|
|
Home Ames Test Mouse Lymphoma Assay Chromosome Aberration Test Micronucleus Test Rat Liver UDS Test Comet Assay ICH and FDA recommendations Links |
Introduction to Rat Liver UDS Test Some mutagens/carcinogens may be too chemically reactive to reach the bone marrow, others may be absorbed by the gut and metabolized by the liver without reaching the systemic blood system. These chemicals will not be active in the bone marrow micronucleus test, but most will have genotoxic effects on the liver (a prime target organ for carcinogens). Note only is the rat liver capable of metabolizing many compounds into active carcinogens, but is it also directly exposed to the test substance. The rat liver UDS test is viewed as complementary to the micronucleus test, and a combination of the two is generally considered likely to detect the majority of human carcinogens.DNA damage normally results in either cell death, or attemps at DNA repair. This process is called unscheduled DNA synthesis (UDS) and can be quantified autoradiographically by measuring the amount of radioactive thymidine incorporated into the nuclear DNA of cells not in the S-phase of the cell cycle. To measure DNA repair induction in the liver, animals are normally treated orally with the test substance. The time between treatment and the peak level of UDS is throught to vary between compounds so that two "expression periods" are normally used for each test substance. After the appropriate expression time (2 to 14 hours), during which the DNA damage may accumulate, the animals are killed and the liver cells are isolated by enzymatic dissociation, then cultured with radioactive thimidine in vitro to optimize labelling of newly synthsized DNA. The cells are washed, fixed and stained, followed by autoradiographic quantification of thymidine incorporation. The incorporated radioactive thymidine results in the depositiion of silver grains in the overlying autoradiographic emulsion. Increases in the nuclear grain density of cells from treated animals are indicative of DNA repair and therefore DNA damage. Normally, only male rats are used, with four animals being assessed at each experimental point. 
|