specific activity of iron 59 in the livers of ir- 

 radiated and control fish was similar through- 

 out the experiment. Since mammalian liver 

 has a regulatory function in iron metabolism, 

 the absence of wide fluctuations in the specific 

 activity of the liver of irradiated pinfish indi- 

 cated that the liver may function in iron mobili- 

 zation and storage and was relatively insensi- 

 tive to the radiation dose. 



The size distribution of blood cells in the 

 pinfish was altered by radiation (fig. 31). On 

 the 1st day, the size distribution of cells was 

 about the same as for the controls, but the 

 peak shifted to a lower value on the 3d and 5th 

 days after irradiation. On the 7th day, size 

 distribution neared that of the controls. From 

 the 14th through the 28th days, most of the blood 

 cells in irradiated fish had a larger volume 

 than in the controls. The last shift in distri- 

 butions of cell size coincided with the increase 

 in the specific activity of the blood. The in- 

 crease in cell size at 14 days indicates that 

 larger younger cells were released into the 

 circulation. 



Comparison with Earlier Studies on 

 Irradiation and Iron Metabolism 



A comparison of the changes in specific 

 activity of iron 59 in the blood with changes in 

 erythrocyte numbers after irradiation shows 

 (fig. 32) that the dose of 2,000 rads caused tenn- 

 porary reduction in erythrocyte production 

 but little change in erythrocyte numbers. The 

 small change in numbers of erythrocytes after 

 irradiation observed in previous experiments 

 may have been due to the long life of the ery- 

 throcyte. Although erythrocyte production was 

 temporarily arrested, the numbers of cells 

 did not decrease. The mean life span of fish 

 blood cells is about 150 days, so the period of 

 depressed erythrocyte production, which ex- 

 tended no longer than 30 days after irradiation, 

 was not long enough to produce significant 

 reductions in numbers of erythrocytes in the 

 blood. 



The effects of radiation on the iron 59 me- 

 tabolism of the pinfish differed somewhat 

 from the results observed with the fresh- water 

 tench and from those which have been observed 

 with mannmals. When the tench was irradiated 

 with 1,500 rads and maintained at 18° C, the 

 specific activity of iron 59 in the erythrocytes 



reached its lowest point 7 days after irradi- 

 ation. The specific activity of pinfish blood 

 reached its lowest point 22 days after irradi- 

 ation, but the pinfish had received a large 

 radiation dose and were maintained at slightly 

 higher temperatures (20° to 25 C). The dif- 

 ferences in the time required to reach maxi- 

 mum depression of specific activity may be 

 a reflection of differences in radiation doses 

 or simply species differences. If temperature 

 were the controlling factor, the maximum de- 

 pression of specific activity of iron 59 should 

 have occurred earlier in the pinfish than in 

 the tench. The maximum depression of ery- 

 throcyte production occurs much more rapidly 

 in mammals than in pinfish. The period of time 

 required for the effect to appear after irradi- 

 ation may be a function of turnover time of 

 red blood cells, which varies with metabolic 

 rate. The mean life span of erythrocytes in 

 the mouse is known to be about 60 days, as 

 compared to 150 days for fish. 



The shift in cell volume from a low point on 

 the 3d and 5th days after irradiation to volumes 

 which were greater than those of the control 

 fish indicated that larger and younger cells 

 were being released into the circulation of the 

 pinfish from the 14th through the 28th days 

 after irradiation. A shift in distribution of cell 

 size has also been demonstrated in mice 

 following bone-marrow arrest. The curves 

 are biphasic at 15 days and then shift to a 

 more normal distribution at 34 days after 

 irradiation. The distributions of cell volumes 

 in pinfish showed some indication of becom- 

 ing biphasic, but the resolution of our sys- 

 tem was not as precise as that used in the 

 mouse work. 



Both kidney and spleen are intimately con- 

 nected with erythrocyte production in fish; 

 the kidney is considered the major site of pro- 

 duction. Incorporation of iron 59 into blood and 

 kidney indicated that following initial arrest 

 of erythrocyte production, the kidney recovered 

 its function and released large numbers of 

 new erythrocytes into the circulation between 

 the 22d and 36th days. The decrease in the 

 specific activity of the kidney below the con- 

 trol level on the 36th and 50th days was caused 

 by the replacement of damaged erythrocytes 

 with newly formed cells. Most of the iron ac- 

 cumulated by the kidney was used immediately 

 to form new erythrocytes, thus making the 

 turnover of iron 59 in the kidney very rapid 



57 



