DNA repair by articular chondrocytes. II. Direct measurements of repair of ultraviolet and X-ray damage in monolayer cultures.

The abilities of human and rabbit articular chondrocytes to repair ultraviolet (UV) and X-ray damage were measured in terms of the removal of UV--endonuclease-sensitive sites (pyrimidine dimers) and single-strand breaks, respectively. The initial 3-h rate of dimer repair in human cells, incubated in medium containing 10% human serum, was about 2.5 times as large as in rabbit cells incubated in medium containing 10% or 20% fetal bovine serum. Similar results have been previously reported for unscheduled DNA synthesis (UDS), indicating that UDS is a valid quantitative measure of repair in this cell system. The repair of single-strand breaks was rapid (approx. 50% completed in less than 10 min). An estimate, from the measured numbers of lesions and patch sizes, indicated that the amount of UDS following 20 krad would be 100 to 300 times less than that in 3 h following 10 J/m2 of 254 nm and hence would not be detectable radioautographically.