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Dietary Crude Protein Levels and the Effect of Isoproterenol on In Vitro Lipogenesis in the Chicken¹

U.S. Department of Agriculture-Agricultural Research Service, Beltsville Agricultural Research Center, Beltsville, MD 20705

Experiments were conducted with male broiler breeder chickens to determine the metabolic effects of two levels of dietary crude protein fed during a protein-reversal regimen on subsequent responses to a ß-adrenergic agonist (isoproterenol) in vitro. Day-old chickens were fed diets containing either 12 or 20% crude protein until 14 d of age and then switched to the opposite diet until 28 d. In experiment 1, chickens were killed at 28 d, and in experiment 2, at intervals following the switch at 14 d. In vitro lipogenesis and lipolysis were determined. The hepatic enzymes isocitrate dehydrogenase, malic enzyme and fatty acid synthetase were determined during the 14 to 28 d of age growth period. Both in vitro lipogenesis and lipolysis were greater at 28 d when chickens were fed the 12% rather than the 20% protein diet. Neither 0 to 14 d of age dietary treatment influenced 28-d metabolic parameters. In contrast, the 12% protein diet during the first 14-d period depressed body weight (160 g vs. 320 g; p < 0.05) and weight gain during the 14 to 28 d of age period regardless of dietary crude protein levels (508 g vs. 626 g; p < 0.05). Lipogenesis and lipolysis were at their highest points 4 d following the dietary switch. Isoproterenol decreased in vitro lipogenesis 70% in all dietary treatment groups. Stimulated lipolysis was not affected by nutritional status and was twice the unstimulated rate. Both malic enzyme and fatty acid synthetase activities increased (p < 0.05) following the switch from the 20% to the 12% protein diet and decreased (p < 0.05) following the switch from the 20% to the 12% protein diet. This change also occurred within 4 d. It can be concluded from this study that most metabolic responses to severe dietary challenges occur within 1 to 3 d and that a steady state is reached within 4 d.

KEY WORDS: • protein repletion • metabolism • adaptation • chickens

¹ Mention of a trade name, proprietary product or specific equipment is not a guarantee or a warranty by the U.S. Department of Agriculture and does not imply an approval to the exclusion of other products.

² Address correspondence to Dr. R. W. Rosebrough, NANL-LPSI, Rm 212, Building 200, BARC-EAST, Beltsville, MD 20705.

Manuscript received 13 March 1990. Revision accepted 15 June 1990.

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