Biochemical and electrical aspects of the tracheal relaxant action of AH 21-132

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Effects of inhaled glaucine on pulmonary responses to antigen in sensitized guinea pigs

The alkaloid (S)-(+)-1,2,9,10-tetramethoxyaporphine (glaucine) is a phosphodiesterase 4 inhibitor with bronchodilator and anti-inflammatory activity in vitro. In this study, we examined the in vivo effects of glaucine on an animal model of asthma. In ovalbumin sensitized guinea pigs, inhaled glaucine (10 mg ml(-1), 3 min) inhibited the acute bronchoconstriction produced by aerosol antigen (antigen response was 256+/-42 and 95+/-14 cm H(2)O l(-1) s(-1) in control and glaucine-treated animals, respectively; P<0.05). Pretreatment with glaucine (10 mg ml(-1), 10 min inhalation, 30 min pre- and 3 h post-antigen exposure) markedly reduced airway hyperreactivity to histamine, eosinophil lung accumulation, and increased eosinophil peroxidase activity in bronchoalveolar lavage fluid 24 h after exposure of conscious guinea pigs to aerosol antigen. In addition, inhaled glaucine (5-10 mg ml(-1), 3 min) inhibited the microvascular leakage produced after inhaled antigen at all airway levels. These data support the potential interest of phosphodiesterase 4 inhibitors in asthma treatment.

Phosphodiesterase 4 (PDE4) inhibitors in asthma and chronic obstructive pulmonary disease (COPD)

Phosphodiesterases (PDE) are a family of enzymes responsible for the metabolism of the intracellular second messengers cyclic AMP and cyclic GMP. PDE4 is a cyclic AMP specific PDE that is the major if not sole cyclic AMP metabolizing enzymes found in inflammatory and immune cells, and contributes significantly to cyclic AMP metabolism in smooth muscles. Based on its cellular and tissue distribution and the demonstration that selective inhibitors of this isozyme reduce bronchoconstriction in animals and suppress the activation of inflammatory cells, PDE4 has become an important molecular target for the development of novel therapies for asthma and COPD. This chapter will review the evidence demonstrating the ability of PDE4 inhibitors to modify airway obstruction, airway inflammation and airway remodelling and hyperreactivity, will present some preliminary findings obtained with theses compounds in clinical trials and and will discuss experimental approaches designed to identify novel compounds that maintain the beneficial activity of the initial selective PDE4 inhibitors but with a reduced tendency of elicit the gastrointestinal side effects observed with this class of compounds.

Effects of SCA40 on bovine trachealis muscle and on cyclic nucleotide phosphodiesterases

While UK-93,928 (1-[[3-(6,9-dihydro-6-oxo-9-propyl-1H-purin-2-yl)-4-ethoxyphenyl] sulfonyl]-4-methylpiperazine; 5 nM-5 microM) was devoid of relaxant activity, benzafentrine, isoprenaline, levcromakalim and SCA40 (6-bromo-8-methylaminoimidazo[1,2-a]pyrazine-2-carbonitrile) each relaxed histamine (460 microM)-precontracted bovine isolated trachealis. Each of these relaxants was antagonised by a K+-rich (80 mM) medium. Except in the case of levcromakalim, nifedipine (1 microM) offset this antagonism. Charybdotoxin (100 nM) antagonised isoprenaline in a nifedipine-sensitive manner but did not antagonise SCA40 or benzafentrine. Iberiotoxin (100 nM) did not antagonise SCA40. Acting on tissue precontracted with carbachol, SCA40 potentiated isoprenaline but did not potentiate sodium nitroprusside. While levcromakalim (1 and 10 microM) induced hyperpolarisation, SCA40 (1 and 10 microM) induced little change in the membrane potential of bovine trachealis. In trachealis preloaded with 86Rb+, levcromakalim (1 and 10 microM) promoted efflux of the radiotracer while SCA40 (1 and 10 microM) had no effect. Tested as an inhibitor of isoenzymes of cyclic nucleotide phosphodiesterase, SCA40 was most potent against the type III, less potent against the type IV and least potent against the type I isoenzyme. It is concluded that neither inhibition of phosphodiesterase type V nor the promotion of BKCa channel opening explains the tracheal smooth muscle relaxant activity of SCA40. This compound relaxes bovine tracheal smooth muscle mainly by inhibiting phosphodiesterase isoenzyme types III and IV.

The relaxant action of osthole isolated from Angelica pubescens in guinea-pig trachea

The effect of osthole, isolated from Angelica pubescens, on the contraction of guinea-pig trachea was studied. Osthole (25-100 mumol/l), theophylline (10-1000 mumol/l) and higher concentrations of nifedipine (0.1-100 mumol/l) suppressed the contraction response curves of tracheal smooth muscle caused by carbachol, prostaglandin F2 alpha (PGF2 alpha), U46619 (thromboxane A2 analogue) and leukotriene C4 (LTC4) in a concentration-dependent manner. The contraction caused by high K+ (120 mmol/l) and cumulative concentrations of CaCl2 (0.03-3 mmol/l) was also inhibited concentration-dependently by osthole (25-100 mumol/l), theophylline (10-1000 mumol/l) and lower concentrations of nifedipine (0.01-0.1 mumol/l). The relaxant actions of osthole were not affected by propranolol (1 mumol/l), glibenclamide (10 mumol/l) or removal of tracheal epithelium. Osthole (100 mumol/l) was still effective in causing tracheal relaxation in the presence of nifedipine (1 mumol/l). In Ca(2+)-free- and EGTA (0.2 mmol/l)-containing medium, the relaxing effect of osthole was more potent than in normal Krebs solution. Osthole (25 and 50 mumol/l) caused 2.9 and 6.5, or 3.0 and 5.6 fold, respectively, increase in potency of forskolin or sodium nitroprusside in causing tracheal relaxation but did not affect that by cromakalim. Osthole (50 mumol/l) enhanced the increase in tissue cAMP and cGMP levels induced by forskolin and sodium nitroprusside, respectively, and in higher concentrations (100 and 250 mumol/l), itself increased markedly tissue cAMP and cGMP contents. Osthole (10-250 mol/l) inhibited the activity of cAMP and cGMP phosphodiesterases in a concentration-dependent manner. It is concluded that osthole exerts a non-specific relaxant effect on the trachealis by inhibiting the cAMP and cGMP phosphodiesterases.

The relaxant actions on guinea-pig trachealis of atherosperminine isolated from Fissistigma glaucescens

The pharmacological activity of atherosperminine, isolated from Fissistigma glaucescens, was determined in isolated guinea-pig trachealis. Atherosperminine (25-100 microM) and theophylline (10-1000 microM) both inhibited the contractile response caused by carbachol, prostaglandin F2 alpha (PGF2 alpha), U46619 (thromboxane A2 analogue), leukotriene C4 (LTC4) and Ca2+ (in the presence of 120 mM KCl) in a concentration-dependent manner. The inhibition was characterized by a rightwards shift of the concentration-response curves with suppression of the maximal contraction. Propranolol (1 microM), glibenclamide (10 microM) and removal of tracheal epithelium did not modify the relaxant action of atherosperminine. Atherosperminine (25 and 50 microM) caused a 2.4- and 5.0-fold, respectively, potentiation of the action of forskolin to cause tracheal relaxation but did not potentiate the action of sodium nitroprusside or cromakalim. Atherosperminine (50 microM) potentiated the action of forskolin to increase tissue cAMP content and, in higher concentrations (100 and 250 microM), itself increased tissue cAMP but not cGMP content. Atherosperminine markedly inhibited cAMP phosphodiesterase but not cGMP phosphodiesterase in homogenates of guinea-pig trachealis. It is concluded that atherosperminine exerts a non-specific relaxant effect on the trachealis. Its major mechanism of action appears to be inhibition of cAMP phosphodiesterase, perhaps with a minor effect on cGMP phosphodiesterase at higher concentrations.

Trials of the bronchodilator activity of the isoenzyme-selective phosphodiesterase inhibitor AH 21-132 in healthy volunteers during a methacholine challenge test

1. An approximately steady-state reduction of specific airway conductance was induced in normal human subjects by means of a methacholine individualized loading+maintenance dose regime. Tested against this background bronchoconstriction, the mixed type III/IV phosphodiesterase inhibitor AH 21-132, ingested in doses up to 90 mg, had no detectable bronchodilator activity. 2. AH 21-132, infused intravenously over 15 min, evoked short-lived bronchodilatation at doses of 20 and 40 mg, without affecting blood pressure or heart rate. 3. AH 21-132, mixed 1:18.5 by weight with sucrose, dissolved in saline, nebulized and inhaled in doses between 2 and 24 mg of AH 21-132, produced dose-dependent bronchodilation. The ED50 was estimated as 9.2 mg AH 21-132. The peak relief of imposed bronchoconstriction was 80% and the apparent half-time of removal of AH 21-132 from its site of action was 25 min. 4. Inhaled, nebulized, hypertonic sucrose had a minor bronchodilator action. 5. AH 21-132, by intravenous and inhaled routes of administration, provides relief of methacholine-induced bronchoconstriction.

Selective inhibition of a high affinity type IV cyclic AMP phosphodiesterase in bovine trachealis by AH 21-132. Relevance to the spasmolytic and anti-spasmogenic actions of AH 21-132 in the intact tissue

The ability of a papaverine-derived bronchodilator, AH 21-132, to inhibit cyclic nucleotide hydrolysis and to increase the cAMP content and the activity of cAMP-dependent protein kinase (A-kinase) was evaluated in bovine tracheal smooth muscle (BTSM) and related to the mechanical effects elicited by this compound in vitro. AH 21-132 (100 nM-1 mM) produced a concentration-related relaxation of BTSM pre-contracted with methacholine (MCh) that was subject to marked functional antagonism. AH 21-132 (100 microM) also displayed anti-spasmogenic activity preventing the generation of tone induced by low, but not high, concentrations of MCh. Neither the spasmolytic nor anti-spasmogenic effects of AH 21-132 were antagonized by the beta 2-adrenoceptor blocking drug ICI 118551 (50 nM). Three Ca(2+)- and calmodulin-independent cyclic nucleotide phosphodiesterases (PDE) were resolved from the soluble fraction of BTSM homogenates by Q-Sepharose anion exchange chromatography. These PDEs were identified by kinetic and inhibitor sensitivity criteria as the Type II (cGMP-stimulated), Type IV (Ro 20-1724-inhibited) and Type V (cGMP-specific) isoenzymes. A small amount (approximately 5%) of a Type III PDE seemed to be present but this was not identified with certainty. AH 21-132 selectively inhibited Type IV PDE in a competitive manner with an IC50 and KI of 3.7 and 2.7 microM, respectively. AH 21-132 similarly increased the cAMP content (from 5.3 to 23.1 pmol/mg protein after 1 mM AH 21-132) and activated A-kinase (from 29.6% to 53.5% after 1 mM AH 21-132) in intact BTSM over the same concentration range at which this compound influenced tone. In addition, AH 21-132 in high concentrations (greater than 100 microM), while exerting no direct effect on A-kinase itself, markedly potentiated (ca. four-fold at 3 mM AH 21-132) the ability of cAMP to activate A-kinase without affecting the affinity of cAMP for this enzyme. It is concluded that the spasmolytic and anti-spasmogenic effects of AH 21-132 in BTSM may be related, in part, to its ability to inhibit Type IV PDE, increase the intracellular cAMP content and so activate A-kinase. A cyclic nucleotide-dependent mechanism is therefore proposed. In addition, the ability of AH 21-132 to augment cAMP-dependent phosphorylation in a cell-free system, when Type IV PDE is inhibited fully, provides the possibility that the observed relaxation elicited by high concentrations of AH 21-132, while cAMP-dependent, does not require any further increase in the intracellular cAMP concentration.