Radiation damage. Effects of radiation exposure (Part 2)

Radiation damage. Effects of radiation exposure (Part 2)

The temperature change during irradiation had no effect on the rate of development of further damage. However, there is evidence in the literature that temperature not only delays the development of the lesion, but also completely eliminates it[132, 89].

This phenomenon was detected by irradiation of ascarid eggs at a temperature of 25 ° with a dose of 50,000 g: in this case, the survival rate was only 1-9%. Those eggs that were irradiated and then kept for 7 weeks at a temperature of 5 °, and then transferred to a medium with a temperature of 25 °, gave a much higher survival rate (up to 40%).

Therefore, during the cooling period, there was an almost complete delay in the reaction and at the same time the number of active molecules capable of continue the reaction, decreased. Most likely, due to diffusion, they have moved from the foci of the reaction (living cells) to points where there is no living substrate capable of reacting.

Studies conducted on large tadpoles of Rana cubetshiana, in which the damaging effect of radiation was studied by morphological parameters (pyknosis, homogenization and fragmentation of nuclei), are very indicative [89].

The effect of temperature on the effects of radiation exposure

It was found that a temperature change in the range of 0-20 ° during irradiation with doses of about 500 g has no effect on the rate of development of further damage. At the same time, the content of tadpoles after irradiation at different temperatures had a strong effect on the rate and degree of damage in the following days.

With an increase in temperature from 0° to 20°, the number of affected cells increased; the temperature coefficient of increase was equal to 4. With an increase in the temperature of the irradiated tadpoles previously kept at a low temperature, as in the above studies, rapid development of the lesion occurred; this shows that the reaction is only delayed, but not eliminated, and the products that arose during irradiation persist for a long time at a low temperature.

Results of practical studies of the effects of radiation

It was found that after irradiation, degenerative cells can be found in the eye and brain of tadpoles; those tadpoles that

were kept at a reduced temperature after irradiation did not have degenerative cells for a long period, but they quickly appeared with an increase in temperature.

Figure 25 shows the survival curves for salamander eggs irradiated with a dose of 3,000 r, which were kept and irradiated at a temperature of 22 degrees Celsius. The survival curve has an S-shape, and on the 6th day after irradiation, the maximum number of organisms died.

Picture 25. Survival curves for salamander eggs irradiated with a dose of 3,000 g at various temperature conditions.

In the second group of tadpoles, which were kept at a temperature of 4 degrees Celsius, weak signs of damage could be noticed only on the 24-26 th day.

In the case when the irradiation was carried out in the cold, and later the tadpoles were kept at a temperature of 22 degrees Celsius, the survival curve almost completely coincided with the curve for the first group.

Figure 26 shows the results of Durie’s experiments with frog eggs [46], which were exposed to different doses at different temperature conditions. These experiments show that in cases where the doses were clearly fatal, a decrease in temperature after irradiation completely delayed the effect of the lesion; when the temperature rose to 22 degrees Celsius, the tadpoles died quickly.

Practical studies of the effects of radiation in higher animals

When irradiating chicken embryos with a dose of 270 r, it was found that after 2 hours cell division was completely suspended. If the embryos are exposed to radiation for 24 hours at a temperature of 5 degrees Celsius, then when the temperature rises, they develop quite normally.

Fig.26

Lowering the temperature reduces the rate of lesion development in higher animals. However, due to the fact that it is difficult to carry out a prolonged decrease in temperature in warm-blooded animals [54], this effect was detected only in newborn rats and mice before the formation of a thermoregulatory apparatus in them [106, 54]. For example, when the temperature of irradiated newborn mice decreased, an increase in resistance was observed.

A dose of 1,500 r, which caused 100% mortality, was not fatal for chilled animals. Studies in which attempts were made to reduce the temperature during and after irradiation in adult animals did not yield results [19]; a noticeable decrease in temperature could be caused in rats and mice for very short periods of time — from 30 minutes to 1 hour. This period was too short compared to the life span of animals (about 10-20 days) to be able to get any result.

On warm-blooded animals, the effect of temperature changes sometimes had the opposite effect due to the fact that a decrease in external temperature did not cause a change in body temperature, but stimulated an increase in metabolism. In those mammals whose temperature decreased during hibernation (bats, marmots), the radiation effect was weakened [21].

These data show with sufficient clarity that the reaction at the time of irradiation differs in nature from the reaction that develops after irradiation. According to the type of its development, the latter belongs to reactions that develop with acceleration. This character is inherent in reactions that develop autocatalytically.

Dependence of primary reactions on temperature during radiation exposure

As already mentioned above, proponents of the target theory (Lee et al.) postulated the position that in cases where the death of the simplest organisms is caused by a single ionization act, the reaction will not depend on temperature, as opposed to reactions caused by several ionization acts. The direct experimental material confirming this position concerns the influence of temperature at the time of irradiation; in these experiments, the aftereffect reaction is not considered.

The above studies show that the temperature at the time of irradiation does not affect the primary reaction at all and does not affect the rate of further damage, regardless of how many ionization acts can lead to death. Of course, it is possible to talk about the complete absence of temperature influence on the first link (photochemical reaction) only in the first approximation. The temperature, of course, has an effect, but the temperature coefficient is very low (about 1.1) and it can be detected with strong cooling.

It is natural to assume that a change in the metabolic rate affects the rate of development of the secondary reaction of the lesion, But a number of observations call this effect into question. For example, immediately before irradiation, mice were forced to perform strenuous physical work: swim in water for 15-60 minutes. However, it was impossible to detect any difference between the experimental animals and the control animals that were in a calm state.

This is also confirmed by experiments aimed at influencing the course of development of the lesion after irradiation with various pharmacological agents. The use of pharmacological drugs, both reducing oxidation and increasing it, sometimes changes only the duration of life, but does not affect the final result.

Response of biological objects to radiation action

The response of biological objects to the radiation effect consists, as already indicated, of two stages: a reaction occurring at the time of irradiation, and a subsequent reaction that develops independently after irradiation for long periods of time [91].

The nature of the secondary reaction occurring during the incubation period is still unclear; many attempts are being made to approach this issue by reproducing the reaction on biochemical compounds outside the body. The phenomenon of the subsequent development of the reaction after irradiation was observed in some chemical and biochemical reactions [6].

For example, it was noticed that the change in the enzymatic activity of trypsin in an aqueous solution continues for some time after irradiation with ionizing radiation. The rate of development of this subsequent inactivation depends on temperature (Table 12) [85],

A similar nature of the aftereffect is observed for pepsin solutions. At the same time, it was established that inactivation occurs as a result of a reaction occurring at a high speed at the time of irradiation and a much slower reaction that proceeds independently after irradiation.

Table 12

Relative change in trypsin activity (in percent) under irradiation at different temperatures in vitro

 

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The previous part of the article:  https://radiation-info.com/en/2022/05/03/patterns-of-radiation-damage-effects-of-radiation-exposure/

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