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

Stimulation of adenylyl cyclase mediates the effects of beta-adrenergi c agonists and prostaglandin E(2) (PGE(2)) on tracheobronchial epithel ial cell function by increasing intracellular cyclic adenosine monopho sphate (cAMP). In turn, increases in cAMP affect airway function by mo dulating ciliary beating, chloride and water transport, mucus secretio n, and release of bronchoactive substances. This study examined the fu nction and regulation of the beta-adrenergic receptor-adenylyl cyclase system (beta AR-AC) in tracheal epithelial cells isolated from the ra bbit, a frequently used animal model of airway reactivity, inflammatio n, and electrolyte transport. beta AR number, assessed by ligand bindi ng 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 de termined by competitive antagonist displacement of iodopindolol by sel ective 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 epithel ial cells for 30 min with either isoproterenol (10 mu M) or the peptid e inflammatory mediator, bradykinin (100 mu M), markedly depressed sub sequent isoproterenol-stimulated cAMP synthesis. Isoproterenol-induced beta AR-AC desensitization appeared to be homologous since cAMP respo nses 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 gl yceride 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, brady kinin, which activates protein kinase C.