Inducible nitric oxide synthase modulates cyclooxygenase-2 activity in the heart of conscious rabbits during the late phase of ischemic preconditioning

Cyclooxygenase-2 (COX-2) is known to mediate the cardioprotective effects of the late phase of ischemic preconditioning (PC); however, the signaling pathways involved in COX-2 induction following ischemic PC are unknown. In addition, although inducible nitric oxide synthase (iNOS) has been identified as a co-mediator of late PC together with COX-2, the interaction between iNOS and COX-2 in the heart is unknown. Using conscious rabbits, we found that the induction of COX-2 expression 24 hours after ischemic PC was blocked by pretreatment with inhibitors of protein kinase C (PKC), Src protein tyrosine kinases (PTKs), and nuclear factor-κB (NF-κB) but not by inhibitors of NOS or scavengers of reactive oxygen species (ROS). The selective iNOS inhibitors SMT and 1400W, given 24 hours after PC, abrogated the increase in myocardial prostaglandin E₂ (PGE₂) and 6-keto-PGF_1α, whereas the selective soluble guanylate cyclase inhibitor ODQ had no effect. COX-2 selective inhibitors (celecoxib and NS-398) did not affect iNOS activity. These results demonstrate that (i) ischemic PC upregulates cardiac COX-2 via PKC-, Src PTK-, and NF-κB-dependent signaling pathways, whereas generation of NO and ROS is not necessary, and (ii) the activity of newly synthesized COX-2 following PC requires iNOS-derived NO whereas iNOS activity is independent of COX-2-derived prostanoids, indicating that COX-2 is located downstream of iNOS in the protective pathway of late PC. The data also indicate that iNOS modulates COX-2 activity via cGMP-independent mechanisms. To our knowledge, this is the first demonstration that iNOS-derived NO drives prostanoid synthesis by COX-2 in the heart. NO-mediated activation of COX-2 may be a heretofore unrecognized mechanism by which NO exerts its salubrious effects in the late phase of PC.