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

Abstract

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 β₂-adrenoceptor agonists such as salbutamol, an effect postulated to operate via intracellular cyclic AMP. In this study, we show that salbutamol closes iK_Ca1 in mast cells derived from human lung and peripheral blood. Salbutamol (1–10 µM) inhibited iK_Ca1 currents following activation with both anti-IgE and the iK_Ca1 opener 1-EBIO, and was reversed by removing salbutamol or by the addition of the selective β₂-adrenoceptor antagonist and inverse agonist ICI 118551. Interestingly, ICI 118551 consistently opened iK_Ca1 in quiescent cells, suggesting that constitutive β2-receptor signaling suppresses channel activity. Manipulation of intracellular cAMP, Gαi, and Gαs demonstrates that the β₂-adrenergic effects are consistent with a membrane-delimited mechanism involving Gαs. This is the first demonstration that gating of the iK_Ca1 channel is regulated by a G protein-coupled receptor and provides a clearly defined mechanism for the mast cell “stabilizing” effect of β₂-agonists. Furthermore, the degree of constitutive β₂-receptor “tone” may control the threshold for human mast cell activation through the regulation of iK_Ca1.