FUNCTIONAL-BEHAVIOR OF THE BETA-ADRENERGIC-RECEPTOR ADENYLYL-CYCLASE SYSTEM IN RABBIT AIRWAY EPITHELIUM

Stimulation of adenylyl cyclase mediates the effects of beta-adrenergic agonists and prostaglandin E(2) (PGE(2)) on tracheobronchial epithelial cell function by increasing intracellular cyclic adenosine monophosphate (cAMP). In turn, increases in cAMP affect airway function by modulating ciliary beating, chloride and water transport, mucus secretion, and release of bronchoactive substances. This study examined the function and regulation of the beta-adrenergic receptor-adenylyl cyclase system (beta AR-AC) in tracheal epithelial cells isolated from the rabbit, a frequently used animal model of airway reactivity, inflammation, and electrolyte transport. beta AR number, assessed by ligand binding using the non-subtype-specific beta-antagonist [I-125]iodopindolol, averaged similar to 10,700 beta AR/cell (400 fmol/mg membrane protein). Greater than 85% of the receptors were of the beta(2) subtype as determined by competitive antagonist displacement of iodopindolol by selective beta(1)-(betaxolol) and beta(2)-(ICI 118,551) antagonists. cAMP synthesis was stimulated with isoproterenol, PGE(2), and forskolin in a time- and concentration-dependent fashion. Preincubation of epithelial cells for 30 min with either isoproterenol (10 mu M) or the peptide inflammatory mediator, bradykinin (100 mu M), markedly depressed subsequent isoproterenol-stimulated cAMP synthesis. Isoproterenol-induced beta AR-AC desensitization appeared to be homologous since cAMP responses to PGE(2) and forskolin, a direct activator of adenylyl cyclase, were not reduced. The effect of bradykinin on isoproterenol-stimulated cAMP response was mimicked by preincubation with either dioctanoyl glyceride or phorbol myristate acetate, activators of protein kinase C. We conclude that the beta AR-coupled adenylyl cyclase system in rabbit tracheal epithelial cells can be desensitized by short-term exposure to either beta-adrenergic agonists or the inflammatory mediator, bradykinin, which activates protein kinase C.