β2-Adrenoceptor regulation of the K+ channel iKCa1 in human mast cells

Human mast cells express the intermediate conductance Ca2+-activated K+ channel iK(Ca)1, 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 beta(2)-adrenoceptor agonists such as salbutamol, an effect postulated to operate via intracellular cyclic AMP. In this study, we show that salbutamol closes iK(Ca)1 in mast cells derived from human lung and peripheral blood. Salbutamol (1-10 mu M) inhibited iK(Ca)1 currents following activation with both anti-IgE and the iK(Ca)1 opener I-EBIO, and was reversed by removing salbutamol or by the addition of the selective beta(2)-adrenoceptor antagonist and inverse agonist ICI 118551. Interestingly, ICI 118551 consistently opened iK(Ca)1 in quiescent cells, suggesting that constitutive beta(2)-receptor signaling suppresses channel activity. Manipulation of intracellular cAMP, G alpha i, and G alpha s demonstrates that the beta(2)-adrenergic effects are consistent with a membrane-delimited mechanism involving G alpha s. This is the first demonstration that gating of the iK(Ca)1 channel is regulated by a G protein-coupled receptor and provides a clearly defined mechanism for the mast cell "stabilizing" effect Of beta(2)-agonists. Furthermore, the degree of constitutive beta(2)-receptor "tone" may control the threshold for human mast cell activation through the regulation of iK(Ca)1.