Revision 1
March 19, 1998

by Radiation, Science, and Health, Inc.,
Edited by J. Muckerheide

1.3
Animal & Plant Biology

1.3.1
Mammals

References

"Lorenz et al. surprised radiobiologists with reports that chronic, low level gamma ray exposure from a filtered radium source increased growth rates in mice, rabbits, and guinea pigs. (Lorenz 1950, Lorenz et al 1946, 1951, 1954, 1955, 1955b) The exposure periods were 8 h/d from 1 month of age to death. When compared with controls, the growth rates of animals exposed to the lowest doses were consistently and significantly faster, p <0.01, during much of the growth period (Figure 3.1).

"Stimulation of the growth rate of young rabbits was particularly strong. The growth rates of mice exposed to 1.1 cGy/d were consistently faster than that of controls. Male mice exposed to 1.1 cGy/d showed early growth rates which were 150% that of controls in both first and second experiments (Lorenz 1954). In both experiments, the exposed males grew larger than their controls and maintained this weight advantage until late maturity, about 100 weeks of age. With somewhat higher doses, 8 cGy/d, growth rates of exposed and control mice were comparable; this approximated the ZEP for chronic experiments.

"Lorenz (1954) found the greatest mature weight of mice occurred in the second experiment where the males lived as long as the females, an unusual occurrence (Table 3.1). The mature weights of male and female guinea pigs was somewhat greater in those exposed to 1.1 cGy/d than in controls at the time of the greatest control weight. Rabbits showed no mature weight differences."

"Difficulty with interpretation of how the control mice were treated in the Lorenz protocols led us to repeat the experiment one decade later. (Morris 1963, 1980) Since handling rodents can increase growth rates, control mice were set under the X-ray machine without current each day to provide a proper control.(Reugamer 1954)

"Growth rates of lightly exposed animals were greater than those of sham exposed controls, p<0.01, from 4 to 8 weeks of age, the breeding age for mice (Figure 3.1). (Morris 1963, 1980) Comparison of our data with those of Lorenz illustrates that growth stimulation was maximum at about the same dose, 1 to 2 cGy/d (Figure 3.2).

"Bustad et al. found that 140 mice exposed to 0.8 cGy/d gamma radiation grew somewhat faster than controls.(Bustad, 1965) They also noted less dermatitis and alopecia in chronically exposed mice.

"Further confirmation of the effect of radiation on growth rates was reported by Ely, who demonstrated faster growth of rats exposed to 0.11 cGy/d from fast neutrons. (1967)

"Intravenous injection of 20 to 60 kBq of U-238 or Ra-226 significantly increased the growth rate of mice, p<0.01. (Finkel, 1953)

"Injection of 300 kBq of polonium decreased growth in mice. (Blair 1964)

"Mice from dams given tritiated water produced offspring which grew at a faster rate than controls, a result confirmed by Laskey et al. (Figure 3.3). (Cahill 1969, 1975, Laskey 1973)

"Experiments which showed no radiation hormesis in growth are usually found to have started with young adults. (Boche 1967, Carlson 1957, 1962, McDonald 1967, Sacher 1964, Spalding 1982)"

"The group at Washington University provided good protocols for their chronic exposure to low levels of ionizing radiation in experiments involving over one million salmon, the "unreachable" goal of radiobiologists working with mice and rats. Following exposure during hatch and growth of alevins, the fingerlings were identified by clipping a scale on one side; controls received comparable identification on the other side. (Bonham 1966, Donaldson 1964, Hershberger 1978) The megafish experiment provides data which confirms the results with mammals (Figure 3.4).

"Following 3 months of continuous exposure to gamma radiation, about 650 salmon were used to estimate the mean weight of the 80,000 fingerlings in each group. (Bonham 1966, Donaldson 1964, Hershberger 1978). Growth stimulation was noted with 0.54 cGy/d, p<0.005. The length of the 1960 fingerlings exposed to 1.3 cGy/d was not greater than that of controls. ZEP was about 4 cGy/d and levels >5 cGy/d were detrimental.

"All fish were released into their natural habitat, from which they migrated to the Pacific Ocean until their return to spawn 3 to 5 years later. (Bonham 1966, Donaldson 1964, Hershberger 1978). The use of natural conditions gives greater confidence for the interpretation of results when compared with the usual experiments with inbred strains and laboratory conditions. The weight of individual, mature fish which returned to breed after 4 years in the Pacific Ocean was greater in the exposed fish; males were 115% and females were 116% of the controls weights. Had the groups not been disturbed, one may presume that the return of more exposed than control fish would result in more offspring from the irradiated animals, a measure of the growth of populations.

"Data from more complex experiments suggest benefits from exposure to ionizing radiation may persist into the next generation. Hershberger and associates (1978) reported that when unexposed fish from lightly exposed fathers returned to spawn after three years in the Pacific Ocean, the mean weight was 117% more than controls. Again, a greater proportion of exposed fish returned; one would presume the total population of the lightly exposed fish would quickly become much greater than that of controls. This transferral of something to the second generation was noted in duckweed. (Feldman 1975)"

"Brues and Sacher found rats showed immediate and sustained increments in growth rate following exposure to 2 Gy of X-rays. (1952)

"Lindop and Rotblot reported increased growth in mice following exposure to 0.5 Gy of X rays. (Lindop 1959)

"Kholin found that exposure to 0.5 to 2.0 cGy of X-rays increased growth rate and maturation in rats. (1968) The best results were obtained with exposures soon after weaning.

"The height of burro foals exposed to 2.5 Gy of gamma radiation was significantly greater than controls at 2 years. P<0.05; no difference had been noted during the first year. (Knapka 1967) Increases noted in weight and heart girth were not significant.

"Chick growth was accelerated by exposure to 5 to 10 Gy of gamma rays, (Kashiwabara 1967) a result confirmed by Shebaita et al. who administered 6.4 Gy of gamma radiation. (1975; 1979) When eggs were exposed to 6.4 Gy of gamma rays, chicks grew faster than chicks from unexposed eggs. (1975)

"When fish embryos were exposed to 0.15 to 15 Gy of X-rays, they grew at a faster rate than controls. (Wadley 1971)

"Exposure of carp sperm to 5 Gy gamma rays increased the survival of young to marketable size; they were 128% larger than controls. (Bakos 1980) The average weight was also increased by exposure of sperm to 10 or 20 Gy; a few abnormalities were found following exposure to these high doses.

"Following extensive research on the growth rates of invertebrates, Stebbing extrapolated to vertebrate homeostatic mechanisms. (1987) The short-term feedback mechanisms suggested may be major factors in radiation hormesis in fast growth rates. These are most important in fast regenerating systems such as intestinal epithelium, liver, hematopoietic tissues, and seminiferous epithelium where injured cells can be replaced by new cells. (Fabrikant 1987) Increased production of mature cells from such systems results from shortened cell-cycle times and increased numbers of dividing cells within the generative compartment of the tissue. Excess stress, such as exposure to large doses of ionizing radiation, can harm these systems and decrease cell renewal and tissue function. "

"The statistically significant results of Lorenz have been confirmed in several studies. The well-designed and executed experiments of Donaldson (1964), Bonham (1990), and Hershberger (1978) leave little doubt that chronic exposure of young animals to low doses of ionizing radiation increases the growth rate. Exposure to acute doses is less pertinent; however, such data add evidence to the concept of radiation hormesis. A chronologic perspective of radiation hormesis in growth (Table 3.2) suggests this is a general phenomenon.

"Increased growth rates, development, and survival in one generation provide potentially greater reproduction and increased numbers in subsequent generations. This provides the capability for increased growth rates in subsequent colonies or populations.

"Preliminary work indicates that exposed animals show increased neurologic acuity, neuro- muscular activity, and early maturation. The reports of increased learning ability and better memory provide the basis for speculation and intriguing areas for full research.

"Information on development following exposure to low doses of ionizing radiation was not focused on any parameter. Muscular development appeared to be accelerated. Much of the data concerned stimulation of neurologic functions. The data include simple physiologic activities such as cerebral blood flow, sensitive activities such as nerve and brain excitability, increased visual and auditory acuity, and complex activities such as increased learning and memory. The chronologic listing of reports of radiation hormesis in neurologic development indicates this should become an exciting area for future research (Table 3.3). "