Effects of dexamethasone, glucose infusion, adrenocorticotropin, and propylthiouracil on plasma leptin concentrations in horses☆

In experiment 1, nine light horse geldings (three 3×3 Latin squares) received dexamethasone (DEX; 125μg/kg BW, i.m.), glucose (0.2g/kg BW, i.v.), or nothing (control) once per day for 4 days. DEX increased (P<0.001) glucose, insulin, and leptin concentrations and resulted in a delayed increase (P<0.001) in IGF-I concentrations. In experiment 2, mares were similarly treated with DEX (n=6) or vehicle (n=6). DEX again increased (P<0.01) glucose, insulin, and leptin concentrations; the delayed elevation in IGF-I concentrations occurred on day 10, 12, and 19, relative to the first day of treatment. In experiment 3, six light horse geldings received either 200IU of adrenocorticotropin (ACTH) i.m. or vehicle twice daily for 4 days. ACTH increased (P<0.001) cortisol concentrations. Further, ACTH resulted in increases (P<0.01) glucose, insulin, and leptin concentrations. In experiment 4, plasma samples from four light horse stallions that were fed 6-n-propyl-2-thiouracil (PTU) at 6mg/kg BW for 60 days to induce hypothyroidism were compared to samples from control stallions. On day 52, stallions receiving PTU had lower concentrations of thyroxine (P<0.05) and triiodothyronine (P<0.01) and higher (P<0.01) concentrations of TSH. Leptin concentrations were higher (P<0.01) in PTU-fed stallions from day 10 through 52. In conclusion, circulating concentrations of leptin in horses was increased by administering DEX. Treatment with ACTH increased cortisol and resulted in lesser increases in leptin, glucose, and insulin. In addition, PTU feeding results in lesser increases in leptin concentrations.