Splenomegaly with Increased Erythroblast Numbers During Erythropoietic Stress Caused by Ambient Low-Temperature in Mice.

Abstract 4602

Erythropoietic stress such as hypoxia has been well described in mammals. These conditions decrease oxygen supply and then enhance production of erythropoietin (EPO) regulates production of red blood cells. Moreover, spleen becomes the main organ of erythropoiesis due to limited marrow space. Here, we describe a new erythropoietic stress, ambient low-temperature. It has been reported that peripheral blood cell counts are affected by ambient low-temperature in several vertebrates. Cold-acclimated rat and chicken exhibit polycythemia (Dveci et al, J Comp Physiol, 2001; Yahav S, Poult Sci, 1997). The response is considered to increase demand of tissues for oxygen, and then enhance metabolic rate and capacity for heat production to acclimate to ambient low-temperature. However, the physiological mechanisms had not been investigated. First, we examined peripheral erythrocyte levels in C57BL/6 mice putting into 5°C ambient. Hematocrits increased from 48% to a plateau of 53% after fourteen days. Likewise, hemoglobin concentration, initially 15 g/dl, rose to 17 g/dl. Reticulocyte production index significantly increased from 4% to 8% after seven days. These data suggested that mice exposed to low-temperature enhanced production of erythrocytes, so we next examined the anatomy and cell composition of their spleens. On day 5, spleens were about 6 mg/g of body weight, two-fold those on day 0. They gradually decreased to their initial weights on day 14. Flow cytometry showed 38% more Ter119+ splenocytes and four-fold more CD71 high Ter119+ early erythroblasts than normal. These values also gradually declined to their initial numbers by day 14. The results suggested the elevated red blood cell counts were due to an increase in production. To test erythropoietic activity in serum, we used an erythrocyte colony-forming assay. Serum from mice kept at low-temperature showed no ability to stimulate CFU-E colony formation in vitro. However, a combination of the serum with EPO (0.5 U/ml) increased CFU-E numbers 1.5 to 2 times that of a combination of normal serum plus EPO or EPO alone. Whether inducible factors account for these effects is the focus of future investigation. Our findings suggest a low-temperature environment is an erythropoietic stress that may offer insights into the adaptive physiology of red blood cell production in mice.