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Opioids may trigger the apoptotic death of widely ranging cell types, and apoptosis contributes to the immune deficiency of critically ill patients and subjects experiencing surgical trauma. There is evidence that an altered mitochondrial membrane potential constitutes an early and irreversible step in the death-signaling pathway of apoptosis. This study investigated whether fentanyl, a opioid widely used in the management of these patients, may induce apoptosis of T cells by altering their mitochondrial membrane potential.Peripheral blood lymphocytes were cultured in the presence of 30 ng fentanyl for 60 (time 1), 90 (time 2), and 120 (time 3) minutes, respectively. The cells then were processed for assessment of mitochondrial membrane potential by means of flow cytometry and confocal scanning microscopy. Furthermore, production of reactive oxygen species, expression of the Fas-Fas L pro-apoptotic pathway, and apoptosis frequency were measured by means of flow cytometry. Control cells were incubated for the same times in the complete culture medium without the drug.Flow cytometry analysis showed a significantly increased rate (p < 0.05) of lymphocytes with disrupted mitochondrial membrane potential after incubation with fentanyl for 90 and 120 minutes, as compared with both control cells and lymphocytes cultured in the presence of fentanyl for 60 minutes. In addition, as early as 60 minutes after exposure to fentanyl, cells displayed a disrupted mitochondrial membrane potential when this was assayed by means of confocal laser scanning. These findings were associated with increased production of reactive oxygen species. The frequency of apoptotic lymphocytes was markedly increased (p < 0.05) after 120 minutes of incubation, as compared with untreated cells and cells exposed to fentanyl for only 60 and 90 minutes. Expression of Fas-FasL was not substantially affected by exposure to fentanyl.Fentanyl may induce a time-dependent apoptosis of lymphocytes by altering their mitochondrial redox metabolism.