Year of Publication

2018

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Animal and Food Sciences

First Advisor

Dr. Kristine L. Urschel

Abstract

Insulin plays a vital role in whole-body metabolism and provides a major anabolic stimulus for cellular signaling pathways, including those involved in the metabolism of glucose and protein. Consequently, insulin dysregulation (ID) is known to alter molecular signal transduction in insulin-sensitive tissues such as skeletal muscle, thereby disrupting glucose metabolism and compromising protein synthetic capacity. Our first objective was to induce ID in healthy horses by administering dexamethasone (DEX), a potent glucocorticoid, for 21 days. We evaluated the effects on insulin-stimulated muscle protein signaling components involved in the mammalian target of rapamycin (mTOR) pathway. DEX-induced ID reduced insulin-stimulated activation of downstream (rpS6, 4EBP-1) mTOR signaling and increased atrogin-1 abundance, a marker for protein breakdown (P < 0.05). Additionally, 21 days of DEX elevated plasma amino acids levels in insulin-stimulated conditions, indicative of reduced uptake or increase release into circulation (P < 0.05). The second objective was to evaluate the short-term effects of DEX treatment in healthy horses. Plasma insulin, glucose and amino acid dynamics and activation of mTOR signaling pathways following an oral sugar test (OST) or intake of a high protein meal were evaluated before and after 7 days of DEX treatment, and after 7 days of no treatment. Seven days of DEX treatment increased basal levels of glucose, insulin and several amino acids (P < 0.05). Additionally horses treated with DEX had an exacerbated insulin response to the OST and consumption of the high protein meal in comparison to control horses (P < 0.05). The majority of blood metabolites returned to basal levels after 7 days of recovery from DEX treatment, indicating these effects were transient. Short-term DEX treatment decreased overall activation of mTOR and FoxO3 but increased total FoxO3 and IRS-1 abundance (P < 0.05). Postprandial activation of rpS6 was greater in horses treated with DEX for 7 days but was lower in those horses after 7 days of recovery from treatment (P < 0.05). Postprandial activation of ULK and AMPK tended to be greater in DEX treated horses (P < 0.1). Akt phosphorylation and mysotatin abundance were lower after the OST in DEX treated horses (P < 0.05). The final objective was to evaluate whether similar changes in postprandial metabolic responses would be seen in horses with naturally occurring ID. Plasma insulin, glucose and amino acid responses following ingestion of a high protein meal were determined in mature horses with equine metabolic syndrome (EMS). Horses with EMS had higher basal plasma insulin concentrations but lower levels of aspartate, glutamate, asparagine and plasma urea nitrogen in comparison to healthy controls (P < 0.05). Consumption of a high protein meal resulted in a 9-fold greater insulin response and higher postprandial levels of various amino acids (P < 0.05). Together this research indicates that ID affects whole body protein metabolism by altering cellular signaling pathways in healthy and diseased horses.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2018.048

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