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Figure 6 (Left) Effects of nitroglycerin (NTG) treatment on (¹⁴C)-PGH₂–dependent prostanoid production in rabbit aortas. Results are expressed as the mean value ± SEM from five experiments. *p < 0.05 vs. control by analysis of variance. Solid bars = without NTG treatment; open bars = with NTG treatment. (Right) Schematic diagram depicting the effects of NTG-induced formation of peroxynitrite (ONOO^–) on arachidonic acid metabolism. ONOO^– inhibits the conversion of the cyclooxygenase product PGH₂ (prostacyclin endoperoxide) by the prostacyclin synthase via tyrosine nitration. This effect can be mimicked in vitro by using the prostacyclin synthase inhibitor U51605. The remaining PGH₂ will act directly or indirectly (via formation of thromboxane A₂ [TXA₂]) on the thromboxane receptor (TxR), leading to enhanced constriction.