Leptin secretion in horses: effects of dexamethasone, gender, and testosterone.

Five experiments were performed to evaluate the effects of dexamethasone (DEX), gender, and testosterone on plasma leptin concentrations in horses. In experiment 1, plasma leptin, insulin, glucose, and IGF-1 concentrations were increased (P < 0.01) in stallions following five daily injections of DEX (125 microg/kg BW). In experiment 2, leptin concentrations increased (P < 0.01) in mares, geldings, and stallions following a single injection of DEX, and the response was greater (P < 0.01) in mares and geldings than in stallions. The gender effect was confounded by differences in body condition scores and diet; however, based on stepwise regression analysis, both BCS and gender were significant sources of variation in the best fit model for pre-DEX leptin concentrations (R(2) = 0.65) and for maximum leptin response to DEX (R(2) = 0.75). In experiment 3, in which mares and stallions were pair-matched based on age and body condition and fed similar diets, mares again had higher (P < 0.01) leptin concentrations than stallions after DEX treatment as used in experiment 2. In experiment 4, there was no difference (P > 0.1) in plasma leptin response in mares following four single-injection doses of DEX from 15.6 to 125 microg/kg BW. In experiment 5, treatment of mares with testosterone propionate every other day for 5 days did not alter (P > 0.1) plasma leptin concentrations or the leptin response to DEX. In conclusion, multiple injections of DEX increase leptin concentrations in stallions, as does a single injection in mares (as low as 15.6 microg/kg BW), geldings and stallions. The greater leptin levels observed in mares and geldings relative to stallions were due partially to their greater body condition and partially to the presence of hyperleptinemic individuals; however, even after accounting for body condition and diet, mares still had greater leptin concentrations than stallions after DEX administration. Elevation of testosterone levels in mares for approximately 10 days did not alter leptin concentrations in mares.