THE ROLE OF DOSE-RATE IN THE INDUCTION OF MICRONUCLEI IN DEEP-LUNG FIBROBLASTS IN-VIVO AFTER EXPOSURE TO CO-60 GAMMA-RAYS

To evaluate the influence of low-dose-rate exposures on biological dam age, it is necessary to have cells that can be maintained in the same stage of the cell cycle for long periods. Normal rat lung fibroblasts represent a stable cell type with a slow turnover rate in vivo. These cells can be stimulated to divide by placing them in tissue culture. T herefore, a constant cell population can be exposed over a protracted time and stimulated to divide, and the cytogenetic damage can be evalu ated at the first cell division after exposure. By placing rats at dif ferent distances from a Co-60 source, they were exposed to graded dose s of gamma rays - 0.0, 3.9, 7.4 and 11.3 Gy - protracted over either 4 or 67 h. Fibroblasts were isolated from the lung and cultured for 24 h; after cytochalasin B was added, the cells were cultured for an addi tional 69 to 72 h. The percentage binucleated cells in fibroblasts of animals exposed for 4 or 67 h was 47.1 +/- 4.3 and 62.1 +/- 3.9. There was no influence of dose on the percentage binucleated cells, but the fraction of cells that divided at 67 h was significantly higher (P < 0.05) than observed at 4 h. Cells were scored for micronuclei on coded slides. The dose-response data from animals exposed for 4 and 67 h we re fitted to the following linear dose-response relationships, where D = dose: micronuclei/binucleated cell = 0.02 +/- 0.03 + 2.38 +/- 0.44 x 10(-2) D, and micronuclei/binucleated cell = 0.01 +/- 0.06 + 1.01 +/ - 0.10 x 10(-2) D, respectively. The r(2) values for the two curves we re 0.67 and 0.91, indicating the goodness of fit for the data for the 4- and 67-h treatments. The slopes were different from zero and each o ther at the P < 0.05 level of significance. The effectiveness of the C o-60 exposure decreased as the dose rate decreased. At dose rates belo w 0.17 Gym, the effectiveness remained constant over the range of dose s and dose rates used. Comparing the slope of the dose response for th e lowest exposure rate to that from information published previously, the dose-rate effectiveness factor was 6.14 +/- 0.65 for the induction of micronuclei in deep-lung fibroblasts. This paper describes how the se data, derived at low dose rates, were used to estimate that the rel ative biological effectiveness (RBE(M)) of Co-60 and radon was 65.2 +/ - 8.4 when the exposures were delivered over similar lengths of time. These studies emphasize the importance of using the proper dose rate f or low-LET radiation in estimation of RBE. (C) 1995 by Radiation Resea rch Society