FUNCTIONAL DESENSITIZATION OF BETA-AGONIST RESPONSES IN HUMAN LUNG MAST-CELLS

The beta adrenergic agonist isoprenaline inhibited the IgE-triggered release of the preformed mediator histamine from human lung mast cells (HLMC) in a dose-dependent fashion. After prolonged (greater than or equal to 4 h) pre-exposure of HLMC to isoprenaline, there was a subsequent diminution in the effectiveness of a second exposure of isoprenaline to inhibit the release of histamine from activated HLMC. This induced hypo-responsiveness to isoprenaline was both concentration and time dependent, Although maximal levels of desensitization were obtained after an initial prolonged (14-h) preincubation with a high (10(-5) M) concentration of isoprenaline, exposure of HLMC for a shorter (4-h) time period with a lower (3 X 10(-7) M) concentration of isoprenaline was also effective at inducing a functional desensitization to isoprenaline. The inhibitory activity of the beta(2) agonist fenoterol was attenuated after a prolonged (14-h) pretreatment step with isoprenaline (10(-5) M), whereas the inhibitory properties of other adenylate cyclase activators, prostaglandin E(2) and forskolin, were not affected appreciably, Prolonged (12-h) exposure of HLMC to the beta agonists fenoterol, salbutamol, and terbutaline also induced hyporesponsive states to beta agonists, qualitatively similar to that obtained with isoprenaline. The beta receptor antagonist propranolol, if coincubated with isoprenaline during the prolonged pretreatment step, protected against the subsequent refractoriness of the HLMC to isoprenaline. The glucocorticoid dexamethasone failed to prevent the isoprenaline-induced functional desensitization. In total, these results indicate that prolonged exposure of HLMC to beta agonists induces a state of selective hyporesponsiveness to agonists that act at beta adrenoreceptors. Should similar processes be operative in vivo, then the use of high-dose beta agonists in asthma may lead to a worsening of airways inflammation due to a diminished ability of beta agonists to inhibit the release of mediators from mast cells.