Influence of chlortetracycline and dietary protein level on visceral organmass of growing beef steers

Thirty-two beef steers (285 +/- 3 kg BW) were used to determine the effects of chlortetracycline and dietary protein level on visceral tissue mass, ch emical composition, intestinal morphology, and proliferation rate indices. Steers were allot;ted randomly by weight to a factorial arrangement of diet ary treatments consisting of either 10 or 13% CP diets top-dressed with a c orn meal carrier (500 g/d) containing either 0 or 350 mg of chlortetracycli ne. After 84 d, steers were slaughtered and visceral organs removed and sep arated. Rinsed wet tissue mass was recorded; total RNA, total DNA, tissue D M, and tissue N content were determined; and tissue sections were prepared for immunohistochemical analysis. Thin tissue sections were evaluated to de termine crypt depth and villus height as well as proliferation rate by immu nohistochemical detection of the nuclear antigen Ki67. Rumen and abomasum w eights and small intestinal length were greater (P < 0.04) in steers fed th e 13% CP diet than in those fed the 10% CP diet on both an absolute weight basis and a percentage of empty BW. Chemical composition of the small intes tinal and ruminal segments were largely unaffected by increased dietary pro tein. Increasing the dietary CP also increased the villus height in duodena l (P = 0.02) and the crypt depth of jejunal (P = 0.03) sections. Dietary ad ministration of chlortetracycline decreased (P < 0.01) small intestinal wei ght both on absolute and empty BW bases. Nitrogen and RNA concentrations of the small intestinal segments were unaffected (P > 0.1) by dietary adminis tration of subtherapeutic levels of chlortetracycline; however, because of increases (P < 0,05), or tendencies for an increase (P < 0.1), in the tissu e content of DNA, the ratio of N to DNA was decreased (P < 0.05) or tended to be decreased (P < 0.1) in the small intestinal segments of the chlortetr acycline-treated animals. The observed decrease in small intestinal epithel ial mass does not appear to be due to alterations in cell proliferation rat e but rather cell size. Consistent with this finding, cell proliferation, a s determined by Ki67 antigen staining, was not affected by dietary treatmen t. Chlortetracycline administration decreased small intestinal mass that ma y be a result of decreased cell size.