Human mast cells express the intermediate conductance Ca2+-activated K+ channel iKCa1, which opens following IgE-dependent activation. This results in cell membrane hyperpolarization and potentiation of both Ca2+ influx and degranulation. Mast cell activation is attenuated following exposure to beta2-adrenoceptor agonists such as salbutamol, an effect postulated to operate via intracellular cyclic AMP. In this study, we show that salbutamol closes iKCa1 in mast cells derived from human lung and peripheral blood. Salbutamol (1-10 microM) inhibited iKCa1 currents following activation with both anti-IgE and the iKCa1 opener 1-EBIO, and was reversed by removing salbutamol or by the addition of the selective beta2-adrenoceptor antagonist and inverse agonist ICI 118551. Interestingly, ICI 118551 consistently opened iKCa1 in quiescent cells, suggesting that constitutive beta2-receptor signaling suppresses channel activity. Manipulation of intracellular cAMP, Galphai, and Galphas demonstrates that the beta2-adrenergic effects are consistent with a membrane-delimited mechanism involving Galphas. This is the first demonstration that gating of the iKCa1 channel is regulated by a G protein-coupled receptor and provides a clearly defined mechanism for the mast cell "stabilizing" effect of beta2-agonists. Furthermore, the degree of constitutive beta2-receptor "tone" may control the threshold for human mast cell activation through the regulation of iKCa1.