Increased level of circulating cell free mitochondrial DNA due to a single bout of strenuous physical exercise.

Physical exercise has been reported to affect the immune response in various ways. The levels of pro-inflammatory cytokines as well as the presence of circulating leukocytes are changed. In this study the presence of circulating cell free mitochondrial DNA (cf-mtDNA) and nuclear DNA (nDNA) was investigated in connection with a single bout of strenuous physical exercise. The number of circulating leukocytes was also measured, as well as their secretion of the inflammatory marker soluble urokinase activator receptor (suPAR).

The level of cf-mtDNA was continuosly increased due to exercise, compared to baseline values, significantly after 30 and 90 minutes of rest. The number of total circulating leukocytes was initially increased but returned to baseline levels during rest and the level of lymphocytes even decreased below initial values 90 minutes after exhaustion. Surface expression of uPAR on neutrophils decreased significantly during exercise. The concentration of suPAR tended to increase during exercise but only significantly after 90 minutes of rest, corresponding to the decreased levels of cell surface uPAR.

The results indicate that a single bout of exercise has a significant effect on the inflammatory response, as seen from both increased values of cf-mtDNA and suPAR, markers of active inflammation. The increased concentration of cf-DNA indicate that cell damage takes place during high intensity training. Hypoxia and tissue damage, due to over-load, are likely causes for the liberation of cf-mtDNA from muscle cells. The levels of cf-mtDNA remains high even during initial rest, due to the decreasing numbers of leukocytes, normally clearing the plasma from cf-DNA. The increased levels of suPAR further emphasize that strenuous physical exercise causes a reaction similar to an active inflammation. Further studies are needed to investigate what really causes the increased levels of cf-mtDNA and the corresponding cleavage of suPAR from the cell surface of circulating leukocytes.