Effects of Oral Chlortetracycline and Dietary Protein Level on Plasma Concentrations of Growth Hormone and Thyroid Hormones in Beef Steers Before and After Challenge with a Combination of Thyrotropin-Releasing Hormone and Growth Hormone-Releasing Hormone1,2,3

The objective of this study was to determine the effect of a subtherapeutic level of chlortetracycline (CTC) fed to growing beef steers under conditions of limited and adequate dietary protein on plasma concentrations of GH, thyroidstimulating hormone (TSH), and thyroid hormones before and after an injection of thyrotropin-releasing hormone (TRH) + GHRH. Young beef steers (n = 32; average BW = 285 kg) were assigned to a 2 × 2 factorial arrangement of treatments of either a 10 or 13% crude protein diet (70% concentrate, 15% wheat straw, and 15% cottonseed hulls) and either a corn meal carrier or carrier + 350 mg of CTC daily top dressed on the diet. Steers were fed ad libitum amounts of diet for 56 d, and a jugular catheter was then placed in each steer in four groups (two steers from each treatment combination per group) during four consecutive days (one group per day). Each steer was injected via the jugular catheter with 1.0 mg/kg BW TRH + .1 mg/kg BW GHRH in 10 mL of saline at 0800. Blood samples were collected at −30, −15, 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, and 360 min after releasing hormone injection. Plasma samples were analyzed for GH, TSH, thyroxine (T4) , and triiodothyronine (T3) . After 84 d on trial, the steers were slaughtered and the pituitary and samples of liver were collected and analyzed for 5′-deiodinase activity. Feeding CTC attenuated the GH response to releasing hormone challenge by 26% for both area under the response curve ( P < .03) and peak response ( P < .10). Likewise, CTC attenuated the TSH response to releasing hormone challenge for area under the response curve by 16% ( P < .10) and peak response by 33% ( P < .02), and attenuated the T4 response for area under the curve by 12% ( P < .08) and peak response by 14% ( P < .04). Type II deiodinase activity in the pituitary was 36% less ( P < .02) in CTC-fed steers than in steers not fed CTC. The results of this study are interpreted to suggest that feeding subtherapeutic levels of CTC to young growing beef cattle attenuates the release of GH and TSH in response to pituitary releasing hormones, suggesting a mechanism by which CTC may influence tissue deposition in cattle.