Dose-dependent and time-dependent changes in the choroid plexus of the irradiated rat brain

A histological assessment has been made of both time- and dose-related changes in the choroid plexus after the local irradiation of the rat brain with single doses of 17.5-25 Gy of X rays. These investigations involved the serial killing of animals 1-52 weeks after irradiation and the quantitative and semiquantitative evaluation of histological sections. Counts of the relative number of cells in the choroid plexus of the lateral ventricles showed an atrophy of the epithelial layer after 13 weeks. However, this was not as marked as the reduction in the number of endothelial cells in the wall of blood vessels. Moreover, the epithelium had recovered by 39 weeks after irradiation, while the dose-related depletion in endothelial cells tended to be progressive. A highly correlated group of changes in the vascular-connective tissue was used to produce a numerical "factor". This represented a combined score of radiation damage which was both time- and dose-related. These data suggest that, as an expression of late radiation damage to the choroid plexus, the effect on the endothelium was more important than that to the epithelial cells.     

Experimental evidence to support the hypothesis that damage to vascular endothelium plays the primary role in the development of late radiation-induced CNS injury

Experimental evidence has been obtained to support the view that late necrosis in the brain, after irradiation, is a consequence of damage to the vascular system. Rats were locally irradiated to the brain with a single dose of 25 Gy of X-rays and their response was compared with rats given the same treatment after administration of the radioprotector, Gammaphos. Time-dependent changes in endothelial cell number were determined for up to 65 weeks after irradiation. The complex pattern of changes in endothelial cell number, seen after irradiation alone, was not found in animals receiving Gammaphos prior to irradiation. The initial marked loss of endothelial cells, seen after 24 h in unprotected animals, was effectively prevented by the pre-administration of Gammaphos. The subsequent slow decline in endothelial cell density in Gammaphos treated animals was insufficient to induce an abortive attempt at endothelial cell re-population. This occurred between 26 and 52 weeks after irradiation in unprotected animals. By 65 weeks after irradiation <10% of animals receiving Gammaphos showed necrosis on histological evaluation. This compared with approximately 50% of the animals showing necrosis that had not received the radioprotector. Since the radioprotector is restricted to the vasculature of the brain these data indicate that endothelium is the primary target cell population, damage to which leads to the development of late radiation-induced necrosis in the brain.     

Cell kinetic changes in the follicular epithelium of pig skin after irradiation with single and fractionated doses of X rays

Changes in the cell kinetics of the follicular epithelium of the pig have been studied after irradiation with single and fractionated doses (30 fractions/39 days) of X rays and the results compared with previously published data for the epidermis. In the follicular epithelium there was an initial degenerative phase, during which the rate of cell depletion was independent of the radiation dose and the mode of administration. Evidence for repopulation was seen between the 14th and 18th days after single doses (15 or 20 Gy) and by the 28th day after the start of irradiation with fractionated doses (52.3-80.0 Gy). However, the degree of cell depletion and the subsequent rate of repopulation were independent of dose. The regenerative phase was characterized by an increased cell proliferation as indicated by an elevation of the labelling index. Islands of cells (colonies), with an appearance similar to cells in the normal follicular epithelium, were seen 18 days after a single dose of 20 Gy and 42 days after the start of fractionated irradiation. When compared with the epidermis, the follicular epithelium exhibited considerably less evidence of damage after both single and fractionated doses of X rays. There was a lower incidence of degenerate cells and reduced levels of cell depletion in the follicular epithelium, suggesting that cells from this region play an important role in the repopulation of the epidermis after high-dose irradiation.     

In vivo radioprotection of mouse brain endothelial cells by Hoechst 33342

Radiation-induced loss of mouse brain endothelial cells has been examined in mice given an intravenous injection of the DNA-binding radioprotector Hoechst 33342 (80 mg kg-1). At the time of irradiation, 10 min after injection, Hoechst fluorescence in the brain was confined to the endothelial cells. Endothelial cell density was measured using a histochemical fluorescence technique that had been used previously to monitor post-irradiation changes in endothelial cell density in rat brain, in which it was shown that a sensitive subpopulation comprising about 15% of the endothelial cells was lost within 24 h of radiation exposure. The present study shows a similar dose-response for the control mice, with depletion of the sensitive subpopulation to 85% being almost complete after a dose of 2.5 Gy gamma-rays. However, in mice irradiated 10 min after Hoechst 33342 administration, doses between 12 Gy and 20 Gy were required to ablate these cells. The kinetics of cell loss and the rather large dose modification factor suggests that Hoechst 33342 may be suppressing an apoptotic response in this subpopulation. Whatever the mechanism involved, Hoechst 33342 clearly provides substantial protection against early radiation-induced endothelial cell loss. Further studies are necessary to determine the extent to which this initial protection translates into an improved long-term survival of the "protected" cells and, especially, to see whether this endothelial cell protection can ameliorate the later consequences of central nervous system irradiation, namely necrosis and paralysis.     

宫颈癌细胞株和正常间质细胞株受X线照射后的辐射效应

目的用宫颈癌、正常血管内皮和正常成纤维细胞株体外模拟宫颈癌实质和间质组织,研究其受到不同剂量放射线照射后的辐射效应。方法采用宫颈腺癌细胞株HeLa,宫颈鳞癌细胞株SiHa、C33A、Caski,人脐静脉内皮细胞株ECV304,及小鼠成纤维细胞株NIH/3T3细胞,分别用6MV的X线（300cGy/min）照射6和10Gy,24和48h后收集细胞,行PI染色,流式细胞术检测凋亡率和细胞周期变化。结果细胞株受到照射后,C33A凋亡率最高,而SiHa凋亡率最低,其余细胞株（包括ECV304与NIH/3T3细胞）均介于两者之间;各宫颈癌细胞株及NIH/3T3照射10Gy后48h的凋亡率较6Gy稍高（P〉0.05）,而ECV304照射10Gy后48h的凋亡率（13.04%±1.08%）比照射6Gy（6.51%±0.61%）明显升高（P=0.001）;各细胞株受到不同剂量照射后,均表现出明显的G2/M期阻滞,且阻滞细胞百分比随照射剂量增加而增加;受到照射后一般在24-48hG2/M期阻滞细胞百分比最高。结论高剂量照射可以提高血管内皮细胞的凋亡率,并导致剂量依赖性的G2/M期阻滞,为预测宫颈癌的高剂量放疗提供理论依据。     

Influence of dose-rate, post-irradiation incubation time and growth factors on interphase cell death by apoptosis and clonogenic survival of human peripheral lymphocytes.

PURPOSE: To investigate the effects of dose-rate, post-irradiation incubation time and growth factors on radiation-induced interphase cell death by apoptosis and reproductive cell death in human peripheral lymphocytes. MATERIALS AND METHODS: Lymphocytes in G0-phase were exposed in vitro to 1-3Gy 137Cs gamma-radiation at a high- (HDR, 45Gy/h) or a low dose-rate (LDR, 0.024Gy/h). HDR exposures were performed either on day 1 (HDR1) simultaneously with the start of the 3 Gy LDR exposure, or on day 6 (HDR6) when the LDR exposures ended. Apoptosis was studied at different times after irradiation by measuring (1) cellular membrane integrity, (2) morphological changes and (3) cell size reduction. Clonogenic survival was analysed for cells plated directly after irradiation (LDR, HDR6, HDR1 day 1) or after 5 days post-irradiation incubation (HDR1 day 6). RESULTS: A significant decrease in reproductive cell death was observed after 3 Gy LDR exposure as compared with the HDR1 exposure for cells plated day 6. For the lower doses applied, the dose-rate effect could not be statistically verified. A decrease in apoptosis for all three doses applied (i.e. 1, 2 and 3Gy) was observed when the LDR exposures were compared with the HDRI analysed day 6, although not of statistical significance. Radiation-induced apoptosis was efficiently counteracted by growth factors up to 24-48 h after 3 Gy HDR exposure. The prevention of radiation-induced cell death by growth factors was dependent on dose and post-irradiation time in G0. When the growth factors were added after a prolonged post-irradiation incubation in G0 (HDR 1 cells plated day 6), a significant increase in reproductive cell death was found (3 Gy) as compared with HDR protocols where the growth factors were added directly after irradiation (HDR 1 plated day 1 and HDR6). CONCLUSIONS: A dose-rate effect on radiation-induced apoptosis was indicated but not statistically verified. A significant dose-rate effect on reproductive cell death was observed. This dose-rate effect was, however, inverted when growth factors were added directly after the HDR irradiations.     

Effects of Irradiation on the [Methyl-3H]Choline Uptake in the Human Prostate Cancer Cell Lines LNCaP and PC3

BACKGROUND AND PURPOSE: Choline positron emission tomography (PET) can help to optimize radiation treatment strategy of prostate cancer. Therefore, the aim of this study was to elucidate the effects of ionizing radiation on the choline uptake in an androgen-dependent (LNCaP) and an androgen-independent (PC3) prostate cancer cell line. MATERIAL AND METHODS: Uptake of [methyl-(3)H]choline chloride was investigated between 4 and 96 h after irradiation with 6 Gy. Dose dependence of choline uptake was examined following irradiation with 2-12 Gy, and cell survival was analyzed via the clonogenic assay. Michaelis-Menten kinetics was determined 24 h (PC3) and 48 h (LNCaP) after irradiation with 6 Gy. RESULTS: PC3 cells showed a significant transitory increase of [methyl-(3)H]choline uptake with a maximum at 24 h after irradiation. In LNCaP cells irradiation induced a significant decrease with a minimum at 48 h. Changes in choline uptake in both cell lines were almost dose-independent up to 12 Gy. Following irradiation with 6 Gy, transport capacity (v(max)) increased and Michaelis-Menten constant (K(M)) decreased in PC3 cells, while in LNCaP cells the two parameters behaved vice versa. CONCLUSION: Changes in choline uptake following irradiation might be due to metabolic changes associated with initiation of processes that finally cause cell death. Thus, changes in tumor choline uptake monitored by PET after radiotherapy might not exclusively reflect therapeutic success but also altered tracer uptake as a consequence of irradiation.     

Mononuclear cell adhesion and cell adhesion molecule liberation after X-irradiation of activated endothelial cells in vitro

Purpose : Increased expression of cell adhesion molecules on endothelial cells is an important early event in inflammation. Low-dose radiotherapy is very effective anti-inflammatory treatment. The hypothesis that it may act by modulation of cell adhesion molecule expression in activated endothelial cells and the subsequent adhesion of mononuclear cells onto the activated endothelial cells was tested. Materials and methods : EA.hy.926 endothelial cells were irradiated with 0.3-10 Gy X-rays at different times before or after stimulation with TNFα. ICAM-1 or E-selectin expression was measured by ELISA and FACS. Isolated peripheral blood mononuclear cells were incubated with an activated and irradiated confluent monolayer of endothelial cells 4 h, 12 h or 24 h after stimulation, and adhesion was determined in dynamic and static adhesion assays. Results : In the static adhesion assay, where integrin-mediated adhesion dominates, radiation doses of 0.3-0.6 Gy reduced the adhesion of mononuclear cells onto EA.hy.926-EC in vitro by 25-40% and 15-25% of the control level 4 h and 24 h after stimulation, respectively, but increased adhesion 12 h after stimulation. In the dynamic adhesion assay, where selectin-mediated adhesion dominates, radiation doses of 0.3-0.6 Gy reduced the adhesion events by 40-50% and 30-40% of the control level 4 h and 24 h after stimulation, respectively, and again increased adhesion 12h after stimulation. X-ray doses of ≤5 Gy did not induce ICAM-1 expression, or modulate TNF α -induced ICAM-1 expression. E-selectin expression was, however, increased in a dose-dependent way 6 h after irradiation. In contrast, X-irradiation 2-5 h before stimulation decreased the characteristic transient expression of E-selectin after TNF α stimulation. Conclusions : Modulation of E-selectin liberation on activated endothelial cells may be one mechanism to decrease leukocyte adhesion after low-dose irradiation in vitro, and could be involved in the therapeutic action of anti-inflammatory radiotherapy.     

Mononuclear cell adhesion and cell adhesion molecule liberation after X-irradiation of activated endothelial cells in vitro

PURPOSE: Increased expression of cell adhesion molecules on endothelial cells is an important early event in inflammation. Low-dose radiotherapy is very effective anti-inflammatory treatment. The hypothesis that it may act by modulation of cell adhesion molecule expression in activated endothelial cells and the subsequent adhesion of mononuclear cells onto the activated endothelial cells was tested. MATERIALS AND METHODS: EA.hy.926 endothelial cells were irradiated with 0.3-10 Gy X-rays at different times before or after stimulation with TNFalpha. ICAM-1 or E-selectin expression was measured by ELISA and FACS. Isolated peripheral blood mononuclear cells were incubated with an activated and irradiated confluent monolayer of endothelial cells 4 h, 12 h or 24 h after stimulation, and adhesion was determined in dynamic and static adhesion assays. RESULTS: In the static adhesion assay, where integrin-mediated adhesion dominates, radiation doses of 0.3-0.6 Gy reduced the adhesion of mononuclear cells onto EA.hy.926-EC in vitro by 25-40% and 15-25% of the control level 4 h and 24 h after stimulation, respectively, but increased adhesion 12 h after stimulation. In the dynamic adhesion assay, where selectin-mediated adhesion dominates, radiation doses of 0.3-0.6 Gy reduced the adhesion events by 40-50% and 30-40% of the control level 4 h and 24 h after stimulation, respectively, and again increased adhesion 12h after stimulation. X-ray doses of < or =5 Gy did not induce ICAM-1 expression, or modulate TNFalpha-induced ICAM-1 expression. E-selectin expression was, however, increased in a dose-dependent way 6 h after irradiation. In contrast, X-irradiation 2-5 h before stimulation decreased the characteristic transient expression of E-selectin after TNFalpha stimulation. CONCLUSIONS: Modulation of E-selectin liberation on activated endothelial cells may be one mechanism to decrease leukocyte adhesion after low-dose irradiation in vitro, and could be involved in the therapeutic action of anti-inflammatory radiotherapy.     

Influence of dose-rate,post-irradiation incubation time and growth factors on interphase cell death by apoptosis and clonogenic survival of human peripheral lymphocytes

Purpose: To investigate the effects of dose-rate, post-irradiation incubation time and growth factors on radiation-induced interphase cell death by apoptosis and reproductive cell death in human peripheral lymphocytes. Materials and methods : Lymphocytes in G0-phase were exposed in vitro to 1-3Gy 137Cs gamma-radiation at a high- (HDR, 45Gy/h) or a low dose-rate (LDR, 0.024Gy/h). HDR exposures were performed either on day 1 (HDR1) simultaneously with the start of the 3Gy LDR exposure, or on day 6 (HDR6) when the LDR exposures ended. Apoptosis was studied at different times after irradiation by measuring (1) cellular membrane integrity, (2) morphological changes and (3) cell size reduction. Clonogenic survival was analysed for cells plated directly after irradiation (LDR, HDR6, HDR1 day 1) or after 5 days post-irradiation incubation (HDR1 day 6). Results: A significant decrease in reproductive cell death was observed after 3Gy LDR exposure as compared with the HDR1 exposure for cells plated day 6. For the lower doses applied, the dose-rate effect could not be statistically verified. A decrease in apoptosis for all three doses applied (i.e. 1, 2 and 3Gy) was observed when the LDR exposures were compared with the HDR1 analysed day 6, although not of statistical significance. Radiation-induced apoptosis was e fficiently counteracted by growth factors up to 24-48h after 3Gy HDR exposure. The prevention of radiation-induced cell death by growth factors was dependent on dose and post-irradiation time in G0. When the growth factors were added after a prolonged post-irradiation incubation in G0 (HDR1 cells plated day 6), a significant increase in reproductive cell death was found (3Gy) as compared with HDR protocols where the growth factors were added directly after irradiation (HDR1 plated day 1 and HDR6). Conclusions: A dose-rate effect on radiation-induced apoptosis was indicated but not statistically verified. A significant dose-rate effect on reproductive cell death was observed. This dose-rate effect was, however, inverted when growth factors were added directly after the HDR irradiations.     

In vitro response of human and porcine vascular cells exposed to high dose-rate γ-irradiation

Aim : The objective of this study was to compare the in vitro response of human and pig endothelial cells, smooth muscle cells and fibroblasts exposed to conventional high dose-rate γ-irradiation. Materials and methods : Clonogenic cell survival and growth responses were obtained after irradiation of plateau-phase cells with a 60 Co source at a dose-rate of 1.5 Gy/min. DNA single-strand breaks were also evaluated using an alkaline filter elution technique. Results : Overall, both the pig and human cell lines showed a similar response to conventional high dose-rate irradiation. Using clonogenic assays, the human aortic smooth muscle cell line was more sensitive than the fibroblast and endothelial cell lines, whereas the pig endothelial cell line was more sensitive than smooth muscle cells and fibroblasts. Shortly after irradiation (10 days) there was a temporary growth arrest, which was similar for endothelial, smooth muscle cells and fibroblasts with doses above 6 Gy. There was also a non-linear, dose-dependent growth delay up to 4 weeks after irradiation. This effect was also consistent between the different cell lines. Using alkaline filter elution, there was no significant difference in relative elution between endothelial cells, smooth muscle cells and fibroblasts, indicating similar DNA damage among the different cell lines. Conclusion : The in vitro response of human and pig endothelial cells, smooth muscle cells and fibroblasts exposed to high doserate irradiation appeared similar. The pig model seems well suited to evaluate the short- and long-term effects of ionizing radiation in the prevention of restenosis after vessel injury.     

Effect of subsequent acute-dose irradiation on cell survival in vitro following low dose-rate exposures

Purpose : Following acute irradiation, excess radiosensitivity is generally seen at doses <1 Gy, a phenomenon termed 'low-dose hyper-radiosensitivity' (HRS). A very strong, HRS-like inverse dose-rate effect has also been described following continuous low dose-rate (LDR) irradiation at <30 cGy h -1. We report on the sequential irradiation of a cell line by such LDR exposures followed by low acute doses, where either treatment individually would elicit a hypersensitive response. The aim was to determine if a prior LDR exposure would remove the HRS normally seen in response to very small acute radiation doses. Materials and methods : T98G human glioma cells were given single continuous LDR exposures of 5-60 cGy h -1 using a 60 Co γ-source. At intervals of 0 or 4 h following LDR irradiation, cells were further irradiated with a range of acute doses using 240-kVp X-rays. The response to the combined treatment was assessed using high-precision clonogenic cell survival assays, and the amount of HRS at acute doses <1 Gy was determined. Results : LDR at ≥60 cGy h -1 to total doses up to 5 Gy in asynchronously growing cells did not remove HRS in the subsequent acute-dose survival curve. In confluent cultures, subsequent acute-dose HRS was not present after an LDR dose of 5 Gy at either 60 or 30 cGy h -1, but returned if a 4-h interval was left between LDR and acute-dose irradiation. In confluent cultures, acute-dose HRS remained for LDR treatments at 5 or 10 cGy h -1 or if the total dose was 2 Gy. Taking all cultures and dose-rates together, the 'degree' of acute-dose HRS, as measured by α s, was significantly greater in cells irradiated at LDR to a total dose of 2 than of 5Gy. Conclusions : Initial LDR exposure can affect a subsequent HRS response. HRS is reduced after LDR exposures at greater dose intensity, but can recover again within 4 h of completion of LDR exposure. This suggests that processes determining increased resistance to small acute doses (removal of HRS) might be governed by the level of repairable DNA lesions.     

镧示踪法观察常规分割外照射后血脑屏障的超微结构改变

目的 观察Sprague-Dawley(SD)大鼠全脑常规外照射后血脑屏障(BBB)的超微结构变化特点及其规律。方法 SD大鼠20只,分为5个实验组,60Coγ射线全脑常规分割外照射,各组受照后16h以硝酸镧为示踪剂,用透射电镜观察BBB超微结构变化。结果 对照组镧颗粒分布在血管腔内,BBB结构完整;10Gy组少量镧颗粒通过开放的毛细血管内皮细胞间的紧密连接,沉积于基底膜,但未入脑组织;20Gy组见大部分紧密连接开放,镧颗粒弥漫性分布于神经细胞间隙,细胞内无镧颗粒沉积;30Gy神经细胞质内见大量、核内见少量镧颗粒沉积;40Gy组部分毛细血管内皮细胞及神经细胞失去正常形态,结构破坏,大量镧颗粒沉积在细胞核内、外。结论 放射线具有降低及破坏BBB功能的作用,且与照射剂量呈正相关;30Gy后BBB完全开放,此时进行化疗为最佳时机;照射40Gy后不易再全脑追加剂量,否则会导致不可逆性放射性脑损伤。     

Early modifications of outgrowth and metallo-endopeptidase activity in fibroblasts from rat cutaneous explants after local gamma-irradiation: dermis cellularity and collagen fibre alterations

The initial response to local gamma-irradiation of skin was investigated in fibroblasts from cutaneous explants after doses of 4, 8, 12, 16 or 20 Gy. On the day of irradiation, fibroblast outgrowth was inhibited in a dose-dependent manner, but by day 7 post-irradiation, cell restoration occurred especially in explants exposed to 4 or 8 Gy. The dose-dependent inhibition of fibroblast outgrowth correlated with the decrease in cellular metallo-endopeptidase (MEP) activity against succinyl trialanine paranitroanilide. However, the secretion of this MEP activity was 10-fold higher in the culture medium after the lowest irradiation dose (4 Gy). Its inhibition profile was not modified after local irradiation, whatever the dose. In vivo, the cell density of mastocytes, pericytes and endothelial cells decreased after irradiation. Moreover, damaged collagen was observed in the superficial dermis after local irradiation. These results strongly suggest that this MEP may be involved in the alterations occurring in dermal connective tissue components after skin irradiation. The rapid decrease with the dose in fibroblast outgrowth and MEP activity also suggests that these two parameters may provide useful tools for dosimetric assay of the heterogeneity and extent of irradiated areas.     

辐射对EL-4、J774A.1细胞CD2、CD48表达的影响

目的　通过时程及量效研究观察不同剂量X射线照射对EL 4和J774A 1细胞株中表面分子CD2、CD4 8的影响。方法　采用荧光免疫流式细胞术检测蛋白表达的变化。结果　 (1)EL 4细胞CD2分子表达结果显示 ,0 0 75GyX射线照射后CD2合成在照射后 4h即开始升高 ,至 8～ 16h达峰值 (P <0 0 5～ 0 0 1) ,2Gy照射后则从照射后 4h开始下降 ,至 8h达最低点 (P <0 0 1) ,一直持续较低至 4 8h恢复 ;8h量效结果显示 ,在低剂量区 0 0 5 0 ,0 0 75Gy使CD2表达上调 ,而高剂量范围中 1,2Gy使CD2表达下调。 (2 )J774A 1细胞CD4 8分子表达结果显示 ,0 0 75GyX射线照射后CD4 8合成在照射后 2h即开始升高 ,至 4h达峰值 (P <0 0 5 ) ,随后急剧下降 ,8h恢复至假照射水平 ,16～4 8h下降明显低于假照射水平 (P <0 0 5 ) ,2Gy照射后则从照射后 2h开始下降 ,至 8h达最低点(P <0 0 1) ,随后有所恢复 ,但直至 4 8h仍未能恢复至假照射水平。 4h量效结果显示 ,在低剂量区0 0 5 0 ,0 0 75 ,0 10 0Gy使CD4 8表达上调 ,而高剂量范围中 1～ 6Gy均使CD4 8表达下调 (P <0 0 5～0 0 1)。结论　X射线照射可引起CD2、CD4 8不同的辐射效应 ,两者的相互作用体现出低剂量辐射具有不同于高剂量辐射的兴奋效应。     

Effect of subsequent acute-dose irradiation on cell survival in vitro following low dose-rate exposures

PURPOSE: Following acute irradiation, excess radiosensitivity is generally seen at doses <1 Gy, a phenomenon termed "low-dose hyper-radiosensitivity" (HRS). A very strong, HRS-like inverse dose-rate effect has also been described following continuous low dose-rate (LDR) irradiation at <30 cGy h(-1). We report on the sequential irradiation of a cell line by such LDR exposures followed by low acute doses, where either treatment individually would elicit a hypersensitive response. The aim was to determine if a prior LDR exposure would remove the HRS normally seen in response to very small acute radiation doses. MATERIALS AND METHODS: T98G human glioma cells were given single continuous LDR exposures of 5-60 cGy h(-1) using a (60)Co gamma-source. At intervals of 0 or 4 h following LDR irradiation, cells were further irradiated with a range of acute doses using 240-kVp X-rays. The response to the combined treatment was assessed using high-precision clonogenic cell survival assays, and the amount of HRS at acute doses <1 Gy was determined. RESULTS: LDR at > or = 60 cGy h(-1) to total doses up to 5 Gy in asynchronously growing cells did not remove HRS in the subsequent acute-dose survival curve. In confluent cultures, subsequent acute-dose HRS was not present after an LDR dose of 5 Gy at either 60 or 30 cGy h(-1), but returned if a 4-h interval was left between LDR and acute-dose irradiation. In confluent cultures, acute-dose HRS remained for LDR treatments at 5 or 10 cGy h(-1) or if the total dose was 2 Gy. Taking all cultures and dose-rates together, the "degree" of acute-dose HRS, as measured by alpha(s), was significantly greater in cells irradiated at LDR to a total dose of 2 than of 5Gy. CONCLUSIONS: Initial LDR exposure can affect a subsequent HRS response. HRS is reduced after LDR exposures at greater dose intensity, but can recover again within 4 h of completion of LDR exposure. This suggests that processes determining increased resistance to small acute doses (removal of HRS) might be governed by the level of repairable DNA lesions.     

Dose and dose-splitting effects of X-rays on lung tumour induction in mice

This paper reports lung-tumour induction 12 months after single or split doses of X-rays in C3H/He male mice. The early proliferative response of lung cells after doses which induced lung tumours was also examined after single X-irradiation. The lung-tumour incidence tended to increase with increasing dose after a single irradiation and peaked at 5 Gy. At more than 10 Gy it decreased sharply to the control level. The mean tumour diameters tended to increase with doses up to 7.5 Gy and then decreased beyond 10 Gy. These results suggested that suppression of tumour growth reduced the tumour incidence at doses of over 10 Gy. The lung-tumour incidence decreased with increasing intervals between two equal doses of 2.5 or 5 Gy. The decrease was thought to be caused by the repair of the tumorigenic injury. However, the tumour incidence after two 2.5 Gy irradiations at 1 day intervals or two 5 Gy irradiations at 6 h intervals was higher than that observed after a single dose. This phenomenon was regarded as a progression of the tumorigenic injury. The labelling indices of the lung cells, determined using tritiated thymidine after a single irradiation, were higher than those of non-irradiated control cells. This response was delayed as the dose increased. The responses versus days after irradiation could be classified into three patterns on the basis of their peaks. The possibility that the larger delay after higher doses had some relation to the reduction of target cells for tumour incidence is suggested.     

Changes in the cell kinetics of pig epidermis after single doses of X rays

Changes in the cell kinetics of pig epidermis have been investigated after irradiation with single doses of 15 Gy and 20 Gy of X rays. The epidermis exhibited an initial degenerative phase when the rate of cell depletion was independent of radiation dose. Changes consistent with repopulation were evident between the 14th and 18th day after irradiation. The severity of cell depletion and the rate of recovery of the epidermis were dose dependent. The regenerative phase was characterized by an increased cell proliferation; values for the labelling index (LI) were greater than those in the non-irradiated epidermis, from 14 days after 15 Gy and from 18 days after 20 Gy. The LI was still elevated at the end of the observation period, i.e. Day 56. No change in the time for DNA synthesis was found. Eighteen days after 15 Gy and 22 days after 20 Gy, islands of cells (colonies), with an appearance similar to the cells in the normal epidermis, were seen. The minimum turnover time (TT) for the proliferating cells of the basal layer of the epidermis in radiation-damaged skin was 61-68 h as compared with 125 h in unirradiated skin. For the basal cells in the colonies, TT was 16-22 h.     

Dose and Dose-splitting Effects of X-rays on Lung Tumour Induction in Mice

Summary

This paper reports lung-tumour induction 12 months after single or split doses of X-rays in C3H/He male mice. The early proliferative response of lung cells after doses which induced lung tumours was also examined after single X-irradiation. The lung-tumour incidence tended to increase with increasing dose after a single irradiation and peaked at 5 Gy. At more than 10 Gy it decreased sharply to the control level. The mean tumour diameters tended to increase with doses up to 7·5 Gy and then decreased beyond 10 Gy. These results suggested that suppression of tumour growth reduced the tumour incidence at doses of over 10 Gy. The lung-tumour incidence decreased with increasing intervals between two equal doses of 2·5 or 5 Gy. The decrease was thought to be caused by the repair of the tumorigenic injury. However, the tumour incidence after two 2·5 Gy irradiations at 1 day intervals or two 5 Gy irradiations at 6 h intervals was higher than that observed after a single dose. This phenomenon was regarded as a progression of the tumorigenic injury. The labelling indices of the lung cells, determined using tritiated thymidine after a single irradiation, were higher than those of non-irradiated control cells. This response was delayed as the dose increased. The responses versus days after irradiation could be classified into three patterns on the basis of their peaks. The possibility that the larger delay after higher doses had some relation to the reduction of target cells for tumour incidence is suggested.     

The circadian rhythm for the number and sensitivity of radiation-induced apoptosis in the crypts of mouse small intestine

Apoptosis is a mode of cell death involving nuclear pycnosis, cytoplasmic condensation and karyorrhexis. Changes in the number of apoptotic cells at various times (3-12 h) after a single dose of either 0.5 or 9.0 Gy given at 09.00, 21.00 or 03.00 h were studied in histological sections of small intestinal crypts of mice. The incidences of apoptosis were examined 3 or 6 h after irradiation at different times of day with different doses of gamma-rays ranging from 0.15 to 9.0 Gy. Survival curves were constructed from the dose-incidence curves for apoptosis, using the number of apoptotic cells after high doses (NM) as the maximum cell population size. The mean lethal doses (Do) for the dose range 0-0.5 Gy were calculated for each time of day. A circadian rhythm in both Do and NM values was detected, indicating that both the number and sensitivity of radiation-induced apoptosis were changing throughout the day. A possible explanation based on the cell-cycle states of the target cell population for apoptosis (presumably functional stem cells) was drawn. Most of the target cells were assumed to be in an extended G1 phase. Around 21.00 h a transition from G1 to S phase takes place in some of these cells (approximately seven or eight cells per whole crypt). The S phase then lasts till around 06.00 h. They may be at G2 and M around 06.00-09.00 h, and then they re-enter G1. The circadian rhythm for the number and sensitivity of the cells susceptible to apoptosis obtained in the present report agrees well with this pattern of cell-cycle phases of target cells.