The regulation of pollen tube growth in Douglas-fir following high doses of ionizing radiation

The relationship between temperature and sensitivity to gibberellin A₃ (GA₃) was studied in lettuce seedlings (Lactuca sativa L. cv. Arctic). Dose/response curves for hypocotyl elongation (10^-4 mol l^-1 to 10^-8 mol l^-1) were constructed for a range of temperatures and the slope of the linear portion of the plots used as an indication of the sensitivity to GA₃. Hypocotyls were unresponsive to GA₃ below 13°C but above this temperature sensitivity increased linearly. Plots of growth rate against temperature had inflexions between 12°C and 13°C, with slopes above this point which increased with increasing GA₃ concentration. The Q₁₀ value for response increased in a similar manner. Reaction rates of NAD-dependent malate dehydrogenase and peroxidase extracted from hypocotyls varied linearly with temperature whilst nonspecific tetrazolium reduction, a membrane based activity, showed an abrupt rate change above 14°C. Pre-exposure to GA₃ had no effect on the temperature responses of soluble or particulate enzymes.Abbreviation GA₃ gibberellin A₃     

Carcinogenesis in laboratory mice after low doses of ionizing radiation

Experimental data on the incidence of solid tumors from various long-term mouse studies performed at the Casaccia laboratories over several years were reconsidered, limiting the analysis to the results available for doses equal to or less than 17 cGy of neutrons and 32 cGy of X rays since these dose limits are reasonably close to the generally accepted low-dose levels for high- and low-LET radiation (i.e. D(high-LET) < 5 cGy and D(low-LET) < 20 cGy, respectively). The following long-term experiments with BC3F1 mice were reviewed: (a) females treated with single doses of 1.5 MeV neutrons or 250 kVp X rays, (b) males treated with fractionated doses of fission neutrons, and (c) mice of both sexes irradiated in utero 17.5 days post coitus with single doses of fission neutrons or X rays. An experiment with CBA mice of both sexes treated with single doses of fission neutrons was also included in this study. Analysis was done on animals at risk; thus all incidences of tumor-bearing animals were expressed as the percentage excess incidence with respect to the controls. Ovarian tumors and other solid neoplasms were considered. The percentage frequencies and mean survival times of tumor-free mice were also recalculated. The results indicate the existence of a region at low doses where the final incidence of solid neoplasms is indistinguishable from the background incidence. These data reinforce the idea that at low doses the effectiveness of ionizing radiation in inducing solid neoplasms in laboratory mice is very low.     

Effects of morphine and naloxone on pain sensitivity after radiation injuries in rats

Radiation in doses 150 Gy induces different changes in pain sensitivity in rats by thermal (analgesia) and electrical (hyperalgesia) stimuli. Naloxone (0.1 and 1 mg/kg) and morphine (5 mg/kg) show, that analgesia is realized due to opioid mechanisms.     

A two compartment model for cell survival after irradiation with ionizing radiation

A theory of cell survival after irradiation has been developed, considering the cell as composed of two compartments with different sensitivities and taking into account recovery phenomena. Expressions are obtained for the probabilities that the cell will be in a survival state or damaged state or will function with reduced efficiency.     

Immunochemical determination of DNA damage induced by high doses of ionizing radiation

It was shown by the immunochemical method that DNA of X-irradiated E. coli cells of a radiosensitive mutant ABA88uvr A6 can react with the antiserum to thymine dimers which, in all appearance, are induced by ionizing radiation in bacterial DNA. The number of thymine dimers in DNA of E. coli AB1886uvr A6 increased with the dose increase. No dimers were detected in radioresistant cells of M. radioproteolyticus probably due to the effective excision thereof.     

Early effects of low doses of ionizing radiation on the fetal cerebral cortex in rats

Pregnant rats were exposed to gamma radiation from a 137Cs irradiator on gestational Day 15. Fetuses that received 0.25, 0.5, 0.75, or 1.0 Gy were examined 24 h after irradiation for changes in the cells of the cerebral mantle of the developing brain. The extent of changes following 0.5 Gy was studied at 3, 6, 12, or 24 h after exposure. Cortical thickness of the cerebral mantle was not significantly altered. The number of pyknotic cells, number of macrophages, nuclear area, and number of mitotic cells were altered in a dose-related way. The number of pyknotic cells was significantly increased at all doses. A positive correlation between the number of pyknotic cells and the number of macrophages developed with time. At 3 h after irradiation about 60% of pyknotic cells were found in the subventricular zone and about 25% in the intermediate zone and cortical plate. The number of such cells in the upper layers of the cortex steadily increased up to 24 h, at which time about 70% of pyknotic cells were in these two layers. The relationship of the movement of pyknotic cells to migration of postmitotic neuroblasts is discussed.     

The stimulating action of low doses of ionizing radiation on glucokinase synthesis in the liver of developing rats

Taking spontaneous glucokinase synthesis as an example the authors showed an increase in the rate of enzymatic differentiation against the insignificantly increased natural radiation background, which might serve as a supporting evidence of the previously obtained data confirming the necessity of the natural radioactive background for the development of mammals.     

Synthesis of antibody by spleen cells after exposure to kiloroentgen doses of ionizing radiation

Mouse spleen cells in diffusion chambers were studied in an effort to assess the radiation survival curves of lymphoid cells during the different serological phases of immune response and to characterize morphologically and metabolically these radiation-resistant cells. The results showed that the capacity of immunocompetent progenitor cells to proliferate and differentiate into antibody-synthesizing cells was highly sensitive to x-irradiation. Their fully differentiated progenies, which were made up mainly of mature plasma cells, were resistant in that they were able to synthesize antibody effectively for as long as several days after their exposure to radiation doses up to 10,000 R. As a result of these studies, a method with a high recovery yield was devised for obtaining a highly purified preparation of dispersed monospecific antibody-synthesizing cells. This was done by simply exposing primed spleen cells to 10,000 R at the end of the log phase of response and harvesting the culture several days later.     

Characteristic cytogenetic effects of small doses of ionizing radiation

A study was made of the frequency of chromosome aberrations in human lymphocyte culture after gamma-irradiation (60Co) with doses ranging from 0.05 to 1.0 Gy at dose--rates of 0,005, 0.05 and 0.5 Gy/min. The frequency of structural changes in chromosomes at low doses was higher than it was expected in the case of extrapolating the effect produced by high to low doses of radiation; within the dose range from 0.1 to 0.5 Gy a plateau was registered for aberrations of the exchange type (dicentrics and rings). The abnormal character of the dose dependence of the yield of chromosome aberrations persisted with all three dose - rates under study.     

Gene expression profiles of normal human fibroblasts after exposure to ionizing radiation: a comparative study of low and high doses

Several types of cellular responses to ionizing radiation, such as the adaptive response or the bystander effect, suggest that low-dose radiation may possess characteristics that distinguish it from its high-dose counterpart. Accumulated evidence also implies that the biological effects of low-dose and high-dose ionizing radiation are not linearly distributed. We have investigated, for the first time, global gene expression changes induced by ionizing radiation at doses as low as 2 cGy and have compared this to expression changes at 4 Gy. We applied cDNA microarray analyses to G1-arrested normal human skin fibroblasts subjected to X irradiation. Our data suggest that both qualitative and quantitative differences exist between gene expression profiles induced by 2 cGy and 4 Gy. The predominant functional groups responding to low-dose radiation are those involved in cell-cell signaling, signal transduction, development and DNA damage responses. At high dose, the responding genes are involved in apoptosis and cell proliferation. Interestingly, several genes, such as cytoskeleton components ANLN and KRT15 and cell-cell signaling genes GRAP2 and GPR51, were found to respond to low-dose radiation but not to high-dose radiation. Pathways that are specifically activated by low-dose radiation were also evident. These quantitative and qualitative differences in gene expression changes may help explain the non-linear correlation of biological effects of ionizing radiation from low dose to high dose.     

Influence of mitotic delay on the results of biological dosimetry for high doses of ionizing radiation

The purpose of this study was to systematically investigate how high doses of sparsely and densely ionizing radiations influence the proliferation time of lymphocytes in short-term cultures and, consequently, the observed frequencies of dicentric and centric ring chromosomes. Peripheral blood samples from five volunteers were irradiated with high doses of 200 kV X-rays and with neutrons with a mean energy of <E _n>=2.1 MeV. First division metaphase cells were collected after different culture times of 48, 56, and 72 h and dicentrics, centric ring chromosomes, and acentric fragments were determined. The data hint at considerable mitotic delay. The main increase in the number of chromosome aberrations occurred between 48 and 72 h after an X-ray exposure and between 56 and 72 h after neutron exposure. When the data were used for a calibration of aberration frequency versus dose, subsequent dose estimations resulted, however, in comparable values. Thus, in spite of the influence of mitotic delay on observable chromosome aberrations, at least for the radiation types investigated here, a culture time of 48 h is acceptable for biological dosimetry.     

Effect of ionizing radiation at high doses on the cyclic nucleotide content of the rat brain

The study of the rat brain after irradiation with superlethal doses of 100 and 500 Gy has revealed a decrease in the cAMP content at early times after exposure. The most considerable decline in the cGMP content was noted in the cerebellum and cerebral hemisphere cortex where low cGMP level was retained for 24 h following irradiation. It is suggested that the observed changes play an important role in the development of the CNS syndrome.