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Title:Effects of ractopamine hydrochloride on nutrient digestibility, environmental N excretion, regulation of skeletal muscle growth, and beta-receptor subtypes in finishing beef steers
Author(s):Harsh, Bailey N.
Director of Research:Boler, Dustin D.
Doctoral Committee Chair(s):Boler, Dustin D.
Doctoral Committee Member(s):Dilger, Anna C.; Shike, Dan W; McCann, Josh C
Department / Program:Animal Sciences
Discipline:Animal Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Beta-adrenergic receptor
Environmental sustainability
Greenhouse gas emissions
Nitrogen excretion
Protein abundance
Abstract:Beta-adrenergic agonists such as ractopamine hydrochloride (Actogain, Zoetis, Parsippany, NJ) are growth-promoting technologies that when fed at the end of the finishing period in cattle act as repartitioning agents, redirecting energy toward protein accretion and away from lipid deposition. Despite nearly fifty years of research, several questions still exist surrounding beta-adrenergic receptors and the agonists to which they bind. The primary objective of this research was to evaluate these areas where relatively little literature exists. Targeted toward two independent areas of focus, this research was divided into two distinct studies. The objective of the first study was to evaluate the effects of ractopamine hydrochloride (Actogain) on nitrogen excretion and nutrient digestibility. The objective of the second study was to compare protein abundance of beta-adrenergic receptor (β-AR) subtypes between bovine muscle, organ, and adipose tissues through western blotting procedures. In a randomized complete block design, twelve Simmental × Angus steers were assigned dietary treatments including a control (without Actogain; CON) or 400 mg·steer-1·d-1 ractopamine hydrochloride (RAC) for 35 d before slaughter. Two 5 d sampling periods were conducted on each block for total collection of feed, orts, feces and urine. No interaction (P > 0.10) between treatment and collection period was observed for any parameter evaluated. Dietary treatment had no effect (P = 0.38) on DMI, but RAC-fed steers had decreased fecal DM output (2.1 vs. 2.5 kg DM/d; P = 0.04) compared with CON-fed steers. Steers fed RAC had greater apparent total tract DM digestibility (72.8 vs. 68.9%; P = 0.02), NDF digestibility (59.2 vs. 53.3%; P < 0.01), and ADF digestibility (53.8 vs. 47.9%; P = 0.05) than CON-fed steers. Although dietary treatment did not affect nitrogen intake (P = 0.51) or nitrogen digestibility (P = 0.14), RAC-fed steers excreted less total nitrogen (120.8 vs. 138.2 g/d; P = 0.02) than CON-fed steers due to a tendency for decreased fecal nitrogen output (61.5 vs. 71.8 g/d; P = 0.09) in RAC-fed steers compared with CON-fed steers. However, dietary treatment had no effect (P = 0.31) on urinary nitrogen output or percentage of nitrogen excreted as urea. An in vitro experiment was conducted to validate the effects of RAC on nutrient digestibility using a contemporary group of heifers (N = 19). Rumen fluid was collected by stomach tube from CON- and RAC-fed heifers to inoculate tubes containing a CON or RAC substrate in a split-plot design. No interaction between rumen fluid source and in vitro substrate was observed (P = 0.44). Greater IVDMD (64.0 vs. 58.6%; P = 0.01) was observed in tubes containing rumen fluid from RAC-fed heifers compared with rumen fluid from CON-fed heifers. Inclusion of RAC in the in vitro substrate increased IVDMD (63.1 vs. 59.5%; P < 0.01). Results suggest feeding RAC may impact microbial digestion of the finishing diet to increase total tract digestion and reduce nitrogen excretion in the 35 d period prior to slaughter. In the second study evaluating β-AR protein abundance, muscle (longissimus lumborum and psoas major), organ (bronchial, atrium, ventricle, and liver), and adipose tissues (visceral, subcutaneous, and intramuscular) from CON-fed steers (N = 9) were collected at slaughter and flash frozen in liquid nitrogen. Tissue samples were normalized to equal protein concentrations and protein abundance was determined by measuring protein band density relative to a standard sample present on every gel. Beta 1-AR protein was more highly enriched in muscle tissue than all other tissue types (P < 0.001). Beta 2-AR protein was most prevalent in bovine heart and lung tissue. No differences in β2-AR protein were observed between muscle tissues (P = 0.25) and adipose tissues (P ≥ 0.17). However, a tendency (P = 0.06) was observed for greater β2-AR protein in subcutaneous fat than longissimus tissue. Although differences in β3-AR protein abundance were observed between tissues (P = 0.03), β3-AR was not especially enriched in any tissue evaluated. Overall, data verify the presence of all three β-AR subtypes in the tissues evaluated and provide a first look at β-AR subtype protein abundance between different tissues. Determining the relative abundance of β-AR subtypes between different tissues may provide critical information to aid in the discovery and development of future β-AR-targeting small molecule drug compounds for beef cattle.
Issue Date:2018-07-05
Rights Information:Copyright 2018 Bailey Harsh
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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