Identifier

etd-07022013-185713

Degree

Master of Science (MS)

Department

Animal Science (Animal, Dairy, and Poultry Sciences)

Document Type

Thesis

Abstract

Six insulin sensitive and six insensitive mares were used in a replicated 3 x 3 Latin square design to determine the pituitary hormonal responses (compared to vehicle) to sulpiride and thyrotropin-releasing hormone (TRH), two compounds commonly used to diagnose pituitary pars intermedia dysfunction (PPID) in horses. Mares were classified as insulin sensitive or insensitive by their previous glucose responses to direct injection of human recombinant insulin. Treatment days were February 25 and March 10 and 24, 2012. Treatments were sulpiride (racemic mixture, 0.01 mg/kg BW), TRH (0.002 mg/kg BW), and vehicle (saline, 0.01 mL/kg BW) administered intravenously. Blood samples were collected via jugular catheters at -10, 0, 5, 10, 20, 30, 45, 60, 90, 120 min relative to treatment injection. Plasma ACTH concentrations were variable and were not affected by treatment or insulin sensitivity category. Plasma melanocyte stimulating hormone (MSH) concentrations responded (P < 0.01) to both sulpiride and TRH injection, and were greater (P < 0.05) in insulin insensitive mares than in sensitive mares. Plasma prolactin concentrations responded (P < 0.01) to both sulpiride and TRH injection, and the response was greater (P < 0.05) for sulpiride; there was no effect of insulin sensitivity. Plasma thyroid stimulating hormone (TSH) concentrations responded (P < 0.01) to TRH injection only, and were higher (P < 0.05) in insulin sensitive mares in almost all time periods. Plasma LH and FSH concentrations varied with time (P < 0.05), particularly in the first week of the experiment, but were not affected by treatment or insulin sensitivity category. Plasma GH concentrations were affected (P < 0.05) only by day of treatment. The greater MSH responses to sulpiride and TRH in insulin insensitive mares were similar to, but not as exaggerated as, those observed by others for PPID horses. Also, the reduced TSH concentrations in insulin insensitive mares are consistent with the previous observation of elevated plasma triiodothyronine concentrations in hyperleptinemic horses (later shown to be insulin insensitive as well).

Date

2013

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Thompson Jr., Donald L.

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