Safety_Series_025_1968 - gnssn - International Atomic Energy ...

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This publication is no longer validPlease see http://www.ns-iaea.org/standards/X -r a y s . S im ilar rela tion sh ip s w ere found fo r oth er sy ste m s, fo rexample ascites tumour c e lls , ch rom osom e aberrations, and m itoticdelay in plant tissues and isolated mam malian ce lls.T he oxygen effect is usually le s s with highly ion izin g radiation,such as alpha p a rticles o r slow e le ctro n s.A v ery m arked effect o f hypoxia has been observed with ir r a d iated rats. The LD5o/3c>for rats irradiated in 5% oxygen is tw ice thatfo r anim als exposed in a ir. In a sim ila r way hypotherm ia has aprotective effect, probably due to decreased blood flow with resultinglow er oxygen tension. On the other hand, oxygenation may in creasethe radiosen sitivity. E xperim ents with in crea sin g oxygen p ressu reare being p erform ed in radiotherapy o f tum ours.The oxygen effect during irra d iation must not be confused withthe application o f oxygen after irradiation . In this ca se the oxygeninfluences som e stages in the developm ent o f radiation injury whichare sensitive to the p resen ce o f oxygen.F o r the understanding o f different a fte r-e ffe cts the form ation oforganic p eroxid es is im portant. Inactivation o f phage p a rticle sirradiated in broth is enhanced by the presen ce of organic peroxidesform ed in the broth during irra d ia tion . T his action m ay be ob serv a b lefo r several days after the irradiation has stopped. If inorganic p e roxid es are added b e fo re irra d ia tion , the sen sitivity o f phage o rb a cteria is in cre a se d .1 .2 . 2. 3. P rotectiv e substancesThe fact that it is possible to d ecrease the radiosensitivity o f theorgan ism by outside intervention has led to a search fo r substanceswhich would provide a m easure o f protection against radiation. Therange o f p rotectiv e substances o f this kind is v ery la rge, includingsubstances o f different ch em ical and physical p rop erties. The m ostim portant are sulphur-containing compounds (cysteam ine, cystam ine,am inoethyl-isothiouronium (A E T ), glutathione, e t c .) Some enzym einhibitors (sodium cyanide), alcoh ols, certain m etabolites (gluconate,A T P , pyruvate) have a sim ila r action. A noxia-producing ch em icalslike PAPP (p-am inopropriophenone) which induces methem oglobinemiaenable 72% survival in m ice after a lethal exposure.The p ra ctica l applicability o f ch em ica l p rotection in man up tonow is slight, b ecau se m ost o f the active ra d iop rotectors a re toxicin con cen tration s needed fo r sign ifican t p rotection . The abovementioned ch e m ica l p r o te c to r s a re o f p ra ctica lly no value against30
This publication is no longer validPlease see http://www.ns-iaea.org/standards/ch ron ic ex p o su re . A cute and ch ro n ic radiation in ju ry in volv e d ifferentorgans o r organ groups and they do not receive equal chem icalp rotection . A ll these fa ctors dim inish con siderably the p racticalityo f use o f different drugs in prevention o f radiation injury.1 .2 .2 .4 . R ecov eryRadiation injury is a p ro ce s s com p osed o f many stages, endingin the ex p ression of the p rim a ry dam age. Some o f these stagesare only tem porary and, in the p resen ce o f inhibitory fa cto rs, theycan be restored . It is p ossible that organ ism s have v ery varied r e cov ery p ossib ilities which are at present not w ell known.It is, however, possible to prom ote recov ery by giving recoveryagents o f two types after irradiation.(1) A gents w hich d estroy som e in term ed ia te com pound in thechain o f events a fter irradiation . The e ffe ct o f u ltra v iolet rays inbacteria con sists in form ation o f thymine d im ers in the m olecule ofd esoxyrib on u cleic acid. In radioresistan t bacteria an enzym e ispresen t which is able to e x cise the d im er and re s to re the integrityo f the DNA m o le cu le . In som e ca s e s changes in tem peratu re mayinhibit the e x p re ssio n o f radiation in ju ry.(2) A gents w hich a re able to rep la ce a dam aged com pound o rce ll. This m ode o f re co v e ry o ccu rs som etim es when irradiated andnon-irradiated bacteriophages in fect bacterial ce lls and the damagedparts o f the phage genom e are subsequently replaced by the 'healthy1ones from n on-irradiated phages.E xp erim en ts with la rge m ultinucleate am oeba have shown thatin lethally irradiated individuals vitality could be restored by fusionwith fragm ents o f n on -irradiated individuals.On a la rg e r sca le , whole populations o f dam aged bon e-m a rrowce lls m ay be rep la ced by in jectin g intact b on e-m a rrow c e lls from , an on -irra d ia ted d on or. T h ese experim en ts su cce e d on ly when thein jected b o n e -m a rro w c e lls are iso lo g o u s. A fter a h om ologou s o rheterogous injection an important im m unological reaction takes placedue to in com patibility between the donor ce lls and the re ce p to r anim al.T his 'secon d a ry d isea se' m ay lead to the death o f the anim al. F o rthis reason autologous bon e-m arrow transplants may be used in manto induce m ore rapid repopulation of m arrow sites, although hom ologoustransplants in a few instances have been tolerated.31
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