle tone are still not fully known. We investigated the in vitro effects of these drugs on canine tracheal smooth muscle by measuring the muscle tension and intracellular Ca2+ concentration ([Ca2+]i) and by measuring inward Ca2+ currents (I(Ca)) through voltage-dependent Ca2+ channels. [Ca2+]i was monitored by the 500-nm light emission ratio of preloaded Ca2+ indicator fura-2. Isometric tension was measured simultaneously. Whole-cell patch clamp recording techniques were used to investigate the effects of the drugs on I(Ca) in freshly dispersed smooth muscle cells. Heparin (0.12-120 U/mL), protamine (0.15-150 U/mL), or heparin-protamine complex (4:5 U/U) was introduced into a bath solution. Protamine and heparin-protamine complex dose-dependently inhibited both carbachol-induced contraction of the muscle and increase in [Ca2+]i. These drugs also decreased the I(Ca) of the muscle cells and shifted the inactivation curve to a more negative potential. Heparin itself had a slight enhancing effect on carbachol-induced muscle contraction without changing [Ca2+]i. Protamine and heparin-protamine complex can decrease the agonist-induced increase in [Ca2+]i by the inhibition of voltage-dependent Ca2+ channels both in the activated and inactivated states.

IMPLICATIONS

Protamine and heparin-protamine complex inhibited carbachol-induced canine tracheal smooth muscle contraction by inhibiting the increase in intracellular concentration of free Ca2+. These drugs can decrease the agonist-induced increase in intracellular Ca2+ by the inhibition of voltage-dependent Ca2+ channels in both the activated and inactivated states.

Mechanisms of 5-hydroxytryptamine-induced inhibition in the porcine myometrium

The present experiments were designed to clarify the mechanisms of the inhibitory response of 5-hydroxytryptamine (5-HT) in the porcine uterine circular muscle. Inhibitory responses induced by 5-HT (1 nM-1 microM) were not affected by apamin (1 microM), charybdotoxin (100 nM) or glibenclamide (20 microM) but were significantly attenuated by 4-aminopyridine (3 mM) and tetraethylammonium (3 mM). Imidazole (100 microM) decreased but 3-isobutyl-1-methylxanthine (30 microM), milrinone (30 microM) and Ro 20-1724 (10 and 30 microM) potentiated the 5-HT-induced inhibition. On the other hand, zaprinast (3-30 microM) had no significant effect on the inhibitory response of 5-HT. 5-HT caused a time (0-5 min)-and concentration (1 nM-1 microM)-dependent increase in the tissue cyclic AMP level, but had no effect on the tissue cyclic GMP level. A significant correlation (P < 0.05) was observed between the inhibition of contraction and tissue cyclic AMP level. The effect of 5-HT on contractile force and cytosolic Ca2+ level ([Ca2+]i) was investigated using fura-PE3-loaded myometrial strips. A low concentration of 5-HT (< or = 10 nM) inhibited the spontaneous contraction without changing the amplitude of the spontaneous [Ca2+]i increase, but a higher concentration of 5-HT (> or = 100 nM) decreased the resting [Ca2+]i and inhibited both the spontaneous [Ca2+]i increase and spontaneous contraction. High-K+ (50 mM) caused increases in muscle contractile force and [Ca2+]i. 5-HT concentrationdependently inhibited the high-K(+)-induced contraction (EC50, 45 nM) with only a small decrease in [Ca2+]i increase. Carbachol also caused increases in muscle contractile force and [Ca2+]i. 5-HT significantly decreased both the carbachol-induced contraction and [Ca2+]i increase, but was more potent at inhibition of contractile force than [Ca2+]i. In Ca(2+)-loaded myometrial strips, carbachol, but not caffeine, caused a transient increase in [Ca2+]i and contraction in the absence of external Ca2+ (EGTA, 1 mM). 5-HT inhibited both the carbachol-induced increases in [Ca2+]i release and contractile force. In the beta-escin permeabilized myometrium, 5-HT significantly inhibited the Ca(2+)-induced contraction. The present results indicate that 5-HT stimulates tissue cyclic AMP production, and inhibits the porcine uterine muscle contractility by a reduction in [Ca2+]i and in Ca2+ sensitivity of the contractile elements. Activation of K+ channels might be partially involved in 5-HT-induced inhibition of the myometrial contractility.

Role of M2 muscarinic receptors in airway smooth muscle contraction

Airway smooth muscle expresses both M2 and M3 muscarinic receptors with the majority of the receptors of the M2 subtype. Activation of M3 receptors, which couple to Gq, initiates contraction of airway smooth muscle while activation of M2 receptors, which couple to Gi, inhibits beta-adrenergic mediated relaxation. Increased sensitivity to intracellular Ca2+ is an important mechanism for agonist-induced contraction of airway smooth muscle but the signal transduction pathways involved are uncertain. We studied Ca2+ sensitization by acetylcholine (ACh) and endothelin-1 (ET-1) in porcine tracheal smooth muscle by measuring contractions at constant [Ca2+] in strips permeabilized with Staphylococcal alpha-toxin. Both ACh and ET-1 contracted airway smooth muscle at constant [Ca2+]. Pretreatment with pertussis toxin for 18-20 hours reduced ACh contractions, but had no effect on those of ET-1 or GTPgammaS. We conclude that the M2 muscarinic receptor contributes to airway smooth muscle contraction at constant [Ca2+] via the heterotrimeric G-protein Gi.

Role of G proteins in agonist-induced Ca2+ sensitization of tracheal smooth muscle

Increased sensitivity to intracellular Ca2+ concentration ([Ca2+]) is an important mechanism for agonist-induced contraction of airway smooth muscle, but the signal transduction pathways involved are uncertain. We studied Ca2+ sensitization with acetylcholine (ACh) and endothelin (ET)-1 in porcine tracheal smooth muscle by measuring contractions at a constant [Ca2+] in strips permeabilized with alpha-toxin or beta-escin. The peptide inhibitor G protein antagonist 2A (GP Ant-2A), which has selectivity for Gq over Gi, inhibited contractile responses to ET-1, ACh, and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), but the proportional inhibition of ACh responses was less than that of ET-1. Pretreatment with pertussis toxin reduced ACh contractions but had no effect on those of ET-1 or GTPgammaS. Clostridium botulinum C3 exoenzyme, which inactivates Rho family monomeric G proteins, caused similar reductions in contractile responses to ACh, ET-1, and GTPgammaS. Farnesyltransferase inhibition, which inhibits Ras G proteins, reduced responses to ET-1. We conclude that the heterotrimeric G proteins Gq and Gi both contribute to Ca2+ sensitization by ACh, whereas ET-1 responses involve Gq but not Gi. Both Gq and Gi pathways likely involve Rho family small G proteins. A Ras-mediated pathway also contributes to Ca2+ sensitization by ET-1 in airway smooth muscle.

Modulation of spasmogen-stimulated Ins(1,4,5)P3 generation and functional responses by selective inhibitors of types 3 and 4 phosphodiesterase in airways smooth muscle

1. The effects of isoenzyme-selective inhibitors of phosphodiesterases PDE3 and PDE4 on cyclic AMP concentration, two indices of phosphoinositide hydrolysis, and contractile responses to spasmogens have been investigated in bovine tracheal smooth muscle (BTSM). 2. Neither the PDE3-selective inhibitor ORG 9935, nor the PDE4-selective inhibitor rolipram increased cyclic AMP levels in BTSM. However, rolipram addition in the presence of PDE3 inhibition (ORG 9935; 1 microM) concentration-dependently (-log EC50 (M), 6.55+/-0.15; n = 3) increased cyclic AMP levels to about 70% of the maximal response to the beta-adrenoceptor agonist isoprenaline. 3. Rolipram per se inhibited histamine-stimulated [3H]-inositol (poly)phosphate ([3H]-InsP(X)) accumulation by > 80% (-log EC50 (M), 6.92+/-0.11; n = 3). Although ORG 9935 (1 microM) had little effect on histamine-stimulated [3H]-InsP(X) accumulation alone it greatly facilitated the inhibitory action of rolipram (-log EC50 (M), 8.82+/-0.39; n = 3). The effects of PDE3 and/or PDE4 inhibition on [3H]-InsP(X) accumulation stimulated by muscarinic acetylcholine (mACh) receptor activation were less marked. However, combined PDE3/4 inhibition significantly decreased this response at a submaximal concentration of mACh receptor agonist (carbachol; 1 microM). 4. The greater-than-additive effect of combined PDE3/4 inhibition was also observed at the level of contractile responses to histamine and carbachol. In experiments designed to investigate the effects of PDE3 and/or 4 inhibitors on the carbachol-mediated phasic contraction, additions of rolipram (10 microM) or ORG 9935 (1 microM) were without effect, whereas added together the inhibitors caused a significant (P < 0.01) 40% reduction in the peak phasic contractile response. 5. The effect on contraction correlated with a substantial inhibitory effect of PDE3/4 inhibition on the initial increase in inositol 1,4,5-trisphosphate (InsP3) accumulation stimulated by spasmogen. Thus, in the presence of ORG 9935 (1 microM) rolipram concentration-dependently inhibited carbachol-stimulated InsP3 accumulation by > or = 50% (-log EC50 (M), 6.77+/-0.21; n = 4). 6. Carbachol (100 microM) addition caused a rapid decrease (by 67% at 10 s) in BTSM cyclic AMP level in the presence of PDE3/4 inhibition. However, omission of Ca2+ from the incubation medium prevented the carbachol-evoked decrease in cyclic AMP and this coincided with a greater inhibition (> or = 80%) of the carbachol-stimulated InsP3 response. 7. These data indicate that combined PDE3 and PDE4 inhibition has greater-than-additive effects on second messenger and functional responses to spasmogens in BTSM. Furthermore, the ability of PDE3/4 inhibition significantly to attenuate mACh receptor-mediated contractile responses, may be, at least in part, attributed to an effect exerted at the level of InsP3 generation.

Muscarinic receptor stimulation modulates the effect of halothane on Mn2+ influx in airway smooth muscle

Prior studies suggest that the mechanism of action by which halothane relaxes airway smooth muscle depends on the contractile state of the cell. We hypothesized that halothane would inhibit the influx of Ca2+ into canine airway smooth muscle cells during submaximal, but not maximal, muscarinic stimulation. This hypothesis was tested by using the rate of quenching of fura 2 fluorescence by Mn2+ in strips of canine tracheal smooth muscle as an index of Ca2+ influx. Acetylcholine (ACh) produced a dose-dependent increase in Mn2+ influx. Halothane (0.64 +/- 0.05 microM) significantly decreased Mn2+ influx and intracellular Ca2+ concentration when added to strips stimulated with a submaximal concentration of ACh (0.3 microM) but had no effect on Mn2+ influx or intracellular Ca2+ concentration during maximal stimulation with ACh (100 microM). Similar results were observed when the strips were treated with verapamil. These results demonstrate that anesthetic effects on Ca2+ homeostasis in intact canine tracheal smooth muscle cells may be critically modulated by receptor-linked mechanisms.

Receptor-dependent G protein-mediated Ca2+ sensitization in canine airway smooth muscle

To determine the mechanisms of receptor-dependent Ca2+ sensitization in airway smooth muscle, canine tracheal smooth muscle (CTSM) was permeabilized with alpha-toxin or beta-escin. Although the effects of 5-hydroxytryptamine (100 microM), histamine (100 microM), and the thromboxane A2 analogue U-46619 (100 microM) were negligible, carbachol (100 microM) and endothelin-1 (ET-1, 1 microM) evoked additional contractions of 47.0 +/- 5.90% and 25.0 +/- 5.37% (n = 6) at pCa 6.7 with GTP (3 microM) (normalized to the maximum contraction at pCa 4.5) in alpha-toxin-permeabilized CTSM. GDP-beta-S (1 mM) reversed the carbachol and ET-1 responses completely. GTP-gamma-S (30 microM) and 4 beta-phorbol 12,13-dibutyrate (PDBu, 3 microM) increased the Ca2+ sensitivity (median effective pCa) of contraction by 1.8- and 4.4-fold, respectively (n = 4-11, P < 0.05). The effects of saturating concentrations of GTP-gamma-S and PDBu were additive. A synthetic peptide (T2) corresponding to the actin-binding site of calponin caused a dose-dependent contraction of beta-escin permeabilized CTSM, with the peak effect (25 +/- 4%, n = 4) at 1200 microM, PDBu (3 microM) caused contraction of the T2 peptide-treated CTSM. In conclusion, Ca2+ sensitization of CTSM depends on receptor type and is mediated by G proteins and protein kinase C whose effects are additive, with a partial contribution by calponin.

Stereospecific effects of ketamine enantiomers on canine tracheal smooth muscle

1. Ketamine is a potent bronchodilator which relaxes airway smooth muscle (ASM). Clinically, ketamine is used as a 1:1 racemic mixture of enantiomers that differ in their analgesic and anaesthetic effects. The aim of this study was to determine whether there was a difference between the enantiomers in their ability to relax isolated ASM and to explore mechanisms responsible for any observed differences. 2. Canine tracheal smooth muscle strips were loaded with fura-2 and mounted in a photometric system to measure simultaneously force and [Ca2+]i. Calcium influx was estimated by use of a manganese quenching technique. 3. In strips stimulated with 0.1 microM ACh (EC50) R(-)-ketamine (1-100 microM) caused a significantly greater concentration-dependent decrease in force (P<0.0001) and [Ca2+]i than S(+)-ketamine (1-100 microM) (P<0.0005). In contrast, there was no significant difference between the enantiomers in their ability to inhibit calcium influx (45% decrease in influx rate for R(-)-ketamine and 44% for S(+)-ketamine, P =0.782). In strips contracted with 24 mM isotonic KCI (which activates voltage-operated calcium channels), the enantiomers modestly decreased force and [Ca2+]i; there was no significant difference between the enantiomers in their effects on force (P=0.425) or [Ca2+]i (P=0.604). 4. The R(-)-enantiomer of ketamine is a more potent relaxant of ACh-induced ASM contraction than the S(+)-enantiomer. This difference appears to be caused by differential actions on receptor-operated calcium channels.

Inhibition of voltage-dependent Ca2+ channels of porcine tracheal smooth muscle by the novel Ca2+ channel antagonist RWJ-22108

1. We compared electrophysiological effects of the bronchoselective Ca2+ channel antagonist RWJ-22108 on voltage-dependent Ca2+ channels (VDCs) of porcine tracheal smooth muscle cells to the effects of nicardipine and verapamil. 2. Each of the three Ca2+ channel antagonists tested inhibited inward Ca2+ currents (ICa) measured by whole-cell patch clamp techniques. Inhibition was dose-dependent with approximately 50% inhibition of peak ICa at +20 mV obtained with 3 x 10(-6) M RWJ-22108, 3 x 10(-7) M nicardipine, or 10(-5) M verapamil. 3. Both RWJ-22108 (3 x 10(-6) M) and nicardipine (3 x 10(-7) M) shifted the voltage dependence of steady-state inactivation to more negative potentials; however, the change in the potential of half-maximal inactivation induced by RWJ-22108 (-18 mV) was significantly greater than that induced by nicardipine (-12 mV). Verapamil did not alter the voltage dependence of inactivation. 4. We conclude that inhibition of VDCs by RWJ-22108 is qualitatively similar to that by nicardipine but with a greater stabilizing effect on the inactivated channel state.

Cyclic AMP and cyclic GMP relax phorbol ester-induced contractions of rat aorta by different mechanisms

This study was designed to test the hypothesis that 8-Br-cAMP and 8-Br-cGMP dependent relaxation of phorbol dibutyrate stimulated contractions of intact rat aorta are independent of changes in the level of myosin light chain phosphorylation. Phorbol dibutyrate stimulated contraction with a concomitant increase in myosin light chain phosphorylation in normal tissues and without an increase in myosin light chain phosphorylation in calcium-depleted tissues. Phorbol dibutyrate stimulated contractions in normal CaCl2-containing physiological salt solution were relaxed in a concentration-dependent manner by 8-Br-cAMP and 8-Br-cGMP. Phorbol dibutyrate-induced contractions in the absence of Ca2+ were only relaxed by 8-Br-cGMP; 8-Br-cAMP had no effect. The relaxation induced by 8-Br-cGMP was associated with a decrease in myosin light chain phosphorylation suggesting that cGMP-dependent protein kinase may alter the activity of either the myosin light chain kinase or phosphatase. The relaxation induced by 8-Br-cAMP was not associated with a decrease in phosphorylation suggesting that cAMP-dependent protein kinase may uncouple myosin light chain phosphorylation from force.

Effects of a prostaglandin I2 analog iloprost on cytoplasmic Ca2+ levels and muscle contraction in isolated guinea pig aorta

In the isolated guinea pig aorta, the prostaglandin I2 analog iloprost (0.01-10 microM) inhibited the contractions induced by the thromboxane A2 analog U46619 (9,11-dideoxy-11 alpha,9 alpha-epoxymethanoprostaglandin F2 alpha; 30 nM) and prostaglandin F2 alpha (PGF2 alpha, 1 microM) in a concentration-dependent manner. In contrast, iloprost only partially inhibited the high K+ (65.4 mM)-induced contraction. In the muscle stimulated with high K+, verapamil (0.3 and 10 microM) inhibited [Ca2+]i and muscle tension in parallel, whereas iloprost (1 microM) inhibited muscle tension with only a small decrease in [Ca2+]i. In the muscle stimulated with U46619 (30 nM), verapamil and iloprost decreased both [Ca2+]i and muscle tension. However, as compared with the effect of verapamil, iloprost more strongly inhibited muscle tension than [Ca2+]i. The iloprost (0.1-1 microM)-induced relaxation was accompanied by a concentration-dependent increase in cAMP content. It was further demonstrated that inhibition of the U46619-contractions was augmented in the presence of cycloxygenase inhibitors, such as indomethacin (10 microM), ibuprofen (10 microM) and aspirin (10 microM). In contrast, the inhibition of PGF2 alpha-induced contraction was not affected by indomethacin. Similarly, the inhibitory effect of forskolin on U46619-induced contractions, but not on PGF2 alpha-induced contraction, was enhanced by indomethacin. These results suggest that iloprost inhibits vascular smooth muscle contraction by decreasing [Ca2+]i and the Ca2+ sensitivity of contractile elements through a cAMP-dependent mechanism. The results also suggest that in U46619-stimulated muscle, vasoactive prostaglandins that counterbalance the relaxing action of cAMP may be generated.

Modulation of agonist-induced phosphoinositide metabolism, Ca2+ signalling and contraction of airway smooth muscle by cyclic AMP-dependent mechanisms

1. The effects of increased cellular cyclic AMP levels induced by isoprenaline, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cyclic AMP) on phosphoinositide metabolism and changes in intracellular Ca2+ elicited by methacholine and histamine were examined in bovine isolated tracheal smooth muscle (BTSM) cells. 2. Isoprenaline (pD2 (-log10 EC50) = 6.32 +/- 0.24) and forskolin (pD2 = 5.6 +/- 0.05) enhanced cyclic AMP levels in a concentration-dependent fashion in these cells, while methacholine (pD2 = 5.64 +/- 0.12) and histamine (pD2 = 4.90 +/- 0.04) caused a concentration-related increase in [3H]-inositol phosphates (IP) accumulation in the presence of 10 mM LiCl. 3. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 mM) did not affect the IP accumulation induced by methacholine, but significantly reduced the maximal IP production by histamine (1 mM). However, the effect of isoprenaline was small (15.0 +/- 0.6% inhibition) and insignificant at histamine concentrations between 0.1 and 100 microM. 4. Both methacholine and histamine induced a fast (max. in 0.5-2 s) and transient increase of intracellular Ca2+ concentration ([Ca2+]i) followed by a sustained phase lasting several minutes. EGTA (5 mM) attenuated the sustained phase, indicating that this phase depends on extracellular Ca2+. 5. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 microM) significantly attenuated both the Ca(2+)-transient and the sustained phase generated at equipotent IP producing concentrations of 1 microM methacholine and 100 microM histamine (approx. 40% of maximal methacholine-induced IP response), but did not affect changes in [Ca2+]i induced by 100 microM methacholine (95.2 +/- 3.5% of maximal methacholine-induced IP response). 6. Significant correlations were found between the isoprenaline-induced inhibition of BTSM contraction and inhibition of Ca2+ mobilization or influx induced by methacholine and histamine, that were similar for each contractile agonist. 7. These data indicate that (a) cyclic AMP-dependent inhibition of Ca2+ mobilization in BTSM cells is not primarily caused by attenuation of IP production, suggesting that cyclic AMP induced protein kinase A (PKA) activation is effective at a different level in the [Ca2+]i homeostasis, (b) that attenuation of intracellular Ca2+ concentration plays a major role in beta-adrenoceptor-mediated relaxation of methacholine- and histamine-induced airway smooth muscle contraction, and (c) that the relative resistance of the muscarinic agonist-induced contraction to beta-adrenoceptor agonists, especially at (supra) maximal contractile concentrations is largely determined by its higher potency in inducing intracellular Ca2+ changes.

Calcium mobilization and isometric tension in bovine tracheal smooth muscle: effects of salbutamol and histamine

We determined if decreases in relative free intracellular calcium concentration ([Ca2+]i) caused by salbutamol, a selective beta2-adrenoreceptor agonist, were paralleled by calcium egression from the cytosol in bovine trachealis muscle strips. [Ca2+]i, or tissue-surface extracellular calcium changes (Ts[Ca2+]ext), were monitored using Fluo-3 acetoxymethylester or Fluo-3 pentaammonium salt simultaneously with isometric tension. Salbutamol (1 microM) decreased histamine-induced isometric tension from an average peak tension of 128.5 +/- 18.4 to -4.9 +/- 0.3 mN/mm2, and reduced the associated sustained increases in [Ca2+]i from 100% at peak to 20.4 +/- 7.6%. Both histamine-induced elevation in [Ca2+]i and isometric tension were reversed completely by forskolin (1 microM). In muscle strip at active resting tension, salbutamol caused a decrease (49.6 +/- 12.1%) in [Ca2+]i. Following precontraction with histamine, salbutamol caused an immediate and sustained increase in Ts[Ca2+]ext which was not seen in a Na(+)-free solution. Finally, propranolol (10 microM) blocked both increases in Ts[Ca2+]ext and muscle relaxation caused by salbutamol. These findings indicate that in bovine trachealis muscle, the effect of salbutamol to decrease [Ca2+]i and isometric tension is via a beta2-adrenoceptor, and the changes in [Ca2+]i are by an increase in calcium egression via the Na(+)/Ca2+ exchanger, and reuptake by myoplasmic stores.

Calcium Mobilization and Muscle Contraction Induced by Acetylcholine in Swine Trachealis

The roles of Ca(2+) mobilization in development of tension induced by acetylcholine (ACh, 0.1-100 &mgr;M) in swine tracheal smooth muscle strips were studied. Under control conditions, ACh induced a transient increase in free cytosolic calcium concentration ([Ca(2+)](i)) that declined to a steady-state level. The peak increase in [Ca(2+)](i) correlated with the magnitude of tension at each [ACh] after a single exposure to ACh, while the steady-state [Ca(2+)](i) did not. Removal of extracellular Ca(2+) had little effect on peak [Ca(2+)](i) but greatly reduced steady-state increases in [Ca(2+)](i) and tension. Verapamil inhibited steady-state [Ca(2+)](i) only at [ACh] <1 &mgr;M. After depletion of internal Ca(2+) stores by 10 min exposure to ACh in Ca(2+)-free solution and then washout of ACh for 5 min in Ca(2+)-free solution, simultaneous re-exposure to ACh in the presence of 2.5 mM Ca(2+) increased [Ca(2+)](i) to the control steady-state level without overshoot. The tension attained was the same as control for each [ACh] used. Continuous exposure to successively increasing [ACh] (0.1-100 &mgr;M) also reduced the overshoot of [Ca(2+)](i) at 10 and 100 &mgr;M ACh, yet tension reached control levels at each [ACh] used. We conclude that the steady-state increase in [Ca(2+)](i) is necessary for tension maintenance and is dependent on Ca(2+) influx through voltage-gated calcium channels at 0.1 &mgr;M ACh and through a verapamil-intensitive pathway at 10 and 100 &mgr;M. The initial transient increase in calcium arises from intracellular stores and is correlated with the magnitude of tension only in muscles that have completely recovered from previous exposure to agonists. Copyright 2001 S. Karger AG, Basel

Airway smooth muscle relaxation

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Alteration in Ca2+ availability involved in antigen-induced airway hyperresponsiveness in rats

The origin of Ca2+ contributing to the enhancement of acetylcholine-induced bronchial smooth muscle constriction in airway hyperresponsiveness induced by antigen challenge was investigated. Under Ca(2+)-free (concomitant with 10(-6) M nicardipine) conditions, the contractile responses of bronchial rings to 1 mM acetylcholine were significantly greater in rings from rats with hyperresponsive airways (0.15 +/- 0.04 g) than those of rings from normal rats (0.02 +/- 0.004 g; P < 0.05). The cumulatively administered Ca2+ induced a markedly greater bronchoconstriction in rings from rats with hyperresponsive airways in Ca(2+)-free solution when muscles were pretreated with 1 mM acetylcholine (in the presence of 10(-6) M nicardipine) than in rings from normal rats, whereas no significant difference in Ca(2+)-induced bronchoconstriction was observed between the two groups when muscles were pretreated with 60 mM K+ (in the presence of 10(-6) M atropine). These findings suggest that enhancement of the availability of Ca2+ released from intracellular stores and/or influxed through receptor-operated Ca2+ channels in airway smooth muscles might be involved in the airway hyperresponsiveness to acetylcholine in rats.

Effects of calcitonin gene-related peptide on intracellular calcium concentration in longitudinal muscle of guinea pig ileum

The effect of rat calcitonin gene-related peptide (rCGRP) on intracellular free calcium concentration ([Ca2+]i) and its relationship with muscle relaxation were examined in plexus-free longitudinal muscle (LM) of the guinea pig ileum using a [Ca2+]i-tension simultaneous recording technique. Tissue was stimulated with either histamine (0.5 microM) or KCl (30 mM). rCGRP at a concentration of 263 nM (which displayed maximal relaxation of the LM) caused a small and brief but significant decrease in the [Ca2+]i in histamine-treated tissue. On the other hand, the same concentration of rCGRP relaxed the muscle without affecting the [Ca2+]i in KCl-treated tissue. rCGRP caused a dissociation between the changes in [Ca2+]i and tension. The effects of forskolin 0.5 microM) and nifedipine (10 nM) on both [Ca2+]i and tension were examined in comparison with rCGRP. Forskolin lowered the tension to greater extent than the [Ca2+]i whereas nifedipine (10 nM) diminished both the [Ca2+]i and tension in a parallel manner in histamine-treated tissues. These results suggest that rCGRP may not reduce [Ca2+]i as its primary mechanism of relaxation and it may change the calcium sensitivity of the contractile elements of the smooth muscle.

Inhibitory effects of halothane on high(K+)-induced canine tracheal smooth muscle contraction and intracellular Ca2+ increment

Halothane is a potent bronchodilator. The effects of halothane on isolated canine tracheal smooth muscle contraction and intracellular Ca(2+) increment induced by a high concentration of K+ were investigated to clarify how this anaesthetic decreases intracellular Ca2+ concentration ([Ca2+]i), an important second messenger. The tension of the muscle strips was measured using an isometric transducer, and [Ca2+]i was measured using a surface fluorescence spectroscopy. Exposure to a 72.7 mM K+ solution increased muscle tension and [Ca2+]i. Halothane (1,2,3 and 4% at the vaporiser) was introduced by bubbling in the presence of the 72.7 mM K+, and significantly decreased both this elevated muscle tension and the [Ca2+]i in a concentration-dependent manner. Similarly, slowly decreasing concentrations of K+ (48.5, 36.4, 24.2 and 18.2 mM) significantly decreased both of these variables. There was no significant difference between the slopes of the two regression lines of changes in muscle tension and changes in [Ca2+]i. From the evidence that tonic increase in [Ca2+]i by high concentrations of K+ is due to the influx of Ca2+ via L-type voltage channels, halothane may modify the L-type channels to decrease Ca2+ influx. In conclusion, halothane inhibits the high K(+)-induced canine airway smooth muscle contraction by decreasing [Ca2+]i. The decrease in [Ca2+]i by halothane might be due to inhibition of voltage-operated channels, especially the L-type channels.

Inhibitory effects of nordihydroguaiaretic acid on ETA-receptor-mediated contractions to endothelin-1 in rat trachea

1. It has been shown previously that nordihydroguaiaretic acid (NDGA) inhibits endothelin-1 (ET-1)-induced contractions in rat isolated tracheal smooth muscle. To investigate the underlying mechanisms, this study examined the effects of NDGA on various aspects of the ETA and ETB receptor-effector systems which mediate ET-1-induced contractions in this preparation. 2. NDGA inhibited contractions induced by each of the isoforms of ET (ET-1, ET-2 and ET-3) but not those induced by the ETB receptor-selective agonist, sarafotoxin S6c, the cholinoceptor agonist, carbachol or the depolarizing spasmogen, KCl. 3. Quantitative autoradiographic studies of [125I]-ET-1 binding to rat tracheal smooth muscle indicated that NDGA was not an ET receptor antagonist. 4. NDGA inhibited the ETA receptor-mediated, intracellular Ca(2+)-dependent contractions induced by 100 nM ET-1 in Ca(2+)-free solution (by 75%, P < 0.01). Furthermore, NDGA markedly inhibited the contractions induced by ryanodine and cyclopiazonic acid; contractions purportedly due to Ca2+ release from intracellular stores. 5. Like NDGA, the sarcoplasmic reticulum Ca(2+)-ATPase inhibitors cyclopiazonic acid and thapsigargin inhibited contractions to ET-1, but not carbachol or KCl. However, cyclopiazonic acid, but not NDGA, also (a) induced transient contractions in rat trachea, (b) potentiated contractions induced by KCl, and (c) potentiated the extracellular Ca(2+)-dependent phase of ET-1-induced contractions, indicating that NDGA did not inhibit ET-1-induced contractions through Ca(2+)-ATPase inhibition and depletion of sarcoplasmic reticular Ca2+. 6. In control preparations, ET-1 induced a slowly developing, sustained contraction. However, in the presence of NDGA or the ETA receptor antagonist, BQ123, ET-1-induced contractions resembled the transient contractions induced by sarafotoxin S6c. In nominally Ca2+-free solution, ETA receptor mediated contractions induced by ET-1 developed very slowly and were inhibited by NDGA.7. Additional studies indicated that the inhibitory effects of NDGA on endothelin-1-induced contractions were not the result of any significant actions of NDGA on lipoxygenase, cytochrome P450, L- orT-type Ca2+-channels, Na+-channels or protein kinase C.8. In summary, NDGA selectively inhibited ET-1-induced contractions in rat tracheal smooth muscle via a lipoxygenase-independent mechanism involving inhibition of the ETA but not the ETB, receptor effector system. NDGA did not appear to inhibit the initial events in the ETA signal transduction pathway, such as receptor binding and protein kinase C activation. However, NDGA inhibited the intracellular Ca2+-dependent component of ET-1-induced contraction, possibly by inhibiting mobilisation of intracellular Ca2+. As an apparent direct consequence of inhibiting the ETA receptor-effector system, NDGA markedly changed the time course of ET-1-induced contractions; from a slowly developing and sustained contraction into a transient contraction resembling that induced by sarafotoxin S6c.

Effects of isoprenaline on cytosolic calcium concentrations and on tension in the porcine coronary artery

1. Using front-surface fluorometry and fura-2-loaded medial strips of the porcine coronary artery, cytosolic Ca2+ concentration ([Ca2+]i) and tension development were simultaneously monitored in an attempt to determine the mechanisms of vasorelaxation induced by l-isoprenaline (Iso). 2. Iso actively decreased [Ca2+]i of the strips at rest, both in the presence and absence of extracellular Ca2+. 3. In the presence of extracellular Ca2+, depolarization with high-external K+ solution induced an elevation of [Ca2+]i and tension of the rapid increase and sustained, steady-state type; both levels depended on external K+ concentration. When Iso was applied at the time of steady state of high-K(+)-induced [Ca2+]i elevation, there was an initial transient reduction (the first component) followed by a subsequent sustained reduction (the second component) of [Ca2+]i. For a given [Ca2+]i level during high-K+ depolarization, the tension developed in the presence of Iso was smaller than that in its absence. Thus, the [Ca2+]i-tension relationship during the steady state of high-K(+)-induced contraction was shifted to the right by Iso. Pretreatment with ryanodine, a compound which depletes Ca2+ stored in the sarcoplasmic reticulum, abolished the first component, but not the second sustained decrease in [Ca2+]i by Iso. 4. In the presence of extracellular Ca2+ (1.25 mM), histamine (Hist) induced an abrupt (the first component) and then sustained (the second component) elevations of [Ca2+]i, while the tension rose rapidly to reach the peak, and then, gradually declined. The second, but not the first, component of [Ca2+]i elevation depended on extracellular Ca2+. Iso inhibited both the first and the second components of [Ca2+]i elevation and the contraction induced by Hist, in a concentration-dependent manner (IC50, 2 x 10(-8) M for the first component, and 5 x 10(-8) M for the second component). Cumulative application of Hist (10(-7)-10(-4) M) increased [Ca2+]i and tension with the [Ca2+]i-tension relationship shifting to the left from that observed with high K+. The [Ca2+]i-tension relationship during the Hist-induced contraction shifted to the right by Iso. In contractions induced by a higher concentration (> or = 6 x 10(-5) M) of Hist, despite the negligible decrease in [Ca2+]i, Iso could relax the muscle in a concentration-dependent manner. 5. In the absence of extracellular Ca2+, Hist induced transient elevations of [Ca2+]i and tension, possibly due to a release of Ca2+ from intracellular stores, and with similar time courses to those of the first component observed in the presence of extracellular Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Isoproterenol changes the relationship between cytosolic Ca2+ and contraction in guinea pig taenia caecum

To determine the role of beta-adrenoceptors in the regulation of intestinal smooth muscle, the action of isoproterenol (ISO) on cytoplasmic Ca2+ level ([Ca2+]cyt) and mechanical activity in the isolated guinea pig taenia caecum was examined. Spontaneous changes in [Ca2+]cyt and contraction were inhibited by ISO (0.1-1 microM) without changing resting [Ca2+]cyt. ISO more strongly inhibited the histamine-induced contraction than the high K(+)-induced contraction. ISO inhibited muscle tension more strongly than [Ca2+]cyt stimulated by high K+ and thus shifted the [Ca2+]cyt-tension curve to the lower-right. In the muscle stimulated by histamine, on the other hand, ISO inhibited both [Ca2+]cyt and tension. Salbutamol, a beta 2-selective agonist, showed similar effects as ISO on spontaneous, high K(+)- and histamine-stimulated [Ca2+]cyt and tension. Stimulation of beta-adrenoceptors by ISO increased cyclic AMP content without changing cyclic GMP content. These results suggest that activation of beta 2-adrenoceptors by ISO inhibits the contractions by two mechanisms of action: decrease in Ca2+ sensitivity of contractile elements in the muscle stimulated by K(+)-depolarization and decrease in [Ca2+]cyt in the muscle stimulated by histamine. These effects may be mediated by cyclic AMP.

Ca2+ regulation of the contractile apparatus in canine gastric smooth muscle

1. The relationships between cytosolic Ca2+ ([Ca2+]cyt; expressed as a fluorescence ratio at 400 nm and 500 nm using Indo-1) and contractile force was examined in strips of circular smooth muscles of canine gastric antrum. Rhythmic increases in [Ca2+]cyt were observed and contractions were biphasic. 2. In most muscles (70%), the amplitude of the second phase of the Ca2+ transient was less than or equal to the first phase of the Ca2+ transient, but the second phase of the contraction was much smaller than the first phase, suggesting a decrease in Ca2+ sensitivity during the second contractile phase. In 30% of muscles, the amplitude of the second phase of the Ca2+ transient was 2- to 3-fold greater than the first phase. In these muscles, the second phase of contraction was 10-fold greater than the first phase of contraction. Thus, a non-linear relationship between [Ca2+]cyt and force greatly amplifies force development when [Ca2+]cyt exceeds a threshold level. 3. Acetylcholine (ACh, 0.3-1 microM) increased the amplitudes of Ca2+ transients and basal [Ca2+]cyt between phasic contractions. The increase in basal [Ca2+]cyt did not cause tone to develop. ACh increased the amplitude of Ca2+ transients 2- to 3-fold and this was associated with a 15 to 20-fold increase in the force of phasic contractions. Pentagastrin (0.5 nM) and cholecystokinin octapeptide (CCK, 40 nM) had similar effects on Ca2+ transients and phasic contractions. 4. Bay K 8644 (0.1 microM) and TEA (5 mM) also increased the amplitudes of Ca2+ transients by 2- to 3-fold and phasic contractions by 15- to 30-fold. There was no significant difference observed between the [Ca2+]cyt-force relationships in the presence of agonists (i.e. ACh, pentagastrin and CCK) or when [Ca2+]cyt was increased by Bay K 8644 or TEA. These data suggest that agonist-dependent increases in Ca2+ sensitivity may not significantly regulate the [Ca2+]cyt-force relationship in antral muscles. 5. D600 (5 microM), added during stimulation with ACh (0.3 M), decreased [Ca2+]cyt and force without affecting the [Ca2+]cyt-force relationship. 6. Mechanisms exist for agonist-mediated enhancement of the Ca(2+)-force relationship. In alpha-toxin-permeabilized antrum, ACh (10 microM) with GTP (100 microM) or GTP gamma S (100 microM) increased the Ca(2+)-induced contraction at clamped levels of Ca2+. Phorbol 12,13-dibutyrate (PDBu, 10 microM) also increased the contractile force at a given level of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of relaxing action of the antiasthmatic drug, azelastine, in isolated porcine tracheal smooth muscle

Azelastine (1-300 microM) inhibited contractions of isolated porcine trachea induced by high K+, carbachol and endothelin-1 (ET-1) with a decrease in [Ca2+]cyt (as measured by fura-2-fluorescence). Verapamil (0.1-10 microM) also inhibited the high K(+)-induced increases in [Ca2+]cyt and contraction, although it only partially inhibited the responses evoked by carbachol or ET-1. In the absence of extracellular Ca2+ (with 0.5 mM EGTA), carbachol induced a transient increase in [Ca2+]cyt and force by releasing Ca2+ from cellular stores. Azelastine (100 microns) completely inhibited these contransient changes. In the absence of extracellular Ca2+, carbachol and 12-deoxyphorbol 13-isobutyrate (DPB) induced small sustained contractions without increasing [Ca2+]cyt. Azelastine inhibited these contractions. In muscle permeabilized with alpha-toxin, Ca2+ (0.3-3 microM) induced contraction in a concentration-dependent manner. DPB (without GTP) and carbachol or ET-1 (with GTP) enhanced the Ca(2+)-induced contraction. Azelastine partially inhibited the contraction induced by 0.3 microM Ca2+ but not the contraction induced by 3 microM Ca2+, and strongly inhibited the potentiating effects of DPB, carbachol and ET-1. Azelastine had no effect on the content of cyclic AMP or cyclic GMP. These results suggest that azelastine inhibits smooth muscle contraction by (i) decreasing [Ca2+]cyt, by inhibition of Ca2+ channels, (ii) decreasing agonist-induced Ca2+ release, and (iii) direct inhibition of contractile elements.

Some properties of Ca(2+)-induced Ca2+ release mechanism in single visceral smooth muscle cell of the guinea-pig

1. Late transient outward Ca(2+)-dependent K+ current (ILTO) correlated with Ca(2+)-induced Ca2+ release mechanism was studied in relation to the calcium inward current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum using the whole-cell patch-clamp technique. 2. The voltage dependencies of peak ICa and ILTO were both bell shaped. However, the I-V curve of the outward current was shifted toward more positive potentials by about 60 mV in comparison to that for ICa. 3. Reduction in the external Ca2+ concentration resulted in a decrease of peak amplitude of both ICa and ILTO. However, caffeine-induced outward current was also decreased abruptly suggesting a rapid loss of stored Ca2+ upon lowering the external Ca2+ concentration. 4. Investigation of the relation of ILTO to partially inactivated ICa showed that inactivation of ICa by approximately 65, 80 or 84% of control (produced by prepulse to -20 mV for 2 s, shifting the holding potential to -20 mV for 30 s or by the ramp voltage command from -50 to +10 mV, respectively) was without detectable effect on the ILTO generation. 5. Bath application of the Ca2+ antagonist nifedipine (300 nM) inhibited ICa by 81% without affecting ILTO peak amplitude (92.0 +/- 5.6% of control in six cells). The mean concentration-response curve for ICa inhibition was sigmoidal with the apparent dissociation constant of 86.9 nM, whereas that for the ILTO had a characteristic sharp transition indicating a definite threshold of Ca2+ influx for ILTO generation. 6. Application of Ca(2+)-free external solution during 500 ms of the time when ICa peaked inhibited the current by about 76% whereas the ILTO during such an intervention remained virtually unchanged. 7. In double-pulse experiments, with conditioning and test pulses to +10 mV from -50 mV and an interpulse interval of 600 ms, most of the cells (about 80%) showed larger outward current at the test pulse suggesting continued Ca2+ release triggered by Ca2+ influx during a short (50-200 ms) depolarizing prepulse. The outward current could also be evoked at large positive potentials (presumably near the calcium equilibrium potential) where it did not occur normally by a prepulse to +10 mV for 50 ms. The charge transferred by Ca2+ current necessary to activate Ca2+ release in most of the cells was estimated to be from 6 to 20 pC. 8. The data are interpreted to suggest that the Ca(2+)-induced Ca2+ release mechanism operates in single ileal cells in a regenerative manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of endothelin on cytosolic Ca2+ level and mechanical activity in rat uterine smooth muscle

The effects of endothelin (ET) on cytosolic Ca2+ level ([Ca2+]i) and mechanical activity were examined in isolated rat uterine smooth muscle. ET-1, ET-2, ET-3 and sarafotoxin S6b (STX) induced rhythmic contractions superimposed on an increased muscle tone. The concentration needed to induce a half-maximum contraction (EC50) was 1.6-3.3 nM for ET-1, ET-2 and STX and higher than 200 nM for ET-3, suggesting that the ET(A) receptor is responsible for these contractions. The sensitivity to ET-1 of uterus at day 20 of gestation was higher than that of non-pregnant rat uterus. Contraction induced by ET-1 followed an increase in [Ca2+]i. The relation between [Ca2+]i and muscle tension, an an indicator of Ca2+ sensitivity of contractile elements, in the presence of ET-1 was identical to that in the presence of high K+ in non-pregnant and pregnant rat uteri. The ET-1-induced increases in [Ca2+]i and muscle tension were strongly inhibited by verapamil in non-pregnant rat uterus. In pregnant rat uterus, however, verapamil only partially inhibited the increases. The verapamil-insensitive portions of [Ca2+]i and contraction were inhibited by EGTA. In the absence of external Ca2+, ET changed neither [Ca2+]i nor muscle tension. These results suggest that ET-1 acts on ET(A) receptors, increase [Ca2+]i and induces contraction without changing Ca2+ sensitivity of contractile elements. The increase in [Ca2+]i seemed to be mediated by opening of L-type Ca2+ channels in non-pregnant rat uterus and also of non-L-type Ca2+ channels in pregnant rat uterus, but not by Ca2+ release from intracellular storage sites.