Role of Cyclic Adenosine Monophosphate in Reducing Superoxide Anion Generation in Guinea Pig Alveolar Macrophages

Alveolar macrophages (AM) may contribute to airway inflammation via release of superoxide anion (O–₂). Elevation of intracellular cyclic adenosine monophosphate (cAMP) levels has been shown to suppress O–₂ generation, although the exact mechanism is uncertain. To examine the inhibitory effect of cAMP against different stimuli for O–₂ generation, we compared the effect of cAMP on O–₂ generation caused by phorbol myristate acetate (PMA), a direct protein kinase C activator, and N-formyl-methionyl-leucyl-phenylalanine (FMLP), which couples its membrane receptor and stimulates guanosine triphosphate binding protein, in guinea pig AM. Cyclic nucleotide phosphodiesterase (PDE) isoenzyme inhibitors or CPT-cAMP, a cAMP analogue, were used in order to increase intracellular cAMP levels. The O–₂ generation caused by either PMA or FMLP was reduced by cilostazol (PDE 3 inhibitor) and Ro20-1724 (PDE 4 inhibitor), but not by zaprinast (PDE 5 inhibitor). The degree of reduction in O–₂ generation was not different between PMA and FMLP. Furthermore, CPT-cAMP also reduced PMA- or FMLP-induced O–₂ generation to a similar degree, although only high concentrations (10–⁵ or 10–⁴ mol/l) of this agent were effective in producing significant inhibition. A remarkable elevation of the cAMP level is required to produce the inhibitory effect on O–₂ generation in guinea pig AM. An elevation of cAMP may suppress O–₂ generation by inhibiting plural sites of the intracellular signaling pathways.