Oxygen consumption and mitochondrial activity by HUVEC. Panel A shows the O2 consumption of HUVEC measured by means of a Clark electrode. After a brief termic acclimatization, 2 ml of O2/O3 mixture containing 50 μgO3/ml was gently added to HUVEC medium (time 0). Two min after the addition of O2/O3, the availability of oxygen increased around twofold. The rate of oxygen consumption significantly increased with respect to basal conditions (cells untreated with O2/O3 mixture). The histogram in panel B compares total O2 consumed in 10 min in basal conditions and after the addition of the O2/O3 mixture, displaying a 6 times increase in O2 consumption for HUVEC treated with O2/O3 . Confocal microscope 2D pictures of HUVEC labeled with mito Traker CMX-ROS red and Acridine Orange green are represented in panel C. Image b, compared with image a, which shows the basal signal, displays an increase in both signals paralleling the infusion of O2/O3 at time 0. Image c demonstrates that the increase in mitochondrial respiration and in nuclear activity reverts to basal activities after 30 min from O2/O3 addition.