Abstract
The aim of the current study was to identify associations between pituitary lesions, body condition scores, and mesenteric lipomas in horses with insulin resistance. Necropsy examinations were performed following euthanasia in 30 adult horses designated as insulin resistant (n = 11) or insulin sensitive (n = 19). Insulin sensitivity was determined using the insulin-modified frequently sampled intravenous glucose tolerance test and resting insulin concentrations. At necropsy, mesenteric lipomas were measured. The pituitary and adrenal glands, pancreas, and liver were evaluated histologically; pituitary glands were scored based on published criteria. Insulin-resistant horses had significantly higher pituitary scores (p = 0.0035) and body condition scores (p = 0.0001), even when adjusting for age, and a greater frequency of mesenteric lipomas (p = 0.014) and greater lipoma area (p = 0.0332) than insulin-sensitive horses. Regardless of insulin status, horses with pituitary scores ≥3 (diffuse hyperplasia; n = 25) had higher body condition scores (p = 0.0313) and a greater frequency of mesenteric lipomas (p < 0.0002) than those with lower pituitary scores. High body condition score was not correlated to an increased frequency of mesenteric lipomas. Detection of higher pituitary scores in insulin-resistant horses suggested an association between insulin resistance and pituitary morphology. Horses in the insulin-resistant group and those with high pituitary scores had higher body condition scores and a greater frequency of mesenteric lipomas. These horses might be at increased risk for lipoma-associated colic.
Highlights
Insulin resistance (IR) is broadly defined as a decrease in tissue responsiveness to circulating insulin, resulting in decreased insulin-mediated glucose uptake into skeletal muscle, and adipose and liver tissues [1]
In both IS and IR horses, pituitary scores were positively correlated with increasing age (r = 0.56, p = 0.0013), but gender was not an association
Insulin resistance was associated with a higher body condition scores (BCS) and higher pituitary score in the horses examined in the current study
Summary
Insulin resistance (IR) is broadly defined as a decrease in tissue responsiveness to circulating insulin, resulting in decreased insulin-mediated glucose uptake into skeletal muscle, and adipose and liver tissues [1]. IR and obesity often occur concurrently; the increased insulin secretion that accompanies obesity can result in hyperinsulinemia, which exacerbates IR [2] [3]. Some horses and ponies in the early stages of IR are able to compensate by increasing pancreatic insulin secretion to maintain normoglycemia [4]. In these horses, insulin concentrations can increase dramatically after feeding, and as the condition progresses, fasting hyperinsulinemia develops. Diagnostic tests for IR include one-time measurement of fasting insulin concentration and measuring insulin sensitivity via the insulin-modified, frequently-sampled intravenous glucose tolerance test (FSIGTT) [5]. Fitting of FSIGTT data to a compartmental population model provides sensitivity to insulin (SI) values, which are used to identify IR animals [6]
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