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Evaluation of a Role for Prolactin in Growth and the Photoperiod-Induced Growth Response in Sheep^1,2,

Cornell University and US Department of Agriculture⁴, Ithaca, NY 14853-0281

³ Reprint requests should be sent to this author.

Abstract

An experiment was conducted to determine whether prolactin is involved in growth or in mediating the photoperiod induced growth response in sheep. Prolactin was manipulated by im injections of 2-Br- ergocryptine (CB154) or sc injections of ovine prolactin (oPRL) and by two light:dark regimens (16L: 8D and 8L:16D). Fifty-six wether lambs (two/ pen) were allotted to one of four treatments for a 9-wk growth study. Treatments were: 1) 16L: 8D, placebo injections; 2) 16L:8D, CB154 injections; 3) 8L:16D, placebo injections and 4) 8L:16D, oPRL injections. Daily injections of CB154 (.1 mg/kg body weighf^.75), oPRL (.8 mg/kg body weight^.75) or placebo were in 1 ml volume. Animals were fed ad libitum a complete mixed diet. At wk 8, plasma prolactin concentrations at 3 to 6 h postinjection were 214, 3, 90 and 228 ng/ml for treatments 1, 2, 3 and 4, respectively. Pattern of feed intake, measured at 8-h intervals for a 48-h period, was affected by photoperiod. Animals exposed to the 16L:8D photoperiod consumed 40.0, 42.4 and 17.6% of their total daily feed intake during the first 8 h of light, second 8 h of light and 8 h dark interval, respectively. Those exposed to the 8L:16D regimen consumed 55.2, 22.2 and 22.6% during their 8-h light interval, first 8 h of dark and second 8 h of dark, respectively. Both cumulative gain and feed intake were greater in 16L:8D control animals than in those animals receiving CB154. However, there was no difference in cumulative gain or feed intake between control animals on the 8L:16D regimen and those receiving oPRL. Overall, these results give no definitive support to a role for prolactin in growth or in mediating the photoperiod induced growth response in sheep.

¹ The authors would like to express appreciation to the National Institute of Arthritis, Metabolism & Digestive Diseases and the National Pituitary Agency for providing the oPRL used in this study, and to Sandoz, Inc. for providing the CB154. Also the authors gratefully acknowledge the assistance of W. B. Currie, R. C. Gorewit and W. R. Butler.

² Supported in part by Cornell Univ. Agr. Exp. Sta., Merck Institute for Therapeutic Research and Agway, Inc.

⁴ Dept. of Anim. Sci.

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