Calcium ion mechanisms in airway smooth muscle: potential targets for novel symptomatic drugs for asthma

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.

Effect of different bronchodilators on airway smooth muscle responsiveness to contractile agents

"Functional antagonism" is often used to describe the general relaxant effect of beta 2 agonists and xanthines and their ability to protect the airways against bronchoconstrictor stimuli. This study in guinea pig isolated trachea addresses the question of whether the capacity of these drugs to protect against constrictor stimuli is related to smooth muscle relaxation. Three antimuscarinic drugs were also examined to determine whether antagonism of mediators other than muscarinic agonists might contribute to bronchodilatation by these antimuscarinic drugs. Terbutaline (1.1 x 10(-7), 2.2 x 10(-7) M), theophylline (2.2 x 10(-4), 4.4 x 10(-4) M), and enprofylline (5.2 x 10(-5), 1.0 x 10(-4) M) relaxed the tracheal tension that remained after indomethacin treatment. They did not, however, alter the carbachol concentration-response curve significantly. In addition, neither theophylline (2.2 x 10(-4) M) nor terbutaline (1.1 x 10(-7) M) altered histamine induced contraction. Atropine sulphate, glycopyrrolate, and ipratropium bromide had EC50 values of 10(-9) - 10(-8) M for relaxation of carbachol induced contractions, whereas concentrations of 10(-6) - 10(-3) M or greater were required to relax contractions induced by allergen and nine other non-muscarinic mediators. It is suggested that bronchodilatation by antimuscarinic drugs in vivo is due to inhibition of acetylcholine induced bronchoconstriction alone and that beta 2 agonists and xanthines have poor ability to protect airway smooth muscle against constrictor stimuli. Hence mechanisms other than bronchodilatation and "functional antagonism" should be considered to explain the protection against constrictor stimuli in asthma seen with beta 2 agonists and xanthines.

Tissue selectivity and spasmogen selectivity of relaxant drugs in airway and pulmonary vascular smooth muscle contracted by PGF2 alpha or endothelin

1. The spasmolytic effects of smooth muscle relaxant drugs with different mechanisms of action have been examined on isolated preparations of guinea-pig trachea and rat pulmonary artery. The preparations were contracted with concentrations of prostaglandin F2 alpha (PGF2 alpha) or endothelin selected to give approximately 80% of the agonist maximum response on each tissue. These concentrations also caused similar levels of tone (% of tissue maximum contraction) on each tissue. 2. With endothelin as the spasmogen, the potassium channel opening drug, pinacidil, was more potent on trachea (-log IC50 5.49) than on pulmonary artery (4.39), i.e. was airway-vascular selective, whereas with PGF2 alpha as the spasmogen it was more potent on pulmonary artery (6.01) than on trachea (5.27), i.e. was vascular-airway selective. 3. With endothelin as the spasmogen, fenoterol was also airway-vascular selective (8.35 on trachea; little effect on pulmonary artery), nitroprusside was vascular-airway selective (7.50 on pulmonary artery; 5.99 on trachea) and forskolin was non-selective (6.69 on trachea; 6.70 on pulmonary artery). Thus, the airway-vascular selectivity of the relaxant drugs varied with the drug. 4. On pulmonary artery, pinacidil, nitroprusside and forskolin were all more potent against PGF2 alpha than against endothelin, i.e. 42, 4 and 7 fold respectively. On trachea, these drugs were equipotent against PGF2 alpha and endothelin. 5. The results suggest that, in pulmonary artery, but not in trachea, the relative contribution of protein kinase C activation and calcium influx to the maintenance of tonic contractions to endothelin and PGF2 alpha may be different. If protein kinase C activation should be the predominant mechanism for endothelin in pulmonary artery, then it may be more difficult to reverse this with relaxant drugs that lower intracellular calcium. 6. The study indicates that the airway-vascular selectivity of relaxant drugs can be spasmogen-dependent as well as dependent on the mechanism of action of the relaxant drug. Thus, relaxant drugs, whether of interest for their airway or vascular effects, should be tested against a full range of spasmogens of likely pathophysiological importance.