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Changes in Hormone and Metabolite Concentrations in Plasma of Steers during a Prolonged Fast¹

Iowa State University, Ames 50011

Abstract

Two Brown Swiss and two Holstein steers, average weight of 226 kg, were fasted 8 d. Two days before the fast, jugular vein catheters were installed. Blood samples were collected every 15 min from 0800 to 1400 h on d 0, 2, 5 and 8 of fasting. Plasma from each sample was analyzed for concentrations of growth hormone, and from selected samples for insulin, glucagon, glucose, β-hydroxybutyrate, free fatty acids, urea N and glycerol. Both growth hormone and insulin concentrations decreased by d 2 of the fast and remained at that concentration. Glucagon, however, remained constant. From d 0 to 2, concentrations of β-hydroxybutyrate, free fatty acids and glycerol increased but then changed little for d 5 and 8. From d 0 to 2, glucose decreased and urea N increased. In contrast to the other metabolites, glucose and urea N concentrations stabilized between 3 and 5 d of fasting. The ratio of growth hormone to insulin decreased threefold and the ratio of glucagon to insulin decreased fivefold from d 0 to 2; both ratios remained constant during the rest of the fast. The data indicate that fasting cattle adapt by decreasing concentrations in plasma of growth hormone and insulin but not glucagon. These endocrine changes, therefore, seem responsible for greater rates of free fatty acid mobilization and glucose sparing during an energy deficit.

¹ Journal Paper No. J-11661 of the Iowa Agr. and Home Econ. Exp. Sta., Ames. Project No. 2507. The authors gratefully acknowledge the assistance of Dr. John Klindt in the analysis of the endocrine data.

² Present address: Roman L. Hruska, U.S. Meat Anim. Res. Center, ARS, USDA, Clay Center, NE.

³ Nutr. Physiol. Group, Dept. of Anim. Sci.

⁴ Present address: Dept. Anim. Sci., Univ. of Alberta, Edmonton, Alberta, Canada T6G 2P5.

⁵ Present address: Ralston Purina Co., St. Louis, MO 63164.

⁶ Ruminant Nutr. Group, Dept. of Anim. Sci.

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