Reflections on some aspects of current research in asthma

Airway responsiveness to beta-adrenergic agonist (salbutamol) in asthma

Despite the controversy of airway responsiveness to beta2-agonist drugs in asthma, in a previous study we showed increased responsiveness of asthmatic airways to isoprenaline. Therefore, in the present study of airway sensitivity to other beta2-agonists, salbutamol and its relationship to histamine responsiveness was reexamined. The threshold bronchodilator concentrations of inhaled salbutamol required for a 20% increase in forced expiratory flow in 1 sec (FEV1), (PC20) was measured in 20 normal and 19 asthmatic adults. Airway responsiveness to histamine, as the concentration that caused a 20% decrease in FEV1, was also measured in 11 normal and 12 asthmatic subjects; and the correlation between PC20 salbutamol and PC20 histamine was evaluated. Sensitivity to salbutamol was greater in asthmatics (PC20 = 7.24 mg/L) than in non-asthmatics (PC20 = 124.25 mg/L, p < 0.001). Airway responsiveness to histamine in asthmatics (PC20 = 0.18 g/L) was also significantly greater than in normal subjects (PC20 = 19.46 g/L, p < 0.001). There was a significant correlation between PC20 salbutamol and histamine (Rs = 0.6052, p < 0.005). Maximum response to both salbutamol and histamine and slope of concentration-response curves of both agents were significantly greater in patients with asthma than in normal subjects (p < 0.001 and p < 0.005 for maximum response and slope, respectively). The increased sensitivity of asthmatics to inhaled salbutamol suggests that they also may be more sensitive to their endogenous adrenaline, which may thus dilate and stabilize their airways.

Bronchial responsiveness to beta-adrenergic stimulation and enhanced beta-blockade in asthma

OBJECTIVE

Beta-adrenergic blocking drugs have no effect on the airways of normal subjects but in asthma even small doses may cause severe deterioration. A seemingly obvious explanation for this abnormal sensitivity is that asthmatic airways, but not normal airways, are tonically dilated by the sympatho-adrenal system. However, studies suggest that sympatho-adrenal activity is normal in asthma, as is bronchial sensitivity to beta-agonists. The present study re-examines bronchial beta-adrenergic sensitivity in asthma and also measures antagonism produced in normal and asthmatic bronchi by a beta-blocking drug.

METHODOLOGY

The threshold bronchodilator doses of inhaled isoprenaline (dose required for a 35% increase in specific airway conductance 'PD35') was measured in 11 normal and 14 asthmatic adults on two separate occasions. After administering propranolol (inhaled or intravenously) PD35 was remeasured.

RESULTS

Sensitivity to isoprenaline was greater in symptomatic asthmatics (PD35 = 0.003 micromol) than in asymptomatic asthmatics (PD35 = 0.021 micromol) and in non-asthmatics (PD35 = 0.123 micromol; P < 0.001 in each case). Symptomatic asthmatics also showed 66-fold more antagonism than non-asthmatics when they were given propranolol by inhalation (P < 0.001) and sixfold more with intravenous propranolol (P = 0.005).

CONCLUSIONS

The increased sensitivity of asthmatics to inhaled isoprenaline suggests that they may also be more sensitive to their endogenous adrenaline, which may thus dilate and stabilize their airways. Taken with enhanced antagonism by propranolol, this provides insight into the aggravation of asthma by beta-blocking drugs.

Concomitant glucocorticoid treatment prevents the development of beta-adrenoceptor desensitization in the guinea pig lung

The aim of the present study was to investigate, whether concomitant administration of the synthetic glucocorticoid betamethasone (BM), theophylline (THEO), or the muscarinic antagonist ipratropium bromide (IPRA) could influence the desensitization-associated decrease of beta-adrenoceptors in the guinea pig lung during prolonged in vivo treatment with the beta 2-agonist terbutaline (TER). The animals were sacrificed 20 hrs after the last drug dosage and the lung membrane homogenates were prepared for 3H-dihydroalprenolol (3H-DHA) binding in vitro. Treatment with TER 200 micrograms/kg subcutaneously twice a day for five days decreased by 22% the maximum number of binding sites (Bmax) at saturation in comparison with the saline-treated controls. Concomitant administration of BM 2 mg/kg intraperitoneally abolished this effect of TER, whereas THEO 20 mg/kg or IPRA 5 micrograms/kg failed to modify it. None of the in vivo treatments affected the binding affinity of 3H-DHA. In vitro, TER inhibited in a concentration-dependent manner 3H-DHA binding to the lung membranes of untreated guinea pigs. At high concentrations IPRA, but not THEO or BM, showed some binding to the beta-receptors as well. Thus, it is concluded that glucocorticoids may prevent beta-adrenoceptor desensitization in the lungs via an indirect mechanism, e.g. inhibition of phospholipase A2 enzyme.

Adrenergic receptors: possible mechanism of inverse regulation of alpha- and beta-receptors

Many physiologic and pathologic conditions, including bronchial asthma, are associated with inverse changes in alpha- and beta-receptor-mediated responses in various tissues. The direction of the change elicited by a given stimulus is tissue specific, as exemplified by the actions of thyroid hormones: In the rat heart, hypothyroidism reduces beta- and increases alpha-receptor responses, whereas in the rat liver it has the opposite effects. A similar increase in beta- and decrease in alpha-receptor responses in the rat liver is triggered by a number of different conditions, including glucocorticoid deficiency, that appear to represent lower levels of cellular differentiation. Among these, incubation of isolated hepatocytes in a serum-free buffer triggers the conversion of the receptor response in vitro within 4 hours, without parallel changes in the density or affinity of receptor binding sites. This change can be acutely reversed by an endogenous inhibitor of membrane phospholipase A2, or accelerated by an activator of phospholipase A2, suggesting that changes in the activity of this enzyme are involved in the conversion of the hepatic adrenoceptor response. The glucocorticoid-induced increase in beta-receptors in cultured human lung adenocarcinoma cells also appears to be mediated indirectly through the induction of an endogenous inhibitor (lipomodulin) of membrane phospholipase A2. The possible relevance of altered membrane phospholipid metabolism in the pathomechanism of asthma and in the associated glucocorticoid-sensitive changes in adrenergic receptor mechanisms is discussed.

Immunologic lung disease

The term "immunologic lung disease" comprises a broad spectrum of disease. I have covered a few entities in which recent studies have been particularly helpful in elucidating pathophysiology though not in uncovering the inciting cause. Common to all of these entities is the problem of finding appropriate methods of defining disease activity and response to treatment. As exemplified by the improved outlook for Goodpasture's syndrome with elucidation of its underlying immunopathology, it is likely that better understanding of the immunologic basis of sarcoid and interstitial disease may be helpful in planning more effective treatment strategies.