Fig. 4. H₂O₂ cooperates with [Ca²⁺]_i oscillation frequency to optimize agonist-stimulated gene expression. Intracellular-Ca²⁺-store-depleted ECs were exposed to the conditions generating 0.1, 0.3, 0.5 and 0.7 [Ca²⁺]_i oscillations/minute in the presence of 10 µM H₂O₂, or in the concomitant presence of 10 µM H₂O₂ and 1 µM histamine stimulation, in Rac^–/–-expressing ECs for 60 minutes. Total RNA was isolated for real-time RT-PCR analysis. An external application of 10 µM H₂O₂ reverses the altered bell-shaped graph of histamine-stimulated VCAM1 expression versus [Ca²⁺]_i oscillation frequency that is induced by Rac^–/– expression (see Fig. 3) to that of vector controls. In the absence of histamine stimulation, the external application of 10 µM H₂O₂ cooperates with [Ca²⁺]_i oscillation frequency to optimize VCAM1 gene expression.