Organ weights and fat volume in rats as a function of strain and age

The Fischer 344 (F344) rat and the Sprague-Dawley (SD) rat are used commonl y to evaluate potential adverse health effects resulting from environmental exposure to chemicals. They are also the most common rat strain/stock used in physiologically based pharmacokinetic (PBPK) modeling. Accurate charact erization of model input parameters will improve the usefulness of PBPK mod el predictions. Thus, organ (i.e., liver, kidneys, spleen, stomach, small i ntestine, large intestine, heart, lungs, brain) weights and body fat were m easured in male SD rats of different ages (4 to 40 wk) and in young (9 to 1 0 wk) and old (22 to 23 mo) male F344 rats. Comparison of age-matched (9 to 10 wk) F344 and SD rats revealed that the SD rats weighed significantly mo re and had significantly higher absolute organ weights. These significant d ifferences usually disappeared when organ weights were expressed as a perce ntage of body weight (relative organ weight). Percent body fat was signific antly lower in the age-matched SD rats (6.48%) than in their F344 counterpa rts (8.67%). As expected, both body weight and absolute organ weights were significantly higher in old than in young F344 rats. However, these differe nces were largely reversed when relative organ weights were considered, wit h most relative organ weights significantly lower in the old F344 rats. Bod y fat as a percentage of body weight was 14.02% in the old F344 rats. When SD rats of various ages were examined, relative organ weights declined betw een the ages of 4 and 14 wk. In contrast, significant differences in percen t body fat were not detected among the SD rats of different ages and weight s examined in this study (4 to 40 wk, similar to 75 to similar to 450 g). I n summary, values for physiological input parameters are provided that shou ld prove useful in development and implementation of more accurate PBPK mod els.