Doctor of Philosophy (PhD)
Animal Science (Animal, Dairy, and Poultry Sciences)
The experiments described herein were designed to answer questions that arose from initially attempting to determine whether treatment with dexamethasone increased concentrations of leptin in geldings. Dexamethasone treatment did in fact increase leptin concentrations in mares, geldings, and stallions. Additional experiments were designed to determine leptin's interaction, not only with the adrenal axis, but with the thyroid axis, the growth hormone/insulin-like growth factor-1 axis, as well as glucose and insulin metabolism in geldings, mares, and stallions. During the course of these experiments, differences in leptin concentrations in the horse were attributed to degree of body condition, gender, and feeding time. Additionally, it was found that horses with high body condition fell into two distinct groups based solely on circulating concentrations of leptin (high vs low). The obese, hyperleptinemic horses were found to be hyperglycemic and hyperinsulinemic and had elevated concentrations of triiodothyronine and decreased concentrations of growth hormone, a hormonal profile similar to that of a type II diabetic human. Indeed, this obesity-related hyperleptinemia was associated with a degree of insulin insensitivity evidenced by the increased insulin response to glucose in these horses. Thus, further experiments were conducted to determine the degree to which these horses were insensitive to insulin, as well as whether diet supplementation or feed restriction might alleviate this insulin insensitivity. Chromium propionate supplementation did not decrease plasma insulin or leptin; however, restricted nutrient intake (6 h of grazing per day) was successful in decreasing concentrations of leptin. It was concluded that leptin in the horse is affected by adrenal and thyroid hormones as well as by glucose/insulin metabolism, and that a syndrome of obesity-related hyperleptinemia, hyperglycemia, and hyperinsulinemia exists in the horse that is similar to type II diabetes in humans.
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Cartmill, Joshua Allen, "Leptin in horses: influences of body condition, gender, insulin insensitivity, feeding, and dexamethasone" (2004). LSU Doctoral Dissertations. 2017.
Donald L. Thompson, Jr.