Action of mediators on airway smooth muscle: functional antagonism as a mechanism for bronchodilator drugs

Physiological perspectives of therapy in bronchial hyperreactivity

PURPOSE

This paper reviews the literature on the aetiology and therapy of bronchial hyperreactivity to describe the underlying pathophysiology, identify patients at risk and update knowledge on new and existing therapies.

SOURCE

Information was obtained from monograms on New Drugs for Asthma, Respiratory Medicine: recent advances, Agents and Actions Supplements, Pulmonary Pharmacology, Anesth Analg, the European Journal of Respiration and a Medline literature search.

PRINCIPAL FINDINGS

Reduced airway calibre, increased bronchial contractility, altered permeability of the bronchial mucosa, humoral and cellular mediators, and dysfunctional neural regulation are critical factors for bronchial hyperreactivity, a characteristic feature of hyperreactive airways which results in bronchoconstriction after exposure to varied stimuli. Preoperative anaesthetic considerations in these patients include FEV1 and PEFR testing to assess the severity and for optimal control of the condition. Bronchospasm causing hypoxaemia is the major intraoperative problem anticipated in these patients. Current therapeutic management of bronchoconstriction focuses on the beta 2 agonists, theophylline and steroids. Besides relaxing the airway smooth muscle these agents are all capable of altering bronchial inflammatory responses. Future developments of therapy are directed towards the inflammatory components of the disease.

CONCLUSION

This review has presented background information on physiological mechanisms of smooth muscle contractility, pathophysiological alterations of bronchial contractility and the pharmacological basis of therapy in bronchoconstrictive disease. Information is presented to enable the prompt arrest and reversal of airway constriction, and to maintain prophylactic treatment during the perioperative period. Intraoperative bronchospasm is managed by adequate oxygenation and reversal of bronchoconstriction.

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.

Control of cyclic AMP levels in primary cultures of human tracheal smooth muscle cells

1. [3H]-adenosine 3':5'-cyclic monophosphate ([3H]-cyclic AMP) responses were studied in primary cultures of human tracheal smooth muscle cells derived from explants of human trachealis muscle and in short term cultures of acutely dissociated trachealis cells. 2. Isoprenaline induced concentration-dependent [3H]-cyclic AMP formation with an EC50 of 0.2 microM. The response to 10 microM isoprenaline reached a maximum after 5-10 min stimulation and remained stable for periods of up to 1 h. After 10 min stimulation, 1 microM isoprenaline produced a 9.5 fold increase over basal [3H]-cyclic AMP levels. The response to isoprenaline was inhibited by ICI 118551 (10 nM), (apparent KA 1.9 x 10(9) M-1) indicating the probable involvement of a beta 2-adrenoceptor in this response in human cultured tracheal smooth muscle cells. However, with 50 nM ICI 118551 there was a reduction in the maximum response to isoprenaline. Prostaglandin E2 also produced concentration-dependent [3H]-cyclic AMP formation (EC50 0.7 microM, response to 1 microM PGE2 6.4 fold over basal). 3. Forskolin (1 nM - 100 microM) induced concentration-dependent [3H]-cyclic AMP formation in these cells. A 1.6 fold (over basal) response was also observed following stimulation with NaF (10 mM). 4. The nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (0.1 mM) and the type IV, cyclic AMP selective, phosphodiesterase inhibitor rolipram (0.1 mM) both elevated basal [3H]-cyclic AMP levels by 1.8 and 1.5 fold respectively. IBMX (1-100 microM) and low concentrations of rolipram (< 10 microM), also potentiated the response to 1 microM isoprenaline. Inhibitors of the type III phosphodiesterase isoenzyme (SK&F 94120 and SK&F 94836) were without effect upon basal or isoprenaline-stimulated cyclic AMP responses in these cells.5. Carbachol (1 nM-I 00 microM) produced concentration-dependent inhibition of the [3H]-cyclic AMP response to 1 microM isoprenaline in human cultured tracheal smooth muscle cells (IC50 0.24 JM). Carbachol(1 JM) inhibited the [3H]-cyclic AMP response to 1 JM isoprenaline by 60%. This effect of carbachol was itself inhibited by atropine (50 nM) (KA 2.3 x 109 M-') indicating the involvement of a muscarinic receptor.6. These results show that primary cultures of human tracheal smooth muscle cells demonstrate cyclic AMP responses to direct receptor stimulation, adenylyl cyclase activation and inhibition with nonselective and type IV-selective cyclic AMP phosphodiesterase isoenzyme inhibitors, and that the cyclic AMP response to isoprenaline can be inhibited by muscarinic receptor stimulation.

Human bronchial cyclic nucleotide phosphodiesterase isoenzymes: biochemical and pharmacological analysis using selective inhibitors

1 The aims of the present study were to characterize the cyclic nucleotide phosphodiesterase (PDE) isoenzyme activities present in human bronchi and to examine the ability of selective isoenzyme inhibitors to relax histamine and methacholine precontracted preparations of human bronchi. 2 Three separations of pooled human bronchial tissue samples were performed. Ion-exchange chromatography showed that the soluble fraction of human bronchial preparations contains PDE I, II, III, IV and V isoenzyme activities. Multiple forms of PDE I and PDE IV were observed and PDE IV was the main cyclic AMP hydrolytic activity. 3 3-Isobutyl-l-methylxanthine (IBMX) non-selectively inhibited all separated isoenzyme activities. Zaprinast selectively inhibited PDE V, but also effectively inhibited one of the two PDE I isoforms identified. The PDE IV selective inhibitors rolipram and RO-201724, inhibited the PDE IV activities as did the dual PDE III/IV inhibitor, Org 30029. Org 9935, a PDE III selective inhibitor, potently attenuated part of the PDE IV activity peak in one of three separations performed, indicating that some PDE III activity may co-elute with PDE IV under the experimental conditions employed. 4 PDE IV-selective (rolipram), PDE III-selective (Org 9935) and dual PDE III/IV (Org 30029) inhibitors were effective relaxants of human bronchial smooth muscle. The PDE V/PDE I inhibitor, zaprinast was relatively ineffective. 5 The present study demonstrates in human bronchi, as in animal airways smooth muscle, that inhibitors of PDE III, PDEIV and dual PDE III/IV have potentially useful bronchodilator activity and are worthy of further consideration as anti-asthma drugs.

Cloning and expression of cDNA for a human low-Km, rolipram-sensitive cyclic AMP phosphodiesterase

We have isolated cDNA clones representing cyclic AMP (cAMP)-specific phosphodiesterases (PDEases) from a human monocyte cDNA library. One cDNA clone (hPDE-1) defines a large open reading frame of ca. 2.1 kilobases, predicting a 686-amino-acid, ca. 77-kilodalton protein which contains significant homology to both rat brain and Drosophila cAMP PDEases, especially within an internal conserved domain of ca. 270 residues. Amino acid sequence divergence exists at the NH2 terminus and also within a 40- to 100-residue domain near the COOH-terminal end. hPDE-1 hybridizes to a major 4.8-kilobase mRNA transcript from both human monocytes and placenta. The coding region of hPDE-1 was engineered for expression in COS-1 cells, resulting in the overproduction of cAMP PDEase activity. The hPDE-1 recombinant gene product was identified as a low-Km cAMP phosphodiesterase on the basis of several biochemical properties including selective inhibition by the antidepressant drug rolipram. Known inhibitors of other PDEases (cGMP-specific PDEase, cGMP-inhibited PDEase) had little or no effect on the hPDE-1 recombinant gene product. Human genomic Southern blot analysis suggests that this enzyme is likely to be encoded by a single gene. The presence of the enzyme in monocytes may be important for cell function in inflammation. Rolipram sensitivity, coupled with homology to the Drosophila cAMP PDEase, which is required for learning and memory in flies, suggests an additional function for this enzyme in neurobiochemistry.

Cloning and expression of cDNA for a human low-Km, rolipram-sensitive cyclic AMP phosphodiesterase

We have isolated cDNA clones representing cyclic AMP (cAMP)-specific phosphodiesterases (PDEases) from a human monocyte cDNA library. One cDNA clone (hPDE-1) defines a large open reading frame of ca. 2.1 kilobases, predicting a 686-amino-acid, ca. 77-kilodalton protein which contains significant homology to both rat brain and Drosophila cAMP PDEases, especially within an internal conserved domain of ca. 270 residues. Amino acid sequence divergence exists at the NH2 terminus and also within a 40- to 100-residue domain near the COOH-terminal end. hPDE-1 hybridizes to a major 4.8-kilobase mRNA transcript from both human monocytes and placenta. The coding region of hPDE-1 was engineered for expression in COS-1 cells, resulting in the overproduction of cAMP PDEase activity. The hPDE-1 recombinant gene product was identified as a low-Km cAMP phosphodiesterase on the basis of several biochemical properties including selective inhibition by the antidepressant drug rolipram. Known inhibitors of other PDEases (cGMP-specific PDEase, cGMP-inhibited PDEase) had little or no effect on the hPDE-1 recombinant gene product. Human genomic Southern blot analysis suggests that this enzyme is likely to be encoded by a single gene. The presence of the enzyme in monocytes may be important for cell function in inflammation. Rolipram sensitivity, coupled with homology to the Drosophila cAMP PDEase, which is required for learning and memory in flies, suggests an additional function for this enzyme in neurobiochemistry.

Pig aortic endothelial-cell cyclic nucleotide phosphodiesterases. Use of phosphodiesterase inhibitors to evaluate their roles in regulating cyclic nucleotide levels in intact cells

Two cyclic nucleotide phosphodiesterase (PDE) activities were identified in pig aortic endothelial cells, a cyclic GMP-stimulated PDE and a cyclic AMP PDE. Cyclic GMP-stimulated PDE had Km values of 367 microM for cyclic AMP and 24 microM for cyclic GMP, and low concentrations (1 microM) of cyclic GMP increased the affinity of the enzyme for cyclic AMP (Km = 13 microM) without changing the Vmax. This isoenzyme was inhibited by trequinsin [IC50 (concn. giving 50% inhibition of substrate hydrolysis) = 0.6 microM for cyclic AMP hydrolysis in the presence of cyclic GMP; IC50 = 0.6 microM for cyclic GMP hydrolysis] and dipyridamole (IC50 = 5 microM for cyclic AMP hydrolysis in the presence of cyclic GMP; IC50 = 3 microM for cyclic GMP hydrolysis). Cyclic AMP PDE exhibited a Km of 2 microM for cyclic AMP and did not hydrolyse cyclic GMP. This activity was inhibited by trequinsin (IC50 = 0.2 microM), dipyridamole (IC50 = 6 microM) and, selectively, by rolipram (IC50 = 3 microM). Inhibitors of cyclic GMP PDE (M&B 22948) and of low Km (Type III) cyclic AMP PDE (SK&F 94120) only weakly inhibited the two endothelial PDEs. Incubation of intact cells with trequinsin and dipyridamole induced large increases in cyclic GMP, which were completely blocked by LY-83583. Rolipram, SK&F 94120 and M&B 22948 did not significantly influence cyclic GMP accumulation. Dipyridamole enhanced the increase in cyclic GMP induced by sodium nitroprusside. Cyclic AMP accumulation was stimulated by dipyridamole and trequinsin with and without forskolin. Rolipram, although without effect alone, increased cyclic AMP in the presence of forskolin, whereas M&B 22948 and SK&F 94120 had no effects on resting or forskolin-stimulated levels. These results suggest that cyclic GMP-stimulated PDE regulates cyclic GMP levels and that both endothelial PDE isoenzymes contribute to the control of cyclic AMP.

Inhibitory effects of AH 21-132 in guinea-pig isolated ileum and taenia caeci

1. AH 21-132 is being investigated as a potential chemotherapeutic agent for bronchial asthma. The present experiments were designed to determine whether AH 21-132 shares the activity of theophylline as an antagonist at adenosine A1 receptors and to assess its potency as a relaxant in intestinal smooth muscle. 2. In the transmurally-stimulated guinea-pig ileum, theophylline (1 mM), but not AH 21-132 (1 and 10 microM), antagonized twitch depression induced by adenosine. Higher concentrations (100 microM and 1 mM) of AH 21-132 themselves had a depressant effect. Neither theophylline (1 mM) nor AH 21-132 (1 and 10 microM) antagonized twitch depression induced by noradrenaline. 3. AH 21-132 (100 microM and 1 mM) depressed maximum contractions of ileum induced by both acetylcholine (ACh) and histamine. 4. In ileum treated with hyoscine (1 microM), AH 21-132 (greater than 10 microM) caused a concentration-dependent depression of the log concentration-effect curve for potassium chloride. 5. Simultaneous extracellular electrophysiological and mechanical recording from taenia caeci showed that AH 21-132 (100 microM-1 mM) inhibited spontaneous tension waves and their associated bursts of electrical spike activity. 6. Intracellular electrophysiological recording from taenia caeci showed that the mechano-inhibitory effect of 1 mM AH 21-132 was accompanied by abolition of spontaneous spike activity. Following spike abolition, the membrane potential assumed a value very close to that observed during periods of electrical quiescence prior to drug exposure. 7. AH 21-132 inhibited the activity of cyclic AMP-dependent and cyclic GMP-dependent phosphodiesterases derived from homogenates of ileal smooth muscle. The effective concentration ranges were 0.1-1OOO microM and 1-1000 microM, respectively. Theophylline, too, inhibited these enzymes but in each case was less potent than AH 21-132. 8. It is concluded that AH 21-132 is devoid of antagonist activity at adenosine Al receptors which modulate ACh release from intramural cholinergic nerves in the ileum. At concentrations greater than IO microM, AH 21-132 has a relaxant effect on intestinal smooth muscle characterized by suppression of spontaneous action potentials but by minor change in resting membrane potential. AH 21-132 previously has been reported to depress the spontaneous tone of trachealis muscle with an EC50 value of less than lO microM and the present experiments therefore show that this agent is much less potent in inhibiting intestinal muscle. This potency difference cannot be attributed to a tissuerelated difference in the potency of AH 21-132 as an inhibitor of cyclic AMP- or cyclic GMPdependent phosphodiesterases.

Analysis of the relaxant effects of AH 21-132 in guinea-pig isolated trachealis

1. Experiments have been performed with the dual intent of analysing the mechanism by which AH 21-132 relaxes airways smooth muscle and determining whether the effects of this compound can be distinguished from those of theophylline. 2. AH 21-132 (0.25-8 microM) and theophylline (1-1000 microM) each caused concentration-dependent suppression of the spontaneous tone of guinea-pig isolated trachealis. The maximal effect of AH 21-132 was equivalent to that of theophylline. No evidence was obtained that the tissue became sensitized or desensitized to the action of AH 21-132. 3. Propranolol (1 microM) profoundly antagonized the tracheal relaxant action of isoprenaline but not that of AH 21-132. 4. In indomethacin (2.8 microM)-treated tissues, tone was induced by K+-rich (120 mM) Krebs solution, acetylcholine (ACh, 1 mM) or histamine (200 microM). Log concentration-relaxation curves for AH 21-132, isoprenaline and theophylline were all moved to the right in the presence of the spasmogens, the smallest rightward shift being induced by histamine and the greatest by ACh. While maximal effects of AH 21-132 and theophylline were unaffected by the spasmogens, that of isoprenaline was reduced by KCl and ACh. 5. In tissues treated with indomethacin (2.8 microM), AH 21-132 (0.1-100 microM) inhibited the spasmogenic effects of potassium chloride (KCl), ACh and histamine in a concentration-dependent manner. The inhibition was characterized by rightward shifts in the spasmogen concentration-effect curves with depression of their maxima. 6. In tissues treated with both indomethacin (2.8 microM) and ACh (1 mM), the removal of tracheal epithelium caused a small but significant leftward shift in the log concentration-relaxation curve for AH 21-132 but did not alter that for theophylline. 7. In tissues treated with indomethacin (2.8 microM) and maintained at 12 degrees C, theophylline (0.1-3.2 mM) caused concentration-dependent spasm. This effect was not shared by AH 21-132. 8. AH 21-132 (0.1-1000 microM) more potently inhibited the activity of cyclic AMP-dependent than of cyclic GMP-dependent phosphodiesterase derived from homogenates of guinea-pig trachealis. Theophylline, too, inhibited these enzymes but was less potent in each case than AH 21-132 and did not exhibit selectivity for the cyclic AMP-dependent enzyme. 9. It is concluded that AH 21-132 exerts a non-specific (i.e. effective no matter what agent is used to support tone) relaxant effect on the trachealis muscle which does not involve the activation of beta l-adrenoceptors. The profile of the relaxant action of AH 21-132 more closely resembles that of theophylline than that of isoprenaline. However, AH 21-132 can be differentiated from theophylline in that: (a) its relaxant potency is increased by epithelial removal; (b) it does not cause tracheal spasm; (c) it exhibits selectivity as an inhibitor of cyclic AMP-dependent as opposed to cyclic GMP-dependent phosphodiesterase. It is possible that the relaxant effects of AH 21-132 are related to its ability to inhibit cyclic nucleotide phosphodiesterases.