Lethal and Sub-Lethal Irradiation of Sprague-Dawley Rats Causes Development of Dense Red Blood Cells

Background: In vitro irradiation of blood produces morphologic and rheologic changes in the red blood cells. These changes may have clinicla significance, and if the changes are prolonged beyond hematologic recovery, serve as an indication of radiation exposure. However, no rheological studies have been performed on blood from irradiated animals.

Objective: Determine the effect of lethal and sub lethal radiation treatment (RT) on whole blood rheology in a rodent model.

Methods: 3 SD rats were subjected to lethal doses of irradiation (7.5 Gy), and 3 untreated; ten days following RT, the animals were sacrificed, and their blood analyzed by an ADVIA hematology analyzer, a cone and plate viscometer, and a RheoScan ektacytometer.

Next, following baseline CBC and dense cell measurements, 6 rats were subjected to whole body RT at 6 Gy; 6 were subjected to 7.5 Gy with one leg shielded, and 2 were not irradiated. Percent dense red blood cells (%DRBC), CBC, reticulocyte count, and red cell deformability were measured 4 hours, 5 days and then every 10 days post RT.

11 week old Sprague Dawley (SD) rats were used in all experiments. Peripheral blood smears were prepared and analyzed from each blood draw to assess RBC morphology.

Results: SD rats who received lethal irradiation at 7.5 Gy exhibited a marked rise in %DRBC; 17.1% ±10.5% compared to 0.5%±0.3% in untreated SD rats (p=0.025). The hematocrit to viscosity ratio (HVR), a measure of oxygen carrying capacity, was calculated at two shear rates, 45 s^-1 and 225 s^-1, to mimic venous and arterial circulation. There was no significant difference in HVR at 45 s^-1, but the HVR at 225s^-1 was significantly lower in the RT group, p=0.018, suggesting impairment of oxygen carrying capacity due to intrinsic red cell defects caused by lethal RT. Deformability was reduced in the RT group as well (p=0.039).

In longitudinally followed SD rats subjected to sub lethal RT, %DRBC rose 4 hours after irradiation, peaked at 5 days post-irradiation, and remained elevated 31 days after RT. At 31 days post-RT, hematologic recovery was observed in both the sub lethal and shielded groups, but %DRBC remained elevated. RBC morphology revealed anisocytosis in the RT groups, but few spherocytes were noted.

Conclusions: In vivo RT impairs red cell rheology in SD rats; lethal doses cause a marked rise in %DRBC, reduction in red cell deformability, and reduction in oxygen carrying capacity as measured by HVR. Longitudinal examination of sub lethal RT reveals a rapid rise in %DRBC, without spherocytosis, suggesting that this change may be due to a KCl channel leak rather than spherocytosis from splenic fibrosis. Obese SD rats have higher %DRBC at baseline, and show a significantly higher increase in %DRBC in response to RT, compared to normal weight SD rats. This suggests that obesity may exacerbate the rheological complications of RT.

The persistence of %DRBC after hematologic recovery is a significant observation, suggesting that radiation exposure after the window of hematologic recovery has passed could be detected by a point of care test for %DRBC, already available, as a rapid field test.