Thrombin or Ca++-Ionophore-Mediated Fall in Endothelial ATP Levels Independent of Poly(ADP-Ribose) Polymerase Activity and NAD Levels - Comparison with the Effects of Hydrogen Peroxide

To test the hypothesis that a fall in cellular ATP following stimulation of endothelial cells with thrombin is secondary to a decrease in NAD levels caused by poly(ADP-Ribose)polymerase (PARP), we measured the levels of NAD and ATP in endothelial cells after treatment with thrombin, the Ca⁺⁺-ionophore A23187, or hydrogen peroxide (H₂O₂), and compared the effects of inhibitors of PARP, NAD synthesis, and ADP-ribose breakdown on these responses. Neither thrombin nor A23187 caused a reduction in endothelial NAD levels and A23187 affected ATP levels independently of NAD levels or PARP activity. H₂O₂ induced lowering of NAD caused modest lowering of ATP but marked additional ATP-lowering, independent of PARP and NAD, was also demonstrated. We conclude that in endothelial cells ATP levels are largely independent of NAD and PARP, which do not play a role in thrombin or Ca⁺⁺-ionophore-mediated lowering of ATP. H₂O₂ caused ATP lowering through a similar mechanism as thrombin and A23187 but, additionally, caused a further ATP lowering through its intense stimulation of PARP and marked lowering of NAD.