Single strand breaks in CHO cell DNA induced by ultrasonic cavitation in vitro

Ultrasonic cavitation induces a multiplicity of bioeffects in cell suspensions exposed in a 72 RPM rotating-tube exposure system. Single strand DNA breaks (SSBs) were found in cultured Chinese hamster ovary (CHO) cells exposed directly to 1.61 MHz ultrasound at a continuous 8 W/cm2 spatial peak temporal average (SPTA) intensity with cavitation for 10 min at 2 degrees C. Viability assessed by the trypan blue test was less than 1%, which indicates that these SSBs were in dead cells. Burst mode exposure with 10.5 microseconds bursts repeated each 21 microseconds not only caused SSBs at 5.6 W/cm2 and 8 W/cm2 (SPTA) for 10 min, but also allowed 20% and 7% viability, respectively. In order to determine if any of these breaks resided in the viable fraction of cells, the exposures were repeated with a 30 min postexposure incubation period at 37 degrees C to allow breaks in viable cells to repair. No significant repair occurred, relative to the samples which remained at 2 degrees C to prevent repair. A similar result was obtained with 10.5 microseconds bursts repeated each 42 microseconds at 4 W/cm2 (SPTA) with 46% viability. Thus, the observed ultrasonically induced SSBs reside primarily in the nonviable fraction of cells.