with age in a complex manner, irradiated animals being generally more sensitive
than controls.
Finally, recovery rate tested by split exposures was found to
decline sharply with age:
the rates estimated in previously-irradiated animals
were much lower than those in non-irradiated animals of the same age.
From all
these data Storer [S19] concluded that the tests applied were in fact measuring
the damage inflicted to different cellular systems each of which aged at a different rate, in contrast with the notion of a non-specific life-shortenining
action.
268.
Lindop and Rotblat [L12] made a systematic study of the age factor giving
small single sublethal exposures of MeV x rays to SAS/4 mice.
They found that
radiation given at 4 weeks of age produced an effect that was linear with dose
in the range of 50 ~- 780 R.
The life-shortening produced by 100 R was about 4
weeks for mice irradiated at the age of 1 week;
it increased to about 6 weeks
for 5 weeks-old-animals and then decreased steadily to a minimum of approximately
2 weeks for animals irradiated when 70-90 weeks old.
A given reduction in
life-time in old animals represents a much greater loss of the life to live than
the same reduction produced at a young age.
When the effect was expressed as a
precentage of the remaining life-span the increase in response at 5 weeks of age
was still evident, followed by oscillations of the response between a maximum of
6 per cent and a minimum of 3 per cent reduction of the remaining life-span.
269.
In a subsequent paper [L16] life-shortening was studied as a function both
of the age and of the oxygenation conditions of the animals.
For animals breath-
ing air, assuming linearity of response at all ages, the life-shortening effect
at 1 day and at 1, 4 8 and 30 weeks of age was found to decrease as a function
of age from 7.6 to 2.7 weeks/100 R.
Under hypoxic conditions a considerable re-
duction of the life-shortening effect was found at all doses, amounting to a
factor of three for mice irradiated at 8 and 30 weeks of age.
However, when the
mice were irradiated at 1 day or at 1 week of age hypoxia changed the linearity
of the dose-response relationship to a convex upward curve, such that the protection afforded by hypoxia at low doses was large but at high doses small.
authors could not suggest a firm interpretation for such findings.
270.
Johnson [J2] set up a simulated experiment where he computed the life-
shortening as a function of age at irradiation utilizing parameters and lifefunctions taken from Sacher's
[S2] analysis of the LAF1 male mice exposed to
fission neutrons and gamma rays in the Greenhouse experiments [U5, Fe]. Irra-
The