SG Kelsen, NC Higgins, S Zhou, IA Mardini and JL Benovic
Department of Medicine, Temple University School of Medicine, Philadelphia, Pennsylvania, USA.

Beta-adrenergic agonist-mediated activation of the beta receptor coupled-adenylyl cyclase (beta AR-AC) system expressed by human airway epithelial cells alters airway function. However, little is known about the magnitude of expression, subtype, and function of the beta receptor- adenylyl cyclase (beta AR-AC) system in human airway epithelial cells from healthy, nonsmoking subjects. Therefore, we characterized beta AR number and subtype and the cAMP response to isoproterenol (iso) in acutely dissociated human tracheocytes harvested from 22 healthy, nonsmoking adults during fibroptic bronchoscopy. Moreover, because the regulation of beta AR-AC system function in response to beta-agonists or inflammatory mediators released into the airway in asthma is poorly understood, we examined the cAMP response to iso after 30 min exposure of cells to iso or the protein kinase C activator, sn-1,2-dioctanoyl glycerol (diC8). The beta AR-AC system was highly expressed and functional in human airway epithelial cells. Group mean beta AR density (i.e., Bmax), equilibrium dissociation constant (Kd), and the percentage of beta 2AR subtypes assessed by radioligand binding were approximately 8,900 receptors/cell, 45 pM, and approximately 80%, respectively. Mean maximum cAMP production was approximately 42 pmol/10(5) cells, and the mean EC50 of the response to iso was 131 nM. However, Bmax and cAMP responses to iso varied considerably across subjects. For example, Bmax varied ninefold, and the EC50 of the cAMP response varied 39-fold interindividually. The EC50 was inversely related to beta AR density (r = -0.81, p < 0.05), suggesting that sensitivity of the cAMP response to iso was in part dependent on beta AR density. In all experiments, cAMP responses to iso stimulation were markedly desensitized in dose-dependent fashion by 30 min pretreatment with iso or diC8. For example, pretreatment with iso 10 microM or diC8 100 microM reduced maximum cAMP production to 22 and 63% of control values, respectively. These data indicate that: (1) the beta AR-AC system is highly expressed on acutely dissociated airway epithelial cells from normal adult, but beta AR expression and its functional coupling to adenylyl cyclase vary considerably interindividually; and (2) the beta AR-AC system of normal human airway epithelial cells is rapidly desensitized by exposure to beta-adrenergic agonists or activators of PKC.

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