Effect of X-irradiation dose rate on the clonagenic survival of human and experimental animal hematopoietic tumor cell lines: evidence for heterogeneity

It is a generally accepted principle of radiation biology that hematopoietic progenitor cells demonstrate dose rate independent killing by x-irradiation over the clinically relevant range for total body irradiation (TBI) (5-25 rad/min). To determine whether low dose rate (5 rad/min, or 20 rad/min) compared to conventional dose rate (200 rad/min) x-irradiation altered the clonagenic survival of leukemia and lymphoma cell lines, several permanent cell lines were studied. These included: bg/bg cl 1, mouse basophillic leukemia; LW12, [W/fu rat acute myelogenous leukemia (AML)]; and human cell lines: JY and Daudi (B-cell lymphomas); K45, (T-cell leukemia); K562, (erythroleukemia); HL60 and KG1 (monomyeloid leukemias), and U937 (human histiocytic/monocytic lymphoma). Dose rate independent killing was demonstrated at several plating densities with mouse and rat leukemia lines and all human leukemia lines tested except lines HL60 and U937. With HL60, increased plating density increased the D0 at each dose rate. This effect was not attributable to an increased plating efficiency. With line U937 there was a clear dose-rate effect with increase in D0 from 88 rad, n 4.6 at 200 rad/min, to D0 = 166, n 2.3 at 5 rad/min. The data demonstrate that some human hematopoietic tumor derived cell lines of myeloid/monocyte/macrophage lineage can exhibit atypical repair of irradiation damage in vitro. This repair may be enhanced by conditions relevant to clinical TBI including low irradiation dose-rate and cell to cell interactions by tumor cells in close proximity.

Activated human N-ras oncogene enhances x-irradiation repair of mammalian cells in vitro less effectively at low dose rate. Implications for increased therapeutic ratio of low dose rate irradiation

The effect of x-irradiation dose rate on the clonagenic survival of mouse embryo fibroblast cell line NIH/3T3 and its N-ras human oncogene transformed subline was studied. Both control and N-ras transformed cell lines were maintained in Dulbecco's modified Eagle's medium at 37 degrees C with 5% CO2. These cell lines were passaged twice weekly and the cells were irradiated in log phase on a 250 kVp orthovoltage unit at 5 or 200 rad/min, adjusting filtration and FSD to account for each dose rate. After irradiation, the cells were replated and colonies of greater than or equal to 50 cells were scored on day 7. D0 and n were calculated via linear regression analysis. There was a significant increase in saturation density and plating efficiency of N-ras transformed cells with loss of contact inhibition. There was no significant difference in radiosensitivity between the two cell lines at 5 rad/min. For NIH/3T3 D0 = 336, n = 2.19; for N-ras transformant D0 = 314, n = 2.35 (p = 0.65); however, irradiation at 200 rad/min revealed a significant survival advantage for the transformed line. For NIH/3T3 D0 = 145, n = 9.1 and for the N-ras transformed line D0 = 208, n = 4.05 (p = 0.0018). The data provide evidence that repair factors which govern irradiation survival may differ for high and low dose rate irradiation and that repair of high dose rate irradiation damage is enhanced directly or indirectly by expression of the N-ras oncogene. The data support hyperfractionated (low dose rate) irradiation for improving the therapeutic ratio during control of rapidly proliferating tumors expressing an activated N-ras oncogene.

A Pascal program for weighted least squares regression on a microcomputer

Weighted least-squares regression has been programmed in Pascal for a microcomputer. A double precision Pascal compiler and the Motorola 6809 assembler produce a fast machine-code program occupying 22,000 bytes of memory when appended to the Pascal run-time module. Large data sets fit in the remaining memory. A regression with 72 observations and 24 parameters runs in 7 min, excluding optional print out of large matrices. The maximum dimensions of the design matrix, X, can be altered by modifying two Pascal constants. Minor changes to the Pascal source program will make it compatible with other Pascal compilers. The program optionally orthogonalises the X matrix to detect linearly-dependent columns in X, and/or generate orthogonal parameter estimates. After orthogonalizing X and fitting the model, the parameter estimates for the original X can be retrieved by the program. Regressions on a repeatedly reduced model are performed through elimination of columns in X until the minimum adequate model is obtained.

Induced tolerance to the discriminative stimulus properties of cocaine

Twenty-five male Sprague-Dawley rats were trained in five two-lever operant chambers on a DRL-15 sec schedule of positive food reinforcement to discriminate 10 mg/kg cocaine from 1 ml/kg saline. Following acquistions of discrimination a counterbalanced design of extinction tests was performed before and after repeated administration of 20 mg/kg cocaine or saline (three times a day at five hr intervals for seven days). The extinction tests consisted of testing responses of animals following 1 ml/kg saline, 2.5 mg/kg cocaine, or 5 mg/kg cocaine. The results showed no significant difference in animals' level choice before and after repeated injection with saline. However, the percent cocaine lever choice with the two doses of cocaine was lower after repeated administration of cocaine than before the repeated injections. This indicates tolerance developed to the discriminative stimulus properties of cocaine.