R.H. Rixon

Effects of X-radiation on multiplication and nucleic acid synthesis in cultures of L-strain mouse cells

Abstract 1. 1. Dilution of cultures of strain-L mouse cells growing in suspension resulted in an immediate cessation of multiplication but did not immediately affect DNA synthesis. 2. 2. During the period of multiplication arrest, DNA per cell reached the double (premitotic) level and DNA synthesis stopped until the cell content of DNA was reduced by the resumption of cell division. 3. 3. This showed that inhibition of division alone may result in inhibition of DNA synthesis when the cell content of DNA reaches the premitotic level. 4. 4. Cell multiplication was twice as sensitive to irradiation as DNA and RNA syntheses. DNA synthesis was slightly less sensitive than RNA synthesis to doses between 0 and 600 r but slightly more sensitive between 600 and 2000 r. 5. 5. A dose of 4000 r permanently stopped cell division but only temporarily inhibited DNA synthesis which resumed only to stop again when the cell content of DNA reached the double (premitotic) level. A dose of 1000 r did not cause an initial lag in DNA synthesis but reduced total DNA synthesis. Reduced DNA synthesis was accompanied but a high concentration of DNA in the cells. 6. 6. It is concluded that DNA synthesis is relatively insensitive to X-rays. The principal effect of radiation is inhibition, or slowing, of cell division. In the absence of cell division DNA synthesis proceeds until the content per cell is doubled and then stops. At very high doses (4000 r), immediate, but temporary, inhibition occurs in addition to later inhibition by attainment of the double or premitotic DNA content per cell.

Radiation resistant derivatives of L strain mouse cells

Abstract A radiation resistant colony has been isolated from the survivors of strain L mouse cells after irradiation with a single dose of 1000 r. Further selection by additional exposures to 1000 r did not result in significant changes in radioresistance. Cells from suspension cultures of this strain (R1) and its derivatives (R2 and R3) retained their colony forming ability to a greater extent than cells of strain L after irradiation with doses ranging from 50 to 1000 r; the survival of the resistant lines was 4 to 5 times that of strain L after irradiation with 1000 r. Initial growth in suspension cultures of R3 was much less affected by irradiation with 1000 r than it was in cultures of strain L. In addition, decline in cell numbers in cultures of R3 irradiated with 2000 and 3000 r was less marked than in cultures of strain L.

PREVENTION OF POSTIRRADIATION MITOTIC DELAY IN CULTURES OF L MOUSE CELLS BY CALCIUM SALTS

Calcium salts in sufficientiy high concentrations were found to be effective in preventing postirradiation mitotic delay in irradiated suspension cultures of mouse cells. Possible reaction mechanisms are discussed. (C.H.)

REDUCTION OF NUCLEAR DAMAGE IN X-IRRADIATED RAT THYMOCYTES BY ELEVATED SALT CONCENTRATIONS.

Abstract Nuclear damage observed in X-irradiated thymocytes appears to be an expression of chromatin disaggregation. The development of the typical pycnotic morphology, which has been interpreted as chromatin condensation, is observed when irradiated cells are fixed for histological examination in the presence of a heavy metal. In the absence of heavy metal from the fixation procedure, however, the development of radiation damage in thymocyte nuclei is seen as a loss of the fine chromatin reticulum and replacement by structureless, homogeneous nuclei. These homogeneous nuclei were produced in unirradiated cells by incubation in hypotonic media as well as by suspending irradiated cells in an isotonic medium. Their formation in irradiated cells, however, was prevented or opposed by incubation in media with elevated sodium chloride concentrations (a treatment known to cause chromatin condensation). These hypertonic environments also reversed nuclei with established homogeneity back to a normal morphology. This evidence supports the idea that radiation damage in thymocyte nuclei involves chromatin disaggregation. It is suggested that irradiation causes altered membrane permeability which results in the loss of chromatin condensing agents (inorganic and organic cations) from the nucleus.

Prevention of mitotic delay in irradiated suspension cultures of L mouse cells by agmatine

Abstract An organic amine, agmatine, causes the chromatin of L mouse cells to aggregate. This effect is reversible. Agmatine (0.05 to 0.1 M ) has the additional property of preventing postirradiation mitotic delay in suspension cultures irradiated with 1000 r of X-rays. It is proposed that agmatine prevents mitotic delay by counteracting radiation-induced chromosome uncoiling resulting from increased cellular permeability with loss of accumulated chromosome coiling agents.

Effects of ultraviolet light on multiplication and deoxyribonucleic acid synthesis in cultures of L strain mouse cells

Abstract Cultures of L strain mouse cells were irradiated with doses of ultraviolet light (UV) sufficient to cause a temporary cessation of multiplication or a marked decrease in cell concentration. When the DNA content per cell at the time of irradiation was high (1.8 to 2.3 × 10 −5 μg) there was little or no DNA synthesis. When the DNA content per cell was initially low (1 to 1.5 × 10 −5 μg), DNA synthesis proceeded rapidly during the first 24 hours after irradiation in spite of mitotic arrest, but slowed down when the DNA content of the non-dividing cells reached 1.8 to 2.3 × 10 −5 μg. It is concluded that DNA synthesis in cultures of L mouse cells is relatively insensitive to UV light; the principal effect of the doses studied being the inhibition of cell division. Under these conditions, the rate of DNA synthesis is controlled by the normal feedback mechanism which limits the DNA content per cell to a level which is close to the double or premitotic content.

Cytological consequences of chromosome bridges in X-irradiated L strain mouse cells

Observations are reported on a post-irraniation accumulation of large numbers of cell pairs in suspension cultures of mouse cells in which the nuclei were connected by long strands of chromosome material. The connecting strands were attributed to the persistence of a number of anaphase chromosome bridges into the telophase stage and beyond. A nuclear membrane was observed to regenerate and enclose both telophase nuclei as well as the bridging chromnsomes. Reaction mechanisms involved are discussed.