Selective Inhibition of Histamine-evoked Ca Signals by Compartmentalized CAMP in Human Bronchial Airway Smooth Muscle Cells

Intracellular Ca and cAMP typically cause opposing effects on airway smooth muscle contraction. Receptors that stimulate these pathways are therapeutic targets in asthma and chronic obstructive pulmonary disease. However, the interactions between different G protein-coupled receptors (GPCRs) that evoke cAMP and Ca signals in human bronchial airway smooth muscle cells (hBASMCs) are poorly understood. We measured Ca signals in cultures of fluo-4-loaded hBASMCs alongside measurements of intracellular cAMP using mass spectrometry or [H]-adenine labeling. Interactions between the signaling pathways were examined using selective ligands of GPCRs, and inhibitors of Ca and cAMP signaling pathways. Histamine stimulated Ca release through inositol 1,4,5-trisphosphate (IP) receptors in hBASMCs. β-adrenoceptors, through cAMP and protein kinase A (PKA), substantially inhibited histamine-evoked Ca signals. Responses to other Ca-mobilizing stimuli were unaffected by cAMP (carbachol and bradykinin) or minimally affected (lysophosphatidic acid). Prostaglandin E (PGE), through EP and EP receptors, stimulated formation of cAMP and inhibited histamine-evoked Ca signals. There was no consistent relationship between the inhibition of Ca signals and the amounts of intracellular cAMP produced by different stimuli. We conclude that β-adrenoceptors, EP and EP receptors, through cAMP and PKA, selectively inhibit Ca signals evoked by histamine in hBASMCs, suggesting that PKA inhibits an early step in H receptor signaling. Local delivery of cAMP within hyperactive signaling junctions mediates the inhibition.