Release of intracellular calcium by two different second messengers in airway epithelium

To better understand regulation of Cl- secretion by airway epithelia, we measured the intracellular Ca2+ concentration ([Ca2+]c) using the Ca2+ indicator fura-2 and a fluorescent microscope imaging system. We examined receptor-mediated changes in [Ca2+]c in response to two Cl- secretagogues: the beta-adrenergic agonist isoproterenol and the peptide hormone bradykinin. Isoproterenol increased cell adenosine 3',5'-cyclic monophosphate (cAMP) levels but did not alter cellular accumulation of inositol phosphates. Bradykinin stimulated inositol phosphate accumulation but, in the presence of indomethacin, did not alter cAMP levels. Despite the difference in second messenger pathways, both bradykinin and isoproterenol transiently increased [Ca2+]c. Bradykinin stimulated inositol phosphate accumulation and increased [Ca2+]c with similar potencies, suggesting that bradykinin elevated [Ca2+]c by stimulating inositol phosphate production. The response to isoproterenol was inhibited by a beta-adrenergic antagonist, but not an alpha-adrenergic antagonist, and was mimicked by a membrane permeant analogue of cAMP. Isoproterenol also increased [Ca2+]c and cAMP at similar potencies. These results suggest that isoproterenol increased [Ca2+]c via cAMP. Both agonists increased [Ca2+]c when the extracellular [Ca2+] was reduced, suggesting that they release Ca2+ from intracellular stores. The ability of cAMP to increase [Ca2+]c suggests a mechanism by which cAMP- and Ca2+-activated membrane transport processes can be regulated in a coordinating manner.