Influence of oxygen on radiation-induced DNA damage in testicular cells of C3H mice.

Radiation-induced DNA single-strand break (ssb) induction and rejoining were measured in murine testicular cells using the alkaline comet assay. Individual cells in different stages of differentiation were identified on the basis of DNA content. As expected, induction of DNA ssb in testis cells irradiated on ice was independent of ploidy, and the extent of damage was similar to that produced in cells from other normal tissues. However, in vivo irradiation of air-breathing mice produced more ssb in haploid than tetraploid germ cells, although their rates of rejoining were similar and comparable to repair rates of cells from other normal tissues. In addition, irradiation of testis in situ produced only half as much damage as irradiation in vitro, and this could be explained only in part by the rapid ssb rejoining occurring during irradiation and cell isolation. A lower cellular oxygenation was postulated to account for the apparent resistance of testis cells to induction of breaks and the difference in induction in relation to DNA content. This was confirmed when carbogen inhalation and treatment with nicotinamide not only increased the overall degree of ssb induction in all these cells, but also reduced differences between cells of different ploidies. Results using the hypoxic cell cytotoxin RSU 1069 confirmed that the extent of hypoxia was not as severe in the testis as in the SCCVII murine tumour. It can be concluded from these data that the oxygenation of all testis cells is low enough to confer radioresistance, and that haploid testis cells are less hypoxic than tetraploid spermatocytes.