GROWTH AND PHYSIOLOGICAL-RESPONSES TO TOXICOSIS IN LINES OF MICE SELECTED FOR RESISTANCE OF SUSCEPTIBILITY TO ENDOPHYTE-INFECTED TALL FESCUE IN THE DIET

In three experiments, mice from lines selected for resistance (R) or s usceptibility (S) to growth depression from endophyte-infected fescue seed in the diet were fed diets containing infected (E+) or non-infect ed (E-) seed. Activities of liver enzymes known to participate in oxid ation, reduction, or hydrolysis or in conjugation of xenobiotics were measured in these mice. In all experiments, E+ caused greater reductio n in initial ADG of S than of R mice. In Exp. 1, liver cytochromes P45 0 and b5 activities were not affected by Line, diet, or their interact ion. These enzymes were not evaluated in subsequent experiments. In al l experiments, glutathione-S-transferase (GST) and uridine diphosphate glucuronosyltransferase (GRT) activities differed between lines. Resi stant mice had significantly higher GST activity on both diets in Exp. 1, on E- in Exp. 2, and on E+ in Exp. 3. Resistant mice had higher GR T activities on E+ in Exp. 1, on E- in Exp. 2, but after 4 wk on eithe r diet in Exp. 3. Before test diets were imposed in Exp. 3, GST and GR T activities were higher in R-line mice. Divergent selection created l ines that differed in response Do tall fescue in the diet. Postweaning growth of resistant mice was less severely depressed by E+, although susceptible mice later expressed compensatory gain. Activities of two detoxification enzymes generally were higher in Livers from R-line mic e, suggesting a biochemical mechanism for the difference. Using such t raits, it may be possible to select ruminants for resistance to fescue toxicosis.