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Isoproterenol, cAMP, and bradykinin stimulate diacylglycerol production in airway epithelium.

Previous studies using phorbol esters and cell-free preparations suggest that protein kinase C (PKC) may regulate Cl- secretion and apical membrane Cl- channels in airway epithelium. To determine whether PKC may be involved in receptor-mediated control of secretion, we measured the mass of diacylglycerol (DAG) generated by two Cl- secretagogues, isoproterenol and bradykinin. Bradykinin increased cellular DAG at concentrations similar to those that increase inositol phosphates, suggesting that bradykinin stimulates phosphatidylinositol hydrolysis, as observed in other systems. Isoproterenol also increased cellular DAG at concentrations similar to those that stimulate adenosine 3',5'-cyclic monophosphate (cAMP) accumulation. The beta-adrenergic receptor antagonist, nadolol, blocked and cell-permanent analogues of cAMP mimicked the effect of isoproterenol. However, isoproterenol does not stimulate phosphatidylinositol turnover. Simultaneous addition of maximal concentrations of isoproterenol and bradykinin produced additive increases in DAG. To test the possibility that the isoproterenol-induced increase in DAG came from phosphatidylcholine turnover, we measured the release of water-soluble choline metabolites and the incorporation of choline into cellular lipids. Although phorbol ester and bradykinin stimulated phosphatidylcholine turnover, isoproterenol did not. These results suggest that isoproterenol and bradykinin generate DAG from the following different lipid sources: bradykinin stimulates phosphatidylinositol hydrolysis to produce DAG; isoproterenol stimulates an increase in DAG from unknown sources. The data suggest that simultaneous activation of cAMP-dependent protein kinase and PKC may occur during receptor-mediated stimulation of Cl- secretion.