Doctoral Dissertations

Date of Award

12-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Comparative and Experimental Medicine

Major Professor

Nicholas Frank

Committee Members

Carla Sommardahl, Stephen Kania, Joseph Bartges

Abstract

In horses, a painful and often debilitating disease known as laminitis can result in impaired function and, in severe cases, euthanasia. Equine Metabolic Syndrome (EMS) is a syndrome in horses that results in development of laminitis and is characterized by the presence of general and/or regional adiposity (“cresty neck”), aberrations in blood lipid concentrations, insulin resistance (IR) and/ or hyperinsulinemia. Therapies have focused on improving the state of obesity and insulin resistance with the goal of diminishing the likelihood of laminitis development. A definitive cause for laminitis has not been established, but hyperinsulinemia and IR are likely candidates as experimental states of hyperinsulinemia have been shown to induce laminitis and improvements in insulin sensitivity and obesity have been associated with a decreased risk of laminitis development. This dissertation discusses associations between obesity and IR, as well as potential therapies for alleviating insulin resistance with the ultimate goal of decreasing the risk of developing laminitis. Therapies evaluated included chromium and magnesium, levothyroxine sodium, and metformin hydrochloride. Horses were treated with each supplement for 10 to 36 weeks, depending on the supplement tested, and physical measurements such as body weight, neck circumference, and body condition score were obtained. Throughout each study, blood concentrations of glucose, insulin, and plasma lipids were analyzed. Chromium and magnesium currently do not appear to have any effect on insulin sensitivity, whereas results of levothyroxine administration indicate therapeutic responses, as does metformin, though results indicate further work are required. Research contained in this dissertation focuses on the potential of identifying animals at risk of developing IR and laminitis through measurement of blood biomarkers such as adiponectin and glucagon-like peptide 1. Assays to measure markers included enzyme-linked immunosorbent assays, western blots, and radioimmunoassays. Glucagon-like peptide 1 currently does not appear to differ between healthy and IR animals, but protein band density of high-molecular weight adiponectin does appear to be lower in horses with IR when compared to healthy animals. There is still much to learn about IR in horses, and therapy appears to be dependent on a case by case basis.

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